WO2015135352A1

WO2015135352A1 - Dc fault ride-through control method of hybrid modular multilevel converter

Info

Publication number: WO2015135352A1
Application number: PCT/CN2014/093855
Authority: WO
Inventors: 孔明; 汤广福; 杨杰; 马巍巍; 季兰兰; 李泓志
Original assignee: 国家电网公司; 国网智能电网研究院; 中电普瑞电力工程有限公司; 国网辽宁省电力有限公司大连供电公司
Priority date: 2014-03-13
Filing date: 2014-12-15
Publication date: 2015-09-17
Also published as: CN104917415B; CN104917415A

Abstract

A DC fault ride-through control method of a hybrid modular multilevel converter. The control method dynamically configures the number of added half-bridge submodules (HBSMs) and full-bridge submodules (FBSMs) within the same leg by controlling the addition and removal of HBSMs and FBSMs (3, 4)in the leg and tracking the average of capacitance and voltage of the HBSMs and FBSMs and monitoring the DC side fault state based on the effectively operated HBSMs and FBSMs, thus realizing the balance of capacitance and voltage of the HBSMs and FBSMs in the steady state, and achieving effective control of AC/DC current and voltage during a transient state DC fault. The method ensures continuous operation of a converter and prevents shutdown of the converter due to a fault.

Description

DC fault ride through control method for hybrid modular multilevel converter

Technical field

The invention relates to the technical field of flexible direct current transmission of a power system, in particular to a DC fault ride through control method of a hybrid modular multilevel converter.

Background technique

The Half Hybrid Modular Multilevel Converter (CH-MMC) uses a new multi-level topology that is popular in the world. Its core unit - Sub Module (SM) is divided into two types. One is a half-bridge sub-module (Half Bridge Sub-Module, HBSM, 3 in Figure 1) consisting of two turn-off power electronic switching devices with anti-parallel diodes and a capacitor; the other is Four shut-off power electronic switching devices with anti-parallel diodes and a full bridge sub-module (FBSM, 4 in Figure 1). Several half-bridge sub-modules are cascaded to form a half-bridge sub-module segment (HBSM Valve); several full-bridge sub-modules are cascaded to form a full-bridge sub-module valve segment (FBSM Valve); The structural sub-module valve section, the full-bridge sub-module valve section and one bridge-arm reactor are connected in series to form a converter leg. The upper and lower symmetrical commutating bridge arms form a phase unit (Phase Module or Phase Unite). 2) in Figure 1. Similar to the H-bridge cascading multi-level structure, it consists of three phase units including A, B, C (or a, b, c).

In normal operation, the submodule hybrid modular multilevel converter controls each bridge arm by controlling the turn-on and turn-off during the two switches in the half-bridge and the turn-on and turn-off during the four switches in the full bridge. The input and cutoff of the module yield different bridge arm output voltages. In the same phase, different AC output voltages are obtained by controlling the output voltage of the upper and lower arms. The voltage of the sub-module of the three-phase input and the voltage drop of the bridge arm react together to form a DC voltage. It can be seen that the capacitance voltage balance of the inner half bridge and the full bridge structure submodule of the bridge arm is directly related to the AC and DC output voltage quality of the converter.

When a temporary bipolar short-circuit fault occurs on the DC side, it is realized by controlling and adjusting the output voltage command of the half-bridge and full-bridge sub-module groups in each bridge arm. The difference from the normal operation is that the output voltage command of the half-bridge structure sub-module group will be set to 0 at this stage; the AC-side output voltage will be fully assumed by the full-bridge structure sub-module, and the upper and lower bridge arm full-bridge output in the same phase. The voltage command is half of the inverter output voltage command.

