WO2002052709A1 - A damping system - Google Patents
A damping system Download PDFInfo
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- WO2002052709A1 WO2002052709A1 PCT/GB2000/005003 GB0005003W WO02052709A1 WO 2002052709 A1 WO2002052709 A1 WO 2002052709A1 GB 0005003 W GB0005003 W GB 0005003W WO 02052709 A1 WO02052709 A1 WO 02052709A1
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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
- H02M7/53—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 using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/537—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 using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters
- H02M7/5387—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 using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration
- H02M7/53871—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 using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only, e.g. single switched pulse inverters in a bridge configuration with automatic control of output voltage or current
Definitions
- the invention relates to an active damping arrangement for electronic power conditioning, units .
- Power conditioning units contain inverters or converters and are used to supply AC or DC power either as voltage-drives into a load or as current-drives for example in the AC case into a power distribution network.
- Most power conditioning units operate in switched-mode and contain output filters on one or more output' lines. The purpose of the output filters is to reduce high-frequency components of voltage and/or current at the output terminals produced by the switching action. There are problems associated with such filters, essentially due to lightly damped resonance effects caused by interaction of inductance and capacitance.
- Inverters/converters that operate at substantial power ratings are generally required to maintain good efficiency, and are frequently required also to offer good output waveform quality.
- the term 'waveform' embraces either periodic AC or continuous DC output.
- PWM pulse-width modulation
- the switching frequency is sometimes fixed and sometimes caused to vary with time, as it may for example throughout the cycle of an AC waveform.
- To achieve high waveform quality it is necessary to include an output filter in each electronically driven output line to remove the majority of the high-frequency ripple harmonics in the output voltage and/or current waveform that are associated with the switching action.
- a typical output filter for one line comprises a series inductor LI that is connected to the appropriate output point of the inverter/converter, followed by a parallel capacitor C, the return end of the capacitor being connected either to a neutral point or to one or both rails of a DC link.
- LI series inductor
- C parallel capacitor
- the total capacitance may be split in two and half returned to each rail.
- the capacitors may be connected between pairs of output lines, which produces an effect as though there were an equivalent value of C connected as described above.
- the equivalence is however not exact in all respects, particularly in regard to common-mode components of ripple waveforms, but sufficiently similar to constitute in practice an acceptable alternative manner of connection for some applications.
- the effect of the output filter is to limit the magnitudes of various high- frequency harmonics of current generated by the switching action of the inverter/converter. Additionally, the filter reduces to a low level the magnitude of the corresponding high-frequency harmonics of voltage occurring across C, at the output end of LI.
- the output terminal to an external load may be the common point of LI and C. Alternatively, there may be interposed a further series inductor L2 following C. The primary purpose of this is to ensure that the majority of the high-frequency current that flows through LI is forced to flow through C. Otherwise, a substantial proportion of this high-frequency current could flow into the external load, if L2 is absent and if the inductance of the load is low.
- the inductor LI, and/or L2 may be a separate inductor for each output line that contains an output filter or there may be a multi-winding combined inductor that provides from a single structure the required inductance per line for all or some of the lines.
- the design of output filters of the type described is generally well understood.
- the output voltage of the power conditioning unit normally needs to be 'stiff.
- the voltage waveform needs to have a magnitude that is either constant or controlled so as to vary slightly in a required manner with load (giving a 'voltage regulation' characteristic).
- the shape of the waveform needs to remain close to a demanded shape (usually sinusoidal) under all load conditions.
- the current drawn is determined by the load, both in magnitude and shape of waveform, and so the current characteristic is 'soft'.
- Such a power conditioning unit may be described as a 'voltage-drive'.
- the output current of the power conditioning unit that needs to be stiff.
- the output voltage adapts readily to the magnitude and shape of the voltage waveform that is applied by for example a distribution network to the output terminals of the power conditioning unit.
- the voltage characteristic of the power conditioning unit needs to be soft.
- Such a power conditioning unit may be described as a 'current-drive!.
- the design may be considered for an output filter for a power conditioning unit that contains a three-phase sine-wave AC voltage-drive inverter, operating at a basic frequency of 50 Hz, with a PWM frequency of 5 kHz and a signal sampling frequency of at least 5kHz. It is convenient to place the resonant frequency of LI and C at roughly 500 Hz, one decade above the basic frequency, so that the filter's attenuating effect at basic frequency is small.
- the reactance of LI at 50 Hz may be set at approximately 0.05 per-unit (the unit of reactance in the per-unit system being equal, in accordance with the usual definition, to the quotient of rated phase voltage ⁇ rated phase current).
