阅读说明：本技术 一种基于模组解耦控制的混合型mmc器件损耗优化方法 (Hybrid MMC device loss optimization method based on module decoupling control ) 是由 路茂增 马新喜 赵艳雷 孙标 苏田田 于 2021-09-08 设计创作，主要内容包括：本发明涉及电力电子技术领域,具体为一种基于模组解耦控制的混合型MMC器件损耗优化方法。首先根据MMC基本运行原理计算各桥臂的输出电压。以降低混合型MMC中损耗最高的器件即半桥子模块下部绝缘栅双极型晶体管的功耗为目标,在满足桥臂输出电压需求、模组一个工频周期内的充放电荷守恒、模组输出能力的前提下,计算半桥模组和全桥模组的平均开关函数。根据模组平均开关函数计算桥臂中需要投入的半桥子模块的个数及全桥子模块的个数,在模组内应用电压排序算法投切子模块。应用本发明提供的方法可以保证MMC正常工作的同时改善损耗分布,使热量分布更加均匀,降低因热应力导致的器件损坏的概率。(The invention relates to the technical field of power electronics, in particular to a hybrid MMC device loss optimization method based on module decoupling control. Firstly, calculating the output voltage of each bridge arm according to the MMC basic operation principle. The method aims to reduce the power consumption of a device with the highest loss in the mixed MMC, namely an insulated gate bipolar transistor at the lower part of a half-bridge submodule, and calculates the average switching function of the half-bridge module and the full-bridge module on the premise of meeting the requirements of output voltage of a bridge arm, charge and discharge conservation in one power frequency period of the module and the output capacity of the module. And calculating the number of half-bridge submodules and the number of full-bridge submodules required to be put into a bridge arm according to the average switching function of the module, and switching the submodules in the module by applying a voltage sequencing algorithm. The method provided by the invention can improve loss distribution while ensuring normal operation of the MMC, so that heat distribution is more uniform, and the probability of device damage caused by thermal stress is reduced.)

1. The utility model provides a mixed type MMC device loss optimization method based on module decoupling control, is applied to many level of modularization transverter, there are a plurality of submodule pieces to constitute in each bridge arm of transverter, the submodule piece includes full-bridge submodule piece and half-bridge submodule piece, its characterized in that:

step 1, outputting an internal potential e according to the requirement of a converter_jrefAnd formula 0 calculates the output voltage u of each bridge arm_pjref、u_njref，

Wherein j represents phase information of the power supply, u_pjrefAnd u_njrefAre respectively the instruction values of the output voltages of the j-phase upper bridge arm and the j-phase lower bridge arm, U_dcRepresents the rated voltage of the direct current side;

step 2, calculating the number n of half-bridge submodules needing to be put into a bridge arm_pjfAnd the number n of full-bridge submodules_pjh；

Step 2.1, number n of bridge submodules in steady state_pjfAnd the number n of full-bridge submodules_pjhThe method satisfies the formula 1 that,

n_paf×U_C+n_pah×U_C＝u_paref (1)

wherein, U_CRated voltage is provided for the sub-module capacitor;

submodule capacitor in full-bridge module and half-bridge moduleAverage switching function S of full-bridge module during voltage equalization_pafAnd half-bridge module average switching function S_pahSatisfies formula 2:

S_paf×N/2×U_c+S_pah×N/2×U_c＝u_paref (2)

wherein N is the total number of the submodules in the bridge arm;

step 2.2, the charge and discharge charge of the full-bridge submodule and the half-bridge submodule capacitors in one power frequency period should be conserved, namely, the average switching function of the module should meet the formula 3:

step 2.3, when the module operates normally, the full/half bridge sub-module only outputs a positive level and a zero level, and the average switching function meets the formula 5:

step 3, calculating the number n of half-bridge sub-modules according to the formula in the step 2_pjfAnd the number n of full-bridge submodules_pjh

Step 4, calculating the number n of the full-bridge sub-modules needing to be input in each bridge arm_pjfAnd the number n of half-bridge submodules_njhAnd controlling a full-bridge submodule and a half-bridge submodule in the bridge arm.