Zhao Chengyong, Liu Xinghua, et al., "Submodule Grouping and Voltage Equalization Control Method for Modular Multilevel Converters" (Application No.: 201210451946.0), proposes a control method for voltage balance of bridge arm grouping sub-modules. The method realizes the relative balance of the sub-modules in the same bridge arm by grouping the equal number of each bridge arm sub-module and calculating the energy balance factor of each segment to determine the number of input sub-modules of each group. However, it should be pointed out that the above control method is only applicable when the number of sub-modules in each segment is the same. When the number of sub-modules in the segment is reduced due to factors such as sub-module failure or when there is a large difference in the number of modules between segments. The control scheme proposed in this paper will no longer be applicable and some improvement is needed.

Zhao Chengyong, Liu Xinghua et al., "A Method for Establishing a Hybrid Structural Model of a Modular Multilevel Converter" (Application No. 201210451918.9), proposes a method for a hybrid structure model of a modular multilevel converter . The hybrid structure converter is a hybrid sub-module modular multi-level converter composed of a half bridge and a full bridge structure sub-module. The difference from the inverter of the present invention is that the bridge arm reactor has a discharge path formed by a thyristor. In terms of control methods, the control method of capacitor voltage balance of half-bridge and full-bridge sub-module during steady-state period is not given in detail. At the same time, for transient DC-side fault, the traversing method designed in this paper also needs to lock the converter. achieve.

The fundamental shortcomings of the above two control methods are: First, the group control method is insufficiently robust; second, the DC transient fault still needs a latching converter.

Summary of the invention

In view of the deficiencies of the prior art, the object of the present invention is to provide a DC fault ride through control method for a hybrid modular multilevel converter, which realizes a bridge of a hybrid modular multilevel converter during steady state. The voltage balance control of the inner half bridge and the full bridge structure submodule ensures the effective control of the AC side current during the DC bipolar fault. The proposed control method can ensure the continuous operation of the converter without causing the converter to be blocked due to the failure.

The object of the present invention is achieved by the following technical solutions:

The invention provides a DC fault traversing control method for a hybrid modular multilevel converter. The hybrid modular multilevel converter is composed of three phases, and each phase is composed of upper and lower bridge arms of the same structure in series. Connecting the AC end of the modular multilevel converter at the midpoint of the upper and lower arms;

Each of the upper and lower arms is composed of a reactor, a plurality of cascaded half-bridge submodules, and a plurality of cascaded full-bridge sub-modules; each of the bridges has a cascaded half-bridge structure Module and cascaded full bridge One end of the structural sub-module in series is connected to the AC end of the modular multi-level converter through a reactor; the other end is connected to one end of the cascaded sub-modules of the other two-phase bridge arms to form the modular multi-level switch Positive and negative bus bars of the DC terminal of the current device;

The improvement is that the method comprises the following steps:

(1) Monitor the DC voltage value u _dc and the bridge arm current change rate d(i _jp,n )/dt to determine the DC short-circuit fault signal Sdc value; where j=A, B, C, respectively represent A, B, C Phase; p for the upper arm and n for the lower arm;

(2) determining whether a DC bipolar short circuit fault occurs according to the short circuit fault signal Sdc value;

(3) When there is no DC bipolar short-circuit fault, according to the number of modules of the half-bridge sub-module and the full-bridge sub-module in the bridge arm, the current direction of the bridge arm, and the average value of the capacitor voltage of the sub-module in the bridge arm The relationship between the average value of the capacitor voltage of the full-bridge structure sub-module in the same bridge arm, and the input command of each bridge arm half-bridge structure sub-module and the full-bridge structure sub-module are initially determined;

(4) further comparing the average value of the capacitor voltage average value of the half-bridge structure sub-module with the average value of the capacitor voltage of the full-bridge structure sub-module in the same bridge arm, and determining the correction amount of the input sub-module number instruction;

(5) According to the correction amount calculated in step (4), the number of instructions for the half-bridge structure sub-module input in the bridge arm and the number of inputs of the full-bridge structure sub-module are further corrected to n _hpnj =n _hpnj _{-Δn pnj} ,n _Fpnj = n _fpnj + Δn _pnj ;

(6) When a DC bipolar short-circuit fault occurs, reset the half-bridge sub-module and the full-bridge sub-module input number command in the bridge arm, and set the number of sub-bridge sub-module inputs in the bridge arm to 0, full bridge The number of structural submodule inputs is set to n _fpnj = n _hfpnj ;