- Power conditioning units are used in electrical generators to accept electrical power in whatever form it is made available from a preceding stage and to provide output electrical power in a form suited to the application.
- Figure 1 shows an example of an electrical generator that comprises a prime mover in the form of a gas turbine, mechanically coupled to a permanent-magnet poly- phase alternator. Electrically connected to the alternator is a power conditioning unit for converting the output voltages at the terminals of the alternator to the voltage waveforms required at the output of the generator.
- the power conditioning unit comprises a rectifier that takes the alternator voltages as input and produces a DC link voltage as output. Connected to the output of the rectifier is a PWM inverter that takes the DC link voltage as input and produces at its output three-phase voltages.
- L- C three-phase series-inductance/parallel-capacitance
- the electrical generator of Figure 1 is commonly referred to as a 'micro- turbine-generator', MTG, which is able to provide a relatively small amount of power typically from a few kW to a few MW.
- An advantage of micro-turbine- generators is that instead of having a few generators of very large power rating at fixed locations and transmitting power over large distances for distribution to consumers, the power is generated in a multiplicity of generators of smaller power rating which are spread locally among consumers. These generators may operate to provide power to local loads or to a multiplicity of points in a distribution network or to a parallel combination of local loads and a distribution network. It should be noted that the output characteristic of the generator inverter in this last case is typically current-drive when there is connection to the distribution network, changing rapidly to voltage-drive if this connection is lost.
- inductor/capacitor filters of the type described there are two particular problems with inductor/capacitor filters of the type described, both associated with the fact that there is little inherent damping of resonance effects caused by the interaction of inductance and capacitance.
- the first problem is that, although they reduce high-frequency ripple harmonics associated with PWM action in the output voltage waveform, such filters also act to increase output voltage harmonics at or close to the resonant frequency.
- These harmonics may have their origin in small voltage harmonics produced within the inverter/converter, or in small harmonics of output current drawn by the load, as may typically be caused by non-linearity of the load impedance.
- sudden changes of load (as at starting or switching load in or out) can produce a slowly decaying transient envelope on the output waveform, ringing at the resonant filter frequency.
- Another problem caused by the action of the lightly damped filter is particularly relevant if overall control of the output voltage is achieved by the method of cyclic feedback.
- unwanted distortion in an output AC waveform is reduced, so causing it to follow closely a required waveform, by progressively over a number of cycles building up a demand waveform that differs from the required waveform in such a way that the output waveform is caused to better approximate to the required waveform.
- the complete feedback loop can be designed so that the total sum of phase-lags around the loop is made to increase as nearly as possible proportionately to the frequency, over as large a part as possible of the frequency spectrum extending up to close to the basic switching frequency of the switched-mode operation.
- phase-lag characteristic is that it can be substantially compensated by a simple time-advance of the signal fed back.
- a lightly damped L-C filter causes a phase lag of the output signal of the filter relative to the input signal that varies from zero to 180° lag in a very narrow frequency band. That departs substantially from the desirable near- proportionality of phase lag to frequency, and the effect on the cyclic feedback system is to cause it to be close to an unstable operating condition.
- the rapid and substantial variation of phase caused by a lightly damped resonance is likely also to produce problems for the design of overall feedback arrangements alternative to cyclic feedback, such for example as a classical linear control system.
- Harmonics in the output waveforms that are not reduced to a negligible level at the output of the filter, by the action of the filter, must be reduced by other means, typically in the form of a control system embodying an overall feedback loop around the power conditioning unit. Such a control system typically also controls the magnitudes of the wanted output waveforms. Cyclic feedback is one such control system.
- harmonics that are allowed by the action of the filter to remain unduly large may be reduced by action of the overall control system.
- An object of the invention is to overcome the problems associated with the arrangements described above.
- an active damping arrangement comprising means to apply feedback to a power conditioning unit that includes an inverter or converter operating in switched mode, and in which there are one or more electronically driven output lines that have an inductor- capacitor low-pass output filter interposed between the output point of the inverter/converter for that line and the output terminal that is connected to an external load.
- the purpose of the feedback is to provide damping of resonance effects between inductance and capacitance which can be caused by the presence of the output filter.
- a primary purpose of the output filter is to attenuate high-frequency components of voltage and current that are generated at the output of the inverter/converter as a consequence of its switching action.
- the filter is of such a nature as to effectively comprise, starting from the inverter/converter end of the filter, a series inductor LI, followed by a parallel capacitor C and optionally a further series inductor L2.
- further filter elements such as an EMC filter, that serve other purposes but contribute to the total effective values of LI or L2 or C.
