US20160315548A1 - Improvements in or relating to converters for use in high voltage direct current power transmission - Google Patents
Improvements in or relating to converters for use in high voltage direct current power transmission Download PDFInfo
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- US20160315548A1 US20160315548A1 US15/103,822 US201415103822A US2016315548A1 US 20160315548 A1 US20160315548 A1 US 20160315548A1 US 201415103822 A US201415103822 A US 201415103822A US 2016315548 A1 US2016315548 A1 US 2016315548A1
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- converter
- limb
- limb portion
- current
- controller
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M5/00—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases
- H02M5/40—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc
- H02M5/42—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters
- H02M5/44—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac
- H02M5/453—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a triode or transistor type requiring continuous application of a control signal
- H02M5/458—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/483—Converters with outputs that each can have more than two voltages levels
- H02M7/4835—Converters with outputs that each can have more than two voltages levels comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
- H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
- H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
- H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
- H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/79—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/797—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/0003—Details of control, feedback or regulation circuits
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/0003—Details of control, feedback or regulation circuits
- H02M1/0025—Arrangements for modifying reference values, feedback values or error values in the control loop of a converter
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- H02M2001/0003—
Definitions
- This invention relates to a converter for use in high voltage direct current power transmission.
- alternating current (AC) power is typically converted to direct current (DC) power for transmission via overhead lines and/or under-sea cables.
- DC direct current
- This conversion removes the need to compensate for the AC capacitive load effects imposed by the power transmission medium, i.e. the transmission line or cable, and reduces the cost per kilometre of the lines and/or cables, and thus becomes cost-effective when power needs to be transmitted over a long distance.
- the conversion between DC power and AC power is utilized in power transmission networks where it is necessary to interconnect the DC and AC electrical networks.
- converters are required at each interface between AC and DC power to effect the required conversion; AC to DC or DC to AC.
- a converter for use in high voltage direct current power transmission, comprising:
- Sequentially routing, via each fully-conducting converter limb in turn, a DC current demand between the first and second DC terminals allows the DC current demand to flow continuously between the first and second DC terminals, and thereby permits the converter of the invention to continuously exchange power with a DC network, connected in use with the first and second DC terminals, throughout an operating cycle of the converter.
- Such passive inductors are physically very large, and so omitting them allows for a significant reduction, e.g. of around 20%, in the overall footprint of a converter station including the converter of the invention. This in turn helps to reduce considerably the cost of the converter station.
- omitting the passive inductor from each limb portion also means that it is possible to connect a transformer directly with the converter without the need for interconnecting bushings which are typically large and expensive, and so offers further space- and cost-saving opportunities.
- omitting an inductor from each limb portion allows for an increase in the level of AC voltage that can be generated by the converter, while permitting a reduction in the level of AC current, i.e. the level of individual AC current demand phase waveforms, that must be provided, and so gives rise to an increase in the efficiency of the converter.
- the first controller is programmed to sequentially define fully-conducting converter limbs at regular intervals of around 60 electrical degrees.
- the first controller is additionally programmed, while selectively operating for a given converter limb the primary switching element in each limb portion thereof to simultaneously switch both the first and second limb portions into circuit and thereby define a fully-conducting converter limb, to concurrently operate for each other converter limb the primary switching element in one or both limb portions thereof to switch a single limb portion into circuit and thereby define a partially-conducting converter limb to direct respective AC current demand phase waveforms towards a given AC terminal whereby the respective AC current demand phase waveforms sum to zero.
- Carrying out the aforementioned mathematical optimization i.e. selecting the best individual limb portion current (with regard to chosen criteria) from a set of available alternatives, allows the AC and DC current demands to be controlled independently of one another, e.g. by a higher level controller.
- the second controller is able to carry out steps (a) and (b) in real time so as to permit robust control of the converter of the invention.
- the second controller may be programmed to carry out mathematical optimization by creating an equivalent converter configuration which represents the flow of current through the converter.
- the second controller is programmed to create an equivalent converter configuration which represents the flow of current through the converter by mapping possible current flow paths through the converter.
- Mapping the possible current flow paths through the converter helps the second controller to tailor the mathematical optimization it provides to the topology, i.e. structure, of the converter of the invention.
- the second controller is programmed to carry out mathematical optimization by applying a current weighting to the relative current contribution provided by a plurality of limb portions.
- the second controller may be programmed to determine the or each weighting according to measured operating parameters of the converter.
- Determining the weightings in the aforementioned manner allows the second controller to take into account environmental factors which might affect the healthy operation of the converter, and to alter the optimal limb portion currents that are determined in an effort to overcome the environmental factors and alleviate the associated impact on the operation of the converter.
- environmental factors include the components in one limb portion running hot, or a limb portion suffering component damage or failure such that its performance is degraded.
- the second controller when controlling the converter under a particular operating condition, is programmed to apply a weighting by applying a different weighting to at least one limb portion such that the or each said limb portion provides a different contribution to the other limb portions.
- Such a feature allows the second controller to distinguish between one limb portion and another, e.g. according to how well a given limb portion is performing.
- the second controller is programmed to carry out mathematical optimization to determine one or more minimum individual limb portion currents that the corresponding limb portion must contribute to track the corresponding required AC current demand phase waveform and the required DC current demand.
- Determining one or more minimum individual limb portion currents reduces the conduction and switching losses in each limb portion because ordinarily such losses are proportional to current squared, i.e. I 2 .
- the second controller is further programmed to carry out mathematical optimization to provide optimal limb portion voltage sources.
- FIG. 1 shows a schematic view of a converter according to a first embodiment of the invention
- FIG. 2 shows a preferred switching sequence of primary switching elements within the converter shown in FIG. 1 during an operating cycle of the converter
- FIG. 3 illustrates schematically the selective definition of a fully-conducting converter limb and respective partially-conducting converter limbs in the converter shown in FIG. 1 ;
- FIG. 4 shows a flow diagram that illustrates the principal steps a second controller in the converter shown in FIG. 1 is programmed to carry out;
- FIG. 5 shows a schematic representation of an equivalent converter configuration corresponding to the converter shown in FIG. 1 ;
- FIG. 6( a ) shows a flow diagram that illustrates the principal steps a further second controller may be programmed to carry out
- FIG. 6( b ) shows a schematic view of a feedback loop which forms a part of the principal steps the further controller may be programmed to carry out.
- a converter according to a first embodiment of the invention is designated generally by reference numeral 10 , as shown in FIG. 1 .
- the converter 10 includes three converter limbs 12 A, 12 B, 12 C, each of which corresponds to a respective phases A, B, C of the converter 10 .
- Each converter limb 12 A, 12 B, 12 C extends between first and second DC terminals 14 , 16 , and each converter limb 12 A, 12 B, 12 C includes a first limb portion 12 A+, 12 B+, 12 C+ and a second limb portion 12 A ⁇ , 12 B ⁇ , 12 C ⁇ which are separated by an AC terminal 18 A, 18 B, 18 C.
- the first and second DC terminals 14 , 16 are connected to a DC network 20 , with the first DC terminal 14 carrying a voltage of V DC+ and the second DC terminal 16 carrying a voltage of V DC ⁇ , while the AC terminal 18 A, 18 B, 18 C is connected to a corresponding phase A, B, C of a three-phase AC network 22 and carries a corresponding AC voltage phase waveform V A , V B , V C .
- Each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ includes a chain-link converter 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ that includes a chain of modules 26 connected in series.
- the number of modules 26 in each chain-link converter 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ depends on the required voltage rating of the respective limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ .
- Each module 26 of each chain-link converter 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ includes two pairs of secondary switching elements (not shown) connected in parallel with an energy storage device, in the form of a capacitor (not shown), to define a 4-quadrant bipolar module 26 that can provide negative, zero or positive voltage and can conduct current in two directions.
- the secondary switching elements of the modules 26 of each chain-link converter 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ are operated to enable each chain-link converter 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ to provide a stepped variable voltage source.
- the secondary switching elements are also desirably switched at near to the fundamental frequency of the AC network 22 .
- each module 26 may be bypassed or inserted into the respective chain-link converter 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ by changing the state of the secondary switching elements.
- each module 26 is bypassed when the pairs of secondary switching elements are configured to form a short circuit in the module 26 . This causes current in the corresponding limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ of the converter 10 to pass through the short circuit and bypass the capacitor, and so the module 26 is able to provide a zero voltage.
- each module 26 is inserted into the respective chain-link converter 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ when the pair of secondary switching elements is configured to allow the aforementioned current to flow into and out of the capacitor.
- the capacitor is then able to charge or to discharge its stored energy so as to provide a voltage.
- the bidirectional nature of the 4-quadrant bipolar module 26 means that the capacitor may be inserted into the module 26 in either forward or reverse directions so as to provide a positive or negative voltage.
- a 4-quadrant bipolar module 26 to provide positive or negative voltages means that the voltage across each chain-link converter 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ may be built up from a combination of modules 26 providing positive or negative voltages. The energy levels in individual capacitors may be maintained therefore at optimal levels by controlling the modules 26 to alternate between providing positive or negative voltage.
- insertion and/or bypass of the capacitors of individual modules 26 in the chain-link converter 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ results in the generation of a voltage waveform at a corresponding AC terminal 18 A, 18 B, 18 C.
- insertion of the capacitors of the individual modules 26 may be staggered to generate a sinusoidal waveform.
- Other waveform shapes may be generated by adjusting the timing of switching operations for each module 26 in the chain-link converter 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ .
- chain-link converters 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ are able to facilitate power transfer between the AC and DC networks 22 , 20 .
- each chain-link converter 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ it is possible for each chain-link converter 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ to block a DC fault current in the event of e.g. a pole to pole fault in the DC network 20 , with such blocking being achieved by opening appropriate secondary switching elements in each module 26 to prevent the flow of fault current through each said module 26 .
- Each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ also includes a primary switching element Sw 1 , Sw 4 , Sw 3 , Sw 6 , Sw 5 , Sw 2 that is connected in series with the corresponding chain-link converter 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ .
- each primary switching element Sw 1 , Sw 2 , Sw 3 , Sw 4 , Sw 5 , Sw 6 selectively switches the respective limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ into and out of circuit within the converter 10 .
- each primary switching element may include a plurality of, e.g. series-connected, switching elements depending on the required voltage rating of each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ .
- each primary switching element Sw 1 , Sw 2 , Sw 3 , Sw 4 , Sw 5 , Sw 6 and the corresponding chain-link converter 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ in each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ allows, in other embodiments of the invention, the respective primary switching element Sw 1 , Sw 2 , Sw 3 , Sw 4 , Sw 5 , Sw 6 and corresponding chain-link converter 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ to be connected in a reverse order between the corresponding AC terminal 18 A, 18 B, 18 C and the respective first or second DC terminal 14 , 16 .
- each primary switching element Sw 1 , Sw 2 , Sw 3 , Sw 4 , Sw 5 , Sw 6 and each of the secondary switching elements in the respective chain-link converters 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ is an insulated gate bipolar transistor (IGBT) 28 connected in parallel with an anti-parallel diode 30 .
