CN113224967A - AC-AC isolated modular converter based on low-voltage DC bus - Google Patents

AC-AC isolated modular converter based on low-voltage DC bus Download PDF

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Publication number
CN113224967A
CN113224967A CN202110474999.3A CN202110474999A CN113224967A CN 113224967 A CN113224967 A CN 113224967A CN 202110474999 A CN202110474999 A CN 202110474999A CN 113224967 A CN113224967 A CN 113224967A
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voltage
switching tube
bridge arm
current
isolated
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刘闯
裴忠晨
蔡国伟
宋晓民
朱帝
姜宇
王菁月
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Northeast Electric Power University
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Northeast Dianli University
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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
    • 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
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • 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
    • H02M1/08—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters
    • H02M1/088—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters for the simultaneous control of series or parallel connected semiconductor devices

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Inverter Devices (AREA)

Abstract

The invention discloses an AC-AC isolated modular converter based on a low-voltage direct-current bus, which comprises three phase units connected in parallel to a medium-voltage single-phase alternating-current side port, wherein each phase unit comprises an upper bridge arm and a lower bridge arm which are connected with each other, and the joint of each upper bridge arm and each lower bridge arm is connected with one medium-voltage alternating-current side port through a filter inductor. The single-polarity modulation scheme of the isolated AC-AC matrix converter has three voltage ports of medium-voltage three-phase AC, medium-voltage single-phase AC and low-voltage DC, single-stage power conversion from the low-voltage DC port to the medium-voltage AC port is realized, the medium-voltage single-phase AC port and the medium-voltage three-phase AC voltage port can realize free AC-AC power conversion with the same or different frequencies, and the single-polarity modulation strategy of the single-polarity modulation scheme avoids the problems of voltage spikes and the like in the current conversion transient process of a bidirectional switch tube of the isolated AC-AC matrix converter.

Description

AC-AC isolated modular converter based on low-voltage DC bus
Technical Field
The invention belongs to the technical field of isolated modular multilevel cascade power conversion, and particularly relates to an AC-AC isolated modular converter based on a low-voltage direct-current bus.
Background
With the continuous and rapid development of economy and the annual acceleration of urbanization process in China, the key problem of how to improve the power supply capacity in the existing power transmission corridor, reduce the construction and transformation cost of the power transmission and distribution line and expand and transform the capacity of a power supply system into an urban power grid which meets the power supply requirement is solved. Advanced power transmission and distribution technologies such as direct current power transmission and flexible low-frequency power transmission become key technical means for the development requirements of modern urban power grids.
At present, an urban energy network is still established based on an alternating current distribution network, in direct current transmission and distribution, an urban alternating current cable has a space charge accumulation effect, the existing cable is difficult to change from alternating current to direct current, a direct current cable needs to be laid again, the allowable maximum temperature of the direct current cable is 70 ℃, and the current-carrying capacity of the direct current cable is reduced. Compared with the flexible power transmission system based on MVDC, the low-frequency power transmission has the following advantages: the low-frequency power transmission system can still continue to use a circuit breaker and other control and protection devices in a power frequency power grid, a ring network is easier to form, and the fault bearing capacity and reliability are superior to those of MVDC power transmission; the low-frequency cable power transmission system is compatible with the existing power grid alternating current cable, and has no direct current cable space charge accumulation problem, so that the low-frequency cable power transmission system has higher economy compared with a low-frequency cable power transmission system which is re-laid with a direct current cable; the economy of low frequency transmission is best when the transmission distance is approximately 30km-150 km. Therefore, the flexible alternating-current low-frequency power transmission and distribution is more suitable for the capacity expansion transformation of the existing urban power grid system, and the power transmission capacity of the power transmission and distribution line is increased. On the basis of the existing power transmission and distribution corridor, the advanced power transmission and distribution AC-AC converter becomes key equipment for improving the power transmission and distribution capacity of a line, meeting the power conversion and power supply requirements of a power grid on different loads and improving the power supply reliability and economy. The japanese scholars, akamu thai, generalize multi-level converters in the form of power module cascades, collectively referred to as the family of modular multi-level cascade converters (MMCC). The MMCC is a two-port non-isolated AC-AC converter, is mainly applied to long-distance high-capacity HVAC transmission, and lacks an LVDC port for utilizing urban distributed energy. In the AC-AC low-frequency conversion process of the MMCC, the current passing through the branch circuit has power frequency and low-frequency components, the internal power fluctuation causes the capacitance voltage fluctuation of the power module and makes the circulation phenomenon prominent, the lower the low-frequency side frequency is, the larger the capacitance voltage fluctuation of the sub-module is, and the complex control of maintaining the multi-level voltage balance of the sub-module and the bridge arm is. At present, research and research on low-frequency power transmission technology is concentrated in the field of three-phase power transmission, low-frequency voltage waveforms are three-phase symmetrical sine waves, research on single-phase low-frequency power transmission is little, only two power transmission cables are needed for single-phase low-frequency power transmission, and the single-phase low-frequency power transmission is more advantageous than three-phase power transmission when investment cost reduction and occupied area reduction of a power transmission corridor are considered.
