CN109004866B - Energy-feed three-port cascade converter topology with hexagonal structure and control method - Google Patents

Energy-feed three-port cascade converter topology with hexagonal structure and control method Download PDF

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CN109004866B
CN109004866B CN201811026071.3A CN201811026071A CN109004866B CN 109004866 B CN109004866 B CN 109004866B CN 201811026071 A CN201811026071 A CN 201811026071A CN 109004866 B CN109004866 B CN 109004866B
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alternating current
voltage
bridge
phase
control
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CN109004866A (en
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刘飞
查晓明
张祥静
刘文君
胡兆文
高海祐
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Wuhan University WHU
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
    • H02P5/00—Arrangements specially adapted for regulating or controlling the speed or torque of two or more electric motors
    • H02P5/74—Arrangements specially adapted for regulating or controlling the speed or torque of two or more electric motors controlling two or more ac dynamo-electric motors
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
    • H02P27/00—Arrangements or methods for the control of AC motors characterised by the kind of supply voltage
    • H02P27/04—Arrangements or methods for the control of AC motors characterised by the kind of supply voltage using variable-frequency supply voltage, e.g. inverter or converter supply voltage
    • H02P27/047—V/F converter, wherein the voltage is controlled proportionally with the frequency

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Abstract

The invention relates to the technical field of power electronic converters, in particular to an energy-feedback type three-port cascade converter topology with a hexagonal structure and a control method. Because the front-end input is only added to the module units on part of the bridge arms of the hexagonal converter, the quantity of power electronic devices required by the converter and the quantity of secondary side windings of the phase-shifting transformer can be greatly reduced. Meanwhile, in order to solve the problems of system power imbalance and the like caused by default of input of the front ends of partial units, the internal relation between loop current and the power of a bridge arm of the converter is further analyzed, and the topology can realize stable operation and energy interaction through a power balance method based on loop current control.

Description

Energy-feed three-port cascade converter topology with hexagonal structure and control method
Technical Field
The invention belongs to the technical field of power electronic converters, and particularly relates to an energy-feedback type three-port cascade converter topology with a hexagonal structure and a control method.
Background
With the rapid development of power electronic devices and control techniques thereof, multi-level converters are gradually becoming the main form of high-power conversion devices. The cascade H-bridge converter adopts a structure of connecting a plurality of single-phase full-bridge converters in series to realize high-voltage and multi-level output, has the advantages of less output voltage harmonic waves, small output voltage change rate, easy expansion of a modular structure and the like, provides great convenience for later industrial production and maintenance, and is widely applied to the field of industry, particularly the driving and speed regulation of alternating current asynchronous machines.
The traditional inverter on the market at present mainly has two driving modes for medium and high voltage motor loads, namely a one-to-one mode for driving one motor by one inverter and a one-to-many mode for simultaneously driving a plurality of motors to synchronously run. However, when the system needs to regulate the speed of a plurality of motors, the adoption of the one-to-one mode can cause the proportional increase of the number of required converters and the corresponding increase of the production cost; the mode of one driving more is adopted, and the free work and the frequent speed regulation working condition of a plurality of motors are greatly restricted, so that the multi-port converter is generated to realize the access and energy interaction of a plurality of systems. The hexagonal multiport structure formed by adopting the module multiplexing mode is an important means for realizing high power density, high conversion rate, volume reduction and cost reduction, and under the condition of reasonable control, the energy circulation path between ports can be shortened, and the energy loss is reduced.
At present, the hexagonal converter only has a two-port structure, one port is input and output, and the hexagonal converter is usually connected with two motors respectively to flexibly transmit active power and reactive power generated by one motor to the other motor. However, if the two motors are operated electrically at the same time, the system lacks an energy front end, and if the dual-motor system has surplus energy output, the energy cannot be effectively utilized, so that the topological working form is not flexible enough, and the application range is not wide enough.
Disclosure of Invention
The invention aims to provide a three-port cascade converter topology which only adds front-end input to module units on partial bridge arms of a hexagonal converter and is used for driving two middle-high voltage high-power motors to simultaneously operate. Meanwhile, a power balance method based on loop current control is also provided.
