Medium voltage direct current energy storage system and control method

Abstract

The application provides a medium-voltage direct-current energy storage system and a control method, and the system is composed of multiple modules. Each module has multiple energy storage ports and one grid-connected port. The energy storage ports of the module are connected with multiple energy storage units, and the grid-connected port is connected in series to access a medium-voltage direct-current bus. Each module contains multiple branches, and each branch is composed of four power switch devices in series. The upper end points of the branches are connected to each other as the positive output of the module. The lower end points of the branches are connected to each other as the negative output. The middle connection points of the branches are connected to the positive poles of the energy storage battery units through inductors, and the middle connection points are connected to the negative poles of the energy storage units. The middle connection points are connected to each other to access the midpoint of the output capacitor. The energy storage system adopts a double-layer staggered modulation method. The first layer is the inter-module staggered control, and the second layer is the inter-branch staggered control in the module. The application considers the control strategy of the equalization charging and discharging of the energy storage battery, and avoids the problem of unbalanced residual capacity of the energy storage battery during operation.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of power generation, and particularly relates to a medium-voltage direct-current energy storage system and a control method. BACKGROUND

[0002] With the development of power electronics technology, medium-voltage direct-current systems have gradually been applied in the fields of power generation, power transmission, power utilization, etc. due to their high efficiency, good stability, high power quality, etc. The main application fields of medium-voltage direct-current systems include large renewable energy power generation bases such as the Saiga Desert Base, offshore wind power and photovoltaic power generation systems, regional medium-voltage direct-current distribution network systems in industrial parks, etc. At present, the development of medium-voltage direct-current equipment such as ±30kV photovoltaic medium-voltage direct-current converters and 10kV medium-voltage direct-current transformers has been completed, and has been demonstrated and applied. Large-capacity energy storage, as an important part of medium-voltage direct-current systems, has the functions of stabilizing the power fluctuation of photovoltaic and other renewable energy systems and stabilizing the medium-voltage direct-current bus. In a medium-voltage direct-current system, the energy storage is connected to the medium-voltage direct-current bus, and the voltage level needs to reach tens to hundreds of kilovolts, and the capacity needs to reach tens to hundreds of megawatts. Using the direct-current transformer scheme requires multiple conversion links such as high-frequency inversion, high-frequency transformers, and high-frequency rectification, which has high equipment cost, low efficiency, large volume, and low reliability. Therefore, it is necessary to develop a large-power low-cost medium-voltage direct-current energy storage system.

[0003] Chinese Invention Application CN115663864 discloses a direct-current cascade energy storage system composed of n full-bridge DC / DC power modules. However, the DC / DC module of this technology can only access low input and output voltage levels. This system connects to the medium-voltage direct-current bus through one-stage voltage reduction and one-stage voltage increase, specifically, the energy storage unit first reduces the voltage through DC / DC, and then realizes voltage increase through the series connection of multiple module outputs to connect to the medium-voltage direct-current bus, which causes the energy storage unit and the module output voltage level to be low, and a large number of modules need to be connected in series to meet the medium-voltage direct-current grid connection requirements, thereby increasing the system cost and failure rate. At present, the voltage of large-capacity energy storage will reach 1.5kV or even higher, so the use of this technology requires high-voltage switching tubes, which has high equipment cost. At the same time, due to the inconsistent parameters of the energy storage units connected to each port of the topology, the power of each module is not uniform, which affects the stable operation of the direct-current converter. SUMMARY

[0004] To solve the above technical problems, the application provides a medium-voltage direct-current energy storage system and a control method, the system is of a modular structure, energy storage is incorporated into a medium-voltage direct-current bus through two-stage voltage boosting, the module topology structure in the system has a high voltage boosting ratio, and high-voltage-ratio conversion from energy storage to a grid-connected side can be realized, the module is of a multi-port structure, provides multiple interfaces for a series connection module of energy storage, the system capacity can reach more than 100 MW through the series connection of high-power modules. Meanwhile, the energy storage system adopts a double-layer interleaved modulation method, reduces grid-connected current ripple, and reduces high-voltage grid-connected filter inductance. The first layer is interleaved control between modules, and the second layer is interleaved control between multiple branches in a module. The application proposes a medium-voltage direct-current energy storage system module equalization control technology considering the residual capacity of energy storage batteries, and solves the problem of inconsistent residual capacity of energy storage units caused by inconsistent output characteristics of the energy storage units.

