A high voltage direct current converter topology and method of controlling the same

Abstract

The application discloses a high-voltage direct-current converter topology and a control method thereof, and belongs to the technical field of power generation, power transformation or power distribution. The topology is composed of a high-voltage side filter inductor, a three-phase MMC inverter circuit, a three-phase leakage inductor, a three-phase transformer, a three-phase diode rectifier circuit, a medium-voltage side filter inductor and a medium-voltage side filter capacitor. The topology structure of the application can realize higher reliability, lower cost and smaller size by the relatively mature high-voltage side modular structure and the medium-voltage side diode series connection technology. By precisely controlling the three-phase current to be a positive-negative half-cycle symmetrical trapezoidal current, reliable commutation of the three-phase rectifier diode is realized, and meanwhile, smooth direct-current is synthesized on the high-voltage side and the medium-voltage side, so that the operation efficiency of the converter and the port power quality are significantly improved, and the filter size is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to power electronic application technology, in particular to a high-voltage direct-current converter topology and a control method thereof, and belongs to the technical field of power generation, power transformation or power distribution. BACKGROUND

[0002] With the popularization of renewable energy and the development of direct-current load, direct-current transmission and distribution technology has been rapidly developed due to its flexible power control, easy access to new energy, long transmission distance and other advantages. Direct-current power grid relies on direct-current transformers to realize voltage conversion and electrical isolation, realize the interconnection of direct-current power grids of different voltage levels, and improve the flexibility of direct-current transmission and distribution.

[0003] The existing high-voltage direct-current converter connects the AC sides of two modular multilevel converters (MMC) through an isolation transformer, and adopts a sinusoidal modulation control method. However, in practical applications, there are problems such as large size and low power density. In particular, for special occasions such as large-capacity receiving-end direct-current converters and other single-phase power flows, the topology structure can be further optimized to reduce the number of power devices and passive components of the direct-current converter, the floor area and investment cost, while ensuring system operation reliability and improving operation efficiency. In addition, the AC side of the isolation type direct-current transformer is an internal AC link, which can further optimize the control method of the topology, improve the power quality of the input and output ports, and reduce the operating loss of the converter. SUMMARY

[0004] The present application aims to solve the problems in the prior art. The present application provides an isolation type modular multilevel direct-current converter topology structure and a control method thereof suitable for interconnecting medium and high voltage direct-current power grids. Based on a three-phase MMC converter, a three-phase power frequency / intermediate frequency transformer and a three-phase diode rectifier circuit are added. By adding high-voltage side filter inductance, three-phase transformer leakage inductance and medium-voltage side filter inductance in the topology of the high-voltage side modular direct-current converter structure and the medium-voltage side diode series rectifier circuit, the three-phase current is accurately controlled to be a positive and negative half-cycle symmetrical trapezoid, reliable commutation of the three-phase rectifier diode is realized, the operation efficiency is improved, and smooth direct-current is synthesized on the high-voltage side and the medium-voltage side, which significantly improves the output power quality of the high-voltage side and the medium-voltage side, reduces the volume of the filter, and further realizes the purpose of improving the reliability of the direct-current converter topology while reducing the volume and cost.

[0005] To achieve the above-mentioned application purposes, the present application adopts the following technical solutions:

[0006] A high-voltage direct-current converter topology and a control method thereof,