- IGBT insulated gate bipolar transistor
- one or more of the primary and secondary switching elements Sw 1 , Sw 2 , Sw 3 , Sw 4 , Sw 5 , Sw 6 may include a different semiconductor device, such as a field effect transistor, a gate-turn-off thyristor, an injection gate enhanced thyristor, an integrated gate commutated transistor or another externally-commutated semiconductor switch, i.e. a semiconductor switch which is turned off by one or more external components causing the current flowing through the semiconductor switch to fall to zero.
- Such other externally-commutated semiconductor switches can include so-called ‘forced commutated’ and ‘self commutated’ semiconductor switches.
- the semiconductor device is preferably connected in parallel with an anti-parallel diode.
- each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ omits any form of physical, passive inductor component, which in turns provides considerable benefits in terms of reducing the overall footprint of a resulting converter station in which the converter of the invention is incorporated.
- the converter 10 includes a first controller 32 that is arranged in operative communication with each primary switching element Sw 1 , Sw 2 , Sw 3 , Sw 4 , Sw 5 , Sw 6 .
- the first controller 32 is a programmable device, such as a microcontroller, and more particularly is programmed to operate, for one converter limb 12 A, 12 B, 12 C at a time, the primary switching element Sw 1 , Sw 2 , Sw 3 , Sw 4 , Sw 5 , Sw 6 in each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ thereof to simultaneously switch both the first limb portions 12 A+, 12 B+, 12 C+ and the second limb portions 12 A ⁇ , 12 B ⁇ , 12 C ⁇ into circuit and thereby define a fully conducting converter limb 12 A, 12 B, 12 C.
- the primary switching element Sw 1 , Sw 2 , Sw 3 , Sw 4 , Sw 5 , Sw 6 in each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ thereof to simultaneously switch both the first limb portions 12 A+, 12 B+, 12 C+ and the second limb portions 12 A ⁇ , 12 B ⁇ , 12 C
- the first controller 32 sequentially routes, via each said fully-conducting converter limb 12 A, 12 B, 12 C, a DC current demand I DC (i.e. the DC current that the converter limbs 12 A, 12 B, 12 C are required to track) between the first and second DC terminals 14 , 16 .
- I DC i.e. the DC current that the converter limbs 12 A, 12 B, 12 C are required to track
- the first controller 32 is programmed to sequentially define fully-conducting converter limbs 12 A, 12 B, 12 C at regular intervals 34 1 , 34 2 , 34 3 , 34 4 , 34 5 , 34 6 of around 60 electrical degrees.
- each interval is 60 electrical degrees, although for practical implementation purposes each interval can lie in the ranges 60 ⁇ 1 electrical degrees, or 60 ⁇ 2 electrical degrees.
- the first controller 32 is also additionally programmed, while selectively operating for a given converter limb 12 A, 12 B, 12 C the primary switching element Sw 1 , Sw 2 , Sw 3 , Sw 4 , Sw 5 , Sw 6 in each limb portion thereof 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ to simultaneously switch both the first and second limb portions 12 A+, 12 B+, 12 C+, 12 A ⁇ , 12 B ⁇ , 12 C ⁇ into circuit and thereby define a fully-conducting converter limb 12 A, 12 B, 12 C, to concurrently operate for each other converter limb 12 A, 12 B, 12 C the primary switching element Sw 1 , Sw 2 , Sw 3 , Sw 4 , Sw 5 , Sw 6 in one or both limb portions 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ thereof to switch a single limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ into circuit and thereby
- the first controller 32 is programmed to direct respective AC current demand phase waveforms I A , I B , I C , i.e. respective AC phase currents that the converter 10 is required to track, towards a given AC terminal 18 A, 18 B, 18 C whereby the AC current demand phase waveforms I A , I B , I C sum to zero.
- FIG. 2 shows one example switching sequence of the primary switching elements Sw 1 , Sw 2 , Sw 3 , Sw 4 , Sw 5 , Sw 6 which is implemented by the first controller 32 .
- Other switching sequences may, however, also be implemented.
- the first controller 32 utilises a phase locked loop (PLL) control scheme to coordinate the switching sequence with respective AC voltage phase waveforms V A , V B , V C of the AC network 22 .
- PLL phase locked loop
- the first controller 32 operates the primary switching element Sw 1 , Sw 4 in each limb portion 12 A+, 12 A ⁇ of a first converter limb 12 A to simultaneously switch both the first and second limb portions 12 A+, 12 A ⁇ into circuit and thereby define a fully-conducting converter limb 12 A.
- the first controller 32 operates the primary switching element Sw 6 in the second limb portion 12 B ⁇ of a second converter limb 12 B to switch the second limb portion 12 B ⁇ into circuit and thereby define a partially-conducting converter limb 12 B.
- the primary switching element Sw 3 in the first limb portion 12 B+ of the second converter limb 12 B remains switched out of circuit.
- the first controller 32 also, at the same time, operates the primary switching element Sw 5 in the first limb portion 12 C+ of a third converter limb 12 C to switch the first limb portion 12 C+ into circuit and thereby define another partially-conducting converter limb 12 C.
- the primary switching element Sw 2 in the second limb portion 12 C ⁇ of the third converter limb 12 C remains switched out of circuit.
- the first controller 32 leaves the first limb portion 12 C+ of the third converter limb 12 C switched into circuit while operating the primary switching element Sw 2 in the second limb portion 12 C ⁇ of the third converter limb 12 C to switch the second limb portion 12 C ⁇ into circuit and thereby define a fully-conducting converter limb 12 C.
- the first controller leaves the first limb portion 12 A+ of the first converter limb 12 A switched into circuit while operating the switching element Sw 4 in the second limb portion 12 A ⁇ to switch the second limb portion 12 A ⁇ out of circuit and thereby define a partially-conducting converter limb 12 A.
- the first controller 32 also, at the same time, leaves the second limb portion 12 B ⁇ of the second converter limb 12 B switched into circuit to continue to define a partially-conducting converter limb 12 B.
- the primary switching element Sw 3 in the first limb portion 12 B+ of the second converter limb 12 B remains switched out of circuit.
- the first controller 32 operates the primary switching element Sw 3 in the first limb portion 12 B+ of the second converter limb 12 B to switch the first limb portion 12 B+ into circuit and thereby define, along with the already switched into circuit second limb portion 12 B ⁇ , a fully-conducting converter limb 12 B.
- the first controller 32 continues to leave the first limb portion 12 A+ of the first converter limb 12 A switched into circuit and the second limb portion 12 A ⁇ of the second limb portion 12 A ⁇ switched out of circuit to continue to define a partially-conducting converter limb 12 A.
- the first controller 32 also, at the same time, leaves the second limb portion 12 C ⁇ of the third converter limb 12 C switched into circuit while operating the switching element Sw 5 in the first limb portion 12 C+ of the third converter limb 12 C to switch the first limb portion 12 C+ out of circuit to thereby define a partially-conducting converter limb 12 C.
- the first controller 32 operates the primary switching element Sw 4 in the second limb portion 12 A ⁇ of the first converter limb 12 A to switch the second limb portion 12 A ⁇ into circuit and thereby define, along with the already switched into circuit first limb portion 12 A+, a fully-conducting converter limb 12 A, as shown by way of example in FIG. 3 .
- the aforesaid fully-conducting converter limb 12 A routes a DC current demand I DC between the first and second DC terminals 14 , 16 .
- the first controller 32 continues to leave the first limb portion 12 B+ of the second converter limb 12 B switched into circuit while operating the switching element SW 6 of the second limb portion 12 B ⁇ of the second converter limb 12 B to switch the second limb portion 12 B ⁇ out of circuit to thereby define a partially-conducting converter limb 12 B, as also shown in FIG. 3 .
- the partially-conducting converter limb 12 B i.e. the first limb portion 12 B+ thereof, directs an AC current demand phase waveform I B , i.e. an AC phase current I B , towards a first AC terminal 18 A.
- the first controller 32 also, at the same time, leaves the first limb portion 12 C+ of the third converter limb 12 C switched out of circuit and the second limb portion 12 C ⁇ of the third converter limb 12 C switched into circuit to continue to define a partially-conducting converter limb 12 C, as again shown in FIG. 3 .
- the partially conducting converter limb 12 C i.e. the second limb portion 12 C ⁇ thereof, also directs an AC current demand phase waveform I C , i.e. an AC phase current I C , towards the first AC terminal 18 A.
- Each of the aforementioned AC current demand phase waveforms I B , I C , together with a further AC current demand phase waveform I A sum to zero at the first AC terminal 18 A, and thereby cancel one another out such that they do not impact adversely on the quality, i.e. smoothness, of the DC current demand I DC routed between the first and second DC terminals 14 , 16 .
- the first controller 32 operates the primary switching element Sw 5 in the first limb portion 12 C+ of the third converter limb 12 C to switch the first limb portion 12 C+ into circuit and thereby define, along with the already switched into circuit second limb portion 12 C ⁇ , a fully-conducting converter limb 12 C.
- the first controller 32 continues to leave the second limb portion 12 A ⁇ of the first converter limb 12 A switched into circuit while operating the switching element Sw 1 of the first limb portion 12 A+ of the first converter limb 12 A to switch the first limb portion 12 A+ out of circuit to thereby define a partially-conducting converter limb 12 A.
- the first controller 32 also, at the same time, leaves the first limb portion 12 B+ of the second converter limb 12 B switched into circuit and the second limb portion 12 B ⁇ of the second converter limb 12 B switched out of circuit to continue to define a partially-conducting converter limb 12 B.
- the first controller 32 operates the primary switching element Sw 6 in the second limb portion 12 B ⁇ of the second converter limb 12 B to switch the second limb portion 12 B ⁇ into circuit and thereby define, along with the already switched into circuit first limb portion 12 B+, a fully-conducting converter limb 12 B.
- the first controller 32 continues to leave the second limb portion 12 A ⁇ of the first converter limb 12 A switched into circuit and the first limb portion 12 A+ switched out of circuit to continue to define a partially-conducting converter limb 12 A.
- the first controller 32 also, at the same time, leaves the first limb portion 12 C+ of the third converter limb 12 C switched into circuit while operating the switching element Sw 2 of the second limb portion 12 C ⁇ of the third limb portion 12 C to switch the second limb portion 12 C ⁇ out of circuit and thereby define a partially-conducting converter limb 12 C.
- the first controller 32 sequentially defines single, individual first, second, third, fourth, fifth and sixth fully-conducting converter limbs 12 A, 12 C, 12 B, 12 A, 12 C, 12 B during corresponding first, second, third, fourth, fifth and sixth intervals 34 1 , 34 2 , 34 3 , 34 4 , 34 5 , 34 6 of a complete operating cycle 36 of the converter 10 .
- the converter 10 also includes a second controller 38 that is arranged in communication with the first controller 32 and each of the chain-link converters 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ .
- the second controller 38 is similarly a programmable device, such as a microcontroller.
- the first and second controllers 32 , 38 are shown as separate items, they may in other embodiments of the invention form individual parts or a single part of a larger controller or controller arrangement.
- the second controller 38 is programmed to:
- the second controller 38 is also further programmed to (c) carry out mathematical optimization to provide optimal limb portion voltage sources V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ , with these principal steps (a), (b) and (c) being illustrated in a first flow diagram 40 shown in FIG. 4 .