[0005] To achieve the above object, the application adopts the following technical scheme:

[0006] A medium-voltage direct-current energy storage system adopts a modular scheme and is composed of a first module to an nth module; each module has multiple energy storage ports and one grid-connected port; the energy storage ports of the module are connected to multiple energy storage units, and the grid-connected port is connected in series to a medium-voltage direct-current bus; the voltage of the energy storage port is low-voltage direct-current; when the energy storage units are discharged, the energy storage units realize voltage boosting conversion through a module circuit, realize secondary voltage boosting through the series connection of the grid-connected ports of multiple modules, and are finally boosted to medium-voltage direct-current to be connected to the medium-voltage direct-current bus; when the energy storage units are charged, the voltage is stepped down from the medium-voltage direct-current bus to low-voltage to charge the energy storage units.

[0007] Further, the module contains m branches, wherein the jth branch is composed of power switch devices Qij1-Qij4 in series, wherein i is the module number, and j is the branch number; the upper end points of the first Ai1, Ai2, …, Aim branches are connected to each other as a module output positive electrode Mi; the lower end points of the first Ei1, Ei2, …, Eim branches are connected to each other as an output negative electrode Ni; the middle connection points Bij of the branches are connected to the positive electrodes of energy storage battery units BATij through inductors Lij, and the middle connection points Dij are connected to the negative electrodes of the energy storage battery units BATij; the middle connection points Cij are connected to each other and connected to the middle point Oi of an output capacitor; the capacitor CAPi1 and the capacitor CAPi2 are connected in series, the positive electrode of which is connected to the module positive electrode Mi, and the negative electrode of which is connected to the module negative electrode Ni.

[0008] Further, the medium-voltage direct-current energy storage system adopts a double-layer interleaved modulation method, the first layer is interleaved control between modules, and the second layer is interleaved control between multiple branches in a module.

[0009] Further, when the medium-voltage direct-current energy storage system contains n modules, the reference phases of the first module to the nth module are sequentially different by T / n, wherein T is the switching period of each switch device in the module, so that the interleaved control between modules of the first layer is realized.

[0010] Further, the modules in the second layer are staggered controlled between the branches, and the phases of the branches in each module are sequentially shifted by T / m behind the reference phase of the module, where m is the total number of branches in the module, so as to realize staggered modulation between the modules and the branches.

[0011] Further, the midpoint of the output capacitor CAPi1 and the capacitor CAPi2 is clamped, and i is the module number, so that the maximum voltage borne by each switch tube of the module is 50% of the grid-connected side voltage of the module.

[0012] The application also provides a control method of the medium-voltage direct-current energy storage system, which adopts a double closed-loop mode, the outer ring is an output voltage ring or an output current ring according to actual demand, and the inner ring is a battery current ring, and the method comprises the following steps:

[0013] Step 1, in the constant-voltage grid-connected mode, the grid-connected voltage instruction V o_ref is obtained by subtracting the instantaneous value v o of the grid-connected voltage, and the energy storage system current instruction I ref is obtained by passing through the first adjuster. In the constant-current grid-connected mode, the grid-connected current instruction I o_ref is obtained by subtracting the instantaneous value i o of the grid-connected voltage, and the energy storage system current instruction I ref is obtained by passing through the second adjuster. According to the actual demand mode, the running mode is selected, and the corresponding current instruction is selected as the output instruction of the mode selector.

[0014] Step 2, the residual capacity of each energy storage battery unit is calculated, and the sum is obtained as the total residual capacity of the medium-voltage direct-current energy storage system , wherein SOC_modij represents the residual capacity of each energy storage battery, i is the module number, j is the branch number, the total residual capacity is multiplied by the reciprocal of the module number 1 / n, n is the module number, and the average value SOC_mod of the residual capacity of the module is obtained. ave ; the SOC_mod ave is subtracted from the residual capacity of the module i , and the module residual capacity current instruction I ref_socmodi is obtained by passing through the third_i adjuster.

[0015] Step 3, the energy storage system current instruction I ref is subtracted from the module residual capacity current instruction I ref_modsoci of the module i, and the final module current instruction I ref_modi of the module i is obtained, wherein i is the module number.