- EMC filter For each output line to which the invention is applied, either current in a capacitor or voltage across a capacitor (depending on the character of the inverter/converter and the corresponding type of damping required) is sensed. The sensed signal is multiplied by a coefficient G and fed back with negative sign to a summing- junction at the input to the inverter/converter.
- the return path for the parallel capacitor C may be to a neutral point.
- the total capacitance of C may be split so as to have return paths distributed among two or more neutral points.
- the return path may be to a rail of a DC supply, or the total capacitance of C may be split so as to have return paths distributed among two or more rails.
- some or all of the capacitance of the capacitors may be connected between the output lines rather than being returned in. a path or paths as described. In this case, the manner of connection between lines is such that the capacitors function substantially in the same way that alternative capacitors of suitably chosen values would function if returned in paths as described.
- the inductors LI and L2 may be individual inductors for each output line, or there may be one or more multi-winding combined inductors which effectively serve the purpose of providing values of LI and/or L2 for more than one and possibly all of the output lines.
- the invention comprises means to provide, for a filter consisting of LI and C as described plus the further inductor L2 when appropriate, an electronic feedback loop around the filter and inverter/converter. This feedback loop is so arranged as to produce a damping effect on filter resonance which may be referred to as active damping.
- the effect is as though a resistor were present in association with the filter circuit (though this is not in all cases a precisely correct physical picture), which is however a virtual resistor rather than a real, passive resistor. Consequently the power loss that would occur in this resistor if it were of the passive type, becomes a virtual power loss that results in no significant dissipation of energy or heating effect.
- the exact manner of the feedback and the effective placement of the virtual resistor are different according to whether the power conditioning unit is a voltage-drive or a current-drive.
- the feedback to provide active damping may be achieved by digital or analogue means. However if digital feedback is employed, in conjunction with digital control of the PWM process, it is found that consequent time delays around the feedback loop easily begin to degrade performance. It is necessary to minimise such time delays and study of system performance by computer modelling during the design process is advisable to ensure that undesirable effects are controlled at an acceptable level.
- the invention as described acts, in control system terms, as an inner loop that can be readily combined with an outer loop and/or with other control signals to provide further control functions.
- An example of such an outer loop is cyclic feedback, which combines with active damping in a power conditioning unit without difficulty.
- cyclic feedback or possibly in addition, a conventional linear overall control loop may be added, or alternatively some form of non-linear control loop, and with all of these possibilities the control loop may further be made adaptive in form. Active damping is generally compatible with all these variations, and a skilled person will be able to design an arrangement to combine the damping and other control functions without difficulty.
- third-harmonic injection is also well-known, whereby for three-phase output a larger output voltage may be obtained from a given DC link voltage without gross distortion of the output waveform occurring due to clipping of the voltage signal against the limits of the DC rail voltages.
- Application of third-harmonic injection to a voltage-drive in combination with the invention as described is similarly straightforward, involving simply the addition of a third-harmonic signal waveform of appropriate magnitude and phase to one or more input summing-junctions for the three-phase output lines and/or for an electronically driven neutral line, according to the application, as described in more detail later.
- the inverter/converter may be of voltage-drive form so that a voltage signal at an input point of the inverter/converter produces a corresponding voltage at output.
- the feedback is arranged as follows. The current in the capacitor C is sensed, multiplied by a coefficient G and fed back to a summing-junction at the input to the inverter/converter so as to be added with negative sign to the demand signal waveform that drives the total input to the inverter/converter. This results in the output voltage waveform from the inverter/converter.
- Rf is such as to achieve very roughly critical damping; in other words, Rf is of the same order as XL1 or XC at the undamped Ll-C resonant frequency, these two values being equal.
- the load draws a small output current at that frequency then correspondingly a large voltage would again appear in the output, due to the denominator of the second term.
- the small input voltage may arise, not at the input summing-junction as represented above, but due to small distortion effects within the inverter/converter, and the small harmonics of output current may typically be drawn by the load, due to the load impedance being markedly non-linear.
- Equation (1) shows that damping is achieved without the disadvantage of a very large power loss that there would necessarily be in a real, passive damping resistor, but the arrangement is actually superior to a real resistor in another important respect.
- a real resistor in series with LI, it increases the voltage drop through the filter at the basic AC frequency; in parallel with LI or in series with C it reduces the effectiveness of the filter in attenuating high-frequency ripple voltages associated with the switching action; in parallel with C it increases the current at basic AC frequency that is drawn by the filter in parallel with the output current to the external load. All these difficulties are avoided by the active damping circuit.