Disclosure of Invention
The invention aims to provide an AC-AC isolated modular converter based on a low-voltage direct-current bus, overcomes the limitation and instability of the traditional multilevel cascade converter topological structure for direct conversion from medium-voltage three-phase alternating current to single-phase alternating current equal-frequency or variable-frequency, increases a low-voltage direct-current port to realize multi-energy complementation and provides an interface for urban distributed energy.
The technical scheme adopted by the invention is that the AC-AC isolated modular converter based on the low-voltage direct-current bus comprises three phase units connected in parallel to a medium-voltage single-phase alternating-current side port, each phase unit comprises an upper bridge arm and a lower bridge arm which are connected with each other, and the joint of each upper bridge arm and each lower bridge arm is respectively connected with one medium-voltage alternating-current side port through a filter inductor.
The invention is also characterized in that:
the upper bridge arm and the lower bridge arm respectively comprise n single-stage isolated full-bridge modules, and the input side of each single-stage isolated full-bridge module is connected with a low-voltage direct-current capacitor C in paralleldcLLow voltage DC capacitor CdcLThe low-voltage direct-current power supply is connected, the positive input ports of the n single-stage isolated full-bridge modules are connected to form isolated full-bridge positive input ports, the negative input ports of the n single-stage isolated full-bridge modules are connected to form isolated full-bridge negative input portsThe output ends of n single-stage isolated full-bridge modules are connected in a mode that the adjacent negative output ports are connected with the positive output port, and the negative output port of the tail end single-stage isolated full-bridge module is connected with the smoothing reactor LmThe isolated full-bridge positive input port of the upper bridge arm is connected with the isolated full-bridge positive input port of the lower bridge arm, the isolated full-bridge negative input port of the upper bridge arm is connected with the isolated full-bridge negative input port of the lower bridge arm, the positive electrode of a single-stage isolated full-bridge module at the head end of the upper bridge arm is connected with the positive electrode of a medium-voltage unidirectional alternating current end, and the smoothing reactor L of the upper bridge armmSmoothing reactor L connected with lower bridge armmAnd the output port of the cathode of the single-stage isolated full-bridge module at the tail end of the lower bridge arm is connected with the cathode of the medium-voltage unidirectional alternating-current end.
Each single-stage isolation type full-bridge module comprises two single-stage isolation type chopping modules, input sides of the two single-stage isolation type chopping modules are connected in parallel, and output sides of the two single-stage isolation type chopping modules are connected in series in a reverse mode.
The single-stage isolation type chopping module comprises a switch tube Q1And a switching tube Q2And a switching tube Q3And a switching tube Q4Switching tube Q1The emitter of the transistor is connected with a switch tube Q3Collector electrode of (2), switching tube Q2The emitter of the transistor is connected with a switch tube Q4Collector electrode of (2), switching tube Q1Collector electrode of (2), and switching tube Q2The collector electrodes are all connected with a low-voltage direct current capacitor CdcLThe positive plate forms the positive electrode of the input side and the switch tube Q3Emitter and switching tube Q4The emitting electrodes are all connected with a low-voltage direct current capacitor CdcLThe negative plate forms the negative electrode of the input side and the switching tube Q1The emitter of the high-frequency transformer is connected with one end of the primary side inlet of the high-frequency transformer T and the switching tube Q2The emitter of the high-frequency transformer is connected with the other end of the inlet of the primary side of the high-frequency transformer T;
also comprises a switch tube Q5And a switching tube Q6And a switching tube Q7And a switching tube Q8Switching tube Q5The emitter of the transistor is connected with a switch tube Q7Collector electrode of (2), switching tube Q6The emitter of the transistor is connected with a switch tube Q8Collector electrode of (2), switching tube Q5Collector and switching tube Q6The collector electrode of the switching tube Q is connected to form an output side anode7Emitter and switching tube Q8The emitting electrodes of the first and second transistors are connected to form an output side cathode and a switching tube Q5The emitter of the high-frequency transformer is connected with one end of the secondary side outlet of the high-frequency transformer T, and the switching tube Q6The emitter of the high-frequency transformer is connected with the other end of the outlet of the secondary side of the high-frequency transformer T.
