CN112366745B - Centralized modularized DC Chopper topology and control method - Google Patents

Abstract

The invention provides a centralized modularized DC Chopper topology and a control method, wherein the topology is a half-bridge and full-bridge hybrid centralized modularized DC Chopper topology, and each bridge arm in the topology consists of N half-bridge sub-modules containing balance resistors, M full-bridge sub-modules and energy dissipation resistors. Compared with a centralized modular DC Chopper consisting of a half bridge or a full bridge, the DC Chopper has good economy. When the main network side has short-circuit faults, the flexible direct-current transmission system can safely and stably run under the condition that the MMC is not locked, the system can be quickly recovered after fault removal, and meanwhile voltage balance among the sub-modules can be realized by utilizing the half-bridge sub-modules with balance resistors. The topology is particularly suitable for treating severe short-circuit faults of a high-capacity and high-voltage-class flexible direct current transmission system, can be effectively applied to a wind power grid-connected flexible direct current transmission system, and is arranged on the side of a main network.

Description

A Centralized Modular DC Chopper Topology and Control Method

technical field

The invention belongs to the technical field of power electronic systems, and more specifically, relates to a topology of a half-bridge/full-bridge hybrid centralized modular DC Chopper applied to fault ride-through of the main grid side of a flexible direct current transmission system connected to a wind power grid. DC Chopper refers to a DC chopper circuit.

Background technique

The wind farm is integrated into the power grid through the flexible DC transmission system. When a voltage drop fault occurs on the main grid side, the flexible DC transmission system has a certain isolation effect, which can prevent the wind farm side from being affected by the fault and causing the fan to go off-grid. However, the output power of the Modular Multilevel Converter (MMC) on the grid side is blocked, and the MMC on the wind farm side continues to inject active power into the DC line. At this time, surplus power will be generated on the DC line, and As a result, the voltage of the DC transmission line continues to rise, thereby triggering the DC line overvoltage protection action. If the overvoltage protection of the DC line is activated, it will take a long time for the flexible DC transmission system to resume operation. In order to solve the above problems, it is necessary to connect DC choppers in parallel on the DC line to dissipate the surplus power during the fault, so as to avoid the continuous rise of DC voltage, thereby improving the fault ride-through capability of the flexible DC transmission system in the face of short-circuit faults on the grid side. Based on economic considerations, the DC Chopper is usually installed at the MMC DC outlet on the grid side.

At present, DC Chopper is divided into three types: insulated gate bipolar transistor (Insulated Gate Bipolar Transistor, IGBT) direct series, distributed modular topology and centralized modular topology. Among them, because the IGBT direct-series type has dynamic and static voltage equalization problems between devices, at the same time, it will also cause large power fluctuations in the DC system, which affects the fault ride-through performance; while the distributed modular topology adopts a modular structure to reduce the DC system. power fluctuations, but its cooling device is distributed, which greatly increases the cost of the equipment; the centralized modular topology combines the advantages of both. On the one hand, the topology adopts a modular structure, which reduces the power fluctuation of the DC system. At the same time, the direct series connection of switching tube IGBTs is avoided. On the other hand, this topology only needs to adopt a centralized water cooling device, and the cost is low. However, the disadvantage of this topology is that it needs to use a large number of IGBT devices, which increases the cost.

Contents of the invention

The purpose of the present invention is to provide a topology and a control strategy of a hybrid centralized modular DC Chopper with a half-bridge and a full-bridge applied to a wind power grid-connected flexible direct current transmission system main grid side fault ride-through. Compared with the centralized modular DC Chopper composed of only half-bridge or full-bridge, this topology has good economical efficiency, and at the same time, it can ensure that the HVDC flexible transmission system is safe under the condition that the MMC is not blocked when a short-circuit fault occurs on the main grid side. Stable operation is conducive to the rapid recovery of the system after fault removal. This topology is especially suitable for dealing with severe short-circuit faults in large-capacity, high-voltage HVDC systems.