- the inverter/converter may be of a current-drive form so that a voltage signal proportional to a current applied at an input point of the inverter/converter produces a corresponding current at output.
- For each output line feedback is applied to produce active damping in which voltage is sensed across a capacitor of the output filter.
- the sensed signal is multiplied by a coefficient, passed through a high-pass filter (preferably though not necessarily second-order) and effectively added with negative sign to the input of the inverter/converter.
- phase of the output current from the filter varies relative to the input current from zero electrical degrees at a frequency sufficiently below the resonant frequency up to 180° lag at a frequency sufficiently above. If the filter is only lightly damped, most of this 180° phase change occurs rapidly in a narrow frequency band, as before, and this again poses a problem for the stability of cyclic feedback.
- the feedback is arranged as follows.
- the multiplied signal is then added with negative sign to the demand signal waveform that drives the input to the inverter/converter and results in the output current waveform from the inverter/converter.
- any scale factor that may exist around the feedback loop for example between the magnitude of signal at input and corresponding current at output of the inverter/converter is disregarded in this discussion, it being assumed that the signal fed back is scaled in accordance with such scaling.
- Io Ii ((-jYL2)/(Yf - JYL2 + jYQ) -
- Equation (2) is similar in form to Equation (1), with voltage and current interchanged, and resistance and reactance exchanged for conductance and admittance; however the equation forms are not identical, again as discussed below.
- the two undesirable effects caused by lightly damped resonance, of a large output current being caused by a small input current and/or a small output voltage, are suppressed.
- the small input current may arise, not at the input summing-junction as represented above, but due to small distortion effects within the inverter/converter, and the small harmonics of output voltage are typically applied by the load, the voltage characteristic of the load with current-drive generally being not perfectly sinusoidal.
- Equation (2) it can be seen that there is an effect equivalent to a virtual resistor of conductance value Yf effectively positioned in parallel with C. This effectively damps resonance effects between
- Equation (2) L2 (or in practice L2tot) and C, in a similar fashion to the damping that is seen in Equation (1).
- Equation (2) the virtual resistor is also represented in the numerator of the second term, so that the physical analogy is in this case exact, unlike Equation (1) where there is no virtual resistance effect represented in the numerator.
- Equation (1) shows that at the basic AC operating frequency, the applied output voltage greatly affects the output current when Yf ⁇ 0.
- Equation (1) shows that the output current drawn only marginally affects the output voltage when Rf ⁇ 0.
- the frequency of switching may be selected.
- the corner frequency of the output filter, flo may be set, by selecting values for LI and C (see earlier), sufficiently below the switching frequency (probably by one decade or rather more) to enable the output filter to achieve the required attenuation at output of the high-frequency ripple harmonics caused by switching.
- L2 if required may be selected (see earlier).
- a value of G may be selected (see earlier).
- Figure 2 is a block diagram of a form of voltage-drive
- Figure 3 is a block diagram of an active damping arrangement for the drive of Figure 2;
- Figure 4 is a block diagram of a form of current-drive
- Figure 5 is a block diagram of an active damping arrangement for the drive of Figure 4.
- Figure 2 shows a power conditioning unit for a generator, which generator is, for example, rated at 120 A RMS line current, 277 V RMS maximum phase voltage and 50/60 Hz.
- the power conditioning unit includes a pulse- width- modulated voltage controller, PWMVC. Simple pulse-width modulation is employed with a modulating frequency of about 8 kHz.
- a four-limb output inverter each limb connected between the rails of a DC link, which is preferably 900 V.
- One limb of the inverter is an active neutral N and the other three limbs constitute the three- phase output lines A, B, C.
- Each of the four inverter lines are connected to an output filter OPF.
- OPF output filter
- Each line filter comprises a series inductor LI plus two parallel capacitors, each C/2, that are connected between the two rails of the DC link.
- the main filtering action is provided by LI and C. Connected in series with LI is an inductor L2.
- each output line OA, OB, OC is sensed relative to the neutral line ON.
- the voltage of the neutral is sensed relative to the mid-point of the DC rails and the required waveform for this line is substantially zero.
- the cyclic feedback path for one line is shown in the diagram. It will be appreciated that like feedback paths are provided for each of the other lines.
- the sensed voltage signal passes through a voltage reduction stage VR. The purpose of this stage is simply to scale the voltage signal to a safe level appropriate to the signal circuitry and not to attenuate the signal as part of the signal processing.
- phase-compensator PCI that is operable to alter the phase-frequency characteristic of the fed back signal so as to modify the variation of phase with frequency in selected ranges of frequency in a pre-determined manner.