[0007] The high-voltage direct-current converter topology comprises: a high-voltage side filter inductor, a three-phase MMC inverter circuit, a three-phase leakage inductor, a three-phase transformer, a three-phase diode rectifier circuit, a medium-voltage side filter inductor and a medium-voltage side filter capacitor; one end of the high-voltage side filter inductor constitutes a positive electrode of a high-voltage direct-current port; the three-phase MMC inverter circuit has a positive electrode of a direct-current port connected to the other end of the high-voltage side filter inductor, and has a negative electrode of a direct-current port constituting a negative electrode of the high-voltage direct-current port; each phase bridge arm has a bridge arm inductor connected in series; the three-phase leakage inductor has one end of each phase connected to one phase of the three-phase MMC inverter circuit; the three-phase transformer has a first end of each phase primary winding connected to the other end of each phase leakage inductor; the three-phase diode rectifier circuit has a first end of each phase secondary winding connected to each phase of the three-phase transformer; each phase diode rectifier branch is composed of two diodes connected in series; the second ends of each phase secondary winding are connected to each other; the first ends of each phase primary winding and the first ends of each phase secondary winding are homonymous; the medium-voltage side filter inductor has one end connected to a positive electrode of a direct-current port of the three-phase diode rectifier circuit; and the medium-voltage side filter capacitor has one pole connected to the other end of the medium-voltage side filter inductor to constitute a positive electrode of a medium-voltage direct-current port, and has the other pole connected to a negative electrode of a direct-current port of the three-phase diode rectifier circuit to constitute a negative electrode of the medium-voltage direct-current port.

[0008] The control method of the high-voltage direct-current converter topology modulates the multi-phase square wave voltage output by each phase MMC bridge arm to obtain symmetrical trapezoidal alternating current at the alternating current port of the three-phase MMC inverter circuit.

[0009] As a further optimization scheme of the high-voltage direct-current converter topology and the control method thereof, the specific method of modulating the multi-phase square wave voltage output by each phase MMC bridge arm is as follows:

[0010] In each commutation phase, the output voltage of the current rising phase MMC bridge arm is increased, the output voltage of the current falling phase MMC bridge arm is decreased, and the amplitude of the increased output voltage of the current rising phase MMC bridge arm is equal to the amplitude of the decreased output voltage of the current falling phase MMC bridge arm, and the duration of the increased output voltage of the current rising phase MMC bridge arm is equal to the duration of the decreased output voltage of the current falling phase MMC bridge arm.

[0011] In each freewheeling phase, the output voltage of the current freewheeling phase MMC bridge arm is adjusted to maintain the current of the current freewheeling phase stable at the amplitude set value of the trapezoidal wave alternating current.

[0012] As a further optimization scheme of the high-voltage direct-current converter topology and the control method thereof, the control method of the high-voltage direct-current converter topology modulates the multi-phase square wave voltage output by each phase MMC bridge arm by adjusting the voltage on each phase bridge arm inductor and the voltage on each phase leakage inductor in the three-phase MMC inverter circuit.

[0013] As a further optimization scheme of the high-voltage direct-current converter topology and the control method thereof, in each commutation stage, the amplitude and duration of the increase of the output voltage of the MMC bridge arm in the current rising phase are constrained by the voltage on the inductance and leakage inductance of the MMC bridge arm in the rising phase.

[0014] As a further optimization scheme of the high-voltage direct-current converter topology and the control method thereof, in each commutation stage, the amplitude of the output voltage of the MMC bridge arm in the current rising phase is constrained by the voltage on the inductance and leakage inductance of the MMC bridge arm in the rising phase.

[0015] As a further optimization scheme of the high-voltage direct-current converter topology and the control method thereof, the amplitude and duration of the increase of the output voltage of the MMC bridge arm in the current rising phase satisfy the expression: t D,X =D t,X T t , wherein X phase is the current rising phase, X is A or B or C, V t,X is the amplitude of the increase of the output voltage of the MMC bridge arm in the X phase, t D,X is the duration of the increase of the output voltage of the MMC bridge arm in the X phase, L ARM,X is the inductance of the X phase bridge arm, L K,X is the leakage inductance of the X phase transformer, I M is the output current of the medium-voltage side, n is the transformer ratio of the three-phase transformer, T t is the alternating current cycle, D t,X is the duty cycle of the alternating current cycle of t D,X .

[0016] As a further optimization scheme of the high-voltage direct-current converter topology and the control method thereof, the amplitude of the output voltage of the MMC bridge arm in the current rising phase is , wherein V m,X is the amplitude of the output voltage of the MMC bridge arm in the X phase, and V M is the voltage of the medium-voltage side.