(7) According to the above steps, the number of sub-modules input, the half-bridge sub-module and the full-bridge sub-module corresponding to the valve-based control device will finally determine the half-bridge sub-module and the full-bridge sub-module in the bridge arm Switching state and performing trigger control to ensure the relative balance of the capacitor voltages of the two sub-modules;

(8) According to steps (1)-(7), the capacitor voltage of the half-bridge structure sub-module and the full-bridge structure sub-module in the bridge arm is relatively stable; after the fault occurs, the converter can achieve effective current on the AC side. Control, to ensure that the converter will not be blocked due to transient short-circuit fault on the DC side.

Further, in the step (2), if the DC short-circuit fault signal Sdc=1, it is considered that no DC bipolar short-circuit fault occurs; the number of input modules of the half-bridge sub-module in the bridge arm is initially set to n _hpnj =Round(( n _hfpnj × N _hpnj ) / (N _hpnj + N _fpnj )); the number of input modules of the full-bridge structure sub-module in the bridge arm is initially set to n _fpnj = Round((n _hfpnj × N _fpnj ) / (N _hpnj + N _fpnj )); wherein n _hfpnj is the number of _modules in the j-phase upper arm or the lower arm; and N _hpnj is the number of modules in the j-phase upper arm or the lower-bridge middle half-bridge sub-module valve segment; N _fpnj is The number of modules in the full-bridge structure sub-module of the j-phase upper arm or the lower arm is normal; Round() is the rounding algorithm.

Further, in the step (3), when the half-bridge structure sub-module input module number n _hpnj in the bridge arm and the full-bridge structure sub-module input module number n _{fpnj in} the bridge arm are greater than the half-bridge structure sub-module input module number When n _hfpnj , the bridge arm current i _jpn is determined; if i _jpn >0, the average value of the capacitor voltage of the half-bridge structure sub-module in the bridge arm u _{smhpnj_avg} and the average value of the capacitor voltage of the full-bridge structure sub-module in the same bridge The size relationship of _{smfpnj_avg} ; if u _{smhpnj_avg} >u _{smfpnj_avg} , the number of modules in the half-bridge structure sub-module is corrected to n _hpnj =n _hpnj –1; otherwise, the number of modules in the full-bridge sub-module is corrected to n _fpnj = n _fpnj –1; if i _jpn <0, and if u _{smhpnj_avg} >u _{smfpnj_avg} , the number of modules in the full-bridge submodule valve segment is corrected to n _fpnj =n _fpnj –1; otherwise, the half-bridge sub-module valve segment is input The number of modules is corrected to n _hpnj =n _hpnj -1.

Further, the step (4), the half-bridge structure further comparison sub-module capacitor voltage magnitude relation between the average value of _u smhpnj_avg _u smfpnj_avg with average structural sub-module capacitor voltage within a full-bridge arm, when the absolute difference between the two when the value is greater than the set threshold value △ u _set, the correction amount is calculated to obtain the number of input modules _△ n pnj; correction amount of the number of module inputs the difference △ _△ n pnj half-bridge and full-bridge configuration submodule mean of _{_u} smpnj = u smhpnj_avg -u _{smfpnj_avg is} multiplied by K _p , multiplied by i _jpn , and finally rounded up the three products.

Further, in the step (6), if the DC short-circuit fault signal Sdc=0, it is considered that a DC bipolar short-circuit fault occurs, and at this time, the j-phase upper and lower bridge arm power transmission voltage command will not contain the DC component, and the upper and lower bridge arms will respectively Undertake half of the AC output voltage u _ejref , set the number of sub-modules input in the half-bridge sub-module of the bridge arm n _hpnj =0; at the same time, set the number of sub-modules in the valve section of the full-bridge sub-module n _Fpnj =n _hfpnj ;

When Sdc=0 is detected, the number of full-arm input sub-modules n _fpnj is positive or negative; when it is negative, it indicates that the full-bridge structure sub-module valve segment needs to be reversely input into n _fpnj modules.