- PC 1 is similar to PC2, described later.
- PCI precedes various signal-processing stages compared with PC2, it may be, found that a particular form of compensation is more conveniently or more effectively achieved in either PCI or PC2, as the case may be.
- any appreciable phase shift through PCI at the basic working frequency in this example 50/60 Hz
- phase shift must accordingly be avoided.
- the compensated signal then passes through an analogue-to-digital converter ADC1, which in a preferred embodiment is a 10-bit processor, sampling at 8 kHz.
- ADC 1 The output of ADC 1 is fed to a summing-junction and differenced with the required signal waveform RW, which is fed down time-delayed as represented by the time-lag stage Lagl.
- the time-delay is set to correspond with the time-delay caused by ADC1, so that the two signals at the summing- junction are in correct time-relationship, both lagging in time relative to the output waveform.
- the magnitude of the lag is a matter of practical convenience and is corrected later in the loop as part of the function of Advl, which is adjusted heuristically to give optimum performance.
- the output from the summing-junction is the sampled error signal waveform of the last cycle.
- This passes through a clipping stage Clipl, where the magnitude of any part of the waveform that exceeds a selected maximum limit is reduced in value to that limit, for example about 100 V.
- the signal is passed to a discretisation stage Disci where it is modified so that the overall waveform magnitude is represented by a series of discrete values.
- the discretisation level i.e. the magnitude of the change between each discrete value, is selected for optimum effect and is different from that occurring automatically in the signal sampling.
- the discretisation level may be set at about 5 V steps.
- a process of discarding signal magnitudes below a threshold level of about 7 V may be used.
- Discarding is less effective in some respects, and more so in others, for reducing a tendency in one form or another to instability at higher frequencies; a heuristic process of trial- and-error is accordingly helpful in optimising the design choice.
- Discarding has the advantage that it saves on processor requirement as compared with discretisation.
- the discretised signal (or the signal that has been through the discard process) then passes to an attenuation stage Attl.
- the waveform could be multiplied by a gain coefficient less than unity.
- a constant amount could be subtracted from the modulus of the signal at each point of the waveform and wherever the result of the subtraction is less than zero, the value of the signal could be set to zero.
- a zoned procedure could be used, in which different zones of the AC cycles are attenuated in accordance with differently defined algorithms, the zones being different regions of time in the AC cycle or different ranges of signal magnitude.
- attenuation is by multiplication and set at about times 0.3.
- the attenuated signal waveform then passes to a summing-junction, at which it is added to a signal waveform store PCE, in which is stored the cumulative error waveform of previous cycles.
- the two sampled waveforms are summed, passed through a further clipping stage Clip2, preferably set at about 100V, and returned into the store PCE, ready to be used as the total cumulative error waveform in the next cycle.
- the stored waveform PCE is read out in the next cycle, but advanced in time by a selected amount, as represented by the block Advl.
- This processed total error waveform is differenced from the waveform read out from the required waveform store RW, this difference representing the demand waveform.
- a drive signal waveform for third-harmonic injection, 3HI is added.
- third harmonic injection is employed with the injected third-harmonic voltage set to about 15% of the rated phase voltage.
- the output of the summing-junction passes through a digital-to-analogue converter DAC1, the output of DAC1 being the analogue drive signal, including third-harmonic injection, for the next cycle.
- DAC1 digital-to-analogue converter
- PC2 phase-compensator
- the output of PC2 represents the total demand waveform, which is input to PWMVC as a revised demand signal. This process is continuously repeated so that the output waveform progressively converges towards the required waveform, despite the presence of various undesirable harmonic distorting effects.
- phase-frequency characteristic is meant the characteristic of change of phase with frequency around the entire cyclic feedback loop, except that the time advance feature of Advl is neglected, of a signal at a general frequency f, where f takes all values within the range of frequency of interest. Since, according to the nature of cyclic feedback, the signal referred to forms an input in one cycle and an output in the next cycle of cyclic feedback, the relevant phase difference is that between ouput and input signals expressed on a common time-base represented by the required waveform at basic frequency. This phase-frequency characteristic may be simulated by computational modelling.
- phase-frequency characteristic of the part of the circuit which contributes the most sizeable and complex effect may be measured. This may be done by cutting the feedback loop between DAC1 and PC2 and inputting a signal of frequency f to PC2, and likewise cutting the loop between ADC1 and PCI and measuring the output signal from PCI.
- phase-compensators PCI and PC2 may be designed to compensate for departure of the total phase- frequency characteristic from linearity, which typically occurs in sub-ranges of frequency within the overall pre-determined range.