[0017] As a further optimization scheme of the high-voltage direct-current converter topology and the control method thereof, the maximum value of the output voltage of the MMC bridge arm in the current rising phase is the voltage of the high-voltage direct-current port.

[0018] As a further optimization scheme of the high-voltage direct-current converter topology and the control method thereof, the three-phase transformer is a centralized three-phase transformer.

[0019] As a further optimization scheme of the high-voltage direct-current converter topology and the control method thereof, in the three-phase diode rectifier circuit, each diode rectifier branch is composed of at least two low-voltage diodes in series.

[0020] The application has the following beneficial effects by adopting the technical scheme:

[0021] (1) The application provides an isolated modular multilevel DC converter suitable for high-voltage DC grid interconnection, a three-phase MMC inverter circuit is used on the high-voltage side, a three-phase diode rectifier circuit is used on the low-voltage side, a power frequency / intermediate frequency transformer is used to connect the high-voltage side and the medium-voltage side, the topology structure is simple, the three-phase MMC inverter circuit and the bridge arm inductance and three-phase leakage inductance are used to realize accurate control of the stepped three-phase current and reliable commutation of the three-phase rectifier diode, and the operation efficiency is improved.

[0022] (2) The application can synthesize smooth DC current on the high-voltage side and the medium-voltage side by controlling the three-phase alternating current to be positive and negative half-cycle symmetrical trapezoidal, and the volume of the filter on the medium-voltage side and the high-voltage side is significantly reduced. BRIEF DESCRIPTION OF DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, a brief introduction will be given to the drawings needed in the embodiments or prior art description. Obviously, for those skilled in the art, other drawings can also be obtained without creative labor based on these drawings;

[0024] Figure 1 It is a schematic diagram of the high-voltage DC converter topology structure of the application.

[0025] Figure 2 It is a schematic diagram of the principle of the half-bridge module in the DC converter of the application.

[0026] Figure 3 It is a schematic diagram of the principle of the centralized three-phase transformer of the application.

[0027] Figure 4 It is a schematic diagram of the principle of the series diode in the DC converter of the application.

[0028] Figure 5 It is a schematic diagram of the basic working waveform of the high-voltage DC converter of the application.

[0029] Figure 6 It is a simulation waveform diagram of the high-voltage side of the high-voltage DC converter of the application.

[0030] Figure 7 It is a simulation waveform diagram of the medium-voltage side of the high-voltage DC converter of the application.

[0031] Explanation of reference numerals in the drawings: L HV , high-voltage side filter inductance, L MV , medium-voltage side filter inductance, C MV , medium-voltage side filter capacitor, L KAA-phase leakage inductance, L KB B-phase leakage inductance, L KC C-phase leakage inductance, D1, first diode, D2, second diode, D3, third diode, D4, fourth diode, D5, fifth diode, D6, sixth diode. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] The high-voltage DC converter topology of the present invention is as follows: Figure 1 As shown, the high-voltage side filter inductor L HV Three-phase MMC inverter circuit, three-phase leakage inductance, three-phase transformer, three-phase diode rectifier circuit, medium-voltage side filter inductor L MV and the medium voltage side filter capacitor C MV Composition: Three-phase MMC inverter circuit and high-voltage side filter inductor L HV The three-phase diode rectifier circuit is connected in series to form the high-voltage DC port; the medium-voltage side filter inductor L MV After being connected in series, it is connected to the medium-voltage DC capacitor C. MV The parallel connection forms a medium-voltage DC port; in a three-phase transformer, the primary winding is connected in series with the three-phase leakage inductance and then connected to the AC port of the three-phase MMC inverter circuit, while the secondary winding is connected to the AC port of the three-phase diode rectifier circuit.