For this reason, according to an aspect of the present invention, the present invention adopts following technical scheme:

A centralized modular DC Chopper topology, which is installed on the main grid side of the flexible direct current transmission system, is characterized in that it is a hybrid centralized modular DC Chopper topology of half-bridge and full-bridge, and has the following circuit structure:

One end of each bridge arm is connected to the DC line, and the other end is grounded. The DC line, current-limiting reactor, N half-bridge sub-modules with balancing resistors, M full-bridge sub-modules, energy dissipation resistors, and ground are connected in sequence, where N and M is a natural number, and N≥1, M>1; the positive terminals of the full-bridge sub-module and the half-bridge sub-module are in the same direction, and they are close to the high voltage side between the DC line and the ground, and the full-bridge sub-module and the half-bridge sub-module are in the same direction. The capacitance values of the half-bridge sub-modules are the same; the two ends of the capacitance of the half-bridge sub-modules are connected in parallel with the switching device T3 and the series branch of the balance resistor;

Among them, the switching device on the positive side of the capacitor in the half-bridge sub-module is T11, and the switching device on the negative side is T12; the switching device on the positive side of the capacitor at the current input terminal in the full-bridge sub-module is T21, and the switching device on the negative side is T22. The switching device on the positive side of the capacitor at the output end is T23, and the switching device on the negative side of the capacitor is T24.

In the full-bridge sub-module and the half-bridge sub-module, due to the unidirectionality of the bridge arm current, each full-bridge sub-module omits the switch tube T21 and T24, and each half-bridge sub-module omits the switch tube T11, and only retains Its anti-parallel diode is sufficient.

According to a second aspect of the present invention, the present invention adopts the following technical solutions:

A control method for the aforementioned centralized modular DC Chopper topology, including four processes of starting charging, normal operation, fault ride-through and fault recovery, characterized in that starting charging, fault ride-through and fault recovery include the following steps:

(1) Starting charging includes the following steps:

Step 1-1: During system startup, all full-bridge sub-modules and all half-bridge sub-modules are locked to charge the capacitors of all full-bridge sub-modules and half-bridge sub-modules;

Step 1-2: When the DC line voltage is charged to the rated value, all the full-bridge sub-modules are turned from the locked state to the positive input state, and all the half-bridge sub-modules are turned from the blocked state to the input state;

Step 1-3: Check whether the capacitor voltage of each half-bridge sub-module exceeds the rated value. If it exceeds the rated value, switch the half-bridge sub-module from the input state to the capacitor balance state until the capacitor voltage of the half-bridge sub-module returns to the rated value, and then Restore the half-bridge sub-module to the input state;

(2) Fault ride-through and fault recovery include the following steps:

Step 2-1: Detect whether the DC voltage exceeds the upper limit value V _dcmax or whether a fault signal is detected from the relay protection on the AC side. If no fault is detected, the device is operating in a normal operating state, and step 2-1 is executed cyclically ; If a fault is detected, go to step 2-2;

Step 2-2: Cut off all half-bridge sub-modules;

Step 2-3: According to the power to be dissipated and the total energy of the capacitor of the bridge arm sub-module, generate the PWM wave of the bridge arm during the fault ride-through period for control;

Step 2-4: Calculate the number of full-bridge sub-modules that need forward input and reverse input respectively, sort the full-bridge sub-modules according to the capacitance voltage, and input in the order from small capacitance voltage to large capacitance voltage requires positive input The number of full-bridge sub-modules, the sequence input from the large capacitor voltage to the small capacitor voltage requires reverse input of the number of full-bridge sub-modules;

Step 2-5: Detect whether the DC voltage is lower than the lower limit V _dcmin or whether the fault recovery signal sent by the relay protection on the AC side is detected. If the fault recovery is not detected, go to step 2-3; if the fault recovery is detected , go to step 2-6;

Step 2-6: Put all the half-bridge sub-modules into operation, and check whether the capacitor voltage in the half-bridge sub-module exceeds the rated value. If it exceeds the rated value, make the half-bridge sub-module whose capacitor voltage exceeds the rated value enter the capacitor balance mode. Until the capacitor voltage drops to near the rated value; when the capacitor voltage is near the rated value, go to step 2-1.

Further, the bridge arm voltage during the fault ride-through period is controlled by Pulse Width Modulation (PWM), wherein the negative half-wave amplitude of PWM is -0.2V _dc , and the positive half-wave amplitude is greater than zero; The empty ratio is used to adjust the charge and discharge balance of the bridge arm in one cycle, and the amplitude of the positive half wave is used to adjust the power dissipated by the DC Chopper.

The quantitative relationship between the full-bridge sub-module and the half-bridge sub-module satisfies:

In the formula, M is the number of full-bridge sub-modules, N is the number of half-bridge sub-modules, and V _{armp_pumax} is the per-unit value of the maximum PWM positive half-wave amplitude required.