The single-stage isolation type chopping module further comprises a clamping circuit connected with an outlet of the secondary side of the high-frequency transformer T.
The clamping circuit comprises a diode D1Diode D2Diode D3Diode D4Diode D1Diode D connected to anode3Negative electrode, diode D2Diode D connected to anode4Negative electrode, diode D1Cathode, diode D2Negative electrode connecting capacitor C1Positive plate, diode D3Anode, diode D4Positive electrode connecting capacitor C1Negative plate and capacitor C1Parallel resistor R1Diode D1The anode is connected with one end of the secondary side outlet of the high-frequency transformer T and a diode D2The positive pole is connected with the other end of the outlet of the secondary side of the high-frequency transformer T.
The invention has the beneficial effects that:
(1) the LVDC port is added, meanwhile, the low-voltage direct current port can flexibly control active power flow and can be used as an energy storage or distributed power interface, the limitation and instability of the traditional multilevel cascade converter topological structure for directly converting the medium-voltage three-phase alternating current to the single-phase alternating current at the same frequency or variable frequency are overcome, and the method is suitable for the application scene of urban distributed power;
(2) because the polarity of the rated port voltage of the single-stage isolated chopping module in the converter is positive, two active tubes in opposite directions do not need to be connected in series like a cycle converter, so that a passage can be formed when the port voltage is negative, and the problem of secondary side current conversion of the traditional single-stage converter is avoided;
(3) the LVDC side capacitor voltage fluctuation caused by power fluctuation is mainly related to three-phase power frequency doubling and single-phase low frequency doubling, the I-MMCC collects the three-phase and single-phase frequency doubling fluctuation to the LVDC side capacitor through a high-frequency link, and the three-phase power frequency doubling and the single-phase frequency doubling are mutually offset, so that the influence of the single-phase low frequency doubling on the capacitor voltage is only needed to be considered, only the single variable and the single capacitor voltage are needed to be controlled, and the control difficulty is reduced.
(4) According to the invention, as the secondary side part of the single-stage isolated chopping module is not provided with a capacitor, and the port voltage level of the secondary side is determined by the output voltage of the single-stage isolated full-bridge module, compared with a topological structure of a multilevel cascaded converter, a complex sampling circuit and complex voltage-sharing control are not needed, the control difficulty is reduced, and the reliability of the system is also improved.
Drawings
FIG. 1 is a schematic structural diagram of an AC-AC isolated modular converter based on a low-voltage DC bus;
FIG. 2 is a schematic view of the structure of each phase unit in the present invention;
FIG. 3 is a schematic structural diagram of a single-stage isolated full-bridge module according to the present invention;
FIG. 4 is a schematic structural diagram of a single-stage isolated chopper module according to the present invention;
FIG. 5 is a schematic diagram of a single-stage isolated full-bridge module driving signal modulation strategy of an upper bridge arm;
FIG. 6 is a main waveform diagram of an upper bridge arm single-stage isolation type chopping module;
FIG. 7 is a medium-voltage side equivalent circuit model of a single-phase multi-level cascaded converter topological structure;
FIG. 8 is MVAC in transient condition(T-P)A port voltage current waveform;
FIG. 9 is MVAC in transient condition(S-P)A port voltage current waveform;
FIG. 10 is a waveform of constant frequency voltage and current at two AC ports under steady state conditions;
FIG. 11 is MVAC at 50Hz in transient condition(T-P)A port voltage current waveform;
FIG. 12 is MVAC at 50Hz in transient condition(S-P)Port voltage current waveform diagram.
Detailed Description
The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.
Aiming at the advantages of single-phase low-Frequency transmission in an urban power transmission and distribution system, a High-Frequency Link (HFL) technology is combined with an MMCC (multi-level alternating current-alternating current) converter, and an AC-AC single-stage isolated modular multilevel cascaded converter (I-MMCC) topology based on a common direct-current bus is provided. The topology adopts a single-pole structure, has a three-degree-of-freedom modulation strategy with two alternating current modulation ratios and one direct current modulation ratio, and can complete direct decoupling control among ports. The converter has a medium voltage three phase alternating current (MVAC)(T-P)) Medium voltage single phase alternating current (MVDC)(S-P)) And the LVDC ports are added to realize multi-energy complementation, an interface is provided for urban distributed energy, and the problem of voltage spike caused by secondary side conversion of a single-stage converter is solved. As shown in fig. 1 and 2, the I-MMCC based medium/low voltage, ac/dc distribution system can provide a plug and play interface for dc loads and renewable energy sources, and improve the renewable energy consumption and energy conversion efficiency, and the (converter) topology has an ac/dc high-freedom modulation ratio, and can complete decoupling control between ports. The low-frequency transmission line can realize cross-regional interconnection of the power distribution network, energy is supplied by novel power supplies such as a wind power plant, a hydrogen energy storage system and a photovoltaic power station, the energy is fed into an alternating current network from a low-voltage direct current side port, and the energy is fed into the alternating current network through a single-phase low-frequency alternating current port MVAC(S-P)The power transmission corridor is transmitted to a remote place to provide green energy for urban rail transit.