[0034] In a three-phase MMC inverter circuit, the upper bridge arm A1 of phase A and the lower bridge arm A2 of phase A are connected in series. The connection point between the upper bridge arm A1 and the lower bridge arm A2 of phase A forms the AC port of phase A. The AC port of phase A is connected to the leakage inductance L of phase A. KA Connected to phase A transformer T A The first end of the primary winding, the upper bridge arm B1 of phase B and the lower bridge arm B2 of phase B are connected in series. The connection point of the upper bridge arm B1 and the lower bridge arm B2 of phase B constitutes the AC port of phase B bridge arm. The AC port of phase B bridge arm passes through the leakage inductance L of phase B. KB Connected to phase B transformer T B The first end of the primary winding, the upper arm C1 of phase C and the lower arm C2 of phase C are connected in series. The connection point of the upper arm C1 and the lower arm C2 of phase C forms the AC port of phase C. The AC port of phase C is connected to the leakage inductance L of phase C. KC Connected to C-phase transformer T CThe first end of the primary winding, the upper end of the A-phase upper bridge arm A1, the upper end of the B-phase upper bridge arm B1, and the upper end of the C-phase upper bridge arm C1 are connected to the high-voltage side filter inductor L HV One end of the high-voltage side filter inductor L HV The other end constitutes the positive electrode of the high-voltage direct current port, and the lower end of the A-phase lower bridge arm A2, the lower end of the B-phase lower bridge arm B2, and the lower end of the C-phase lower bridge arm C2 constitute the negative electrode of the high-voltage direct current port. Each bridge arm is composed of a plurality of half-bridge modules connected in series, and a bridge inductor L is connected in series in each bridge arm. ARM .

[0035] In the three-phase diode rectifier circuit, the anode of the first diode D1 is connected to the cathode of the fourth diode D4 and the A-phase transformer T A The first end of the secondary winding is connected, the anode of the third diode D3 is connected to the cathode of the sixth diode D6 and the B-phase transformer T B The first end of the secondary winding is connected, the anode of the fifth diode D5 is connected to the cathode of the second diode D2 and the C-phase transformer T C The first end of the secondary winding is connected, the cathode of the first diode D1, the cathode of the third diode D3, and the cathode of the fifth diode D5 are connected to the high-voltage side filter inductor L MV One end of the high-voltage side filter inductor L MV The other end and the high-voltage side filter capacitor C MV One pole is connected to constitute the positive electrode of the medium-voltage direct current port, and the anode of the fourth diode D4, the anode of the sixth diode D6, the anode of the second diode D2, and the high-voltage side filter capacitor C MV The other pole is connected to constitute the negative electrode of the medium-voltage direct current port.

[0036] In the three-phase transformer, the A-phase transformer T A The second end of the primary winding, the B-phase transformer T B The second end of the primary winding and the C-phase transformer T C The second end of the primary winding is connected, the A-phase transformer T A The second end of the secondary winding, the B-phase transformer T B The second end of the secondary winding and the C-phase transformer T C The second end of the secondary winding is connected, the A-phase transformer T A The first end of the primary winding and the first end of the secondary winding are the same name, the B-phase transformer T B The first end of the primary winding and the first end of the secondary winding are the same name, the C-phase transformer T C The first end of the primary winding and the first end of the secondary winding are the same name.

[0037] Each bridge arm of the three-phase MMC inverter circuit is composed of a plurality of half-bridge modules connected in series as shown in Figure 2 The half-bridge module is composed of an upper switch S N1 and a lower switch SN2 The upper switch S N1 and the lower switch S N2 The two ends of the series branch are connected with a filter capacitor C N .

[0038] As shown in Figure 3 , the three-phase transformer can adopt a centralized three-phase transformer.

[0039] As shown in Figure 4 , the diodes in the three-phase rectifier circuit are formed by a plurality of low-voltage diodes in series.

[0040] The three-phase MMC inverter circuit can be replaced by any high-voltage voltage source type inverter circuit with three-phase alternating voltage output capability.