Compared with the prior art, the present invention improves the centralized modularized DC Chopper starting from the cost, and proposes a hybrid centralized modularized half-bridge and full-bridge under the premise of ensuring the fault ride-through capability of the flexible DC system The topology of DCChopper is proposed, and the control strategy of this topology is proposed. The half-bridge and full-bridge hybrid DC Chopper topology and its control method of the present invention can be effectively applied to a flexible direct current transmission system for wind power grid-connected, installed on the main grid side, and has the following beneficial technical effects:

(1) DC Chopper topology of the present invention needs to use less IGBTs and diode devices compared with the centralized modularized DC Chopper that only consists of full-bridge sub-modules and energy-consuming resistors; Compared with the centralized modular DC Chopper or distributed modular DC Chopper composed of resistors, although it needs more IGBTs and diodes, the cost of cooling devices is lower, and the cost saved on cooling devices will be Over and above the added cost of IGBTs and diodes;

(2) The bridge arm voltage control method proposed by the present invention uses the PWM duty cycle to adjust the energy charge and discharge balance of the bridge arm, and uses the PWM positive half-wave amplitude to adjust the dissipated power. Compared with using the duty cycle to adjust the power consumption Dissipated power, use the corrected voltage to realize the balance of charging and discharging energy of the bridge arm, has a single-valued functional relationship, can be adjusted one by one, and the control system is simple;

(3) The present invention designs a discharge circuit for the half-bridge sub-module, which is conducive to maintaining the balance of capacitor voltage between the half-bridge sub-module and the full-bridge sub-module.

Description of drawings

Figure 1(a) is a diagram of the working state of the half-bridge sub-module with balancing resistors in the cut-off state.

Figure 1(b) is a diagram of the working state of the half-bridge sub-module with balancing resistors in the input state.

Figure 1(c) is a diagram of the working state of the half-bridge sub-module with balanced resistors in a capacitor balanced state.

Figure 2(a) is a half-bridge and full-bridge hybrid centralized modular DC Chopper topology.

Figure 2(b) is a simplified topological diagram of a half-bridge and full-bridge hybrid centralized modular DC Chopper.

Figure 3(a) is the voltage diagram of the bridge arm of the DC Chopper when the system is operating normally.

Figure 3(b) is the bridge arm voltage diagram of DC Chopper during system failure.

Figure 4(a) is the control flow diagram of the DC Chopper topology.

Figure 4(b) is the control block diagram of the DC Chopper topology.

Figure 5(a) is the PWM wave diagram of the DC Chopper bridge arm voltage.

FIG. 5( b ) is a relationship diagram between the duty cycle D and the voltage amplitude V _{armp_pu} of the positive half-wave bridge arm.

Fig. 5(c) is a relationship diagram between the average power dissipation P _{diss_pu} and V _{armp_pu} in one cycle.

Detailed ways

In order to describe the present invention more specifically, the technical solutions and related principles of the present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

The topology of the full-bridge sub-module and half-bridge sub-module is shown in Figure 2(a). The topology of the full-bridge sub-module is a classic full-bridge structure, and the half-bridge sub-module is based on the classic half-bridge structure. The switch device T3 and the balance resistor series branch are connected in parallel; the switch device T3 is connected in series with the balance resistor.

The switching device on the positive side of the capacitor in the half-bridge sub-module is T11, and the switching device on the negative side is T12; the switching device on the positive side of the capacitor at the current input end of the full-bridge sub-module is T21, the switching device on the negative side is T22, and the switching device on the current output end is The switching device on the positive side of the capacitor is T23, and the switching device on the negative side of the capacitor is T24.

The switching devices T21, T22, T23 and T24 in the full-bridge sub-module and the switching devices T11, T12 and T3 in the half-bridge sub-module all use IGBTs.

Switching devices T11 , T12 and T3 in the half-bridge sub-module are connected in antiparallel with diodes D11 , D12 and D3 respectively. Switching devices T21 , T22 , T23 and T24 in the full bridge sub-module are connected in antiparallel with diodes D21 , D22 , D23 and D24 respectively.