- the second controller 38 is programmed to first obtain a respective AC current demand phase waveform I A , I B , I C for each converter limb 12 A, 12 B, 12 C which each converter limb 12 A, 12 B, 12 C is required to track, and then obtain a DC current demand I DC which the converter limbs 12 A, 12 B, 12 C are also required to track.
- the various AC current demand phase waveforms I A , I B , I C and the DC current demand I DC may be obtained directly from a higher-level controller (not shown) within a converter or from some other external entity. Alternatively the converter 10 may obtain it directly by carrying out its own calculations.
- the second controller 38 is also programmed to, as a second step (and as indicated by a first process box 42 in the first flow diagram 40 ), carry out mathematical optimization to determine an optimal limb portion current I A+ , I A ⁇ , I B+ , I B ⁇ , I C+ , I C ⁇ for each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ that the limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ must contribute to track the corresponding required AC current demand phase waveform I A , I B , I C and the required DC current demand I DC .
- the second controller 38 is programmed to carry out such mathematical optimization by creating an equivalent converter configuration 100 , as shown in FIG. 5 , which represents the flow of current through the corresponding converter 10 of the invention.
- the equivalent converter configuration 100 includes similar features to the converter 10 of the invention and these like features share the same reference numerals. To that end the equivalent converter configuration 100 includes three converter limbs 12 A, 12 B, 12 C, each of which corresponds to a respective phase A, B, C of the converter 10 of the invention.
- each converter limb 12 A, 12 B, 12 C similarly extends between first and second DC terminals 14 , 16 , and each converter limb 12 A, 12 B, 12 C includes a first limb portion 12 A+, 12 B+, 12 C+ and a second limb portion 12 A ⁇ , 12 B ⁇ , 12 C ⁇ .
- Each pair of first and second limb portions 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ in each converter limb 12 A, 12 B, 12 C is separated by a corresponding AC terminal 18 A, 18 B, 18 C.
- the equivalent converter configuration 100 also represents the respective AC current demand phase waveforms I A , I B , I C that each converter limb 12 A, 12 B, 12 C is required to track, e.g. match as closely as possible, and the DC current demand I DC that the converter limbs 12 A, 12 B, 12 C are also required to track.
- each converter limb 12 A, 12 B, 12 C also operates within the constraints of the corresponding AC voltage phase waveforms V A , V B , V C of the AC network 22 , as well as a DC voltage V DC of the DC electrical network 20 , to which the converter 10 is, in use, connected, and so the equivalent converter configuration 100 may also represent these elements.
- the second controller 38 is programmed to create an equivalent converter configuration 100 which represents the flow of current through the converter 10 by mapping possible current flow paths through the converter 10 .
- I A ⁇ A + I A+ ⁇ A ⁇ I A ⁇
- I B ⁇ B + I B+ ⁇ B ⁇ I B ⁇
- I C ⁇ C + I C+ ⁇ C ⁇ I C ⁇
- I DC+ ⁇ A + I A+ + ⁇ B + I B+ + ⁇ C + I C+
- I DC ⁇ ⁇ A ⁇ I A ⁇ + ⁇ B ⁇ I B ⁇ + ⁇ C ⁇ I C ⁇
- I A ⁇ A + I A+ ⁇ A ⁇ I A ⁇
- I B ⁇ B + I B+ ⁇ B ⁇ I B ⁇
- I C ⁇ C + I C+ ⁇ C ⁇ I C ⁇
- I DC ⁇ A + I A+ + ⁇ B + I B+ + ⁇ C + I C+
- A is a matrix which maps the possible current flow paths provided by the limb portions 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ .
- the second controller 38 is also programmed to carry out mathematical optimization by applying a current weighting to the relative current contribution provided by each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ .
- the respective current weighting for each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ is determined according to measured operating parameters of the converter 10 during its operation.
- the various current weightings can be determined throughout operation of the said converter 10 so as to permit an updating of the current weightings, e.g. in response to changing environmental conditions. As a result the various current weightings can vary as the converter 10 operates.
- a different current weighting may be applied to the current contribution, i.e. the limb portion current I A+ , I A ⁇ , I B+ , I B ⁇ , I C+ , I C ⁇ , provided by at least one limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ .
- a larger current weighting may be applied to the optimal limb portion current I A+ , I A ⁇ , I B+ , I B ⁇ , I C+ , I C ⁇ that a particular limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C must contribute, so as to reduce an actual limb portion current that the said limb portion contributes relative to an actual current contribution of each of the other limb portions, which are otherwise all the same as one another.
- the second controller 38 is programmed to carry out mathematical optimization to determine a minimum individual limb portion current I A+ , I A ⁇ , I B+ , I B ⁇ , I C+ , I C ⁇ that each of the limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ must contribute so as to track the corresponding required AC current demand phase waveform I A , I B , I C and the required DC current demand I DC -One way in which minimum individual limb portion currents I A+ , I A ⁇ , I B+ , I B ⁇ , I C+ , I C ⁇ , i.e.
- the current weighting matrix Q 1 is determined according to measured operating parameters of the converter 10 , and may be so determined throughout the operation of the converter 10 , such that it can vary as the said converter 10 is controlled in response to changes in the operation of the converter 10 .
- the Lagrangian (or the method of Lagrange multipliers) is a technique for solving the above-identified nonlinear optimization in order to find local minima of the current objective function J Current . It may also be solved using other optimization algorithms, including iterative and programming algorithms.
- the aforementioned nonlinear optimization could additionally include one or more inequality constraints in which case it could be solved by using the further method of Hamiltonian (Pontryagin's minimum principle).
- the minimum individual limb portion currents I A+ , I A ⁇ , I B+ , I B ⁇ , I C+ , I C ⁇ may also be determined by solving a nonlinear optimization of the form
- the second controller 38 is also programmed to carry out mathematical optimization to provide an optimal limb portion voltage source V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ for each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ to achieve the corresponding mathematically optimized minimum limb portion current I A+ , I A ⁇ , I B+ , I B ⁇ , I C+ , I C ⁇ .
- such mathematical optimization of the limb portion voltage sources need not take place.
- the second controller 38 is programmed to carry out mathematical optimization to provide optimal limb portion voltage sources V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ by creating an equivalent converter configuration 100 which represents voltage conditions in the converter 10 .
- Representing the voltage conditions in the converter 10 portrayed in the equivalent converter configuration 100 additionally includes mapping a limb portion voltage source V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ and an inductive component for each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ .
- each limb portion voltage source V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ corresponds to a respective chain-link converter 24 A+, 24 A ⁇ , 24 B+, 24 B ⁇ , 24 C+, 24 C ⁇ which is switchable into and out of the corresponding limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ via operation of the corresponding primary switching element Sw 1 , Sw 2 , Sw 3 , Sw 4 , Sw 5 , Sw 6 .
- Each limb portion voltage source V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ is therefore variable in magnitude between zero (i.e. equivalent to being switched out of the corresponding limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ ) and an upper voltage limit.
- an inductive component for each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ within the equivalent converter configuration 100 represents the inductance associated with the corresponding limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ of the actual converter 10 .
- Such inductances do not include a passive limb inductor within each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ since they are no longer needed to control the level of circulating current between the converter limbs 12 A, 12 B, 12 C.
- the respective inductances take the form of a phase inductance 44 A, 44 B, 44 C and a DC line inductance 46 (each of which may be made up of a physical passive inductor component and any stray inductance within the associated electrical structure of the converter), and a very small remaining stray inductance within the corresponding limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ .
- each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ is represented in the equivalent converter configuration 100 as an inductive voltage portion U A+ , U A ⁇ , U B+ , U B ⁇ , U C+ , U C ⁇ that is made up of the voltage arising from the flow of current through the aforementioned inductance, i.e. phase inductance 44 A, 44 B, 44 C and DC line inductance 46 only, associated with a corresponding limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ .
- Such a resistive component represents the resistance associated with a given limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ , and similarly may take the form of a resistor within a given limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ , i.e. a limb portion resistance, or the form of a resistance electrically associated with a given limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ , e.g. a phase resistance and/or a DC line resistance.
- M V is a matrix which maps the position of the limb portion voltage sources V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ within the particular converter structure;
- M u is a matrix which maps the position of the inductive voltage portions U A+ , U A ⁇ , U B+ , U B ⁇ , U C+ , U C ⁇ within the particular converter structure;
- the second controller 38 is further programmed to carry out mathematical optimization to provide an optimal limb portion voltage source V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ for each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ by reducing any deviation in an actual measured limb portion current I′ A+ , I′ A ⁇ , I′ B+ , I′ B ⁇ , I′ C+ , I′ C ⁇ of a given limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ from the corresponding determined optimal limb portion current I A+ , I A ⁇ , I B+ , I B ⁇ , I C+ , I C ⁇ for the said given limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ .
- the second controller 38 is still further programmed to calculate the inductive voltage portion U A+ , U A ⁇ , U B+ , U B ⁇ , U C+ , U C ⁇ for each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ . This calculation is based on the corresponding determined optimal limb portion currents I A+ , I A ⁇ , I B+ , I B ⁇ , I C+ , I C ⁇ , together with the inductance associated with the corresponding limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ .
- the calculated inductive voltage portion U A+ , U A ⁇ , U B+ , U B ⁇ , U C+ , U C ⁇ is modified to drive the actual measured limb portion current I′ A+ , I′ A ⁇ , I′ B+ , I′ B ⁇ , I′ C+ , I′ C ⁇ to follow the corresponding determined optimal limb portion current I A+ , I A ⁇ , I B+ , I B ⁇ , I C+ , I C ⁇ .
- Such measuring and modification takes the form of a feedback loop which provides closed-loop control, as illustrated schematically by a second process box 48 in the first flow diagram 40 shown in FIG. 4 .
- the feedback loop may additionally include a feed-forward element which seeks to predict desirable future values for one or more of the inductive voltage portions U A+ , U A ⁇ , U B+ , U B ⁇ , U C+ , U C ⁇ in order to improve the performance of the closed-loop control.
- the calculated inductive voltage portion U A+ , U A ⁇ , U B+ , U B ⁇ , U C+ , U C ⁇ of each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ is utilised, as indicated by a third process box 50 in the first flow diagram 40 , when carrying out the aforementioned mathematical optimization to provide the optimal limb portion voltage sources V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ .
- Such mathematical optimization also includes applying a voltage weighting to the relative voltage contribution provided by each limb portion voltage source V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ .
- the voltage weightings are determined according to measured operating parameters of the converter 10 , and may be so determined throughout operation of the said converter 10 . Such potentially repeated determination of the voltage weightings permits the ongoing optimization of the converter operation during, e.g. changing environmental conditions.
- a different voltage weighting can be applied to the limb portion voltage source V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ of one or more limb portions 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ to further alleviate the impact of, e.g. the abnormal operating conditions.
- the second controller 38 is programmed to carry out mathematical optimization to provide an optimal limb portion voltage source V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ for each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ by determining a minimum individual limb portion voltage source V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ for each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ that is required to achieve the corresponding minimum limb portion current I A+ , I A ⁇ , I B+ , I B ⁇ , I C+ , I C ⁇ previously determined.