[0041] The high-voltage side filter inductor L HV , the medium-voltage side filter inductor L MV , and the medium-voltage side filter capacitor C MV can be configured according to the actual ripple current and fault protection requirements.

[0042] For the high-voltage DC converter topology proposed in the application, a multi-phase square wave voltage generated by modulating the three-phase MMC inverter circuit generates a positive and negative half-cycle symmetrical trapezoidal alternating current at the alternating current port of the three-phase MMC inverter circuit, and the A phase, B phase and C phase work for 1 / 3 of the alternating current period. The basic working waveform is shown in Figure 5 .

[0043] At time t0, the A-phase bridge arm output voltage rises V t,A , so that the A-phase current starts to rise smoothly from 0, and the first diode D1 is turned on. The C-phase bridge arm output voltage decreases V t,C , so that the C-phase current decreases smoothly from I M / n, and I M is the medium-voltage side output current, n is the transformer ratio of the three-phase transformer, and V t,A = V t,C .

[0044] At time t1, the A-phase current rises to a set value I M / n, and the C-phase current decreases to 0, and the fifth diode D5 is turned off. The converter commutation stage ends, and enters the current freewheeling stage. By adjusting the A-phase and B-phase bridge arm output voltages, the A-phase and B-phase currents are maintained at a set value. The A-phase and B-phase bridge arm output voltages are realized by adjusting the voltages on the A-phase bridge arm inductor and the A-phase leakage inductor and the voltages on the B-phase bridge arm inductor and the B-phase leakage inductor.

[0045] At time t2, the converter enters the commutation stage again. By increasing the B-phase alternating voltage V t,B and decreasing the C-phase alternating voltage V t,C , the A-phase and B-phase currents are maintained at a set value., the B-phase current amplitude decreases to 0, the sixth diode D6 is turned off, the converter commutation stage ends, and the current freewheeling stage begins. The A-phase and C-phase bridge arm output voltages are adjusted to maintain the A-phase and C-phase currents at the set values. The A-phase and C-phase bridge arm output voltages are adjusted by regulating the voltages on the A-phase bridge arm inductance and A-phase leakage inductance and the voltages on the C-phase bridge arm inductance and C-phase leakage inductance. t,B t,C

[0046] At time t3, the C-phase current reversely increases to the set value -I M / n, and the B-phase current amplitude decreases to 0. The sixth diode D6 is turned off, the converter commutation stage ends, and the current freewheeling stage begins. The A-phase and C-phase bridge arm output voltages are adjusted to maintain the A-phase and C-phase currents at the set values. The A-phase and C-phase bridge arm output voltages are adjusted by regulating the voltages on the A-phase bridge arm inductance and A-phase leakage inductance and the voltages on the C-phase bridge arm inductance and C-phase leakage inductance.

[0047] At time t4, the converter enters the commutation stage again. The A-phase alternating voltage decreases by V t,A , and the B-phase alternating voltage increases by V t,B , so that the A-phase current decreases and the B-phase current increases. The third diode D3 is turned on, and V t,A = V t,B .

[0048] At time t5, the A-phase current decreases to 0, the first diode D1 is turned off, and the B-phase current increases to the set value I M / n. The converter commutation stage ends, and the current freewheeling stage begins. The B-phase and C-phase bridge arm output voltages are adjusted to maintain the B-phase and C-phase currents at the set values. The B-phase and C-phase bridge arm output voltages are adjusted by regulating the voltages on the B-phase bridge arm inductance and B-phase leakage inductance and the voltages on the C-phase bridge arm inductance and C-phase leakage inductance.

[0049] At time t6, the negative half cycle begins, and the working principle is similar to that of the positive half cycle.

[0050] In each commutation stage, the amplitude and duration of the increase in the MMC bridge arm output voltage in the current rising phase and the decrease in the MMC bridge arm output voltage in the current falling phase are equal, the neutral point voltage is maintained unchanged, and the current of the current freewheeling phase is maintained constant.