There are three working states of the half-bridge sub-module with balanced resistors, which are cut-off, input and capacitor balanced states, as shown in Table 1. The three working states are shown in Figure 1(a), 1(b) and 1(c) respectively, and the dotted line in the figure is the working circuit. When the half-bridge sub-module is in the cut-off state, the port voltage is 0, which is equivalent to a short-circuit of the sub-module; when the half-bridge sub-module is in the on-state, the port voltage is equal to the capacitor voltage U _c ; and when the half-bridge sub-module is in the capacitance balance state , the switch tube T3 is turned on, and the excess energy in the capacitor is dissipated through the balance resistor. Since all the sub-modules in the bridge arm share the DC voltage, the capacitor voltage of the half-bridge sub-module is getting lower and lower, and the capacitor voltage of the full-bridge sub-module is getting lower and lower. High until the voltage of the half-bridge sub-module drops to near the rated value, at this time, the voltage of the full-bridge sub-module will also rise to near the rated value, thereby realizing the balance of the capacitor voltage of the sub-module.

Table 1: Operating principle of a half-bridge submodule with balancing resistors

As shown in Figure 2 (a), the topology of the half-bridge and full-bridge hybrid centralized modular DC Chopper according to the present invention is characterized in that:

One end of each bridge arm is connected to the DC line, and the other end is grounded. The DC line, current-limiting reactor, N half-bridge sub-modules (HBSMn, 1≤n≤N) with balancing resistors, and M full-bridge sub-modules (FBSMm , 1≤m≤M), energy dissipation resistors, and grounds are connected in sequence, where N and M are natural numbers, and N≥1, M>1; the positive terminals of the full-bridge sub-module and the half-bridge sub-module have the same direction and are close to The side of the high voltage between the DC line and the ground, and the capacitance value of the full-bridge sub-module and the half-bridge sub-module are the same; both ends of the capacitance of the half-bridge sub-module are connected in parallel with the switching device T3 and the series branch of the balance resistor; the above HBSM is The abbreviation of Half-Bridge SM, FBSM is the abbreviation of Full-Bridge SM.

Due to the unidirectionality of the bridge arm current, T21 and T24 can be omitted for each full-bridge sub-module, and T11 can be omitted for each half-bridge sub-module, and only its anti-parallel diode can be retained, as shown in Figure 2(b) .

The control method of the half-bridge and full-bridge hybrid centralized modular DC Chopper topology of the present invention includes four processes of starting charging, normal operation, fault ride-through and fault recovery. The control block diagram is shown in Figure 4 (a), specifically The working process is as follows:

(1) Starting charging includes the following steps:

Step 1-1: During system startup, all full-bridge sub-modules and all half-bridge sub-modules are locked to charge the capacitors of all sub-modules;

Step 1-2: When the DC line voltage is charged to the rated value, all the full-bridge sub-modules are turned from the locked state to the positive input state, and all the half-bridge sub-modules are turned from the blocked state to the input state;

Step 1-3: Check whether the capacitor voltage of each half-bridge sub-module exceeds the rated value. If it exceeds the rated value, switch the half-bridge sub-module from the input state to the capacitor balance state until the capacitor voltage of the half-bridge sub-module returns to the rated value, and then Restore the half-bridge sub-module to the input state;

(2) Normal operating state As shown in Figure 3(a), all half-bridge sub-modules in the bridge arm are in the input state, all full-bridge sub-modules are in the positive input state, and all sub-modules share the entire DC voltage. At this time, the energy consumption The voltage across the resistor is 0 and no power is dissipated.

(3) Fault ride-through and fault recovery include the following steps:

Step 2-1: Detect whether the DC voltage exceeds the upper limit value V _dcmax or whether a fault signal is detected from the relay protection on the AC side. If no fault is detected, the device is operating in a normal operating state, and step 2-1 is executed cyclically ; If a fault is detected, go to step 2-2;

Step 2-2: Cut off all the half-bridge sub-modules and only keep the full-bridge sub-modules, as shown in Figure 3(b);

Step 2-3: According to the power to be dissipated and the total energy of the capacitors of the full-bridge sub-modules in the bridge arm, generate PWM waves of the bridge arm during the fault ride-through period for control;

Step 2-4: Calculate the number of full-bridge sub-modules that need forward input and reverse input respectively, sort the full-bridge sub-modules according to the capacitance voltage, and input in the order from small capacitance voltage to large capacitance voltage requires positive input The number of full-bridge sub-modules, the sequence input from the large capacitor voltage to the small capacitor voltage requires reverse input of the number of full-bridge sub-modules;