The AC-AC isolated modular converter based on the low-voltage direct-current bus comprises three phase units which are connected in parallel to a medium-voltage single-phase alternating-current side port, each phase unit comprises an upper bridge arm and a lower bridge arm which are connected with each other, and the connection part of each upper bridge arm and each lower bridge arm is connected with one medium-voltage alternating-current side port through a filter inductor.
As shown in FIG. 2, each of the upper arm and the lower arm includes n armsA single-stage isolated full-bridge module, wherein the input side of each single-stage isolated full-bridge module is connected with a low-voltage DC capacitor C in paralleldcLLow voltage DC capacitor CdcLThe low-voltage direct-current power supply is connected, positive input ports of n single-stage isolated full-bridge modules are connected to form an isolated full-bridge positive input port, negative input ports of n single-stage isolated full-bridge modules are connected to form an isolated full-bridge negative input port, output ends of the n single-stage isolated full-bridge modules are connected in a mode that adjacent negative output ports are connected with a positive output port, and a negative output port of the tail single-stage isolated full-bridge module is connected with a smoothing reactor LmThe isolated full-bridge positive input port of the upper bridge arm is connected with the isolated full-bridge positive input port of the lower bridge arm, the isolated full-bridge negative input port of the upper bridge arm is connected with the isolated full-bridge negative input port of the lower bridge arm, the positive electrode of a single-stage isolated full-bridge module at the head end of the upper bridge arm is connected with the positive electrode of a medium-voltage unidirectional alternating current end, and the smoothing reactor L of the upper bridge armmSmoothing reactor L connected with lower bridge armmAnd the output port of the cathode of the single-stage isolated full-bridge module at the tail end of the lower bridge arm is connected with the cathode of the medium-voltage unidirectional alternating-current end.
As shown in fig. 3, each single-stage isolated full-bridge module includes two single-stage isolated chopper modules, input sides of the two single-stage isolated chopper modules are connected in parallel, and output sides of the two single-stage isolated chopper modules are connected in series in a reverse direction.
As shown in fig. 4, the single-stage isolation type chopper module includes a switching tube Q1And a switching tube Q2And a switching tube Q3And a switching tube Q4Switching tube Q1The emitter of the transistor is connected with a switch tube Q3Collector electrode of (2), switching tube Q2The emitter of the transistor is connected with a switch tube Q4Collector electrode of (2), switching tube Q1Collector electrode of (2), and switching tube Q2The collector electrodes are all connected with a low-voltage direct current capacitor CdcLThe positive plate forms the positive electrode of the input side and the switch tube Q3Emitter and switching tube Q4The emitting electrodes are all connected with a low-voltage direct current capacitor CdcLThe negative plate forms the negative electrode of the input side and the switching tube Q1The emitter of the high-frequency transformer is connected with one end of the primary side inlet of the high-frequency transformer T and the switching tube Q2The emitter of the high-frequency transformer is connected with the other end of the inlet of the primary side of the high-frequency transformer T;
also comprises a switch tube Q5And a switching tube Q6And a switching tube Q7And a switching tube Q8Switching tube Q5The emitter of the transistor is connected with a switch tube Q7Collector electrode of (2), switching tube Q6The emitter of the transistor is connected with a switch tube Q8Collector electrode of (2), switching tube Q5Collector and switching tube Q6The collector electrode of the switching tube Q is connected to form an output side anode7Emitter and switching tube Q8The emitting electrodes of the first and second transistors are connected to form an output side cathode and a switching tube Q5The emitter of the high-frequency transformer is connected with one end of the secondary side outlet of the high-frequency transformer T, and the switching tube Q6The emitter of the high-frequency transformer is connected with the other end of the outlet of the secondary side of the high-frequency transformer T.
The single-stage isolation type chopping module further comprises a clamping circuit connected with an outlet of the secondary side of the high-frequency transformer T.