- V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ i.e. the minimum level of voltage a variable voltage source within a given limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ must provide
- x is the transpose of [V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ ]
- the voltage weighting matrix Q V is similarly determined according to measured operating parameters of the converter 10 , and may be so determined throughout the operation of the converter 10 . As such it too can vary as the said converter 10 is controlled.
- An alternative second controller (not shown), which may instead be included in the converter 10 , is similarly programmed to:
- the alternative second controller is programmed to apply a control algorithm to directly establish optimal limb portion voltage sources V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ from each of the corresponding determined minimum limb portion current I A+ , I A ⁇ , I B+ , I B ⁇ , I C+ , I C ⁇ , i.e. as shown by a single fourth process box 62 in the second flow diagram 60 .
- Applying such a control algorithm includes reducing any deviation in the actual measured limb portion current I′ A+ , I′ A ⁇ , I′ B+ , I′ B ⁇ , I′ C+ , I′ C ⁇ of a given limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ from the corresponding determined minimum limb portion current I A+ , I A ⁇ , I B+ , I B ⁇ , I C+ , I C ⁇ for the said given limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ .
- the feedback loop 70 compares a respective actual measured limb portion current I′ A+ , I′ A ⁇ , I′ B+ , I′ B ⁇ , I′ C+ , I′ C ⁇ with the corresponding determined minimum limb portion current I A+ , I A ⁇ , I B+ , I B ⁇ , I C+ , I C ⁇ and calculates a corresponding limb portion error e A+ , e A ⁇ , e B+ , e B ⁇ , e C+ , e C ⁇ .
- the feedback loop 70 then applies a correction factor K to each limb portion error e A+ , e A ⁇ , e B+ , e B ⁇ , e C+ , e C ⁇ to thereby establish directly the corresponding limb portion voltage source V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ which is required to drive the error e A+ , e A ⁇ , e B+ , e B ⁇ , e C+ , e C ⁇ towards zero.
- the correction factor K may take the form of a control system matrix, such as a gain matrix (not shown), which sets out individual correction factors that each limb portion error e A+ , e A ⁇ , e B+ , e B ⁇ , e C+ , e C ⁇ is, e.g. multiplied by in the case of a gain matrix, to establish the corresponding limb portion voltage source V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ .
- a control system matrix such as a gain matrix (not shown)
- One way in which such individual correction factors may be established is by creating an equivalent converter configuration that represents the voltage conditions in the particular three-phase converter structure under control and thereafter considering the dynamics of such an equivalent converter configuration.
- the foregoing steps may be achieved by creating the equivalent converter configuration 100 shown in FIG. 5 and mapping a limb portion voltage source V A+ , V A ⁇ , V B+ , V B ⁇ , V C+ , V C ⁇ and an inductive component for each limb portion 12 A+, 12 A ⁇ , 12 B+, 12 B ⁇ , 12 C+, 12 C ⁇ in the equivalent converter configuration 100 .
- mapping may include conducting a Kirchhoff analysis of the equivalent converter configuration 100 (although other equivalent converter configurations and corresponding analysis techniques are also possible) that applies Kirchhoff's current and voltage laws to describe the dynamics of the equivalent converter configuration 100 as:
- M 10 - 3 ⁇ ( 3 - 2 1 0 1 - 1 2 1 0 1 1 0 3 - 2 1 1 0 - 1 2 1 - 1 2 1 3 - 2 3 - 2 1 ) ;
- N 1 6 ⁇ [ 3 - 4 2 3 4 - 2 3 2 2 3 - 2 - 2 3 2 - 4 3 - 2 4 ] ;
- the feedback loop 70 involves minimal computational effort since in each cycle it is simply required to multiply a given limb portion error e A+ , e A ⁇ , e B+ , e B ⁇ , e C+ , e C ⁇ by the corresponding individual correction factor which has already been determined.
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Abstract
In the field of high voltage direct current (HVDC) power transmission, a converter comprises three converter limbs, each corresponding to a respective phase of the converter, each extending between first and second DC terminals and each including first and second limb portions separated by an AC terminal. Each limb portion includes a chain-link converter that is operable to provide a stepped variable voltage source, and a primary switching element to selectively switch the respective limb portion into and out of circuit. The converter also includes a first controller that is programmed to selectively operate for one converter limb at a time the primary switching element in each limb portion thereof to simultaneously switch both the first and second limb portions into circuit and thereby define a fully-conducting converter limb to sequentially route via each said fully-conducting converter limb a DC current demand (IDC) between the first and second DC terminals.
Description
- This invention relates to a converter for use in high voltage direct current power transmission.
- In high voltage direct current (HVDC) power transmission networks alternating current (AC) power is typically converted to direct current (DC) power for transmission via overhead lines and/or under-sea cables. This conversion removes the need to compensate for the AC capacitive load effects imposed by the power transmission medium, i.e. the transmission line or cable, and reduces the cost per kilometre of the lines and/or cables, and thus becomes cost-effective when power needs to be transmitted over a long distance.
- The conversion between DC power and AC power is utilized in power transmission networks where it is necessary to interconnect the DC and AC electrical networks. In any such power transmission network, converters are required at each interface between AC and DC power to effect the required conversion; AC to DC or DC to AC.
- According to a first aspect of the invention there is provided a converter, for use in high voltage direct current power transmission, comprising:
-
- three converter limbs, each corresponding to a respective phase of the converter, each extending between first and second DC terminals and each including first and second limb portions separated by an AC terminal, each limb portion including a chain-link converter operable to provide a stepped variable voltage source and a primary switching element to selectively switch the respective limb portion into and out of circuit; and
- a first controller programmed to selectively operate for one converter limb at a time the primary switching element in each limb portion thereof to simultaneously switch both the first and second limb portions into circuit and thereby define a fully-conducting converter limb to sequentially route via each said fully-conducting converter limb a DC current demand between the first and second DC terminals.
- Sequentially routing, via each fully-conducting converter limb in turn, a DC current demand between the first and second DC terminals allows the DC current demand to flow continuously between the first and second DC terminals, and thereby permits the converter of the invention to continuously exchange power with a DC network, connected in use with the first and second DC terminals, throughout an operating cycle of the converter.
- Meanwhile, having only one fully-conducting converter limb at a time avoids the creation of current paths between respective converter limbs, and so no transient circulating currents between converter limbs arise. The absence of such circulating currents permits the removal of a passive inductor from each limb portion that would otherwise be needed to limit the level of aforementioned circulating current between the converter limbs.
- Such passive inductors are physically very large, and so omitting them allows for a significant reduction, e.g. of around 20%, in the overall footprint of a converter station including the converter of the invention. This in turn helps to reduce considerably the cost of the converter station.
- In addition, omitting the passive inductor from each limb portion also means that it is possible to connect a transformer directly with the converter without the need for interconnecting bushings which are typically large and expensive, and so offers further space- and cost-saving opportunities.
- In addition, omitting an inductor from each limb portion allows for an increase in the level of AC voltage that can be generated by the converter, while permitting a reduction in the level of AC current, i.e. the level of individual AC current demand phase waveforms, that must be provided, and so gives rise to an increase in the efficiency of the converter.
- Preferably the first controller is programmed to sequentially define fully-conducting converter limbs at regular intervals of around 60 electrical degrees.
- The inclusion of a first controller so programmed results in the DC current demand routed between the first and second DC terminal being smooth and continuous.
- Optionally the first controller is additionally programmed, while selectively operating for a given converter limb the primary switching element in each limb portion thereof to simultaneously switch both the first and second limb portions into circuit and thereby define a fully-conducting converter limb, to concurrently operate for each other converter limb the primary switching element in one or both limb portions thereof to switch a single limb portion into circuit and thereby define a partially-conducting converter limb to direct respective AC current demand phase waveforms towards a given AC terminal whereby the respective AC current demand phase waveforms sum to zero.
- Having the AC current demand phase waveforms sum to zero at a given AC terminal, i.e. within the converter of the invention, eliminates the inclusion of any AC components in the DC current demand routed between the first and second DC terminals, and so avoids the need to filter this current before, e.g. passing it to a DC network connected in use to the first and second DC terminals.
- Any kind of filter in a HVDC installation has major implications with regards to the footprint of a resulting converter station, and so avoiding such filters is very beneficial.
- A converter according to preferred embodiment of the invention further includes a second controller programmed to:
-
- (a) obtain a respective AC current demand phase waveform for each converter limb which the corresponding converter limb is required to track, and a DC current demand which each converter limb is also required to track; and
- (b) carry out mathematical optimization to determine an optimal limb portion current for each limb portion that the limb portion must contribute to track the corresponding required AC current demand phase waveform and the required DC current demand.
- Carrying out the aforementioned mathematical optimization, i.e. selecting the best individual limb portion current (with regard to chosen criteria) from a set of available alternatives, allows the AC and DC current demands to be controlled independently of one another, e.g. by a higher level controller.
- It also permits individual limb portion currents to vary independently of one another to accommodate different current flow paths through the converter, e.g. as occasioned by the sequential definition of a fully-conducting converter limb and respective partially-conducting converter limbs throughout each operating cycle of the converter.
- Moreover, the second controller is able to carry out steps (a) and (b) in real time so as to permit robust control of the converter of the invention.
- The second controller may be programmed to carry out mathematical optimization by creating an equivalent converter configuration which represents the flow of current through the converter.
- Creating an equivalent converter configuration in the aforementioned manner imposes constraints on the way in which the converter can be controlled and so assists in carrying out mathematical optimization to determine each optimal limb portion current.
- Optionally the second controller is programmed to create an equivalent converter configuration which represents the flow of current through the converter by mapping possible current flow paths through the converter.
- Mapping the possible current flow paths through the converter helps the second controller to tailor the mathematical optimization it provides to the topology, i.e. structure, of the converter of the invention.
- Preferably the second controller is programmed to carry out mathematical optimization by applying a current weighting to the relative current contribution provided by a plurality of limb portions.
- Applying such weightings allows variations in the performance of each limb portion to be further accommodated while continuing to optimise the operation of the converter as a whole.
- The second controller may be programmed to determine the or each weighting according to measured operating parameters of the converter.
- Determining the weightings in the aforementioned manner allows the second controller to take into account environmental factors which might affect the healthy operation of the converter, and to alter the optimal limb portion currents that are determined in an effort to overcome the environmental factors and alleviate the associated impact on the operation of the converter. Examples of such environmental factors include the components in one limb portion running hot, or a limb portion suffering component damage or failure such that its performance is degraded.
- In another preferred embodiment of the invention the second controller, when controlling the converter under a particular operating condition, is programmed to apply a weighting by applying a different weighting to at least one limb portion such that the or each said limb portion provides a different contribution to the other limb portions.
- Such a feature allows the second controller to distinguish between one limb portion and another, e.g. according to how well a given limb portion is performing.
- This is useful in circumstances where it becomes desirable to reduce the level of current contributed by a given limb portion, e.g. because the cooling associated with the limb portion is operating at a reduced capacity, and temporarily increase the level of current provided by one or more other limb portions so as to allow the converter to continue to operate and provide a high level of power conversion.
- It can also be used to reduce the limb portion voltage that a given limb portion must provide, e.g. in circumstances where a fault or other damage has degraded the performance of the given limb portion, such that the converter remains able to continue operating and provide a high level of power conversion.