2. The utility model provides a mixed type MMC device loss optimization method based on module decoupling control, is applied to many level of modularization transverter, there are a plurality of submodule pieces to constitute in each bridge arm of transverter, the submodule piece includes full-bridge submodule piece and half-bridge submodule piece, its characterized in that:

step 1, according to the changeInternal potential e of current device required to output_jrefAnd formula 0 calculates the output voltage u of each bridge arm_pjref、u_njref，

Wherein j represents phase information of the power supply, u_pjrefAnd u_njrefAre respectively the instruction values of the output voltages of the j-phase upper bridge arm and the j-phase lower bridge arm, U_dcRepresents the rated voltage of the direct current side;

step 2, calculating the number n of half-bridge submodules needing to be put into a bridge arm_pjfAnd the number n of full-bridge submodules_pjh；

Step 2.1, number n of bridge submodules in steady state_pjfAnd the number n of full-bridge submodules_pjhThe method satisfies the formula 1 that,

n_paf×U_C+n_pah×U_C＝u_paref (1)

wherein, U_CRated voltage is provided for the sub-module capacitor;

average switching function S of full-bridge module when sub-module capacitor voltage in full-bridge module and half-bridge module is balanced_pafAnd half-bridge module average switching function S_pahSatisfies formula 2:

S_paf×N/2×U_c+S_pah×N/2×U_c＝u_paref (2)

wherein N is the total number of the submodules in the bridge arm;

step 2.2, the full-bridge submodule and the half-bridge submodule capacitors are used for charge and discharge energy conservation in a power frequency period, namely the average switching function of the module meets a formula 4:

step 2.3, when the module operates normally, the full/half bridge sub-module only outputs a positive level and a zero level, and the average switching function meets the formula 5:

step 3, calculating the number n of half-bridge sub-modules according to the formula in the step 2_pjfAnd the number n of full-bridge submodules_pjh：

Step 4, calculating the number n of the full-bridge sub-modules needing to be input in each bridge arm_pjfAnd the number n of half-bridge submodules_njhAnd controlling a full-bridge submodule and a half-bridge submodule in the bridge arm.

3. The hybrid MMC device loss optimization method based on module decoupling control of claim 1 or 2, wherein:

in the step 4: and sequencing control and control switching are carried out on the full-bridge sub-modules and the half-bridge sub-modules in the bridge arms.

4. The hybrid MMC device loss optimization method based on module decoupling control of claim 3, characterized in that:

firstly, sorting a half-bridge module neutron module and a full-bridge module neutron module in a bridge arm according to the detected capacitance voltage;

then switching the sub-modules according to the current direction of the bridge arm:

for a full-bridge module, when the bridge arm current is positive, n is input_fThe submodule with the lowest capacitor voltage; otherwise, n is added_fEach submodule with the highest capacitor voltage;

for half-bridge module, when bridge arm current is positive value, n is put in_hThe submodule with the lowest capacitor voltage; otherwise, n is added_hAnd the submodule with the highest capacitor voltage.

5. The hybrid MMC device loss optimization method based on module decoupling control of claim 4, characterized in that:

the charge for controlling the charge and discharge of the half-bridge sub-module in one fundamental frequency period is the maximum, namely the participation degree of the half-bridge module is the maximum.

6. The hybrid MMC device loss optimization method based on module decoupling control of claim 5, characterized in that:

when the bridge arm current is a negative value, all the full-bridge submodules are put into use, namely when the upper bridge arm current is less than or equal to 0, the average switching function of the half-bridge module is 1, and the discharge charge of the half-bridge module in the interval is Q₃；

When the bridge arm current is greater than zero and the bridge arm output voltage command is greater than the full-bridge module output voltage, the half-bridge module output voltage is equal to the difference between the bridge arm output voltage command and the full-bridge module output voltage, namely:

the sum of the sub-module capacitor rated voltages of the sub-modules of which the bridge arm current is greater than zero, the bridge arm output voltage instruction is greater than the full-bridge module output voltage and the instruction value of the bridge arm output voltage is greater than or equal to half; the average switching function of the half-bridge module is-msin ω t, wherein m is the operation modulation ratio, and the charge of the half-bridge module in the interval is Q2;

the charge of the half-bridge module switching function in the previous half period meets the following requirements: Q1-Q3-Q2.

7. The hybrid MMC device loss optimization method based on module decoupling control of claim 5, characterized in that:

when the converter operates in a unit power factor, the loss of the insulated gate bipolar transistor at the lower part of the half-bridge submodule is reduced, and the condition of being beneficial to the balanced distribution of the loss is as follows:

the charging charge Q1 of the previous half-cycle corresponds to a continuous switching function of the half-bridge module in the previous half-cycle, and its midpoint is pi/2.

Technical Field

The invention relates to the technical field of power electronics, in particular to a hybrid MMC device loss optimization method based on module decoupling control.