The clamping circuit comprises a diode D1Diode D2Diode D3Diode D4Diode D1Diode D connected to anode3Negative electrode, diode D2Diode D connected to anode4Negative electrode, diode D1Cathode, diode D2Negative electrode connecting capacitor C1Positive plate, diode D3Anode, diode D4Positive electrode connecting capacitor C1Negative plate and capacitor C1Parallel resistor R1Diode D1The anode is connected with one end of the secondary side outlet of the high-frequency transformer T and a diode D2The positive pole is connected with the other end of the outlet of the secondary side of the high-frequency transformer T.
As shown in fig. 5, in the single-stage isolated full-bridge module (IBC module), each single-stage isolated chopper module (ICC module) has three modulation ratios: d is a DC modulation ratio, danAnd damRespectively, independent ac modulation ratios. dui and duj are ICC in the upper bridge arm single-stage isolated full-bridge module respectively(u)、ICC(l)The modulation signal of (a); dli and dlj are ICC in lower bridge arm single-stage isolated full-bridge module respectively(p)、ICC(n)The modulated signals of dui, duj, dli, dlj are AC/DCThe modulation ratio of the flow mixture is da when D is 0.5n、da m0. ltoreq. dan+damLess than or equal to 0.5. The formula is as follows:
Figure BDA0003046685090000081
the single-stage isolated chopper module in the bridge arm single-stage isolated full-bridge module is taken as an example, the main waveforms of the single-stage isolated chopper module are shown in fig. 6, vut1、vut2，iut2Is the voltage current on both sides of the transformer. In the secondary side circuit of the transformer, a switching tube Su1Trigger pulse signal and Su4Same, Su2And Su3Are the same, and Su1(Su4) And Su2(Su3) Complementary to each other, are fixed duty cycle square wave signals corresponding to the carrier wave. In order to realize bidirectional power flow in a module and ensure that a circuit topology can provide a follow current path at any time, a modulation strategy of an overlap dead zone (overlap) is provided according to the working process of a secondary side switching tube. The modulation strategy ensures that the current is from Su1,4Is reversed to Su2,3And a current channel is always present to avoid voltage impact, so that the working condition that secondary side devices are simultaneously turned off or are completely turned off due to dead time and the like does not exist. In the overlap process, all switches on the secondary side of the module are in a conducting state, and the inductive current can be considered to be kept unchanged due to the action of the bridge arm inductor. The modulation strategy can ensure that the primary side circuit switching tube and the secondary side switching tube work under a high-frequency environment, the turn-on and turn-off of the secondary side circuit switching tube can be ensured to be in a primary side circuit follow current state every time, and the voltage of a high-frequency transformer is zero when the primary side circuit continues current, so that zero-voltage switching of the secondary side switching tube can be realized.
The single-stage isolated full-bridge module is used as a key part of the converter, has bidirectional power transmission capability, and has a wider voltage regulation range and light capacitor loading. As shown in FIG. 3, the main circuit configuration of the single-stage isolated full-bridge module is formed by connecting two single-stage isolated chopper modules in parallel at the primary side and in series at the secondary side of the transformerAnd (4) obtaining. In order to facilitate research and distinguish two single-stage isolated chopping modules in a single-stage isolated full-bridge module, the single-stage isolated chopping modules are named uniformly: two single-stage isolated chopping modules in upper bridge arm single-stage isolated full-bridge module are named as ICC(u)、ICC(l)The output voltage of the corresponding port is Vu、Vl(ii) a Two single-stage isolated chopping modules in lower bridge arm single-stage isolated full-bridge module are named as ICC(p)、ICC(n)The output voltage of the corresponding port is Vp、Vn。
The LVDC port is regarded as a direct current input port, and the alternating current side port MVAC(T-P)And the alternating current output port is regarded as an alternating current output port, and the alternating current side voltage is modulated through the alternating current-direct current hybrid modulation ratio. The port output voltage of the single-stage isolated chopper module in the single-stage isolated full-bridge module can be expressed as LVDC side voltage VdcLAnd the product of the modulation ratio of the single-stage isolated chopping module, and assuming that the transformation ratio of high-frequency transformers (HFT) of all the single-stage isolated chopping modules is k, the output voltage of the single-stage isolated chopping module in the upper bridge arm single-stage isolated full-bridge module can be obtained by the formula (2).