Step 2-5: Detect whether the DC voltage is lower than the lower limit value V _dcmin or whether it is detected that the relay protection on the AC side sends a fault recovery signal. If the fault recovery is not detected, go to step 2-3; if the fault recovery is detected, Then go to steps 2-6;

Step 2-6: Put all the half-bridge sub-modules into operation, and check whether the capacitor voltage in the half-bridge sub-module exceeds the rated value. If it exceeds the rated value, make the half-bridge sub-module whose capacitor voltage exceeds the rated value enter the capacitor balance mode. Until the capacitor voltage drops to near the rated value; when the capacitor voltage is near the rated value, go to step 2-1.

During the fault, after the half-bridge sub-module is removed, only the bridge arm voltage including the full-bridge sub-module is controlled by PWM wave, in which the negative half-wave amplitude of the PWM wave is -0.2V _dc , and the positive half-wave amplitude is greater than zero. The duty cycle of the PWM wave is used to adjust the charge-discharge balance of the bridge arm in one cycle, and the amplitude of the positive half-wave of the PWM is used to adjust the power dissipated by the DC Chopper. The control block diagram is shown in Figure 4(b). Let the period of the PWM wave of the bridge arm be T, and the duty ratio be D, so the time of the negative half-wave of the bridge arm is T·D, and the time of the positive half-wave is T·(1-D). The PWM wave of the bridge arm is shown in Figure 5 ( a) as shown.

The relationship between DC voltage, bridge arm voltage and energy dissipation resistor voltage is

V _arm + V _Rdiss = V _dc (1)

In the formula, V _arm is the bridge arm voltage, V _dc is the DC line voltage, and V _Rdiss is the energy dissipation resistor voltage (the same below).

The charging power P _c of the bridge arm during the positive half wave is

In the formula, V _armp is the positive half-wave amplitude of the bridge arm voltage, and R _diss is the resistance value of the energy-dissipating resistor. The change of the charging power P _c of the bridge arm with the voltage V _armp of the bridge arm is a parabola symmetrical about V _dc /2.

The bridge arm discharge power P _d at negative half-wave is

In order to ensure the balance of charging and discharging energy of the bridge arm in one cycle, formula (4) needs to be satisfied.

The calculated relationship between the duty cycle D and the positive half-wave V _armp of the bridge arm voltage is

In the formula, V _{armp_pu} is the per unit value of the positive half-wave of the bridge arm voltage, that is, V _{armp_pu} =V _armp /V _dc (the same below). As shown in Figure 5(b), when the PWM positive half-wave amplitude varies between 0-1pu, adjusting the duty cycle can achieve energy balance between charging and discharging of the bridge arm within one cycle.

The formula for calculating the average power dissipation P _{diss of} the energy dissipation resistor in one cycle is

In the case of maintaining the energy balance of the bridge arm, substituting equation (5) into equation (6), the relationship between the average dissipated power P _diss and the PWM positive half-wave amplitude V _{armp_pu} in one period is obtained as

Define the rated power dissipation P _s =V _dc ² /R _diss , so the relationship between the per unit value P _{diss_pu} and V _{armp_pu} of the average power dissipation in one cycle is

The relationship between the per unit value P _{diss_pu} and V _{armp_pu} of the average power dissipation in one cycle is shown in Figure 5(c). Therefore, the required maximum bridge arm voltage V _{armp_pumax} can be calculated in combination with the MMC's own fault ride-through capability and fault ride-through requirements, and then the ratio of the full-bridge sub-module to the half-bridge sub-module can be calculated. The relationship is

In the formula, M is the number of full-bridge sub-modules, and N is the number of half-bridge sub-modules.

The above description of the embodiments is for those of ordinary skill in the technical field to understand and apply the present invention. It is obvious that those skilled in the art can easily make various modifications to the above-mentioned embodiments, and apply the general principles described here to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art according to the disclosure of the present invention should fall within the protection scope of the present invention.