In order to achieve the purpose, the invention adopts the technical scheme that: an energy-fed three-port cascade converter topology with a hexagonal structure comprises a first alternating current motor and a second alternating current motor; the bridge arm comprises a hexagonal structure formed by sequentially connecting six groups of bridge arms end to end, each group of bridge arms comprises n H-bridge inverter cascade modules and inductors connected in series with the H-bridge inverter cascade modules, and n is a positive integer larger than or equal to 1;
the vertexes of the hexagon are R, W, S, U, T, V respectively in the clockwise direction, wherein RST is a first group of three-phase alternating current output ports, UVW is a second group of three-phase alternating current output ports, and the first and second groups of three-phase alternating current output ports are connected with a first alternating current motor and a second alternating current motor respectively;
six groups of bridge arms are sequentially from A to F, wherein three bridge arms of the ACE all adopt energy feedback units based on three-phase PWM rectification front ends, and each energy feedback unit comprises a three-phase bridge type full-control converter circuit, a direct-current voltage side capacitor and an H bridge inverter which are connected in parallel; the input end of the three-phase bridge type full-control converter circuit is connected with a local alternating current power grid through a phase-shifting transformer, so that the three bridge arms of the ACE can directly obtain electric energy from the side of the alternating current power grid; the BDF three bridge arms all adopt capacitance units formed by connecting a direct-voltage side capacitor and H bridge inverters in parallel, and output ports of cascade modules of the H bridge inverters are sequentially connected in series to form output of the cascade modules;
the six groups of bridge arms are connected in series to form a ring structure to provide a passage for loop current, electric energy obtained from the local alternating current network side flows in all bridge arms through the loop current to realize energy interaction, and the BDF three bridge arms indirectly obtain the electric energy through the loop current, so that the load is driven to operate through the two output ports.
In the energy-feedback three-port cascade converter topology with the hexagonal structure, the ACE three bridge arm is an active bridge arm, and the BDF three bridge arm is a passive bridge arm; the primary side windings of the phase-shifting transformer connected with the front end of the active bridge arm power supply share, and the phase shifting of the secondary side windings lags pi/3 n in sequence.
In the control method of the energy-fed three-port cascaded converter topology with the hexagonal structure, an inversion side control strategy and a rectification side control strategy are included; the inversion side control strategy adopts constant V/f control and carrier horizontal phase shift PS-PWM modulation; the rectification side control strategy is to adopt three-phase PWM rectification control at the front end of an ACE three-bridge arm; the method comprises the following specific steps:
step 1, maintaining the voltage balance of direct current capacitors in each bridge arm through direct voltage balance integral adjustment and direct voltage balance unit adjustment;
step 2, loop current control is superposed on the basis of voltage control of a RST port of the converter;
step 3, a voltage control outer ring of a double-ring structure adopts a proportional-integral controller, a current control inner ring adopts a proportional controller and a voltage feedforward unit, and zero sequence components are injected at the same time; and the surplus power generated by the first motor and the second motor is fed back to the power grid side.
In the control method of the energy-fed three-port cascaded converter topology with the hexagonal structure, the implementation of the step 1 and the step 2 comprises the following steps:
(1) respectively setting rated frequency and initial phase signals of the first alternating current motor and the second alternating current motor, and obtaining phase voltage amplitude control signals of the first alternating current motor and the second alternating current motor through constant V/f control and direct voltage balance integral adjustment;
(2) superimposing a circulating current control command v on a voltage control command of the first motor alternating currentN *The voltage commands are converted into bridge arm voltage commands through the adjustment of a direct voltage balancing unit, and the switch control signals of the cascade modules are generated through the carrier horizontal phase shift PS-PWM modulation;
(3) the direct voltage balance overall control adopts a maximum voltage error selection method to carry out direct voltage average value and set value E of each bridge arm cascade moduledc *Comparing, selecting the maximum deviation value, sending to a proportional-integral link to obtain a control instruction V1 *And V2 *；
(4) The direct voltage balancing unit controls to compare the detected direct voltage side capacitor voltage of each module with the average value of all cascaded module capacitor voltages on the same bridge arm, then proportional integration is carried out, and the obtained result is multiplied by the bridge arm current to obtain the deviation value of the cascaded module modulation signal; reference voltage commands of all cascade modules of the six groups of bridge arms are modulated by horizontal phase shifting PS-PWM and then are respectively sent to all the cascade modules as switch control signals, and the triangular carrier phases of the n cascade modules are sequentially lagged by pi/n.