Figure BDA0003046685090000091
Output port alternating-current voltage v of upper bridge arm single-stage isolated full-bridge moduleunComprises the following steps:
vun＝Vu-Vl＝2VdcL×(dan+dam) (3)
ICC in lower bridge arm single-stage isolation type full-bridge module(p)And ICC(n)Port voltage V ofp、VnRespectively as follows:
Figure BDA0003046685090000092
output port voltage v of lower bridge arm single-stage isolated full-bridge modulelnComprises the following steps:
vln＝Vp-Vn＝2VdcL×(dan-dam) (5)
taking the single-stage isolated full-bridge module of the bridge arm as an example, VuAnd VlThe controlled direct-current voltage containing alternating-current and direct-current components and output by the single-stage isolation type chopping module can be represented as the sum of a controllable alternating-current voltage source and a controllable direct-current voltage source, the output voltage of the controlled direct-current voltage source depends on the input voltage of the LVDC side, the transformation ratio of the high-frequency transformer and three modulation ratios, and the voltage, V, on the output side of the two cascade single-stage isolation type chopping modules is output through the two cascade single-stage isolation type chopping modulesuAnd VlDC component DV when AC voltage is formed by superpositiondcLThe/k is mutually offset, the alternating current component forms the port output voltage of the single-stage isolated full-bridge module, a bidirectional switch tube is not needed for forming the alternating current voltage, and the problems of voltage spike and the like can be avoided. The port output characteristics of the single-stage isolated full-bridge module show that the number of the upper bridge arm module and the lower bridge arm module is MVAC(T-P)And MVAC(S-P)The voltage level of the single-stage isolated full-bridge module is determined according to the output voltage of the single-stage isolated full-bridge module.
Through the analysis of the port characteristics of the single-stage isolated full-bridge module, an average equivalent model and mechanism analysis are established for the single-phase topological structure of the converter. When the LVDC port is taken as a direct current input side, the total voltage v output by the upper bridge arm and the lower bridge armsu、vslRespectively as follows:
Figure BDA0003046685090000101
in the formula: n is the number of modules of the bridge arm; k is the transformation ratio of the high-frequency transformer. Taking phase a as an example, the equivalent circuit on the medium voltage side of the single-phase I-MMCC shown in fig. 7 can be derived from equation (6). From Kirchhoff's Voltage Law (KVL), MVAC can be derived(S-P)And MVAC(T-P)Phase a output voltage vacM、va：
Figure BDA0003046685090000102
From equation (7) and FIG. 7, dan、damAnd the single-stage isolated full-bridge module alternating current modulation ratio is independent of the single-stage isolated full-bridge module alternating current modulation ratio. The flexible modulation strategy improves the transmission and distribution capacity of the I-MMCC to energy, so that the converter can realize the conversion of common frequency or variable frequency. The three-free modulation ratio is used as a carrier of an energy node, so that the coordinated operation of I-MMCC ports is more flexible, and the power distribution is optimized.
Figure BDA0003046685090000103
As can be seen from equation (7), MVAC(S-P)Voltage v at the portacMModulation ratio da of alternating current onlynCorrelation, equation (8) gives danFrequency f ofn. Input voltage of theoretically constant amplitude and danAny alternating output voltage can be generated, and from this point of view, MVAC(S-P)The port can output single-phase alternating-current voltages with different frequencies and amplitudes, so that the single-phase alternating-current voltages can provide corresponding electric energy for lines and electric equipment needing single-phase variable-frequency transmission. v. ofaAs MVAC(T-P)The output voltage of the middle A phase port shows that the modulation of the three-phase AC output voltage is only damRelated, different phase damThe difference between the modulation signals is 120 degrees, MVAC(T-P)Port output voltage pass regulation damCan generate variable frequency and variable amplitude three-phase voltage.
FIG. 8 and FIG. 9 are MVACs(S-P)20Hz、MVAC(T-P)Fig. 8 shows a switching experimental waveform under a three-phase ac side load step condition when the three-phase ac side load is switched from ∞ Ω to 160 Ω as a result of the 50Hz experiment, and the three-phase power frequency current jumps from zero to the output rated amplitude at the dotted line. As can be seen from fig. 8, the voltage and current amplitude of the single-phase current-intersecting side is always kept constant regardless of whether the three-phase load is switched. Fig. 9 shows the transient experimental waveform under the step condition of the load on the single-phase current side when the load on the single-phase current side is switched from ∞ Ω to 40 Ω, and the single-phase low-frequency current transitions from zero to the rated amplitude at the dashed line. As can be seen from FIG. 9, the amplitude of the voltage and current on the three-phase AC side is always kept constant regardless of whether the single-phase load is switched. Experiments prove that the single-phase alternating current and three-phase alternating current switching have no influence on each other, and the single-phase alternating current and the three-phase alternating current are all good sine waves when low/power frequency voltage and current are output, so that the effectiveness of the topological modulation strategy is verified.
FIG. 10-FIG. 12 are MVACs(S-P)50Hz、MVAC(T-P)Fig. 10 shows the steady-state experimental waveform under the conditions of low-voltage dc input 80V, single-phase, three-phase ac equal-frequency 50 Hz. As can be seen from the graph, the AC side voltage and current waveform was good, and the experimental value was matched with the theoretical value.