Background

The Modular multi-level converter (MMC) is a topological structure of a converter, the equipment comprises six bridge arms, each bridge arm consists of dozens or even hundreds of sub-modules, the Modular multi-level converter is called as the Modular multi-level converter because the output waveform comprises a plurality of levels, and the topology is a research hotspot of the current high-voltage direct-current transmission converter. The sub-module types in one bridge arm of the hybrid MMC are not only one, but also comprise at least two. The full-bridge half-bridge 1:1 mixed MMC bridge arm comprises a half-bridge sub-module and a full-bridge sub-module, and the number ratio of the two sub-modules is 1: 1. For a full-bridge-half-bridge 1:1 mixed MMC working in an inversion state, when a traditional voltage-sharing sorting algorithm is adopted, the problems of loss and unbalanced distribution of thermal stress of a switching device of a full/half-bridge submodule exist, particularly the on-state loss of a T2 tube of the half-bridge submodule is large, and the reliability of long-term operation of equipment can be reduced. In practical application, the voltage-sharing algorithm needs to ensure that the voltage fluctuation of hundreds of sub-modules in a bridge arm is kept consistent as much as possible, and the sub-modules in the bridge arm need to be subjected to closed-loop voltage-sharing control. The traditional voltage-sharing sequencing algorithm firstly sequences the detected capacitor voltages of the full-bridge and half-bridge sub-modules together according to the magnitude; then reasonably switching the sub-modules according to the current direction of the bridge arms: supposing that n submodules are required to be put into the bridge arm at present, and when the bridge arm current is a positive value, putting n submodules with the lowest voltage into the bridge arm; and otherwise, putting the n sub-modules with the highest voltage. During actual work, all the full-bridge sub-modules in a certain bridge arm have basically the same action characteristics, and can be called a full-bridge module for convenient analysis; similarly, the half-bridge sub-modules in the bridge arm have substantially the same operating characteristics, and they may be collectively referred to as a half-bridge module. The device switching function represents the value of the switching action state, and 1 represents conduction; 0 represents off. The module average switching function represents the ratio of the number of sub-modules put into a module to the number of all sub-modules of the type. Module decoupling control: the full-bridge module and the half-bridge module are independently and respectively controlled, and the average switching functions of the full-bridge module and the half-bridge module are independently distributed to optimize the loss distribution characteristics of the switching device.

The modular multilevel converter MMC has the advantages of small switching loss, no device dynamic/static voltage-sharing problem and the like. At present, converters in domestic and foreign high-voltage direct-current transmission projects adopt MMC topologies. When the direct-current short-circuit protection circuit is applied to an overhead line power transmission field, direct-current short-circuit faults are easy to occur due to the fact that the power transmission line is exposed and leaked outside. Considering the direct current short-circuit fault ride-through capability, the construction cost and the operation efficiency, the full-bridge-half-bridge 1:1 mixed type MMC (shown in figure 1) becomes the preferred topology of the high-voltage direct current transmission converter, and the mixed type MMC is successfully applied to the Wudongde direct current engineering in China. Due to the difference of topological structures of a half-bridge submodule and a full-bridge submodule, the mixed MMC working in an inversion state has the problems of device loss and unbalanced distribution of thermal stress, wherein the loss of a half-bridge submodule T2 tube is particularly outstanding, the loss distribution directly determines the model selection of a switching device and the design of a heat dissipation system, and the reliability of long-term operation of the whole device is reduced due to the fact that the half-bridge submodule T2 tube has too much loss. Aiming at the problems, the rated current parameters of the switching device in actual engineering are selected greatly, which can increase the cost of equipment; in addition, the loss maldistribution also affects the design of heat dissipation and thermal stress of the switching device. The loss distribution of the hybrid MMC is improved, the specifications of the switching device can be reduced by the switching device with prominent loss, and the cost is saved; reducing the imbalance of the loss distribution also facilitates the design of the heat dissipation system. Therefore, the improvement of the loss distribution of the hybrid MMC has practical application significance and economic benefit.

At present, although the total loss of the MMC can be reduced to a certain extent by adopting a method of injecting double frequency circulating current and injecting triple frequency voltage, the loss distribution characteristic of a submodule device is not optimized, and the effect of improving the loss distribution characteristic of a submodule switching device is weak. The alternate modulation method can improve the effect of loss distribution of the full-bridge sub-module device. Due to the limitation of a topological structure, when the half-bridge sub-module outputs positive, negative and zero levels, the half-bridge sub-module does not have the alternate modulation effect, and the alternate modulation cannot be used. Therefore, the rotating modulation cannot improve the loss distribution in the hybrid MMC and is more prominent in the half-bridge sub-module. In addition, in the existing hybrid MMC control method, the full-bridge submodule works in the half-bridge submodule, and the strong coupling of the half-bridge submodule and the full-bridge submodule in voltage-sharing control brings great difficulty to optimizing the loss distribution characteristic of the device.