[0051] The amplitude V t,X and duration t D,X of the increase in the X-phase MMC bridge arm output voltage in the commutation stage are determined by the X-phase bridge arm inductance L ARM,X , the X-phase transformer leakage inductance L K,X , the transformer turns ratio n, and the medium voltage side output current I M .

[0052]

[0053] t D,X = D t,X T t ​​

[0054] wherein, X phase is the electric rising phase of the commutation stage, X phase bridge arm inductance L ARM,X determined by the values of the X phase upper bridge arm inductance and the X phase lower bridge arm inductance, X is A or B or C, T t is an AC period, D t,X is the duty ratio of the AC period t D,X .

[0055] the amplitude V m,X of the X phase MMC bridge arm output voltage of the commutation stage is determined by the three-phase transformer transformation ratio n, the medium voltage side voltage V M , the increased amplitude V t,X of the X phase MMC bridge arm output voltage of the commutation stage, and the amplitude V m,X of the X phase MMC bridge arm output voltage of the commutation stage should not be greater than the maximum MMC bridge arm output voltage V H , V H is the high voltage DC port voltage.

[0056]

[0057] Figure 6 and Figure 7 are simulation waveforms of the high voltage side and the medium voltage side of the high voltage DC converter of the application, the rated power is 1200MW, the high voltage side voltage is 500kV, the low voltage side voltage is 120kV, and the intermediate AC link frequency is 50Hz. Among them, Figure 6 is the high voltage side simulation waveform, the high voltage side three-phase current is a positive and negative half cycle symmetrical trapezoid, and the high voltage side line voltage and the high voltage side phase voltage are multiple square wave voltages; Figure 7 is the medium voltage side simulation waveform, wherein the rectifier diode current synthesis is a smooth DC current, the medium voltage side current is 1kA, and the medium voltage side voltage is 120kV.

[0058] The application proposes an isolated DC converter suitable for medium and high voltage DC grid interconnection and having a modular multi-level, the high voltage side adopts a three-phase MMC inverter circuit, the low voltage side adopts a three-phase diode rectifier circuit, and the two are connected through a power frequency / intermediate frequency transformer, the topology structure is simple, the relatively mature high voltage side modular structure and the diode series connection technology of the medium voltage side can realize high reliability, low cost and small size.

[0059] With the help of the three-phase MMC inverter circuit, accurate control of the three-phase current and reliable commutation of the three-phase rectifier diode are realized, and the operation efficiency is improved.

[0060] In the application, by controlling the three-phase AC current to be a positive and negative half cycle symmetrical trapezoid, smooth DC currents can be synthesized on the high voltage side and the medium voltage side, and the filter volume of the medium voltage side and the high voltage side is significantly reduced.

[0061] The foregoing is considered as illustrative only of the principles of the application. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the application to the exact construction and practice described. Accordingly, all such variations and modifications are intended to be included within the scope of the application as defined in the following claims.