Fig. 11 shows an experimental waveform of transition under a three-phase ac side load step condition when the three-phase ac side load is switched from ∞ Ω to 160 Ω, and the three-phase current transitions from zero to the rated amplitude at the dashed line. Fig. 12 shows a transient experimental waveform under the step condition of the load on the single-phase current side when the load on the single-phase current side is switched from ∞ Ω to 40 Ω, and the single-phase power-frequency current rises from zero to the rated amplitude at the dashed line. From the comparison of fig. 10-12, it can be seen that the output frequency variation of the single-phase AC side does not affect the three-phase AC output waveform, further proving that v in the formula (7)acM、vaRespectively with AC modulation ratio dan、damCorrelation proves that the two AC freedom degrees of the converter are independent from each other, the interconversion between the same frequency or variable frequency can be realized, and the result is consistent with the conclusion obtained by theoretical analysis.
In such a way, the AC-AC isolated modular converter based on the low-voltage direct-current bus has medium-voltage three-phase alternating current (MVAC)(T-P)) Medium voltage single phase alternating current (MVAC)(S-P)) And a Low Voltage Direct Current (LVDC) three voltage port. The converter can realize single-stage power conversion from LVDC to Medium Voltage Alternating Current (MVAC)(T-P)And MVAC(S-P)The voltage port can realize free conversion of AC-AC power with the same or different frequencies, and the unipolar modulation strategy avoids the problems of voltage spikes and the like in the current conversion transient process of the bidirectional switch tube of the isolated AC-AC matrix converter. The converter adopts a single-pole structure, eliminates the independent capacitor of the traditional MMCC submodule, has the low-voltage direct-current port connected with an energy storage and distributed power supply device, can flexibly control the active power flow, enhances the running stability of the converter andthe operating mode of the converter is expanded. The converter is single-stage control, has a three-degree-of-freedom modulation strategy with two alternating current modulation ratios and one direct current modulation ratio, does not need a complex control strategy, and can complete direct decoupling control among ports. For the three-phase system of the converter, double-frequency power of each phase is collected to the capacitance on the common low-voltage direct current bus and mutually counteracted, and a smaller capacitance can be selected at the low-voltage direct current side. The topological structure is a three-phase system structure, three phase units are mutually connected in parallel at a side port of a medium-voltage single-phase alternating current, each phase unit consists of an upper bridge arm and a lower bridge arm, and a connection point N from the upper bridge arm and the lower bridge arm of each phase is arranged1，N2，N3After being led out, a filter inductor is connected in series to form medium-voltage three-phase alternating-current side ports a, b and c, and each bridge arm consists of n single-stage isolated full-Bridge Submodules (IBCs) and a bridge arm reactor LmAnd (4) forming. The single-stage isolated chopper sub-module (ICC) includes a primary-side portion, a high-frequency transformer portion, and a secondary-side portion. The IBC sub-module is obtained by connecting two ICC sub-modules in parallel at the primary side part and in series at the secondary side part in the reverse direction.

Claims (6)

1. The AC-AC isolated modular converter based on the low-voltage direct-current bus is characterized by comprising three phase units which are connected in parallel to a medium-voltage single-phase alternating-current side port, wherein each phase unit comprises an upper bridge arm and a lower bridge arm which are connected with each other, and the connection part of each upper bridge arm and each lower bridge arm is connected with one medium-voltage alternating-current side port through a filter inductor.
2. The AC-AC isolated modular converter based on low-voltage DC bus as claimed in claim 1, wherein the upper bridge arm and the lower bridge arm each comprise n single-stage isolated full-bridge modules, and a low-voltage DC capacitor C is connected in parallel to the input side of each single-stage isolated full-bridge moduledcLSaid low voltage DC capacitor CdcLThe device is connected with a low-voltage direct-current power supply, the positive input ports of n single-stage isolated full-bridge modules are all connected to form an isolated full-bridge positive input port, the negative input ports of n single-stage isolated full-bridge modules are all connected to form an isolated full-bridge negative input portThe output ends of the n single-stage isolated full-bridge modules are connected in a mode that the adjacent negative output port is connected with the positive output port, and the negative output port of the single-stage isolated full-bridge module at the tail end is connected with a smoothing reactor LmThe isolated full-bridge positive input port of the upper bridge arm is connected with the isolated full-bridge positive input port of the lower bridge arm, the isolated full-bridge negative input port of the upper bridge arm is connected with the isolated full-bridge negative input port of the lower bridge arm, the positive electrode of a single-stage isolated full-bridge module at the head end of the upper bridge arm is connected with the positive electrode of a medium-voltage unidirectional alternating-current end, and the smoothing reactor L of the upper bridge armmSmoothing reactor L connected with lower bridge armmAnd the output port of the cathode of the single-stage isolated full-bridge module at the tail end of the lower bridge arm is connected with the cathode of the medium-voltage unidirectional alternating-current end.