The invention has the beneficial effects that: the front-end input is only added to the module units on one half of the bridge arms of the hexagonal converter, so that the number of power electronic devices required by the converter and the number of secondary side windings of the phase-shifting transformer are greatly reduced. And the electric energy can be reversely transmitted to the power grid side for other loads in the system to use. Constant v/f control with direct voltage balance control and loop current control as the center is adopted, and energy can be uniformly distributed and transferred among bridge arms by adjusting the magnitude of loop current, so that voltage rise caused by accumulation of energy on a capacitor is avoided. By analyzing the internal relation between the loop current and the power of a bridge arm of the converter, a power balance method based on loop current control is provided, so that the topology can realize stable operation and energy interaction.
Drawings
Fig. 1 is a schematic diagram of a main circuit of an energy-fed three-port cascaded converter topology with a hexagonal structure according to an embodiment of the present invention;
fig. 2(a) is an equivalent model diagram of an energy-fed three-port cascaded converter topology independent triangular power supply with a hexagonal structure according to an embodiment of the present invention;
fig. 2(b) is an equivalent model diagram of an energy-fed three-port cascaded converter topology RST symmetrical triangular power supply with a hexagonal structure according to an embodiment of the invention;
fig. 2(c) is an equivalent model diagram of an energy-fed three-port cascaded converter topology UVW symmetric triangle power supply with a hexagonal structure according to an embodiment of the present invention;
fig. 3 is a flowchart of a control strategy of an energy-fed three-port cascaded converter topology inversion side with a hexagonal structure according to an embodiment of the present invention;
fig. 4 is a flowchart of a control strategy on a rectifying side of an energy-fed three-port cascaded converter topology with a hexagonal structure according to an embodiment of the present invention.
Detailed Description
Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.
The embodiment effectively combines a hexagonal converter and a traditional H-bridge cascade multilevel converter driven by a motor of an existing medium-high voltage system, and then adds front-end input to a hexagonal part bridge arm through a multi-winding phase-shifting transformer, so that the input is expanded into a single-input double-output three-port structure to drive two motors to operate under various working conditions. In the embodiment, energy interaction among three ports is realized, except for the working mode of braking by one motor and electrically operating by one motor, when two motors are simultaneously braked and operated, electric energy can be reversely transmitted to the power grid side for other loads in the system to use.
The energy-feedback three-port cascaded converter topology with the hexagonal structure is realized through the following technical scheme, the topology is formed by sequentially connecting six groups of bridge arms end to form the hexagonal structure, and each group of bridge arms comprises a plurality of H-bridge inverter cascaded modules and series inductors. The six vertices of the hexagon can be divided into two groups, and two output ports are formed to be respectively connected with two alternating current systems, such as RST and UVW in the figure 1. The six bridge arms are numbered from A to F in sequence, and the internal structures of the H-bridge inverter cascade modules on each group of the bridge arms can be divided into two types. Three bridge arms of ACE in the topology all adopt energy feedback units based on three-phase PWM rectification front ends and are formed by connecting a three-phase bridge type full-control converter circuit, a direct-current voltage side capacitor and an H bridge inverter in parallel. The input end of the three-phase bridge is connected with a local alternating current power grid through a phase-shifting transformer, so that the ACE bridge arm can directly obtain electric energy from the alternating current power grid side. Three bridge arms of the BDF in the topology are all capacitor units formed by connecting a direct-voltage side capacitor and H bridge inverters in parallel, and output ports of cascade modules of the H bridge inverters are sequentially connected in series. The six bridge arms are connected in series to form an annular structure which can provide a path for loop current, and electric energy obtained from the network side passes through the loop current icirEnergy interaction is realized by circulating among all bridge arms, so that the bridge arm BDF can indirectly obtain electric energy from the bridge arms BDF, and further, the load is driven to operate through two output ports.
In order to simplify the circuit structure and facilitate power analysis of the bridge arms, each bridge arm is equivalent to a model in which a controllable alternating-current voltage source and an impedance are connected in series, so as to obtain an equivalent model of the converter topology, as shown in fig. 2(a), two groups of adjacent bridge arms in the equivalent model provide line voltages for a group of motors, so that the topology can be further spatially equivalent to two groups of imaginary independent triangular power supplies which respectively supply power to two groups of loads, as shown in fig. 2(b) and fig. 2 (b). For the convenience of current calculation and analysis, the two equivalent triangular power supplies are assumed to be completely symmetrical. In the figure, vaTo vfFor bridge arm voltage, iaTo ifFor bridge arm current, ir、iuEqual to the load phase current, vr、vuIs equal to loadPhase voltage irt、ivuEqual to the equivalent triangular supply current.