- Preferably the second controller is programmed to carry out mathematical optimization to determine one or more minimum individual limb portion currents that the corresponding limb portion must contribute to track the corresponding required AC current demand phase waveform and the required DC current demand.
- Determining one or more minimum individual limb portion currents reduces the conduction and switching losses in each limb portion because ordinarily such losses are proportional to current squared, i.e. I2.
- In a still further preferred embodiment of the invention the second controller is further programmed to carry out mathematical optimization to provide optimal limb portion voltage sources.
- The inclusion of a second controller so programmed assists in the provision, in the most efficient manner possible, of individual limb portion currents that vary independently of one another.
- There now follows a brief description of preferred embodiments of the invention, by way of non-limiting example, with reference being made to the following figures in which:
-
FIG. 1 shows a schematic view of a converter according to a first embodiment of the invention; -
FIG. 2 shows a preferred switching sequence of primary switching elements within the converter shown inFIG. 1 during an operating cycle of the converter; -
FIG. 3 illustrates schematically the selective definition of a fully-conducting converter limb and respective partially-conducting converter limbs in the converter shown inFIG. 1 ; -
FIG. 4 shows a flow diagram that illustrates the principal steps a second controller in the converter shown inFIG. 1 is programmed to carry out; -
FIG. 5 shows a schematic representation of an equivalent converter configuration corresponding to the converter shown inFIG. 1 ; -
FIG. 6(a) shows a flow diagram that illustrates the principal steps a further second controller may be programmed to carry out; and -
FIG. 6(b) shows a schematic view of a feedback loop which forms a part of the principal steps the further controller may be programmed to carry out. - A converter according to a first embodiment of the invention is designated generally by
reference numeral 10, as shown inFIG. 1 . - The
converter 10 includes threeconverter limbs converter 10. - Each
converter limb second DC terminals converter limb second limb portion 12A−, 12B−, 12C− which are separated by anAC terminal - In use, the first and
second DC terminals DC network 20, with thefirst DC terminal 14 carrying a voltage of VDC+ and thesecond DC terminal 16 carrying a voltage of VDC−, while theAC terminal phase AC network 22 and carries a corresponding AC voltage phase waveform VA, VB, VC. - Each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− includes a chain-link converter 24A+, 24A−, 24B+, 24B−, 24C+, 24C− that includes a chain of
modules 26 connected in series. The number ofmodules 26 in each chain-link converter 24A+, 24A−, 24B+, 24B−, 24C+, 24C− depends on the required voltage rating of the respective limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−. - Each
module 26 of each chain-link converter 24A+, 24A−, 24B+, 24B−, 24C+, 24C− includes two pairs of secondary switching elements (not shown) connected in parallel with an energy storage device, in the form of a capacitor (not shown), to define a 4-quadrantbipolar module 26 that can provide negative, zero or positive voltage and can conduct current in two directions. - In use, the secondary switching elements of the
modules 26 of each chain-link converter 24A+, 24A−, 24B+, 24B−, 24C+, 24C− are operated to enable each chain-link converter 24A+, 24A−, 24B+, 24B−, 24C+, 24C− to provide a stepped variable voltage source. The secondary switching elements are also desirably switched at near to the fundamental frequency of theAC network 22. - The capacitor of each
module 26 may be bypassed or inserted into the respective chain-link converter 24A+, 24A−, 24B+, 24B−, 24C+, 24C− by changing the state of the secondary switching elements. - The capacitor of each
module 26 is bypassed when the pairs of secondary switching elements are configured to form a short circuit in themodule 26. This causes current in the corresponding limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− of theconverter 10 to pass through the short circuit and bypass the capacitor, and so themodule 26 is able to provide a zero voltage. - The capacitor of each
module 26 is inserted into the respective chain-link converter 24A+, 24A−, 24B+, 24B−, 24C+, 24C− when the pair of secondary switching elements is configured to allow the aforementioned current to flow into and out of the capacitor. The capacitor is then able to charge or to discharge its stored energy so as to provide a voltage. The bidirectional nature of the 4-quadrantbipolar module 26 means that the capacitor may be inserted into themodule 26 in either forward or reverse directions so as to provide a positive or negative voltage. - It is therefore possible to build up a combined voltage across each chain-link converter 24A+, 24A−, 24B+, 24B−, 24C+, 24C− which is higher than the voltage available from each
individual module 26 via the insertion of the capacitors ofmultiple modules 26, each providing its own voltage, into the chain-link converter 24A+, 24A−, 24B+, 24B−, 24C+, 24C−. - The ability of a 4-quadrant
bipolar module 26 to provide positive or negative voltages means that the voltage across each chain-link converter 24A+, 24A−, 24B+, 24B−, 24C+, 24C− may be built up from a combination ofmodules 26 providing positive or negative voltages. The energy levels in individual capacitors may be maintained therefore at optimal levels by controlling themodules 26 to alternate between providing positive or negative voltage. - It is possible to vary the timing of switching operations for each
module 26 such that the insertion and/or bypass of the capacitors ofindividual modules 26 in the chain-link converter 24A+, 24A−, 24B+, 24B−, 24C+, 24C− results in the generation of a voltage waveform at acorresponding AC terminal individual modules 26 may be staggered to generate a sinusoidal waveform. Other waveform shapes may be generated by adjusting the timing of switching operations for eachmodule 26 in the chain-link converter 24A+, 24A−, 24B+, 24B−, 24C+, 24C−. - In this manner the chain-link converters 24A+, 24A−, 24B+, 24B−, 24C+, 24C− are able to facilitate power transfer between the AC and
DC networks - In addition to the foregoing, the inclusion of 4-quadrant
bipolar modules 26 means that it is possible for each chain-link converter 24A+, 24A−, 24B+, 24B−, 24C+, 24C− to block a DC fault current in the event of e.g. a pole to pole fault in theDC network 20, with such blocking being achieved by opening appropriate secondary switching elements in eachmodule 26 to prevent the flow of fault current through each saidmodule 26. - Each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− also includes a primary switching element Sw1, Sw4, Sw3, Sw6, Sw5, Sw2 that is connected in series with the corresponding chain-link converter 24A+, 24A−, 24B+, 24B−, 24C+, 24C−. In use, each primary switching element Sw1, Sw2, Sw3, Sw4, Sw5, Sw6 selectively switches the respective limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− into and out of circuit within the
converter 10. - In other embodiments of the invention (not shown) each primary switching element may include a plurality of, e.g. series-connected, switching elements depending on the required voltage rating of each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−.
- In addition, the series connection between each primary switching element Sw1, Sw2, Sw3, Sw4, Sw5, Sw6 and the corresponding chain-link converter 24A+, 24A−, 24B+, 24B−, 24C+, 24C− in each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− allows, in other embodiments of the invention, the respective primary switching element Sw1, Sw2, Sw3, Sw4, Sw5, Sw6 and corresponding chain-link converter 24A+, 24A−, 24B+, 24B−, 24C+, 24C− to be connected in a reverse order between the
corresponding AC terminal second DC terminal - In the embodiment shown, each primary switching element Sw1, Sw2, Sw3, Sw4, Sw5, Sw6 and each of the secondary switching elements in the respective chain-link converters 24A+, 24A−, 24B+, 24B−, 24C+, 24C− is an insulated gate bipolar transistor (IGBT) 28 connected in parallel with an
anti-parallel diode 30. - In other embodiments of the invention (not shown) one or more of the primary and secondary switching elements Sw1, Sw2, Sw3, Sw4, Sw5, Sw6 may include a different semiconductor device, such as a field effect transistor, a gate-turn-off thyristor, an injection gate enhanced thyristor, an integrated gate commutated transistor or another externally-commutated semiconductor switch, i.e. a semiconductor switch which is turned off by one or more external components causing the current flowing through the semiconductor switch to fall to zero. Such other externally-commutated semiconductor switches can include so-called ‘forced commutated’ and ‘self commutated’ semiconductor switches. In each instance the semiconductor device is preferably connected in parallel with an anti-parallel diode.
- For the reasons set out hereinabove, each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− omits any form of physical, passive inductor component, which in turns provides considerable benefits in terms of reducing the overall footprint of a resulting converter station in which the converter of the invention is incorporated.