Compared with the prior art, the beneficial effects achieved by the present invention are:

The capacitor voltage balance control method of the hybrid sub-module modular multi-level converter in the steady state period of the invention ensures the relative balance of the capacitor voltages of the half-bridge structure sub-module and the full-bridge structure sub-module; Performance requirements for fault traversal, no need to lock the converter during a fault. Reduce the required switching devices, related drivers and other equipment to achieve a unified economic and equipment performance.

During the transient fault, there is no need to lock the converter, which ensures the continuity of the power control; it can be used for the application of the modular multi-level converter in the field of DC transmission in the overhead line mode.

DRAWINGS

1 is a flow chart of a DC fault ride through control method of a hybrid modular multilevel converter provided by the present invention;

2 is a schematic structural diagram of a submodule hybrid modular multilevel converter provided by the present invention;

3 is a schematic diagram of calculation of the total number of input modules of the bridge arm provided by the present invention;

4 is a schematic diagram of determining the correction amount Δn _pnj provided by the present invention;

5 is a waveform diagram of a capacitor voltage average value of the A-phase upper and lower bridge half-bridge structure sub-modules provided by the present invention;

6 is a waveform diagram of a capacitor voltage average value of a full-bridge structure sub-module of the A-phase upper and lower arms provided by the present invention;

7 is a current waveform diagram of a full-bridge structure sub-module of the A-phase upper and lower bridge arms provided by the present invention;

8 is a waveform diagram of an AC side current before and after a DC bipolar short-circuit fault according to the present invention;

Figure 9 is a waveform diagram of a bipolar DC voltage provided by the present invention.

detailed description

The specific embodiments of the present invention are further described in detail below with reference to the accompanying drawings.

Aiming at the steady state of the submodule hybrid modular multilevel converter and its DC fault ride through problem, the present invention proposes a DC fault ride through control method for the hybrid modular multilevel converter. In the present invention, through The DC voltage value and its rate of change are monitored to determine if a DC bipolar short circuit fault has occurred. If there is no short-circuit fault, determine the number of input sub-modules in each half-bridge sub-module valve segment and full-bridge sub-module valve segment according to the number of half-bridge and full-bridge sub-modules in the bridge arm and the average of the voltages of the two sub-modules; If a DC short-circuit fault occurs, the number of sub-modules input into the bridge half-bridge sub-module is 0, and the output AC voltage is assumed by the full-bridge sub-module. The designed control method has strong robustness, and there is no need to lock the converter during DC fault crossing, thus making up for the shortcomings of the prior art.

The hybrid modular multilevel converter is composed of three phases, each phase is composed of upper and lower bridge arms of the same structure in series; the intermediate ends of the upper and lower bridge arms are connected to the AC end of the modular multilevel converter;

Each of the upper and lower arms is composed of a reactor, a plurality of cascaded half-bridge submodules, and a plurality of cascaded full-bridge sub-modules; each of the bridge-armed half-bridge sub-modules and One end of the cascaded full-bridge structure sub-module is connected to the AC end of the modular multi-level converter through a reactor; the other end is connected to one end of the cascaded sub-module of the other two-phase bridge arm to form the module The positive and negative bus bars of the DC terminal of the multilevel converter. Schematic diagram of the sub-module hybrid modular multilevel converter is shown in Figure 2.

A flow chart of a DC fault ride-through control method for a hybrid modular multilevel converter provided by the present invention is shown in FIG. 1 and includes the following steps:

If the DC short-circuit fault signal Sdc=1, it is considered that no DC bipolar short-circuit fault has occurred; the number of input modules of the half-bridge structure sub-module in the bridge arm is initially set to n _hpnj =Round((n _hfpnj ×N _hpnj )/(N _hpnj + N _fpnj )); the number of input modules of the full-bridge structure sub-module in the bridge arm is initially set to n _fpnj = Round((n _hfpnj × N _fpnj ) / (N _hpnj + N _fpnj )); where n _hfpnj is the j-phase upper bridge The arm or lower arm is put into the module number command; N _hpnj is the number of modules _{working normally} in the j-phase upper arm or the lower-bridge middle half-bridge sub-module valve segment; N _fpnj is the j-phase upper arm or the lower arm The number of modules in the bridge structure sub-module valve segment working normally; Round() is the rounding algorithm.