3. The low-voltage direct-current bus-based AC-AC isolated modular converter according to claim 2, wherein each single-stage isolated full-bridge module comprises two single-stage isolated chopping modules, input sides of the two single-stage isolated chopping modules are connected in parallel, and output sides of the two single-stage isolated chopping modules are connected in series in an inverted manner.
4. The AC-AC isolated modular converter based on low-voltage direct-current bus according to claim 3, wherein the single-stage isolated chopping module comprises a switch tube Q1And a switching tube Q2And a switching tube Q3And a switching tube Q4Said switch tube Q1The emitter of the transistor is connected with a switch tube Q3The collector of (1), the switching tube Q2The emitter of the transistor is connected with a switch tube Q4The collector of (1), the switching tube Q1Collector electrode of (2), and switching tube Q2The collector electrodes are all connected with a low-voltage direct current capacitor CdcLThe positive plate forms the positive electrode of the input side, and the switch tube Q3Emitter and switching tube Q4The emitting electrodes are all connected with a low-voltage direct current capacitor CdcLThe negative plate forms the negative electrode of the input side, and the switching tube Q1The emitter of the high-frequency transformer is connected with one end of the primary side inlet of the high-frequency transformer T, and the switching tube Q2The emitter of the high-frequency transformer is connected with the other end of the inlet of the primary side of the high-frequency transformer T;
also comprises a switch tube Q5And a switching tube Q6And a switching tube Q7And a switching tube Q8Said switch tube Q5The emitter of the transistor is connected with a switch tube Q7The collector of (1), the switching tube Q6The emitter of the transistor is connected with a switch tube Q8The collector of (1), the switching tube Q5Collector and switching tube Q6The collector electrode of the switching tube Q is connected to form an output side anode7Emitter and switching tube Q8The emitting electrodes of the switching tube Q are connected to form a negative electrode at the output side5The emitting electrode of the high-frequency transformer is connected with one end of the secondary side outlet of the high-frequency transformer T, and the switching tube Q6The emitter of the high-frequency transformer is connected with the other end of the outlet of the secondary side of the high-frequency transformer T.
5. The AC-AC isolated modular converter based on the low-voltage direct-current bus according to claim 4, characterized in that the single-stage isolated chopping module further comprises a clamping circuit connected with an outlet of a secondary side of the high-frequency transformer T.
6. The low voltage dc bus based AC-AC isolated modular converter of claim 5, wherein said clamp circuit comprises a diode D1Diode D2Diode D3Diode D4Said diode D1Diode D connected to anode3Negative pole, the diode D2Diode D connected to anode4Negative pole, the diode D1Cathode, diode D2Negative electrode connecting capacitor C1Positive plate, said diode D3Anode, diode D4Positive electrode connecting capacitor C1Negative plate of the capacitor C1Parallel resistor R1Said diode D1The anode is connected with one end of the secondary side outlet of the high-frequency transformer T, and the diode D2The positive pole is connected with the other end of the outlet of the secondary side of the high-frequency transformer T.
CN202110474999.3A 2021-04-29 2021-04-29 AC-AC isolated modular converter based on low-voltage DC bus Pending CN113224967A (en)

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN108155825A (en) * 2018-01-26 2018-06-12 东北电力大学 Isolated form Modular multilevel converter
CN108173442A (en) * 2018-01-18 2018-06-15 东北电力大学 Isolated form Modular multilevel converter based on High Frequency Link technology
CN108847777A (en) * 2018-07-16 2018-11-20 东北电力大学 Isolated form modularization cascade converter based on High Frequency Link technology
CN109639150A (en) * 2018-12-28 2019-04-16 浙江华云清洁能源有限公司 Isolated form Modular multilevel converter topological structure based on mixing module

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN108173442A (en) * 2018-01-18 2018-06-15 东北电力大学 Isolated form Modular multilevel converter based on High Frequency Link technology
CN108155825A (en) * 2018-01-26 2018-06-12 东北电力大学 Isolated form Modular multilevel converter
CN108847777A (en) * 2018-07-16 2018-11-20 东北电力大学 Isolated form modularization cascade converter based on High Frequency Link technology
CN109639150A (en) * 2018-12-28 2019-04-16 浙江华云清洁能源有限公司 Isolated form Modular multilevel converter topological structure based on mixing module

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