- In addition to the foregoing, the
converter 10 includes afirst controller 32 that is arranged in operative communication with each primary switching element Sw1, Sw2, Sw3, Sw4, Sw5, Sw6. - The
first controller 32 is a programmable device, such as a microcontroller, and more particularly is programmed to operate, for oneconverter limb second limb portions 12A−, 12B−, 12C− into circuit and thereby define a fully conductingconverter limb - In this way the
first controller 32 sequentially routes, via each said fully-conductingconverter limb converter limbs second DC terminals - More particularly still, the
first controller 32 is programmed to sequentially define fully-conductingconverter limbs regular intervals ranges 60±1 electrical degrees, or 60±2 electrical degrees. - The
first controller 32 is also additionally programmed, while selectively operating for a givenconverter limb converter limb other converter limb converter limb - In this manner the
first controller 32 is programmed to direct respective AC current demand phase waveforms IA, IB, IC, i.e. respective AC phase currents that theconverter 10 is required to track, towards a givenAC terminal -
FIG. 2 shows one example switching sequence of the primary switching elements Sw1, Sw2, Sw3, Sw4, Sw5, Sw6 which is implemented by thefirst controller 32. Other switching sequences may, however, also be implemented. - In the embodiment shown, the
first controller 32 utilises a phase locked loop (PLL) control scheme to coordinate the switching sequence with respective AC voltage phase waveforms VA, VB, VC of theAC network 22. - More particularly, during a
first interval 34 1 thefirst controller 32 operates the primary switching element Sw1, Sw4 in each limb portion 12A+, 12A− of afirst converter limb 12A to simultaneously switch both the first and second limb portions 12A+, 12A− into circuit and thereby define a fully-conductingconverter limb 12A. - At the same time, i.e. concurrently with the foregoing, the
first controller 32 operates the primary switching element Sw6 in thesecond limb portion 12B− of asecond converter limb 12B to switch thesecond limb portion 12B− into circuit and thereby define a partially-conductingconverter limb 12B. The primary switching element Sw3 in the first limb portion 12B+ of thesecond converter limb 12B remains switched out of circuit. - The
first controller 32 also, at the same time, operates the primary switching element Sw5 in the first limb portion 12C+ of a third converter limb 12C to switch the first limb portion 12C+ into circuit and thereby define another partially-conducting converter limb 12C. The primary switching element Sw2 in the second limb portion 12C− of the third converter limb 12C remains switched out of circuit. - During a
second interval 34 2 thefirst controller 32 leaves the first limb portion 12C+ of the third converter limb 12C switched into circuit while operating the primary switching element Sw2 in the second limb portion 12C− of the third converter limb 12C to switch the second limb portion 12C− into circuit and thereby define a fully-conducting converter limb 12C. At the same time the first controller leaves the first limb portion 12A+ of thefirst converter limb 12A switched into circuit while operating the switching element Sw4 in thesecond limb portion 12A− to switch thesecond limb portion 12A− out of circuit and thereby define a partially-conductingconverter limb 12A. Thefirst controller 32 also, at the same time, leaves thesecond limb portion 12B− of thesecond converter limb 12B switched into circuit to continue to define a partially-conductingconverter limb 12B. The primary switching element Sw3 in the first limb portion 12B+ of thesecond converter limb 12B remains switched out of circuit. - During a
third interval 34 3 thefirst controller 32 operates the primary switching element Sw3 in the first limb portion 12B+ of thesecond converter limb 12B to switch the first limb portion 12B+ into circuit and thereby define, along with the already switched into circuitsecond limb portion 12B−, a fully-conductingconverter limb 12B. At the same time thefirst controller 32 continues to leave the first limb portion 12A+ of thefirst converter limb 12A switched into circuit and thesecond limb portion 12A− of thesecond limb portion 12A− switched out of circuit to continue to define a partially-conductingconverter limb 12A. Thefirst controller 32 also, at the same time, leaves the second limb portion 12C− of the third converter limb 12C switched into circuit while operating the switching element Sw5 in the first limb portion 12C+ of the third converter limb 12C to switch the first limb portion 12C+ out of circuit to thereby define a partially-conducting converter limb 12C. - During a
fourth interval 34 4 thefirst controller 32 operates the primary switching element Sw4 in thesecond limb portion 12A− of thefirst converter limb 12A to switch thesecond limb portion 12A− into circuit and thereby define, along with the already switched into circuit first limb portion 12A+, a fully-conductingconverter limb 12A, as shown by way of example inFIG. 3 . As also shown inFIG. 3 , the aforesaid fully-conductingconverter limb 12A routes a DC current demand IDC between the first andsecond DC terminals - At the same time the
first controller 32 continues to leave the first limb portion 12B+ of thesecond converter limb 12B switched into circuit while operating the switching element SW6 of thesecond limb portion 12B− of thesecond converter limb 12B to switch thesecond limb portion 12B− out of circuit to thereby define a partially-conductingconverter limb 12B, as also shown inFIG. 3 . As additionally shown inFIG. 3 , the partially-conductingconverter limb 12B, i.e. the first limb portion 12B+ thereof, directs an AC current demand phase waveform IB, i.e. an AC phase current IB, towards afirst AC terminal 18A. - The
first controller 32 also, at the same time, leaves the first limb portion 12C+ of the third converter limb 12C switched out of circuit and the second limb portion 12C− of the third converter limb 12C switched into circuit to continue to define a partially-conducting converter limb 12C, as again shown inFIG. 3 . As shown inFIG. 3 , the partially conducting converter limb 12C, i.e. the second limb portion 12C− thereof, also directs an AC current demand phase waveform IC, i.e. an AC phase current IC, towards thefirst AC terminal 18A. - Each of the aforementioned AC current demand phase waveforms IB, IC, together with a further AC current demand phase waveform IA sum to zero at the
first AC terminal 18A, and thereby cancel one another out such that they do not impact adversely on the quality, i.e. smoothness, of the DC current demand IDC routed between the first andsecond DC terminals - During a
fifth interval 34 5 thefirst controller 32 operates the primary switching element Sw5 in the first limb portion 12C+ of the third converter limb 12C to switch the first limb portion 12C+ into circuit and thereby define, along with the already switched into circuit second limb portion 12C−, a fully-conducting converter limb 12C. At the same time thefirst controller 32 continues to leave thesecond limb portion 12A− of thefirst converter limb 12A switched into circuit while operating the switching element Sw1 of the first limb portion 12A+ of thefirst converter limb 12A to switch the first limb portion 12A+ out of circuit to thereby define a partially-conductingconverter limb 12A. Thefirst controller 32 also, at the same time, leaves the first limb portion 12B+ of thesecond converter limb 12B switched into circuit and thesecond limb portion 12B− of thesecond converter limb 12B switched out of circuit to continue to define a partially-conductingconverter limb 12B. - During a sixth and
final interval 34 6 thefirst controller 32 operates the primary switching element Sw6 in thesecond limb portion 12B− of thesecond converter limb 12B to switch thesecond limb portion 12B− into circuit and thereby define, along with the already switched into circuit first limb portion 12B+, a fully-conductingconverter limb 12B. At the same time thefirst controller 32 continues to leave thesecond limb portion 12A− of thefirst converter limb 12A switched into circuit and the first limb portion 12A+ switched out of circuit to continue to define a partially-conductingconverter limb 12A. Thefirst controller 32 also, at the same time, leaves the first limb portion 12C+ of the third converter limb 12C switched into circuit while operating the switching element Sw2 of the second limb portion 12C− of the third limb portion 12C to switch the second limb portion 12C− out of circuit and thereby define a partially-conducting converter limb 12C. - It follows that the
first controller 32 sequentially defines single, individual first, second, third, fourth, fifth and sixth fully-conductingconverter limbs sixth intervals complete operating cycle 36 of theconverter 10. - The
converter 10 also includes asecond controller 38 that is arranged in communication with thefirst controller 32 and each of the chain-link converters 24A+, 24A−, 24B+, 24B−, 24C+, 24C−. Thesecond controller 38 is similarly a programmable device, such as a microcontroller. Although in the embodiment described, the first andsecond controllers - Returning to the embodiment shown, the
second controller 38 is programmed to: -
- (a) obtain a respective AC current demand phase waveform IA, IB, IC for each
converter limb converter limb converter limb - (b) carry out mathematical optimization to determine an optimal limb portion current IA+, IA−, IB+, IB−, IC+, IC− for each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− that the limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− must contribute to track the corresponding required AC current demand phase waveform IA, IB, IC and the required DC current demand IDC.
- (a) obtain a respective AC current demand phase waveform IA, IB, IC for each
- The
second controller 38 is also further programmed to (c) carry out mathematical optimization to provide optimal limb portion voltage sources VA+, VA−, VB+, VB−, VC+, VC−, with these principal steps (a), (b) and (c) being illustrated in a first flow diagram 40 shown inFIG. 4 . - As set out above, the
second controller 38 is programmed to first obtain a respective AC current demand phase waveform IA, IB, IC for eachconverter limb converter limb converter limbs - The various AC current demand phase waveforms IA, IB, IC and the DC current demand IDC may be obtained directly from a higher-level controller (not shown) within a converter or from some other external entity. Alternatively the
converter 10 may obtain it directly by carrying out its own calculations. - The
second controller 38 is also programmed to, as a second step (and as indicated by afirst process box 42 in the first flow diagram 40), carry out mathematical optimization to determine an optimal limb portion current IA+, IA−, IB+, IB−, IC+, IC− for each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− that the limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− must contribute to track the corresponding required AC current demand phase waveform IA, IB, IC and the required DC current demand IDC. - The
second controller 38 is programmed to carry out such mathematical optimization by creating anequivalent converter configuration 100, as shown inFIG. 5 , which represents the flow of current through the correspondingconverter 10 of the invention. - The
equivalent converter configuration 100 includes similar features to theconverter 10 of the invention and these like features share the same reference numerals. To that end theequivalent converter configuration 100 includes threeconverter limbs converter 10 of the invention. - In the
equivalent converter configuration 100 eachconverter limb second DC terminals converter limb second limb portion 12A−, 12B−, 12C−. Each pair of first and second limb portions 12A+, 12A−, 12B+, 12B−, 12C+, 12C− in eachconverter limb corresponding AC terminal - The
equivalent converter configuration 100 also represents the respective AC current demand phase waveforms IA, IB, IC that eachconverter limb converter limbs - In practice each
converter limb AC network 22, as well as a DC voltage VDC of the DCelectrical network 20, to which theconverter 10 is, in use, connected, and so theequivalent converter configuration 100 may also represent these elements. - The
second controller 38 is programmed to create anequivalent converter configuration 100 which represents the flow of current through theconverter 10 by mapping possible current flow paths through theconverter 10. - One way in which the possible current flow paths through the
converter 10 may be mapped is by conducting a Kirchhoff analysis of theequivalent converter configuration 100 obtain the following equations: -
I A=αA + I A+−αA − I A− -
I B=αB + I B+−αB − I B− -
I C=αC + I C+−αC − I C− -
I DC+=αA + I A++αB + I B++αC + I C+ -
I DC−=αA − I A−+αB − I B−+αC − I C− - where
-
- the binary variables αk ± indicate whether a given limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− is switched into circuit (αk ±=1) or out of circuit (αk ±=0), i.e. αA +, αA −, αB +, αB −, αC +, αC − represent the state of the corresponding primary switching element Sw1, Sw2, Sw3, Sw4, Sw5, Sw6 in each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− of the converter 10 (details of which are provided to the
second controller 38 by the first controller 32); - IDC+ is the sum of currents in the first limb portions 12A+, 12B+, 12C+, i.e. as shown in
FIG. 3 ; - IDC− is the sum of currents in the
second limb portions 12A−, 12B−, 12C−, i.e. as also shown inFIG. 3 ; and
- the binary variables αk ± indicate whether a given limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− is switched into circuit (αk ±=1) or out of circuit (αk ±=0), i.e. αA +, αA −, αB +, αB −, αC +, αC − represent the state of the corresponding primary switching element Sw1, Sw2, Sw3, Sw4, Sw5, Sw6 in each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− of the converter 10 (details of which are provided to the
-
I DC+ =I DC− =I DC - The preceding equations are then combined and simplified into
-
I A=αA + I A+−αA − I A− -
I B=αB + I B+−αB − I B− -
I C=αC + I C+−αC − I C− -
I DC=αA + I A++αB + I B++αC + I C+ - The possible current flow paths through the
converter 10 are then mapped by expressing the latter equations in a matrix form, i.e.: -
- such that A is a matrix which maps the possible current flow paths provided by the limb portions 12A+, 12A−, 12B+, 12B−, 12C+, 12C−.
- Other equivalent converter configurations and corresponding analysis techniques are, however, also possible.
- The
second controller 38 is also programmed to carry out mathematical optimization by applying a current weighting to the relative current contribution provided by each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−. The respective current weighting for each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− is determined according to measured operating parameters of theconverter 10 during its operation. The various current weightings can be determined throughout operation of the saidconverter 10 so as to permit an updating of the current weightings, e.g. in response to changing environmental conditions. As a result the various current weightings can vary as theconverter 10 operates. - During normal operation of the
converter 10 an identical current weighting is applied to each limb portion current IA+, IA−, IB+, IB−, IC+, IC−. - However, when the
converter 10 is operating under certain conditions, e.g. an abnormal operating condition, a different current weighting may be applied to the current contribution, i.e. the limb portion current IA+, IA−, IB+, IB−, IC+, IC−, provided by at least one limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−. For example, a larger current weighting may be applied to the optimal limb portion current IA+, IA−, IB+, IB−, IC+, IC− that a particular limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C must contribute, so as to reduce an actual limb portion current that the said limb portion contributes relative to an actual current contribution of each of the other limb portions, which are otherwise all the same as one another. - In addition to the foregoing the
second controller 38 is programmed to carry out mathematical optimization to determine a minimum individual limb portion current IA+, IA−, IB+, IB−, IC+, IC− that each of the limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− must contribute so as to track the corresponding required AC current demand phase waveform IA, IB, IC and the required DC current demand IDC-One way in which minimum individual limb portion currents IA+, IA−, IB+, IB−, IC+, IC−, i.e. x in the A·x=b equation set out above, may be determined, and the aforementioned individual current weightings applied to the minimal individual limb portion currents IA+, IA−, IB+, IC+, IC−, is by solving a nonlinear optimization of the general form: -
- subject to the equality constrained equation of the form:
-
A·x=b - where
-
- JCurrent is the current objective function to be minimized;
- Ψ is the current weighting at time t1;
- f is the current cost function which in the embodiment described includes a current weighting matrix Q1;
- x is the transpose of [IA+, IA−, IB+, IB−, IC+, IC−], i.e. [IA+, IA−, IB+, IB−, IC−] reflected in a column vector;
- t0 is the time at which a particular period of control of the
converter 10 starts; and - t1 is the time at which a particular period of control of the
converter 10 ends.