(3) When there is no DC bipolar short circuit fault, according to the bridge half inner bridge submodule and full bridge The module number of the structure sub-module, the current direction of the bridge arm, and the average value of the capacitor voltage of the half-bridge structure sub-module in the bridge arm and the average value of the capacitor voltage of the full-bridge structure sub-module in the same bridge arm, and the half-bridge structure of each bridge arm is initially determined. Sub-module and full-bridge structure sub-module input number instructions;

When the sub-arm half-bridge module into n number of modules and the arm _hpnj full-bridge structure into sub-modules the number of modules _n fpnj greater than the sum of the number of half-bridge module into submodules when n _hfpnj, it is determined that the arm current i _jpn size; if i _jpn> 0, the half-bridge arm submodules capacitor voltage magnitude relation between the average value of _u smhpnj_avg _u smfpnj_avg with average structural sub-module capacitor voltage within a full-bridge arm comparator; if _{_u} smhpnj_avg> u smfpnj_avg , the half-bridge structure sub-module valve segment input module number is corrected to n _hpnj =n _hpnj -1; otherwise, the full-bridge structure sub-module valve segment input module number is corrected to n _fpnj =n _fpnj -1; if i _jpn <0, And if u _{smhpnj_avg} >u _{smfpnj_avg} , the number of modules of the full-bridge structure sub-module valve segment is corrected to n _fpnj =n _fpnj –1; otherwise, the number of modules of the half-bridge structure sub-module valve segment is corrected to n _hpnj =n _hpnj –1 . The schematic diagram of the calculation of the total input module number of the bridge arm is shown in Fig. 3.

(4) Further compare the average value of the capacitor voltage average value u _{smhpnj_avg of the} half-bridge structure sub-module with the average voltage u _{smfpnj_avg} of the full-bridge structure sub-module in the same bridge arm, if the absolute difference between the two is greater than the set threshold _Δu _set when, the correction amount is calculated to obtain the number of input modules _△ n pnj; correction amount of the number of inputs of the difference between _△ n pnj module half and full bridge configuration submodule mean of _{_{_△}} u smpnj = u smhpnj_avg -u smfpnj_avg multiplied by K _p, Multiply by i _jpn and finally round up the three products. A schematic diagram of the determination of the correction amount Δn _pnj is shown in FIG.

(6) When a DC bipolar short-circuit fault occurs, reset the half-bridge sub-module and the full-bridge sub-module input number command in the bridge arm, and set the number of sub-bridge sub-module inputs in the bridge arm to 0, full bridge The number of structural submodule inputs is set to n _fpnj =n _hfpnj ;

If the DC short-circuit fault signal Sdc=0, it is considered that a DC bipolar short-circuit fault occurs. At this time, the j-phase upper and lower arm transmission voltage commands will not contain the DC component, and the upper and lower bridge arms will respectively bear half of the AC output voltage u _ejref and set The number of _submodules input in the valve section of the half-bridge structure sub-module of the bridge arm n _hpnj =0; at the same time, the number of input sub-modules in the valve section of the full-bridge structure sub-module is set to n _fpnj =n _hfpnj ; bipolar DC voltage waveform diagram As shown in Figure 9.

(8) According to steps (1)-(7), the capacitor voltage of the half-bridge structure sub-module and the full-bridge structure sub-module in the bridge arm is relatively stable; after the fault occurs, the converter can achieve effective current on the AC side. Control (the current waveform diagram of the A-phase upper and lower bridge arm full-bridge structure sub-module is shown in Figure 7), ensuring that the inverter will not be blocked due to the transient short-circuit fault on the DC side. The average voltage waveforms of the capacitor voltages of the A-phase upper and lower arm half-bridge structure sub-modules and the full-bridge structure sub-modules are shown in Figures 5 and 6, respectively.