The invention provides a device loss distribution characteristic optimization control method based on full/half bridge module decoupling control, which improves the device loss distribution characteristic between/in sub-modules by independently distributing average switching functions of a full bridge and a half bridge on the premise of ensuring the output power quality of a current converter so as to improve the reliability of long-term operation of equipment.

Disclosure of Invention

The technical problem to be solved by the invention is as follows: the hybrid MMC device loss optimization method based on module decoupling control is provided, and the device loss distribution characteristics between/in submodules are improved, so that the long-term operation reliability of equipment is improved.

The technical scheme of the technical problem to be solved by the invention is as follows: the utility model provides a mixed type MMC device loss optimization method based on module decoupling control, is applied to many level of modularization transverter, there are a plurality of submodule pieces to constitute in each bridge arm of transverter, the submodule piece includes full-bridge submodule piece and half-bridge submodule piece, its characterized in that:

step 1, outputting an internal potential e according to the requirement of a converter_jrefAnd formula 0 calculates the output voltage u of each bridge arm_pjref、u_njref，

Wherein j represents phase information of the power supply, u_pjrefAnd u_njrefAre respectively the instruction values of the output voltages of the j-phase upper bridge arm and the j-phase lower bridge arm, U_dcRepresents the rated voltage of the direct current side;

step 2, calculating the number n of half-bridge submodules needing to be put into a bridge arm_pjfAnd the number n of full-bridge submodules_pjh；

Step 2.1, under the stable state, the bridge submoduleNumber n of blocks_pjfAnd the number n of full-bridge submodules_pjhThe method satisfies the formula 1 that,

n_paf×U_C+n_pah×U_C＝u_paref (1)

wherein, U_CRated voltage is provided for the sub-module capacitor;

average switching function S of full-bridge module when sub-module capacitor voltage in full-bridge module and half-bridge module is balanced_pafAnd half-bridge module average switching function S_pahSatisfies formula 2:

S_paf×N/2×U_c+S_pah×N/2×U_c＝u_paref (2)

wherein N is the total number of the submodules in the bridge arm;

step 2.2, the charge and discharge charge of the full-bridge submodule and the half-bridge submodule capacitors in one power frequency period should be conserved, namely, the average switching function of the module should meet the formula 3:

step 2.3, when the module operates normally, the full/half bridge sub-module only outputs a positive level and a zero level, and the average switching function meets the formula 5:

step 3, calculating the number n of half-bridge sub-modules according to the formula in the step 2_pjfAnd the number n of full-bridge submodules_pjh

Step 4, calculating the number n of the full-bridge sub-modules needing to be input in each bridge arm_pjfAnd the number n of half-bridge submodules_njhAnd controlling a full-bridge submodule and a half-bridge submodule in the bridge arm.

The utility model provides a mixed type MMC device loss optimization method based on module decoupling control, is applied to many level of modularization transverter, there are a plurality of submodule pieces to constitute in each bridge arm of transverter, the submodule piece includes full-bridge submodule piece and half-bridge submodule piece, its characterized in that:

step 1, outputting an internal potential e according to the requirement of a converter_jrefAnd formula 0 calculates the output voltage u of each bridge arm_pjref、u_njref，

Wherein j represents phase information of the power supply, u_pjrefAnd u_njrefAre respectively the instruction values of the output voltages of the j-phase upper bridge arm and the j-phase lower bridge arm, U_dcRepresents the rated voltage of the direct current side;

step 2, calculating the number n of half-bridge submodules needing to be put into a bridge arm_pjfAnd the number n of full-bridge submodules_pjh；

Step 2.1, number n of bridge submodules in steady state_pjfAnd the number n of full-bridge submodules_pjhThe method satisfies the formula 1 that,

n_paf×U_C+n_pah×U_C＝u_paref (1)

wherein, U_CRated voltage is provided for the sub-module capacitor;

average switching function S of full-bridge module when sub-module capacitor voltage in full-bridge module and half-bridge module is balanced_pafAnd half-bridge module average switching function S_pahSatisfies formula 2:

S_paf×N/2×U_c+S_pah×N/2×U_c＝u_paref (2)

wherein N is the total number of the submodules in the bridge arm;

step 2.2, the full-bridge submodule and the half-bridge submodule capacitors are used for charge and discharge energy conservation in a power frequency period, namely the average switching function of the module meets a formula 4:

step 2.3, when in normal operation, the full/half-bridge submodule only outputs a positive level and a zero level, and the average switching function meets the public

Formula 5:

step 3, calculating the number n of half-bridge sub-modules according to the formula in the step 2_pjfAnd the number n of full-bridge submodules_pjh：

Step 4, calculating the number n of the full-bridge sub-modules needing to be input in each bridge arm_pjfAnd the number n of half-bridge submodules_njhAnd controlling a full-bridge submodule and a half-bridge submodule in the bridge arm.

Preferably, in the step 4: and sequencing control and control switching are carried out on the full-bridge sub-modules and the half-bridge sub-modules in the bridge arms.

Preferably, the neutron modules of the half-bridge module and the neutron modules of the full-bridge module in the bridge arm are respectively sequenced according to the detected capacitor voltage; then switching the sub-modules according to the current direction of the bridge arm: for a full-bridge module, when the bridge arm current is positive, n is input_fThe submodule with the lowest capacitor voltage; otherwise, n is added_fEach submodule with the highest capacitor voltage; for half-bridge module, when bridge arm current is positive value, n is put in_hThe submodule with the lowest capacitor voltage; otherwise, n is added_hAnd the submodule with the highest capacitor voltage.

Preferably, the charge for controlling the charging and discharging of the half-bridge sub-module in one fundamental frequency period is the maximum, i.e. the participation degree of the half-bridge module is the maximum.

Preferably, when the bridge arm current is negative, the bridge arm is put into full operationPartial full-bridge submodule, i.e. when the current of upper bridge arm is less than or equal to 0, the average switching function of half-bridge module is 1, and the discharge charge of half-bridge module in this time interval is Q₃(ii) a When the bridge arm current is greater than zero and the bridge arm output voltage command is greater than the full-bridge module output voltage, the half-bridge module output voltage is equal to the difference between the bridge arm output voltage command and the full-bridge module output voltage, namely: the sum of the sub-module capacitor rated voltages of the sub-modules of which the bridge arm current is greater than zero, the bridge arm output voltage instruction is greater than the full-bridge module output voltage and the instruction value of the bridge arm output voltage is greater than or equal to half; the average switching function of the half-bridge module is-msin ω t, wherein m is the operation modulation ratio, and the charge of the half-bridge module in the interval is Q2; the charge of the half-bridge module switching function in the previous half period meets the following requirements: Q1-Q3-Q2.

Preferably, when the converter operates in a unit power factor, the loss of the insulated gate bipolar transistor at the lower part of the half-bridge submodule is reduced, and the condition of being favorable for the balanced distribution of the loss is as follows: the charging charge Q1 of the previous half-cycle corresponds to a continuous switching function of the half-bridge module in the previous half-cycle, and its midpoint is pi/2.

The invention has the beneficial effects that:

(1) for the existing sequencing voltage-sharing control algorithm, the longer time for the bridge arm to flow through the half-bridge submodule T2 tube can be deduced when the current of the bridge arm is larger, and the loss is closely related to the current, so that the on-state loss and the switching loss of the device are large. Based on the analysis, the idea of module decoupling control is to reduce the conduction time of the T2 tube in the half-bridge submodule by changing the current path when the bridge arm current is large, so as to reduce the on-state loss of the half-bridge submodule.

(2) The conventional alternate modulation mechanism of the full-bridge submodule can improve the device loss distribution of the full-bridge submodule but cannot improve the device loss distribution of the half-bridge submodule. The module decoupling control reduces the loss of a T2 tube of a half-bridge submodule by increasing the average switching function of the half-bridge module when the current of a bridge arm is larger; because the average switching function of the full-bridge module is reduced, the output zero level time is increased, but because the modulation mechanism is rotated, the loss distribution characteristic of the full-bridge sub-module device can be considered on the basis of improving the loss distribution characteristic of the half-bridge sub-module device.

(3) Analysis shows that the charge quantity of the half-bridge submodule for charging or discharging is the maximum at the moment when the bridge arm current is larger, and the on-state loss and the switching loss of the half-bridge submodule T2 are reduced most favorably. When the hybrid MMC is operated under different conditions, for example, by changing the modulation ratio or changing the power factor, the average switching functions of the full-bridge and half-bridge modules, which are most beneficial to reducing the on-state and switching losses of the T2 transistor, can be obtained according to the iterative calculation.