- PCI and PC2 are shown at stages in the feedback loop where the signal path is analogue in form. They are typically constructed ;out of passive components and operational amplifiers. It will be appreciated, however, that PCI and PC2 could equally be placed where the feedback path is digital and digital processing used. However, this has the disadvantage that the compensators PCI and PC2 then add to the burden of signal processing in the loop and may introduce signal-distorting effects associated with digital filters.
- the performance of the cyclic feedback system may be further improved by introducing active damping.
- a signal from the output filter OPF is fed back to an input of the inverter, which signal is proportional to the current in the capacitor C.
- the active damping arrangement of Figure 2 is shown in more detail in Figure 3. As before, the ouput filter for only one line is shown.
- Figure 3 shows a sensor that is provided for sensing current in the capacitor C.
- the sensed current signal is adjusted in magnitude, equivalent in effect to multiplying the current value by a coefficient G, to provide a damping signal.
- G may vary within a range, according to the particular application. Typically, it may have a value that is substantially of the same order as the reactance of the total capacitance C or inductance LI at their undamped resonant frequency (these values being equal).
- the damping signal is fed back to a summing-junction at an input to the inverter, as shown in Figure 3, this summing-junction being between PC2 and the input to the pulse-width modulation stage (not shown in detail) of PWMVC. In this way, the output of the combined inverter/converter and output filter is actively damped.
- active damping is introduced with the feedback coefficient G (see Figure 3) set to 1.5 ohms.
- This degree of active damping yields a phase-frequency characteristic for the filter, which in combination with all other contributory sources of phase- frequency effect around the loop yields a total characteristic for the loop that has acceptably low deviation from linearity.
- the deviation may be so low as to make it possible to omit the phase-compensators PCI and/or PC2 from the cyclic feedback loop.
- the output waveform is sampled, stored and compared with a pre-determined required waveform, the difference between the output and the required waveforms representing an error waveform.
- the error waveform is processed by clipping, discretising and attenuating, to produce a processed error waveform. Once this is done, the processed error waveform is added to a cumulative sum of processed error waveforms, with further clipping at that addition stage. The new cumulative sum of processed waveforms is advanced in time relative to the output waveform and subtracted from the required waveform to produce a revised demand waveform for use in the next AC cycle.
- the active damping loop which is a fast-responding loop
- current through the capacitor is multiplied by a coefficient G and fed to an input of the inverter as described, producing an improved phase-frequency characteristic for the closed loop of the cyclic feedback arrangement.
- the active damping also acts to reduce the uncorrected harmonic distortion at the output terminals of the power conditioning unit, by which is meant, the distortion that would be present in the absence of cyclic feedback.
- the task of the cyclic feedback is eased, which tends further to improve the performance of the generator.
- the combination of qualities of level of harmonic distortion, settling time, and robustness of the output waveform purity and stability to changes in load condition may be further improved over what is available using only cyclic feedback.
- the generator output substantially settles in 2 - 3 cycles following a sudden disturbance. This is advantageous.
- the system has been shown to provide better than 3% THD into a wide range of linear and non-linear loads.
- Figure 4 shows a power conditioning unit for a generator, which generator is, for example, rated at 120 A RMS line current, 277 V RMS maximum phase voltage and 50/60 Hz.
- the preferred DC link is 900V.
- the power conditioning unit is a current- drive, as opposed to a voltage-drive.
- a main inverter block which is a bang-bang hysteresis-type current controller BBHCC for driving three output lines A,B,C, which BBHCC is preferably provided with a variable-width hysteresis-band feature.
- the aim of this is to keep the switching frequency roughly constant at around 8 kHz throughout the AC cycle; the band is set to vary from approximately 80A near the zero crossovers of voltage to 40A near the voltage peaks. Also provided is an active neutral limb N.
- the drive for the neutral N is a pulse-width modulated voltage controller drive PWMVC as previously described.
- an output filter OPF Connected to each of the lines in the controller is an output filter OPF, the form of which is the same as for Figure 2.
- OPF output filter
- an active damper may be incorporated around the filter and inverter, through the feedback path ADFB.
- line N the nature of the active damper would be the same as that shown in Figure 3.
- the arrangement is slightly different. This will be described later in more detail.
- the current in each output line OA, OB, OC is sensed and the voltage of the neutral is sensed relative to the mid-point of the DC rails, the required waveform for the neutral line being zero.
- the feedback paths for one output line are shown in Figure 4. Concentrating on the output lines OA, OB, OC, the current in LI is fed back to the bang-bang controller, as shown, which accordingly exercises direct control on that current and not the output current in L2.