If there is no correction amount Δn _pnj obtained in step (4), the balance of the capacitor voltages of the inner bridge half bridge and the full bridge sub-module will not be effectively guaranteed, which will affect the control effect of the AC/DC side voltage of the converter, resulting in power. Oscillation and other phenomena. If there is a correction amount Δn _{pnj as} described in step (4), the half-bridge and full-bridge sub-module capacitor voltage errors will be controlled within a certain range (as shown in Figures 5 and 6).

In particular, if there is no switching of the modulation strategy described in step (6), the current on the AC side of the converter cannot be controlled, and the traversal control of the DC fault cannot be realized. If there is a switching of the modulation strategy described in the step (6), even under the condition that the DC voltage is 0, the effective control of the alternating current can be realized (as shown in FIG. 8).

The capacitor voltage balance control strategy of the hybrid multi-module modular multi-level converter in the steady state period of the invention ensures the relative balance of the capacitor voltages of the half bridge submodule and the full bridge submodule; and simultaneously satisfies the DC transient fault Performance requirements for traversal, no need to lock the converter during a fault. Reduce the required switching devices, related drivers and other equipment to achieve a unified economic and equipment performance.

Finally, it should be noted that the above embodiments are only for explaining the technical solutions of the present invention and are not limited thereto, although the present invention has been described in detail with reference to the above embodiments, and those skilled in the art should It is to be understood that the invention may be modified or equivalently modified without departing from the spirit and scope of the invention.

Claims

A DC fault traversing control method for a hybrid modular multilevel converter, wherein the hybrid modular multilevel converter is composed of three phases, each phase consisting of two upper and lower bridge arms of the same structure in series; Connecting the AC end of the modular multilevel converter at the midpoint of the bridge arm;

Each of the upper and lower arms is composed of a reactor, a plurality of cascaded half-bridge submodules, and a plurality of cascaded full-bridge sub-modules; each of the bridges has a cascaded half-bridge structure One end of the module and the cascaded full-bridge structure sub-module is connected to the AC end of the modular multi-level converter through a reactor; the other end is connected to one end of the cascaded sub-module of the other two-phase bridge arm to form a The positive and negative bus bars of the DC end of the modular multilevel converter;

The method is characterized in that the method comprises the following steps:

(1) Monitor the DC voltage value u dc and the bridge arm current change rate d(i jp,n )/dt to determine the DC short-circuit fault signal Sdc value; where j=A, B, C, respectively represent A, B, C Phase; p for the upper arm and n for the lower arm;

(2) determining whether a DC bipolar short circuit fault occurs according to the short circuit fault signal Sdc value;

(3) When there is no DC bipolar short-circuit fault, according to the number of modules of the half-bridge sub-module and the full-bridge sub-module in the bridge arm, the current direction of the bridge arm, and the average value of the capacitor voltage of the sub-module in the bridge arm The relationship between the average value of the capacitor voltage of the full-bridge structure sub-module in the same bridge arm, and the input command of each bridge arm half-bridge structure sub-module and the full-bridge structure sub-module are initially determined;

(4) further comparing the average value of the capacitor voltage average value of the half-bridge structure sub-module with the average value of the capacitor voltage of the full-bridge structure sub-module in the same bridge arm, and determining the correction amount of the input sub-module number instruction;

(5) According to the correction amount calculated in step (4), the sub-module sub-module input number command and the full-bridge structure sub-module input number command in the bridge arm are further corrected to n hpnj =n hpnj -Δn pnj , n fpnj , respectively =n fpnj +Δn pnj ;

(6) When a DC bipolar short-circuit fault occurs, reset the half-bridge sub-module and the full-bridge sub-module input number command in the bridge arm, and set the number of sub-bridge sub-module inputs in the bridge arm to 0, full bridge The number of structural submodule inputs is set to n fpnj = n hfpnj ;