(4) Through analyzing the device loss distribution in different switching submodule states, the module decoupling control does not influence the on-state loss of the device; meanwhile, all the submodules in the bridge arm have the states of being put into and bypassed all the time, so that the switching loss is reduced.

(5) The distribution of loss of the submodule devices is improved, particularly the on-state loss and the switching loss of a half-bridge submodule switching device T2 are reduced, the size of a device radiator is favorably reduced, the cost is favorably reduced, and the power density of the submodule is favorably improved.

(6) For the faults of the converter, a considerable proportion is caused by serious heating of devices, so that the loss distribution characteristics of full-bridge and half-bridge submodule devices are improved, particularly the on-state loss and the switching loss of a half-bridge submodule switching device T2 are reduced, the fault occurrence probability of the converter is favorably reduced, and the long-term running reliability of equipment is favorably improved.

Drawings

Fig. 1 hybrid MMC topology.

Fig. 2 is a control block diagram of the proposed module-based decoupling control.

Fig. 3 is a schematic diagram of the average switching function of the half-bridge module when the participation of the half-bridge module is maximum.

Fig. 4 shows the results of a half-bridge sub-module loss distribution calculation using a conventional sorting algorithm.

Fig. 5 uses the full-bridge sub-module loss distribution calculation result of alternate modulation.

FIG. 6 shows sub-module device loss calculations based on optimal decoupling control.

Fig. 7 uses the full-bridge sub-module loss distribution calculation result of alternate modulation.

Detailed Description

In order to make the technical solution and the advantages of the present invention clearer, the following explains embodiments of the present invention in further detail.

A hybrid MMC device loss optimization method based on module decoupling control is applied to a converter control system in the prior art. The method comprises the following steps:

step 1, as shown in fig. 2, is a control block diagram based on module decoupling control in the prior art. The phase-locked loop is used for calculating the real-time phase theta of the voltage of the alternating-current power grid; the power outer ring and the current inner ring are used for realizing the control of the output power of the converter, and the output quantity is three-phase internal potential e_jref(j ═ a, b, c); the circulation current suppression ring is used for suppressing higher harmonic circulation current in the converter, and the output quantity of the circulation current suppression ring is the common mode voltage adjustment quantity u of three phases_diffjref(j ═ a, b, c); the above control scheme can still adopt the existing mature control scheme. According to the variables, the six bridge arm output voltage commands can be calculated by the following formula:

wherein u is_pjrefAnd u_njrefThe instruction values U of the output voltages of the upper bridge arm and the lower bridge arm of the phase_dcIndicating the dc side rated voltage. Where j is a or b or c, i.e. a three-phase source of three-phase alternating current.

Step 2, the following steps list the conditions for realizing the balance and reduction of power consumption, namely the number n of half-bridge submodules needing to be put into the lower bridge arm_pjfAnd the number n of full-bridge submodules_pjhThe conditions of power consumption equalization and power consumption reduction are to be realized.

Step 2.1

After the output voltage command of each bridge arm is calculated, the number of half-bridge and full-bridge submodules required to be put into the bridge arm is calculated according to the following three aspects and the constraint of five formulas.

The bridge arm output voltage requirements need to be met first.

Taking the phase A upper bridge arm as an example, the number n of full-bridge and half-bridge submodules input in a steady state_pafAnd n_pahIt should satisfy:

n_paf×U_C+n_pah×U_C＝u_paref (1)

wherein, U_CAnd (4) rated voltage of the sub-module capacitor.

Average switching function S of full-bridge module when sub-module capacitor voltage in full-bridge module and half-bridge module is balanced_pafAnd half-bridge module average switching function S_pahIt should satisfy:

S_paf×N/2×U_c+S_pah×N/2×U_c＝u_paref (2)

and N is the total number of the submodules in the bridge arm.

Step 2.2

And secondly, the capacitor voltage balance requirement needs to be met.

The charge and discharge charge of the full-bridge submodule or half-bridge submodule capacitor in one power frequency period should be conserved, namely, the average switching function of the module should meet the following requirements:

taking phase A as an example:

wherein i_paRepresents the current of the upper bridge arm of the A phase, i under steady state_paCan be expressed as

Wherein, I_dcRated current for the dc side; phi is the power factor angle.

Or step 2.2

The conditions met are calculated in an energy conservation manner:

the full-bridge submodule and the half-bridge submodule capacitors are used for charge and discharge energy conservation in a power frequency period, namely, the average switch function of the module is satisfied:

step 2.3

And finally, the voltage output capability of the sub-modules needs to be met. When the full/half-bridge submodule normally operates, only positive level and zero level are output, and the average switching function is satisfied:

in the case of the phase A,

the other phases need to satisfy the same conditions.