- The current weighting matrix Q1 is determined according to measured operating parameters of the
converter 10, and may be so determined throughout the operation of theconverter 10, such that it can vary as the saidconverter 10 is controlled in response to changes in the operation of theconverter 10. - When subject only to an equality constrained equation, as mentioned above, the Lagrangian (or the method of Lagrange multipliers) is a technique for solving the above-identified nonlinear optimization in order to find local minima of the current objective function JCurrent. It may also be solved using other optimization algorithms, including iterative and programming algorithms.
- As a general optimal control problem, the aforementioned nonlinear optimization could additionally include one or more inequality constraints in which case it could be solved by using the further method of Hamiltonian (Pontryagin's minimum principle).
- One example of such an inequality constraint is:
-
- where
-
- C is a matrix which maps possible maximum current flow paths provided by the limb portions 12A+, 12A−, 12B+, 12B−, 12C+, 12C−; and
- d is a vector representing of the maximum desired current in each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−.
- In either case the minimum individual limb portion currents IA+, IA−, IB+, IB−, IC+, IC− may also be determined by solving a nonlinear optimization of the form
-
- Meanwhile, as mentioned above, the
second controller 38 is also programmed to carry out mathematical optimization to provide an optimal limb portion voltage source VA+, VA−, VB+, VB−, VC+, VC− for each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− to achieve the corresponding mathematically optimized minimum limb portion current IA+, IA−, IB+, IB−, IC+, IC−. In other embodiments of the method of the invention, however, such mathematical optimization of the limb portion voltage sources need not take place. - The
second controller 38 is programmed to carry out mathematical optimization to provide optimal limb portion voltage sources VA+, VA−, VB+, VB−, VC+, VC− by creating anequivalent converter configuration 100 which represents voltage conditions in theconverter 10. - Representing the voltage conditions in the
converter 10 portrayed in theequivalent converter configuration 100 additionally includes mapping a limb portion voltage source VA+, VA−, VB+, VB−, VC+, VC− and an inductive component for each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−. - In the embodiment described, each limb portion voltage source VA+, VA−, VB+, VB−, VC+, VC− corresponds to a respective chain-link converter 24A+, 24A−, 24B+, 24B−, 24C+, 24C− which is switchable into and out of the corresponding limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− via operation of the corresponding primary switching element Sw1, Sw2, Sw3, Sw4, Sw5, Sw6. Each limb portion voltage source VA+, VA−, VB+, VB−, VC+, VC− is therefore variable in magnitude between zero (i.e. equivalent to being switched out of the corresponding limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−) and an upper voltage limit.
- Meanwhile an inductive component for each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− within the
equivalent converter configuration 100 represents the inductance associated with the corresponding limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− of theactual converter 10. Such inductances do not include a passive limb inductor within each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− since they are no longer needed to control the level of circulating current between theconverter limbs phase inductance - The aforementioned inductive component of each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− is represented in the
equivalent converter configuration 100 as an inductive voltage portion UA+, UA−, UB+, UB−, UC+, UC− that is made up of the voltage arising from the flow of current through the aforementioned inductance, i.e.phase inductance DC line inductance 46 only, associated with a corresponding limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−. - In other embodiments of the invention representing of the voltage conditions in the
converter 10 may additionally include mapping a resistive component for each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−. - Such a resistive component represents the resistance associated with a given limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−, and similarly may take the form of a resistor within a given limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−, i.e. a limb portion resistance, or the form of a resistance electrically associated with a given limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−, e.g. a phase resistance and/or a DC line resistance.
- Mapping the limb portion voltage sources VA+, VA−, VB+, VB−, VC+, VC− and inductive voltage portions UA+, UA−, UB+, UB−, UC+, UC− again similarly includes conducting a Kirchhoff analysis of the
equivalent converter configuration 100, although other equivalent converter configurations and corresponding analysis techniques are also possible. In applying the Kirchhoff analysis the following equation, in matrix form, is obtained: -
- where:
-
- i.e. MV is a matrix which maps the position of the limb portion voltage sources VA+, VA−, VB+, VB−, VC+, VC− within the particular converter structure;
-
- i.e. Mu is a matrix which maps the position of the inductive voltage portions UA+, UA−, UB+, UB−, UC+, UC− within the particular converter structure;
-
- VDC is the DC voltage, i.e. the voltage difference between the first and
second DC terminals - VAB is the voltage difference between the first and
second converter limbs - VCB is the voltage difference between the third and
second converter limbs 12C, 12B.
- VDC is the DC voltage, i.e. the voltage difference between the first and
- The
second controller 38 is further programmed to carry out mathematical optimization to provide an optimal limb portion voltage source VA+, VA−, VB+, VB−, VC+, VC− for each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− by reducing any deviation in an actual measured limb portion current I′A+, I′A−, I′B+, I′B−, I′C+, I′C− of a given limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− from the corresponding determined optimal limb portion current IA+, IA−, IB+, IB−, IC+, IC− for the said given limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−. - The
second controller 38 is still further programmed to calculate the inductive voltage portion UA+, UA−, UB+, UB−, UC+, UC− for each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−. This calculation is based on the corresponding determined optimal limb portion currents IA+, IA−, IB+, IB−, IC+, IC−, together with the inductance associated with the corresponding limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−. - Thereafter the calculated inductive voltage portion UA+, UA−, UB+, UB−, UC+, UC− is modified to drive the actual measured limb portion current I′A+, I′A−, I′B+, I′B−, I′C+, I′C− to follow the corresponding determined optimal limb portion current IA+, IA−, IB+, IB−, IC+, IC−.
- Such measuring and modification takes the form of a feedback loop which provides closed-loop control, as illustrated schematically by a
second process box 48 in the first flow diagram 40 shown inFIG. 4 . The feedback loop may additionally include a feed-forward element which seeks to predict desirable future values for one or more of the inductive voltage portions UA+, UA−, UB+, UB−, UC+, UC− in order to improve the performance of the closed-loop control. - The calculated inductive voltage portion UA+, UA−, UB+, UB−, UC+, UC− of each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− is utilised, as indicated by a
third process box 50 in the first flow diagram 40, when carrying out the aforementioned mathematical optimization to provide the optimal limb portion voltage sources VA+, VA−, VB+, VB−, VC+, VC−. Such mathematical optimization also includes applying a voltage weighting to the relative voltage contribution provided by each limb portion voltage source VA+, VA−, VB+, VB−, VC+, VC−. The voltage weightings are determined according to measured operating parameters of theconverter 10, and may be so determined throughout operation of the saidconverter 10. Such potentially repeated determination of the voltage weightings permits the ongoing optimization of the converter operation during, e.g. changing environmental conditions. - For example, during normal operation of the said particular converter structure an identical voltage weighting is applied to the limb portion voltage source VA+, VA−, VB+, VB−, VC+, VC− of each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−.
- However during, e.g. abnormal operating conditions, a different voltage weighting can be applied to the limb portion voltage source VA+, VA−, VB+, VB−, VC+, VC− of one or more limb portions 12A+, 12A−, 12B+, 12B−, 12C+, 12C− to further alleviate the impact of, e.g. the abnormal operating conditions.
- More particularly, the
second controller 38 is programmed to carry out mathematical optimization to provide an optimal limb portion voltage source VA+, VA−, VB+, VB−, VC+, VC− for each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− by determining a minimum individual limb portion voltage source VA+, VA−, VB+, VB−, VC+, VC− for each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− that is required to achieve the corresponding minimum limb portion current IA+, IA−, IB+, IB−, IC+, IC− previously determined. - One way in which minimum individual limb portion voltage sources VA+, VA−, VB+, VB−, VC+, VC− (i.e. the minimum level of voltage a variable voltage source within a given limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− must provide) may be determined, and the aforementioned individual voltage weightings applied thereto, is by solving for x (where x is the transpose of [VA+, VA−, VB+, VB−, VC+, VC−]) a nonlinear optimization of the general form:
-
- subject to an equality constrained equation MV·x=b, where b is known, of the form:
-
- and where
-
- JVoltage is the voltage objective function to be minimized;
- Ψ is the voltage weighting at time t1
- f is the voltage cost function which in the embodiment described includes a voltage weighting matrix QV;
- t0 is the time at which a particular period of control of the
converter 10 starts; and - t1 is the time at which a particular period of control of the
converter 10 ends.
- The voltage weighting matrix QV is similarly determined according to measured operating parameters of the
converter 10, and may be so determined throughout the operation of theconverter 10. As such it too can vary as the saidconverter 10 is controlled. - Solving the nonlinear optimization mentioned above may also be made subject to an inequality equation of the form:
-
C·x≦d - where
-
- C is a matrix which maps the position of possible maximum limb portion voltage sources in the limb portions 12A+, 12A−, 12B+, 12B−, 12C+, 12C−; and
- d is a vector representing of the maximum desired voltage in each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−.
- An alternative second controller (not shown), which may instead be included in the
converter 10, is similarly programmed to: -
- (a) obtain a respective AC current demand phase waveform IA, IB, IC for each
converter limb converter limb converter limb - (b) carry out mathematical optimization to determine an optimal limb portion current IA+, IA−, IB+, IB−, IC+, IC− for each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− that the limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− must contribute to track the corresponding required AC current demand phase waveform IA, IB, IC and the required DC current demand IDC.
- (a) obtain a respective AC current demand phase waveform IA, IB, IC for each
- The aforementioned steps are again, similarly indicated by a
first process box 42 in a second flow diagram 60 shown inFIG. 6(a) . - Thereafter, however, the alternative second controller is programmed to apply a control algorithm to directly establish optimal limb portion voltage sources VA+, VA−, VB+, VB−, VC+, VC− from each of the corresponding determined minimum limb portion current IA+, IA−, IB+, IB−, IC+, IC−, i.e. as shown by a single
fourth process box 62 in the second flow diagram 60. - Applying such a control algorithm includes reducing any deviation in the actual measured limb portion current I′A+, I′A−, I′B+, I′B−, I′C+, I′C− of a given limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− from the corresponding determined minimum limb portion current IA+, IA−, IB+, IB−, IC+, IC− for the said given limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C−.