Claims

1. A high-voltage direct-current converter topology and a control method thereof, characterized in that, the high-voltage direct-current converter topology comprises: a high-voltage side filter inductor, one end of which constitutes a positive pole of a high-voltage direct-current port, a three-phase MMC inverter circuit, a direct-current port positive pole of which is connected to the other end of the high-voltage side filter inductor, and a direct-current port negative pole of which constitutes a negative pole of the high-voltage direct-current port, and a bridge inductor is connected in series in each phase bridge arm, a three-phase leakage inductor, one end of each phase leakage inductor being connected to one phase alternating-current port of the three-phase MMC inverter circuit, a three-phase transformer, the first end of each phase transformer primary winding being connected to the other end of one phase leakage inductor, and the second ends of the phase transformer primary windings being connected to each other, a three-phase diode rectifier circuit, each phase alternating-current port being connected to the first end of one phase transformer secondary winding, each phase diode rectifier branch being composed of two diodes connected in series, the second ends of the phase transformer secondary windings being connected to each other, and the first ends of the phase transformer primary windings and the first ends of the phase transformer secondary windings being homonymous terminals, a medium-voltage side filter inductor, one end of which is connected to the positive pole of a direct-current port of the three-phase diode rectifier circuit, and a medium-voltage side filter capacitor, one pole of which is connected to the other end of the medium-voltage side filter inductor to constitute a positive pole of a medium-voltage direct-current port, and the other pole of which is connected to the negative pole of the direct-current port of the three-phase diode rectifier circuit to constitute a negative pole of the medium-voltage direct-current port; the control method of the high-voltage direct-current converter topology, the method comprising: In each commutation stage, the output voltage of the MMC bridge arm in the current rising phase is increased, the output voltage of the MMC bridge arm in the current falling phase is decreased, and the amplitude of the increased output voltage of the MMC bridge arm in the current rising phase is equal to the amplitude of the decreased output voltage of the MMC bridge arm in the current falling phase, the duration of the increased output voltage of the MMC bridge arm in the current rising phase is equal to the duration of the decreased output voltage of the MMC bridge arm in the current falling phase, and the amplitude and the duration of the increased output voltage of the MMC bridge arm in the current rising phase satisfy the expression: , , wherein, is the current rising phase, is A or B or C, is the amplitude of the increased output voltage of the MMC bridge arm in the current rising phase, is is the duration of the increased output voltage of the MMC bridge arm in the current rising phase, is is the inductance of the bridge arm in the current rising phase, is is the leakage inductance of the transformer, is the output current of the medium voltage side, is the transformer ratio of the three-phase transformer, is the AC period, is is the duty cycle of the AC period, modulating a multi-phase square wave voltage output by each phase MMC bridge arm to obtain a trapezoidal alternating-current current with symmetrical positive and negative half cycles at the alternating-current port of the three-phase MMC inverter circuit, and 2. The topology of high voltage direct current converter and the control method thereof according to claim 1, characterized in that, the specific method of modulating the multi-phase square wave voltage output by each phase MMC bridge arm comprising:

3. The topology of high voltage direct current converter and the control method thereof according to claim 2, characterized in that, in each freewheeling phase, adjusting the output voltage of the current freewheeling phase MMC bridge arm to maintain the current of the current freewheeling phase stable at a set value of the amplitude of the trapezoidal wave alternating-current current.

4. The topology of high voltage direct current converter and the control method thereof according to claim 3, characterized in that, the control method of the high-voltage direct-current converter topology, the method comprising:

5. The topology of high voltage DC converter and the control method thereof according to claim 1, characterized in that, The amplitude of the output voltage of the phase MMC bridge arm is wherein, is the amplitude of the output voltage of the phase MMC bridge arm, is the medium voltage side voltage.

6. The topology of high voltage DC converter and the control method thereof according to claim 1, characterized in that, modulating the multi-phase square wave voltage output by each phase MMC bridge arm by adjusting the voltage on each phase bridge arm inductor and the voltage on each phase leakage inductor in the three-phase MMC inverter circuit.

7. The converter topology and its control method according to any one of claims 1 to 6, characterized in that, in each commutation phase, the amplitude and duration of the increase in the output voltage of the current rising phase MMC bridge arm are constrained by the voltage on the rising phase MMC bridge arm inductor and the voltage on the rising phase leakage inductor.

8. The converter topology and its control method according to any one of claims 1 to 7, characterized in that, in each commutation phase, the amplitude of the output voltage of the current rising phase MMC bridge arm is constrained by the medium-voltage side voltage, the amplitude of the increase in the output voltage of the current rising phase MMC bridge arm, and the maximum value of the MMC bridge arm output voltage. the maximum value of the MMC bridge arm output voltage is the high-voltage direct-current port voltage. the three-phase transformer is a centralized three-phase transformer. in the three-phase diode rectifier circuit, each phase diode rectifier branch is composed of at least two low-voltage diodes connected in series.

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