Claims (4)

1. A control method of a centralized modularized DC Chopper topology, wherein the centralized modularized DC Chopper topology is a half-bridge and full-bridge mixed centralized modularized DC Chopper topology, is installed on a main network side of a flexible direct current transmission system, and has the following circuit structure: one end of each bridge arm is connected to a direct current circuit, the other end of each bridge arm is grounded, and the direct current circuit, the current limiting reactor, N half-bridge sub-modules containing balance resistors, M full-bridge sub-modules, the energy consumption resistor and the ground are sequentially connected, wherein N and M are natural numbers, N is more than or equal to 1, and M is more than 1; the directions of the positive ends of the capacitors of the Quan Qiaozi module and the half-bridge submodule are consistent, the capacitors are close to one side with high voltage between the direct-current line and the ground, and the capacitance values of the full-bridge submodule and the half-bridge submodule are the same; the two ends of the capacitor of the half-bridge submodule are connected with a switching device T3 and a balancing resistor series branch in parallel; the switching device at the positive side of the capacitor in the half-bridge submodule is T11, and the switching device at the negative side of the capacitor is T12; in the Quan Qiaozi module, a switching device at the positive side of a capacitor at a current input end is T21, a switching device at the negative side of the capacitor is T22, a switching device at the positive side of the capacitor at a current output end is T23, and a switching device at the negative side of the capacitor is T24;

the control method comprises four processes of starting charging, normal operation, fault ride-through and fault recovery, and is characterized in that the starting charging, the fault ride-through and the fault recovery respectively comprise the following steps:

the step (1) of starting charging comprises the following steps:

step 1-1: in the starting process of the system, the full-bridge sub-module and the half-bridge sub-module are locked, and the capacitors of all the full-bridge sub-module and the half-bridge sub-module are charged;

step 1-2: when the direct current line voltage is charged to the rated value, the full-bridge submodule is switched from a locking state to a forward input state, and the half-bridge submodule is switched from the locking state to the input state;

step 1-3: detecting whether the capacitance voltage of each half-bridge submodule exceeds a rated value, if so, switching the half-bridge submodule from a throw-in state to a capacitance balance state until the capacitance voltage of the half-bridge submodule is recovered to the rated value, and recovering the half-bridge submodule to the throw-in state;

(II) fault ride-through and fault recovery comprising the steps of:

step 2-1: detecting whether the DC voltage exceeds an upper limit value V _dcmax Or whether the relay protection at the alternating current side sends out a fault signal is detected, if the fault is not detected, the system operates in a normal operation state, and the step 2-1 is circularly executed; if a fault is detected, the step 2-2 is carried out;

step 2-2: cutting off all the half-bridge sub-modules;

step 2-3: generating a bridge arm PWM wave for control in the fault crossing period according to the power required to be dissipated and the total energy of the bridge arm submodule capacitor;

step 2-4: the number of full-bridge submodules needing forward input and reverse input is calculated respectively, the full-bridge submodules are ordered according to the capacitor voltage, the full-bridge submodules needing forward input are input from the sequence of small capacitor voltage to large capacitor voltage, and the full-bridge submodules needing reverse input are input from the sequence of large capacitor voltage to small capacitor voltage;

step 2-5: detecting whether the DC voltage is lower than the lower limit value V _dcmin Or whether a fault recovery signal sent by the relay protection of the alternating current side is detected, if the fault recovery signal is not detected, the step 2-3 is carried out; if the fault recovery is detected, the step 2-6 is entered;

step 2-6: putting all the half-bridge submodules into the system, detecting whether the capacitance voltage in each half-bridge submodule exceeds a rated value, and if so, entering a capacitance balance mode until the capacitance voltage is reduced to be close to the rated value; when the capacitor voltage is near the nominal value, go to step 2-1.

2. The control method according to claim 1, wherein each full-bridge submodule omits switching devices T21 and T24, and each half-bridge submodule omits switching device T11, but retains its antiparallel diode, among the Quan Qiaozi modules and the half-bridge submodules, due to the unidirectional nature of the bridge arm current.

3. The control method according to claim 1, wherein the bridge arm voltage during the fault ride through is controlled using PWM, wherein the negative half-wave amplitude of PWM is-0.2V _dc The positive half-wave amplitude is greater than zero; the duty ratio of the PWM wave is used for adjusting the charge-discharge balance of the bridge arm in one period, and the amplitude of the positive half wave is used for adjusting the power dissipated by the DC Chopper.

4. A control method according to claim 3, wherein the number relationship between the full-bridge sub-modules and the half-bridge sub-modules satisfies:

wherein M is a module of Quan QiaoziThe number N is the number of half-bridge sub-modules, V _{armp_pumax} Is the per unit value of the maximum PWM positive half-wave amplitude required.

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