- the cyclic feedback loop acts to correct the situation by forming an error correction term in PCE, which alters the current controlled in LI so as to make the current in L2 accord with the required current waveform, RW.
- the signal fed back in the outer cyclic feedback loop is the current in L2, formed by differencing at a summation stage, the sensed currents in LI and C.
- a sensor is provided for sensing the current passing through LI, the sensed current being added into a summing-junction with positive sign.
- Another sensor is provided for sensing the current passing through the capacitor C, the sensed current being added into the summing-junction with negative sign.
- the difference between the two sensed currents provides an indirect measure of the current in L2 at the output.
- the current waveform is then passed through a phase-compensator PCI, and an analogue-to-digital converter ADC1, whose descriptions and functions are as previously given.
- the output of ADC 1 is fed to a summing-junction and differenced with the required waveform signal RW, which is fed down time-delayed as represented by the time-lag stage Lagl.
- the time-delay is set to correspond with that caused by ADC1, as before, so that the two signals at the summing-junction are in correct relative time-phase, both lagging in time relative to the output waveform.
- the output from the summing-junction is the sampled error signal waveform of the last cycle.
- This passes through a clipping stage Clipl, where the magnitude of any part of the waveform that exceeds a selected maximum limit is reduced in value to that limit, for example about 20A.
- the signal is passed to a discretisation stage Disci where it is modified so. that the overall waveform magnitude is represented by a series of discrete values.
- the discretisation level may be set at about 1A steps.
- a procedure of discarding signal magnitudes below a threshold level of a few amperes may be acceptable, the choice in this matter being influenced both by overall effectiveness in practice, and demand on processing power.
- the discretised signal (or the signal that has been through the discard process) then passes to an attenuation stage Attl.
- Attl an attenuation stage
- various options for attenuating the waveform magnitude are possible.
- the attenuation may be by straight multiplication and set at about times 0.02.
- the processed signal waveform from Attl then passes to a summing-junction, at which it is added to the signal waveform store PCE, which is the cumulative error waveform of previous cycles.
- the two sampled waveforms are summed, passed through a further clipping stage Clip2, preferably set at about 20A, and returned into the store PCE, ready to be used as the total cumulative error waveform in the next cycle.
- the stored waveform PCE is read out in the next cycle, but read out advanced in time by a selected amount, as represented by the block Advl.
- This processed total error waveform is subtracted from the waveform read out from the required waveform store RW, the difference representing the demand waveform.
- a drive-signal waveform for third- harmonic injection, 3HI is added. In contrast to the voltage-drive of Figure 2, this is done only for the neutral line.
- the third-harmonic injection signal waveform, 3 HI is a voltage waveform.
- the voltages on the lines OA, OB, OC, relative to the neutral, are set by the voltages of the connected load, the terminal voltage characteristic of the inverter being 'soft' as noted earlier. Therefore, these lines follow the third-harmonic voltage of the neutral, so that third-harmonic injection is automatically applied to all four lines.
- the output of the summing-junction passes through a digital-to-analogue converter DAC1 and then to another phase-compensator PC2.
- the output of PC2 is the analogue drive signal representing the total revised demand waveform, including third-harmonic injection, for the next cycle, which is input to BBHCC.
- an active damping arrangement is provided in each of the output lines to further improve the stability and output waveform quality of the power conditioning unit.
- the active damping arrangement for lines A,B,C of Figure 4 is shown in more detail in Figure 5. As before, the ouput filter for only one line is shown.
- Figure 5 shows an active damping arrangement for a current-drive having a sensor that is provided for sensing voltage across the capacitor C.
- the sensed voltage is adjusted in magnitude, equivalent to multiplying it by a coefficient G, to provide a damping signal.
- G may vary within a range to suit particular applications as previously described.
- the coefficient is set to 0.7 inverse-ohms.
- the signal is then fed back to a summing- junction at an input of the inverter, this summing-junction being between PC2 and the input to the bang-bang control stage of BBHCC (not shown in detail). In this way, the output of the combined inverter/converter and output filter is actively damped.
- the chosen value of G gives a degree of damping of the L2tot-C resonance that is substantial though less than critical, depending on the characteristics of the external load and the effective value of L2tot taking account of the contribution from the load inductance. With the embodiment shown values of L2tot up to about 200 micro-H are acceptable. It should be noted that the phase- frequency characteristic of the actively damped filter is further influenced by the high-pass filter HPF.
- this degree of active damping yields a phase-frequency characteristic for the filter which, in combination with all other contributory sources of phase-frequency effect around the loop, yields a total characteristic that has acceptably low deviation from linearity.