In order to achieve stable voltage of each bridge arm and flexibly control the power flow direction, so that two motors can simultaneously operate under different working conditions, the embodiment establishes a dual-motor driving control strategy method based on the traditional constant V/f control through mathematical modeling and power analysis, as shown in fig. 3 and 4, and specifically comprises the following steps: maintaining the direct-voltage balance integral adjustment and the direct-voltage balance unit adjustment of the direct-current capacitor voltage balance in each bridge arm; loop current control superimposed on the basis of converter RST port voltage control; and three-phase PWM rectification control is arranged at the front end of the ACE three-bridge arm for realizing energy feedback.
In specific implementation, fig. 1 is a diagram showing a topology structure of an energy-feedback three-port cascaded converter with a hexagonal structure, where six sets of bridge arms are connected in series to form the hexagonal structure, each set of bridge arm includes N H-bridge inverter cascaded modules and a series inductor, six vertices of the hexagon can be divided into two sets to form two output ports, where RST is one set, the formed port is connected to a first ac motor, UVW is one set, the formed port is connected to a second ac motor, and N is one set1、N2Which are the voltage neutral points of the two motors respectively. The six bridge arms are numbered from A to F in sequence, wherein the ACE bridge arm is provided with a rectification front end and is called an active bridge arm for short, and the BDF bridge arm is not provided with a rectification front end and is called a passive bridge arm for short correspondingly. The primary side windings of the phase-shifting transformer connected with the front end of the power supply of the active bridge arm are shared, and the phase shifting of the secondary side windings lags pi/3 n in sequence. By adopting the mixed structure, only part of bridge arms are provided with the PWM rectification front ends, the number of secondary side windings of the phase-shifting transformer and the number of switching devices in the rectification circuit can be reduced, and the mixed structure is suitable for the requirements of different working conditions of the motor.
According to different working states of the first alternating current motor and the second alternating current motor, the power flow direction of the topology has four conditions: when the first alternating current motor and the second alternating current motor are both in an electric state, both active power and reactive power are obtained from the local power grid side; when the reverse output power of the first alternating current motor is smaller than that of the second alternating current motor, the first alternating current motor obtains electric energy from the local power grid side and the load 1; when the reverse output power of the first alternating current motor is larger than that required by the second alternating current motor, part of power emitted by the first alternating current motor is sent to the load 2 for driving, and the rest output power is fed back to the alternating current network side in a reverse direction; when the first alternating current motor and the second alternating current motor simultaneously generate power, all the power generated by the first alternating current motor and the second alternating current motor is reversely transmitted to the local alternating current power grid. In order to realize the stable operation of the motor under the four working conditions and adjust the power flow direction in the topology, the control of the converter comprises an inversion side control strategy and a rectification side control strategy.
The inversion side control strategy is based on constant V/f control and carrier horizontal phase shift PS-PWM modulation, the direct voltage balance control adopts a maximum error selection method to correct a voltage control signal, and a control block diagram is shown in figure 3. The rated frequency and the initial phase signals of the first alternating current motor and the second alternating current motor are respectively given, and the motor phase voltage amplitude control signals are obtained through constant V/f control and direct voltage balance integral adjustment. Because the bridge arm ACE in the topology has a power supply front end, and the bridge arm BDF does not have a power supply front end, when the motor stably runs, the power on the bridge arm BDF is kept to be zero, and the voltage of the capacitor on the direct voltage side is kept constant. As shown in the loop current control section (a) of fig. 3, the present embodiment achieves this object by superimposing the loop current control command v on the voltage control command of the first alternating current motor at the RST end, i.e., superimposing the loop current control command v on the voltage control command of the first alternating current motor at the RST endNAnd then the voltage is regulated by the direct voltage balance unit, then the voltage is converted into a voltage instruction of each bridge arm, and a switching control signal of each cascade module is generated through carrier phase-shifting modulation.