Step 3, obtaining the formula in step 2

I.e. number n of full-bridge submodules_pjfAnd the number n of half-bridge submodules_njh。

Step 4, calculating the number n of the full-bridge sub-modules needing to be input in each bridge arm_pjfAnd the number n of half-bridge submodules_njhAnd controlling a full-bridge submodule and a half-bridge submodule in the bridge arm. Specifically, the method comprises the following steps:

in step 2, the number n of full-bridge submodules to be put into each bridge arm is calculated_pjfAnd the number n of half-bridge submodules_njh(j ═ a, b, and c, and represents three phases), and then the three phases are passed through the arms of the bridgeThe full-bridge submodule and the half-bridge submodule are used for sequencing control and control switching. Taking the switching of the full-bridge sub-module as an example, the specific method is as follows: firstly, sequencing full-bridge submodules in a bridge arm according to the detected capacitor voltage; then according to the direction of bridge arm current, when the bridge arm current is positive value, then n is put in_pjfThe full-bridge submodule with the lowest capacitor voltage; otherwise, n is added_pjfAnd the submodule with the highest capacitor voltage.

Preferably, to reduce the on-state loss and the switching loss of the half-bridge submodule switching device T2, the charge for charging and discharging the half-bridge submodule in one fundamental frequency cycle should be maximized, i.e. the participation degree of the half-bridge module is maximized. Similarly, taking the phase a upper bridge arm as an example, on the premise that the conditions are met, that is, firstly, the requirement of bridge arm output voltage, the requirement of capacitor voltage balance and the capability of sub-module voltage output are met, the average switching function of the half-bridge module is met:

1) when the bridge arm current is negative (interval 3 in fig. 3), all the full-bridge submodules are put into use, i.e. when i is_paWhen the concentration is less than or equal to 0, S_pahWhen the half-bridge module discharges charge Q in the interval 1₃。

2) When the bridge arm current is greater than zero and the bridge arm output voltage command is greater than the full-bridge module output voltage (interval 2 in fig. 3), the half-bridge module output voltage should be equal to the difference between the bridge arm output voltage command and the full-bridge module output voltage, i.e. when i_paU is less than or equal to 0_paref≥U_C*N/2，S_pahWhere m is the running modulation ratio. At this time, the half-bridge module in this interval charges to Q₂。

3) Half-bridge module switching function S_pahThe charge in the previous half cycle should satisfy:wherein t1 and t2 are the starting time and the ending time of the half-bridge module in the previous half-cycle.

When the above conditions are satisfied, the number n of the full-bridge sub-modules is calculated_pjfAnd the number n of half-bridge submodules_njhThe power consumption being effected after switching-onReduction and balancing of power consumption.

Further, in order to reduce the on-state loss and the switching loss of the half-bridge submodule switching device T2 to the maximum extent, taking the a-phase upper bridge arm as an example, on the premise that the condition (3) is satisfied, the average switching function S of the half-bridge module in the interval 1, that is, the previous half period_pahThe following two cases are included:

s in the first half period when the inverter is operating at unity power factor_pahShould be continuous and have a midpoint of π/2; with increasing modulation ratio of operation, S_pahThe angle increases continuously in interval 1. The charge Q of the first half cycle is charged when the inverter is operating at unity power factor₁When the switching functions of the half-bridge modules in the corresponding first half period are continuous and the midpoint is pi/2, the loss of an insulated gate bipolar transistor (T2 tube for short) at the lower part of the half-bridge submodule is reduced most favorably, and the loss distribution is balanced most favorably.

As the modulation ratio of operation increases, the optimum half-bridge module switching function continues during the first half-cycle, at a midpoint of pi/2, but for an increasing duration during the first half-cycle.

S in the first half period when the inverter is operating at non-unity power factor_pahContinuous and with the centre no longer being pi/2, shifted from pi/2. When the converter operates at a non-unity power factor, the optimal half-bridge module switching function is still continuous in the first half cycle, but the midpoint is no longer pi/2, and is shifted from pi/2.

Based on the technical scheme, the effect simulation is as follows:

(1) improvement of modular mean switching function

Under the fixed operation modulation ratio and the power factor, in order to verify the influence of the module decoupling control on the device loss distribution characteristics, the loss distribution characteristics of the hybrid MMC device based on the traditional sorting algorithm and the optimal module decoupling control with the maximum half-bridge module participation degree are respectively calculated, and are respectively shown in fig. 4-7.