- this deviation may be sufficiently low as to make it possible to omit the phase-compensators PCI or PC2 from the cyclic feedback loop.
- stability can only be maintained over a substantial range of load conditions with a small value of the attenuation coefficient Attl, such as to cause the settling time to occupy a considerable number of cycles. This is, however, acceptable for typical current-drive applications.
- Both Figures 2 and 4 depict drives in which the signal processing is sufficient to provide an acceptable level of harmonic distortion, stability, and robustness, suitable for many applications.
- the active damping serves a dual purpose, as described earlier. It improves the phase-frequency characteristic of the cyclic feedback loop, and it reduces the harmonic distortion at the output terminals of the power conditioning unit in the absence of cyclic feedback, thus making the task of the cyclic feedback easier and so further improving the performance of the generator.
- a third-harmonic injection waveform may be added, the output of the summing-junction then being further signal-processed and made available as an input, point 107.
- the third-harmonic injection signal may be added after the further signal- processing, immediately prior to point 107, if that arrangement results in significantly less unwanted phase-shifting of the third-harmonic signal relative to the required waveform; this alternative is not shown in the Figures.
- the input 107 is to the sub-component of the system that comprises the inverter or converter and the output filter. The output from that sub-component is sensed, and the sensed signal closes the loop back to point 101.
- Power conditioning units essentially similar to those described with reference to Figures 2 and 4 have a three-phase output (with or without a fourth neutral line), and may have a power rating in a range from a few kW to a few MW.
- the method of control is equally applicable to single-phase supply or to supply with any other phase-number and to power ratings above and below the stated range.
- the neutral line Whilst the units of Figures 2 and 4 show neutral lines, it is possible in principle that a neutral line is not required, although this may require some modification to signal processing procedures. Alternatively, the neutral line may be a passive connection to an appropriate point in the power conditioning unit. As described, it is possible for the amplitude of the AC waveform to be zero at an output line of the power conditioning unit, as may be the case with an active neutral line, or the said AC waveform may contain only third-harmonic frequency, and the method in which the invention is embodied may still be applicable to the line.
- the cyclic feedback loops of Figures 2 and 4 work to reduce unwanted harmonics in the output waveform, for all harmonics whose frequencies are integral multiples of the basic AC operating frequency, so that the distortion produced by the harmonics remains in constant phase relation to the AC cycle in successive cycles of operation. It is well known that such harmonics, of substantial size, are generated and/or modified both by internal effects within the power conditioning unit and by external effects due to the applied load. Distortion of the AC waveform by the occurrence of a DC component is also reduced by the feedback.
- the period of the AC cycle is not constrained to be constant and may vary in accordance with the variation of output frequency, for example as the output follows the frequency of a power distribution network to which the power conditioning unit is connected. It may be assumed however that significant change in the period occurs only over a time that is long in comparison with one period, in which case no difficulties of principle are caused for the working of the invention. However, means must be provided tor sense with sufficient accuracy the period of the AC cycle at any particular time.
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- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Inverter Devices (AREA)
Abstract
Description
Claims
Priority Applications (2)
Application Number | Priority Date | Filing Date | Title |
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EP00987577A EP1344307A1 (en) | 2000-12-22 | 2000-12-29 | A damping system |
US10/433,879 US7035124B2 (en) | 2000-12-22 | 2000-12-29 | Damping system |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
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GB0004973 | 2000-12-22 | ||
GBPCT/GB00/04973 | 2000-12-22 |
Publications (1)
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WO2002052709A1 true WO2002052709A1 (en) | 2002-07-04 |
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PCT/GB2000/005003 WO2002052709A1 (en) | 2000-12-22 | 2000-12-29 | A damping system |
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US (1) | US7035124B2 (en) |
EP (1) | EP1344307A1 (en) |
WO (1) | WO2002052709A1 (en) |
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US6979914B2 (en) | 2003-02-20 | 2005-12-27 | Ebara Corporation | Power generating apparatus |
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US9722511B2 (en) * | 2012-12-07 | 2017-08-01 | General Electric Company | Systems and methods for controlling an electrical power supply |
US11289940B2 (en) | 2013-06-14 | 2022-03-29 | Abb Schweiz Ag | Systems and methods for multi-use multi-mode ups |
TWI499193B (en) * | 2013-08-16 | 2015-09-01 | Univ Nat Cheng Kung | Power conversion apparatus and control method thereof |
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US7035124B2 (en) | 2006-04-25 |
US20040080966A1 (en) | 2004-04-29 |
EP1344307A1 (en) | 2003-09-17 |
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