(7) According to the above steps, the number of sub-modules input, the half-bridge sub-module and the full-bridge sub-module corresponding to the valve-based control device will finally determine the half-bridge structure sub-module and the full-bridge sub-module in the bridge arm The switching state of the block and trigger control to ensure the relative balance of the capacitor voltages of the two sub-modules;

(8) According to steps (1)-(7), the capacitor voltage of the half-bridge structure sub-module and the full-bridge structure sub-module in the bridge arm is relatively stable; after the fault occurs, the converter can achieve effective current on the AC side. Control, to ensure that the converter will not be blocked due to transient short-circuit fault on the DC side.
The control method according to claim 1, wherein in the step (2), if the DC short-circuit fault signal Sdc=1, it is considered that no DC bipolar short-circuit fault occurs; the half-bridge sub-module of the bridge arm is put into the module. The initial setting is n hpnj =Round((n hfpnj ×N hpnj )/(N hpnj +N fpnj )); the number of modules of the full-bridge structure sub-module in the bridge arm is initially set to n fpnj =Round((n hfpnj) ×N fpnj )/(N hpnj +N fpnj )); wherein n hfpnj is the j-phase upper arm or the lower arm input module number command; N hpnj is the j-phase upper arm or the lower arm middle half-bridge structure sub-module The number of modules in which the valve segment works normally; N fpnj is the number of modules in the j-phase upper bridge or the lower bridge arm in the full-bridge structure sub-module valve segment; Round() is the rounding algorithm.
The control method according to claim 1, wherein in the step (3), when the half-bridge structure sub-module of the bridge arm is put into the module number n hpnj and the full-bridge structure sub-module of the bridge arm is put into the module number n fpnj is greater than the sum of the number of half-bridge module into submodules when n hfpnj, it is determined that the arm current i jpn size; if i jpn> 0, the half-bridge capacitor voltage averaged over the sub-module comparator arm with the same bridge arm u smhpnj_avg The size relationship of the capacitor voltage average u smfpnj_avg of the internal full-bridge structure sub-module; if u smhpnj_avg >u smfpnj_avg , the number of modules of the half-bridge structure sub-module segment is corrected to n hpnj =n hpnj -1; otherwise, the full-bridge structure The number of modules in the module segment is corrected to n fpnj =n fpnj –1; if i jpn <0, and if u smhpnj_avg >u smfpnj_avg , the number of modules in the full-bridge submodule valve segment is corrected to n fpnj =n fpnj –1 Otherwise, the number of modules in the half-bridge structure sub-module segment is corrected to n hpnj =n hpnj -1.
The control method according to claim 1, wherein in the step (4), the capacitor voltage average value u smhpnj_avg of the half bridge structure submodule and the capacitor voltage average value of the full bridge structure submodule in the same bridge are further compared. smfpnj_avg the magnitude relation, if the difference between the two absolute value greater than the set threshold value Δu set, the correction amount is calculated to obtain the number of input modules Δn pnj; correction amount Δn pnj number of input modules as a half bridge and full bridge configuration submodule mean The difference Δu smpnj =u smhpnj_avg -u smfpnj_avg is multiplied by K p , multiplied by i jpn , and finally the three products are rounded up and rounded.
The control method according to claim 1, wherein in the step (6), if the DC short-circuit fault signal Sdc=0, it is considered that a DC bipolar short-circuit fault occurs, and at this time, the j-phase upper and lower bridge arm power transmission voltage command Will not contain DC component, the upper and lower arms will bear half of the AC output voltage u ejref , set the number of submodules in the half-bridge structure sub-module of the bridge arm n hpnj =0; at the same time, set the full bridge structure The number of submodules input in the submodule valve segment n fpnj = n hfpnj ;

When Sdc=0 is detected, the number of full-arm input sub-modules n fpnj is positive or negative; when it is negative, it indicates that the full-bridge structure sub-module valve segment needs to be reversely input into n fpnj modules.