- One way in which the deviation in the actual measured limb portion current I′A+, I′A−, I′B+, I′B−, I′C+, I′C− of a given limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− from the corresponding determined minimum limb portion current IA+, IA−, IB+, IB−, IC+, IC− may be reduced, and preferably eliminated, is by establishing a
feedback loop 70 as shown schematically inFIG. 6(b) . - In the embodiment shown the
feedback loop 70 compares a respective actual measured limb portion current I′A+, I′A−, I′B+, I′B−, I′C+, I′C− with the corresponding determined minimum limb portion current IA+, IA−, IB+, IB−, IC+, IC− and calculates a corresponding limb portion error eA+, eA−, eB+, eB−, eC+, eC−. Thefeedback loop 70 then applies a correction factor K to each limb portion error eA+, eA−, eB+, eB−, eC+, eC− to thereby establish directly the corresponding limb portion voltage source VA+, VA−, VB+, VB−, VC+, VC− which is required to drive the error eA+, eA−, eB+, eB−, eC+, eC− towards zero. - The correction factor K may take the form of a control system matrix, such as a gain matrix (not shown), which sets out individual correction factors that each limb portion error eA+, eA−, eB+, eB−, eC+, eC− is, e.g. multiplied by in the case of a gain matrix, to establish the corresponding limb portion voltage source VA+, VA−, VB+, VB−, VC+, VC−.
- One way in which such individual correction factors may be established is by creating an equivalent converter configuration that represents the voltage conditions in the particular three-phase converter structure under control and thereafter considering the dynamics of such an equivalent converter configuration.
- More particularly, in relation to the embodiment described hereinabove, the foregoing steps may be achieved by creating the
equivalent converter configuration 100 shown inFIG. 5 and mapping a limb portion voltage source VA+, VA−, VB+, VB−, VC+, VC− and an inductive component for each limb portion 12A+, 12A−, 12B+, 12B−, 12C+, 12C− in theequivalent converter configuration 100. - Thereafter such mapping may include conducting a Kirchhoff analysis of the equivalent converter configuration 100 (although other equivalent converter configurations and corresponding analysis techniques are also possible) that applies Kirchhoff's current and voltage laws to describe the dynamics of the
equivalent converter configuration 100 as: -
- where:
-
- v is the transpose of [VA+, VA−, VB+, VB−, VC+, VC−],
- M is a coupled-inductance matrix which maps the inductive component of each limb portion, and more particularly maps each of the phase and DC line inductances associated with each limb portion; e.g.
-
-
- I is the transpose of [I′A+, I′A−, I′B+, I′B−, I′C+, I′C− ], i.e. the transpose of a currents vector representing the actual measured limb portion currents I′A+, I′A−, I′B+, I′B−, I′C+, I′C−;
- N is an input voltage matrix which maps the position of various input voltages within the particular converter structure, e.g.
-
- and
-
- ξ is an input voltages vector representing external disturbances, e.g.
-
- where
-
- VDC is the DC voltage, i.e. the voltage difference between the first and
second DC terminals - VAB is the voltage difference between the first and
second converter limbs - VCB is the voltage difference between the third and
second converter limbs 12C, 12B.
- VDC is the DC voltage, i.e. the voltage difference between the first and
- In this way, conducting the aforesaid Kirchhoff analysis makes it possible to take into account all of the factors mentioned above relating to the
converter 10, i.e. M, I, N, ξ, when considering what impact a change in one or more individual limb portion voltage sources VA+, VA−, VB+, VB−, VC+, VC− will have, e.g. on the actual measured limb portion current I′A+, I′A−, I′B+, I′B−, I′C+, I′C−. This ability renders the alternative second controller robust against controller uncertainties and modelling errors. - Moreover, as a result it is possible thereafter to establish each of the individual correction factors by considering what change needs to be made to a given individual limb portion voltage source VA+, VA−, VB+, VB−, VC+, VC− to desirably alter the corresponding limb portion current provided by the
converter 10 under control, i.e. the corresponding actual measured limb portion current I′A+, I′A−, I′B+, I′B−, I′C+, I′C−, in order to drive the actual measured limb portion current I′A+, I′A−, I′B+, I′B−, I′C+, I′C− towards the determined minimum limb portion current IA+, IA−, IB+, IB−, IC+, IC−, i.e. in order to reduce the corresponding limb portion error eA+, eA−, eB+, eB−, eC+, eC− towards zero. - Once such individual correction factors have been established for the
converter 10, e.g. at initial design and commissioning stages, there is not normally a need to determine them again. As a result thefeedback loop 70 involves minimal computational effort since in each cycle it is simply required to multiply a given limb portion error eA+, eA−, eB+, eB−, eC+, eC− by the corresponding individual correction factor which has already been determined.
Claims (11)
1. A converter, for use in high voltage direct current power transmission, comprising:
three converter limbs, each corresponding to a respective phase of the converter, each extending between first and second DC terminals and each including first and second limb portions separated by an AC terminal, each limb portion including a chain-link converter operable to provide a stepped variable voltage source and a primary switching element to selectively switch the respective limb portion into and out of circuit; and
a first controller programmed to selectively operate for one converter limb at a time the primary switching element in each limb portion thereof to simultaneously switch both the first and second limb portions into circuit and thereby define a fully-conducting converter limb to sequentially route via each said fully-conducting converter limb a DC current demand between the first and second DC terminals.
2. A converter according to claim 1 wherein the first controller is programmed to sequentially define fully-conducting converter limbs at regular intervals of around 60 electrical degrees.
3. A converter according to claim 1 wherein the first controller is additionally programmed, while selectively operating for a given converter limb the primary switching element in each limb portion thereof to simultaneously switch both the first and second limb portions into circuit and thereby define a fully-conducting converter limb, to concurrently operate for each other converter limb the primary switching element in one or both limb portions thereof to switch a single limb portion into circuit and thereby define a partially-conducting converter limb to direct respective AC current demand phase waveforms towards a given AC terminal whereby the respective AC current demand phase waveforms sum to zero.
4. A converter according to claim 1 further including a second controller programmed to:
(a) obtain a respective AC current demand phase waveform for each converter limb which the corresponding converter limb is required to track, and a DC current demand which each converter limb is also required to track; and
(b) carry out mathematical optimization to determine an optimal limb portion current for each limb portion that the limb portion must contribute to track the corresponding required AC current demand phase waveform and the required DC current demand.
5. A converter according to claim 4 wherein the second controller is programmed to carry out mathematical optimization by creating an equivalent converter configuration which represents the flow of current through the converter.
6. A converter according to claim 5 wherein the second controller is programmed to create an equivalent converter configuration which represents the flow of current through the converter by mapping possible current flow paths through the converter.
7. A converter according to claim 1 wherein the second controller is programmed to carry out mathematical optimization by applying a current weighting to the relative current contribution provided by a plurality of limb portions.
8. A converter according to claim 7 wherein the second controller is programmed to determine the or each weighting according to measured operating parameters of the converter.
9. A converter according to claim 7 wherein when controlling the converter under a particular operating condition the second controller is programmed to apply a weighting by applying a different weighting to at least one limb portion such that the or each said limb portion provides a different contribution to the other limb portions.
10. A converter according to claim 4 wherein the second controller is programmed to carry out mathematical optimization to determine one or more minimum individual limb portion currents that the corresponding limb portion must contribute to track the corresponding required AC current demand phase waveform and the required DC current demand.
11. A converter according to claim 4 wherein the second controller is further programmed to carry out mathematical optimization to provide optimal limb portion voltage sources.
Applications Claiming Priority (3)
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EP13275311.2 | 2013-12-12 | ||
EP13275311.2A EP2884653B1 (en) | 2013-12-12 | 2013-12-12 | Improvements in or relating to the control of converters |
PCT/EP2014/077636 WO2015086837A1 (en) | 2013-12-12 | 2014-12-12 | Improvements in or relating to converters for use in high voltage direct current power transmission |
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US20160315548A1 true US20160315548A1 (en) | 2016-10-27 |
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ID=49880638
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US15/103,792 Active US10027217B2 (en) | 2013-12-12 | 2014-12-12 | Converter and method of controlling a converter |
US15/103,822 Abandoned US20160315548A1 (en) | 2013-12-12 | 2014-12-12 | Improvements in or relating to converters for use in high voltage direct current power transmission |
US15/103,800 Abandoned US20160329833A1 (en) | 2013-12-12 | 2014-12-12 | Improvements in or relating to converters for use in high voltage direct current power transmission |
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US15/103,792 Active US10027217B2 (en) | 2013-12-12 | 2014-12-12 | Converter and method of controlling a converter |
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US15/103,800 Abandoned US20160329833A1 (en) | 2013-12-12 | 2014-12-12 | Improvements in or relating to converters for use in high voltage direct current power transmission |
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US (3) | US10027217B2 (en) |
EP (3) | EP2884653B1 (en) |
CN (3) | CN105993124A (en) |
WO (3) | WO2015086842A1 (en) |
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US20180166966A1 (en) * | 2015-05-28 | 2018-06-14 | General Electric Technology Gmbh | Improvements in or relating to the control of voltage source converters |
US10027217B2 (en) | 2013-12-12 | 2018-07-17 | General Electric Technology Gmbh | Converter and method of controlling a converter |
US10380314B1 (en) * | 2017-05-10 | 2019-08-13 | Cadence Design Systems, Inc. | System and method for estimating current in an electronic circuit design |
US10978951B2 (en) * | 2017-04-12 | 2021-04-13 | Kyoto University | Passivity-based switching power supply system, controller, and control method |
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GB2541007B (en) * | 2015-08-05 | 2017-12-13 | General Electric Technology Gmbh | Voltage source converter |
GB2545023B (en) * | 2015-12-04 | 2018-06-06 | General Electric Technology Gmbh | Improvements in or relating to converters |
EP3614552B1 (en) | 2018-08-24 | 2021-05-19 | General Electric Technology GmbH | Voltage source converter |
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- 2014-12-12 EP EP14823931.2A patent/EP3080900A1/en not_active Withdrawn
- 2014-12-12 US US15/103,792 patent/US10027217B2/en active Active
- 2014-12-12 US US15/103,822 patent/US20160315548A1/en not_active Abandoned
- 2014-12-12 EP EP14812466.2A patent/EP3080905A1/en not_active Withdrawn
- 2014-12-12 CN CN201480075215.8A patent/CN105993124A/en active Pending
- 2014-12-12 WO PCT/EP2014/077647 patent/WO2015086844A1/en active Application Filing
- 2014-12-12 US US15/103,800 patent/US20160329833A1/en not_active Abandoned
- 2014-12-12 WO PCT/EP2014/077636 patent/WO2015086837A1/en active Application Filing
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- 2014-12-12 CN CN201480075214.3A patent/CN105993121A/en active Pending
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Also Published As
Publication number | Publication date |
---|---|
EP3080905A1 (en) | 2016-10-19 |
EP2884653A1 (en) | 2015-06-17 |
WO2015086837A1 (en) | 2015-06-18 |
WO2015086842A1 (en) | 2015-06-18 |
CN105981287B (en) | 2019-03-08 |
WO2015086844A1 (en) | 2015-06-18 |
CN105981287A (en) | 2016-09-28 |
CN105993121A (en) | 2016-10-05 |
EP2884653B1 (en) | 2022-10-26 |
US20160329833A1 (en) | 2016-11-10 |
US20160329804A1 (en) | 2016-11-10 |
CN105993124A (en) | 2016-10-05 |
EP3080900A1 (en) | 2016-10-19 |
US10027217B2 (en) | 2018-07-17 |
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