Setting rated frequencies of the first and second AC motors as f1、f2Calculating the formula p by using the instantaneous powerx＝vx·ix(x ═ a, b … f) the respective arm powers were calculated, and it was found that the respective arm powers were composed of seven frequency components, i.e., a direct current component and a frequency f1Component, frequency f2Component, frequency 2f1Component, frequency 2f2Component, frequency f1+f2Component, frequency f1-f2And (4) components. Direct current component, frequency f of ACE three-bridge arm power1-f2The components are equal, the phases of the other frequency components are delayed by 2 pi/3 in sequence, and the components can be eliminated after superposition. This rule is forThe BDF bridge arm is also applicable, but the power difference frequency component of the B bridge arm is the same as that of the A bridge arm in size and opposite in direction. Therefore, the direct current component and the difference frequency component in the instantaneous power are two key factors influencing the topological energy distribution. In combination with the instantaneous power formula of the load, taking the B-arm as an example, the power dc component thereof can be expressed as:
Figure GDA0003268551300000081
wherein p ism1、pm2、qm1、qm2The instantaneous active power and the reactive power of the first alternating current motor and the second alternating current motor can be obtained through real-time monitoring. Since bridge arm BDF does not have the power supply front end, when the motor runs stably, it should be:
Figure GDA0003268551300000082
neutral point voltage difference v of motor obtained by combining formulas (1) and (2)NAnd a circulation flow icirThe relation of (1):
Figure GDA0003268551300000083
to find the neutral point voltage difference vNAnd a circulation flow icirTaking the topology of the cascade module of the x bridge arm n H-bridge inverters as an example, the voltage and current analysis is performed. Let total output voltage of bridge arm be vxWherein the output voltage of the nth H-bridge inverter cascade module is vo(xn)All H-bridge inverter cascade module output voltages sum to vo(x)For the bridge arm output voltage, there are
Figure GDA0003268551300000084
For the whole hexagonal topology, the method can be obtained according to kirchhoff's voltage law
Figure GDA0003268551300000091
Calculating formula i by combining loop currentcir＝(ia+ib+ic+id+ie+if) 6 formula (5) can be simplified to
Figure GDA0003268551300000092
The SPWM of the bridge arm x is set as the modulation signal wave vm(x)Carrier amplitude of VCmThe voltages of the capacitors of the units on the same bridge arm are approximately equal and are marked as Edc(x)In a switching period TcIn the method, the voltage of a bridge arm port containing the cascade module of the n-stage H-bridge inverter can be expressed as
Figure GDA0003268551300000093
Adding port voltages of six bridge arms, adopting an average value model analysis method and combining a relational expression vm(a)+vm(c)+vm(e)＝3v* N，vm(b)+vm(d)+vm(f)＝-3v* NThe simplified expression can be obtained:
Figure GDA0003268551300000094
wherein the content of the first and second substances,
Figure GDA0003268551300000095
the average values of the capacitor voltages of the ACE bridge arm and the BDF bridge arm are respectively. Combining formulas (6) and (8) and introducing a proportionality coefficient k1The neutral point voltage difference v can be obtainedNAnd loop current icirThe relation of (1):
Figure GDA0003268551300000096
as can be seen from the above equivalent modeling of the topology and power analysis, the control of the loop current is a key factor for maintaining the stability of the system. In order to realize energy interaction between an active bridge arm and a passive bridge arm, a neutral point voltage instruction vNThe loop current value is adjusted to realize power balance according to the change of the operation state of the system, and the calculation formula is obtained by arranging the following formulas (3) and (9):
Figure GDA0003268551300000097
wherein k is2The average value of the DC voltage of the ACE bridge arm and the BDF bridge arm is a proportionality coefficient
Figure GDA0003268551300000098
Can be obtained by detection.
As shown in the integral direct voltage balance adjusting part (b) in fig. 3, the integral direct voltage balance control adopts the maximum voltage error selection method to select the direct voltage sampling value of the BDF bridge arm unit and the set value E thereofdcComparing, selecting the maximum deviation value, and sending it to proportional-integral link to obtain control instruction V1A and V2*。
As shown in the direct voltage balance unit control part (c) of fig. 3, the detected direct voltage side capacitance voltage E of each moduledc(xk)(x ═ a, b, …, f, k ═ 0,1, …, n) and the average E of the module capacitor voltages on the same armdc(x)Compared, then Proportional Integral (PI) is carried out, and the obtained result is compared with the bridge arm current ixAnd multiplying to obtain the deviation value of the module modulation signal. Reference voltage commands of all module units of the six bridge arms are modulated by horizontal phase shifting PS-PWM and then are respectively sent to all the units to serve as switch control signals, and the triangular carrier phases of the cascade modules of the n H-bridge inverters are sequentially lagged by pi/n.