Focusing on the on-state loss and the total loss of a half-bridge submodule T2 in a graph, and finding that the on-state loss of the half-bridge submodule T2 after decoupling is reduced by 50.83W, is reduced by 7.21% compared with that before decoupling, and is consistent with theoretical calculation; the total loss of the half-bridge submodule T2 is reduced by 71.66W, 8.62 percent compared with that before decoupling, and is consistent with theoretical calculation. Other power devices may vary accordingly. This indicates that the proposed optimal half-bridge module based participation in the strongest module decoupling control can effectively improve the loss distribution characteristics of the hybrid MMC device.

(2) Influence of the modulation ratio of operation

The transmission power, the dc side voltage and the unit power factor are kept constant, and the ac side voltage is changed so that the modulation ratio m is 0.8, 0.8267, 0.85, 0.9 and 0.95 in this order. Table 1 shows the analysis calculation results and simulation results of the on-state loss of the T2 tube before and after the decoupling control under different modulation ratios. According to the data in the table, it can be found that: 1) under an allowable error, before and after decoupling control, a simulated value of the T2 tube loss is matched with a theoretical calculated value; 2) as m increases, the effect of the decoupling control on the improvement of the T2 loss profile decreases.

Table 1 comparison of simulation and calculation results of on-state loss of T2 tube before and after decoupling control under different modulation ratios

(3) Influence of Power factor

The apparent power, the dc side voltage and the ac side voltage were kept constant, and the transmission power was changed so that the power factors were 0.97 (lag), 0.98 (lag), 0.99 (lag), 1, 0.99 (lead), 0.98 (lead) and 0.97 (lead) in this order. Table 2 shows the on-state loss analysis calculation results and simulation results of the T2 tube before and after the decoupling control under different power factors. According to the data in the table, it can be found that: 1) under an allowable error, before and after decoupling control, a simulated value of the T2 tube loss is matched with a theoretical calculated value; 2) as the power factor decreases, the effect of the decoupling control on the improvement of the T2 loss profile increases.

TABLE 2 comparison of simulation and calculation results of on-state loss of T2 tube before and after decoupling control under different operating power factors

In summary, due to the difference of the full-bridge and half-bridge sub-modules in operation, the hybrid MMC operating in an inversion state has the problem that the loss distribution of the sub-module switching devices is seriously unbalanced; among them, the T2 tube loss of the half-bridge submodule is especially prominent. In order to solve the problem, the invention provides a control method for improving loss distribution characteristics of a sub-module device based on full/half-bridge module decoupling control. Firstly, summarizing a general method for module decoupling control according to controllable bridge arm output voltage, full/half-bridge submodule capacitor voltage balance and submodule output voltage capability; secondly, analyzing the improvement mechanism analysis of the module decoupling control on the device loss distribution characteristics, and providing a concrete implementation method of the module decoupling control. The quantitative analysis module then decoupled the effect of the control on reducing the on-state loss of the T2 tube. And finally, a simulation model is built by utilizing PLECS and MATLAB/Simulink simulation software, the effectiveness of the decoupling control of the module is verified by a simulation result, and the correctness of the analysis of the decoupling control on the improvement effect of the loss distribution characteristics of the device is verified by a device loss simulation result. Further, the specific conclusions are as follows:

(1) the half-bridge module has the strongest participation and is most beneficial to reducing the on-state loss and the switching loss of the T2 tube. Taking a certain engineering example where m is 0.8267, cos is 1, and P is 200MW, the module decoupling control can reduce the on-state loss of T2 in the half-bridge sub-module by 7.22%, and the total loss of T2 by 8.63%.

(2) Compared with the decoupling control, when the voltage, the transmission power and the power factor on the direct current side are unchanged, along with the increase of the operation modulation ratio, the improvement effect of the module decoupling control on the loss distribution characteristic of the T2 tube is weakened.

(3) Compared with the decoupling control, when the voltage, the transmission power and the operation modulation ratio on the direct current side are unchanged, along with the reduction of the power factor, the improvement effect of the module decoupling control on the loss distribution characteristic of the T2 tube is enhanced.

The above description is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention, and various changes and modifications can be made by workers in the field without departing from the technical spirit of the present invention. The technical scope of the present invention is not limited to the content of the specification, and all equivalent changes and modifications in the shape, structure, characteristics and spirit described in the scope of the claims of the present invention are included in the scope of the claims of the present invention.