In order to feed back surplus power generated by the first and second ac motors to the grid side, a block diagram of a control strategy on the rectification side adopted in this embodiment is shown in fig. 4. In the figure, a voltage control outer ring of a double-ring structure adopts a Proportional Integral (PI) controller, a current control inner ring adopts a proportional controller and a voltage feedforward unit, and zero sequence components are injected to improve the control precision.
It should be understood that parts of the specification not set forth in detail are well within the prior art.
Although specific embodiments of the present invention have been described above with reference to the accompanying drawings, it will be appreciated by those skilled in the art that these are merely illustrative and that various changes or modifications may be made to these embodiments without departing from the principles and spirit of the invention. The scope of the invention is only limited by the appended claims.

Claims (1)

1. The control method of the energy-feedback three-port cascade converter based on the hexagonal structure comprises a first alternating current motor and a second alternating current motor; the bridge arm structure is characterized by also comprising a hexagonal structure formed by sequentially connecting six groups of bridge arms end to end, wherein each group of bridge arms comprises n H-bridge inverter cascade modules and inductors connected with the N H-bridge inverter cascade modules in series, and n is a positive integer larger than or equal to 1;
the vertexes of the hexagon are R, W, S, U, T, V respectively in the clockwise direction, wherein RST is a first group of three-phase alternating current output ports, UVW is a second group of three-phase alternating current output ports, and the first and second groups of three-phase alternating current output ports are connected with a first alternating current motor and a second alternating current motor respectively;
six groups of bridge arms are sequentially from A to F, wherein three bridge arms of the ACE all adopt energy feedback units based on the front end of three-phase PWM rectification, and the three bridge arms comprise a three-phase PWM rectification circuit, a direct-current voltage side capacitor and an H bridge inverter which are connected in parallel; the input end of the three-phase PWM rectification circuit is connected with a local alternating current power grid through a phase-shifting transformer, so that the three bridge arms of the ACE can directly obtain electric energy from the side of the alternating current power grid; the BDF three bridge arms all adopt capacitance units formed by connecting a direct-voltage side capacitor and H bridge inverters in parallel, and output ports of cascade modules of the H bridge inverters are sequentially connected in series to form output of the cascade modules;
the six groups of bridge arms are connected in series to form a ring structure to provide a passage for loop current, electric energy obtained from the local alternating current network side flows in all bridge arms through the loop current to realize energy interaction, and the BDF three bridge arms indirectly obtain the electric energy through the loop current so as to drive a load to operate through two output ports; the method is characterized in that: the control method comprises the following steps:
the H-bridge inverter control includes: the direct voltage balance overall control adopts a maximum voltage error selection method to carry out BDF bridge arm unit direct voltage sampling value and the set value E thereofdcComparing, selecting the maximum deviation value, and sending the maximum deviation value to a proportional-integral link to obtain a control instruction; the control instruction and the V/f control output are superposed to generate a phase voltage amplitude control signal of the first alternating current motor and the second alternating current motor; according to the reference phase calculation, the reference phase calculation is combined with the phase voltage amplitude control signals of the first alternating current motor and the second alternating current motor to synthesize a first group of voltage signals and a second group of voltage signals of a three-phase alternating current output port; loop current control: collecting instantaneous active power and reactive power of the first alternating current motor and the second alternating current motor, and generating neutral point offset voltage v by the average value of capacitor voltages of an ACE bridge arm and a BDF bridge armN *And the voltage signals are superposed with the voltage signals of the first group of three-phase alternating current output ports; the voltage signals of the second group of three-phase alternating current output ports are compared with vN *Synthesizing the bridge arm voltage with the signal superposed with the voltage signal of the first group of three-phase alternating current output ports; adjusting a direct-voltage balancing unit: detecting the capacitance voltage of a direct voltage side of each module to be compared with the mean value of the capacitance voltages of all cascade modules on the same bridge arm, then generating deviation values of modulation signals of the cascade modules by adopting a proportional controller, superposing the deviation values with the voltage signals of the bridge arms, respectively sending the deviation values to all the cascade modules as switch control signals through horizontal phase-shifting PS-PWM modulation, and sequentially lagging the triangular carrier phases of the cascade modules of the n H-bridge inverters by pi/n;
the three-phase PWM rectification circuit adopts double-loop control: the voltage control outer ring of the double-ring structure adopts a proportional-integral controller, the current control inner ring adopts a proportional controller and a voltage feedforward unit, and zero-sequence components are injected simultaneously; and the surplus power generated by the first motor and the second motor is fed back to the power grid side.
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