[0016] Step 4, the final module current instruction I ref_modi and the residual capacity SOC_mod modi1 , SOC_mod modi2 …… of the energy storage battery of each branch of the module i are compared.modim The current command I for each branch of module i is obtained through the 4_i regulator. ref_bati1 I ref_bati2 ...I ref_batim , where i is the module number and m is the number of branches within the module;

[0017] Step 5: Transfer the current command I of each branch of module i. ref_bati1 I ref_bati2 ...I ref_batim Subtract the energy storage port sampling current I of the corresponding branch respectively ref_i1 I ref_i2 ...I ref_im Through the 5th i1 Regulator, 5th i2 Regulator...5_ im The regulator obtains the duty cycle D_ of each branch of module i. i1 D_ i2 ...D_ im ;

[0018] Step 6: The reference phase of modules 1 to n is shifted backward by T / n. The phase of each branch of a module is shifted backward by T / m based on the reference phase of that module, where m is the total number of branches of the module, thereby realizing inter-module and inter-branch intermodulation to form triangular waves in each branch;

[0019] Step 7, Duty cycle D_ of each branch of module i i1 D_ i2 ...D_ im The triangular waves of each branch are compared with the i1th comparator, i2th comparator, ..., imth comparator to obtain the PWM signals PWM_i1, PWM_i2, ..., PWM_im of each branch, and the switching transistors of the control module are turned on and off.

[0020] Beneficial effects:

[0021] (1) The system of the present invention has a modular structure, which realizes the energy storage and connection to the medium-voltage DC bus through two-stage boost. The DC-DC converter module has a high boost ratio. The first stage realizes the DC boost of the energy storage unit through the module; the second stage realizes the secondary DC boost through the output of the DC-DC converter module in series. Therefore, the system has a high boost ratio. Under the same DC bus voltage, the number of series modules can be effectively reduced, the system reliability can be improved and the cost can be reduced.

[0022] (2) The DC boost module of the present invention realizes the high boost ratio conversion of energy storage to grid connection. The module topology can withstand high input and output voltage levels through inductor boost and capacitor clamping, thereby improving the access voltage of energy storage battery.

[0023] (3) The module of the present application is a multi-port structure, which provides multiple interfaces for the energy storage series module, avoids the energy storage circulating current problem caused by connecting the energy storage unit in parallel first and then connecting the DC conversion module, and realizes a system capacity of more than 100 MW through the series connection of high-power modules.

[0024] (4) The branches between and in the modules of the present application adopt the interleaved modulation method, which effectively reduces the grid-connected current ripple and the high-voltage filter inductance on the grid side.

[0025] (5) The present application proposes a medium-voltage DC energy storage system module equalization control technology considering the remaining capacity of the energy storage battery, solves the problem of inconsistent remaining capacity of the energy storage units caused by inconsistent output characteristics of the energy storage units, and effectively improves the reliability and availability of the energy storage system. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 The structure block diagram of the medium-voltage DC energy storage system of the present application is shown in the figure;

[0027] Figure 2 The modulation waveform diagram of the medium-voltage DC energy storage system module of the present application is shown in the figure;

[0028] Figure 3 The flowchart of the control method of the medium-voltage DC energy storage system of the present application is shown in the figure. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application will be further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other.

[0030] As shown in the figure, Figure 1 The medium-voltage DC energy storage system of the present application adopts a modular scheme, which is composed of the 1st module to the nth module. Each module has multiple energy storage ports and one grid-connected port. The energy storage ports of the module are connected to multiple energy storage units, and the grid-connected port is connected in series to the medium-voltage DC bus. The voltage of the energy storage port is low-voltage DC, and the low-voltage DC port can be connected to a voltage of several hundred volts to several thousand volts. When the energy storage unit is discharged, the energy storage unit realizes voltage conversion through the module circuit, realizes secondary voltage conversion through the series connection of the grid-connected ports of multiple modules, and finally is raised to medium-voltage DC to be connected to the medium-voltage DC bus. The voltage of the medium-voltage DC bus is generally tens of kilovolts. Conversely, when the energy storage unit is charged, the voltage is reduced from the medium-voltage DC bus to the low voltage for charging the energy storage unit.

[0031] The following is an example of the present application. Figure 1The first module in the middle is taken as an example to illustrate the composition of the module in the medium voltage direct current energy storage system. The first module includes m branches, wherein the ith branch is formed by power switching device Q1i1 to power switching device Q1i4 in series, wherein i is an integer from 1 to m. The upper end points of the A11, A12, …, A1m branches are connected to each other as the module output positive electrode M1. The lower end points of the E11, E12, …, E1m branches are connected to each other as the output negative electrode N1. The middle connection point B1i is connected to the positive electrode of the energy storage battery unit BAT1i through the inductor L1i, and the middle connection point D1i is connected to the negative electrode of the energy storage battery unit BAT1i. The middle connection points C1i are connected to each other and connected to the output capacitor middle point O1. The capacitor CAP11 and the capacitor CAP12 are connected in series, the positive electrode of which is connected to the module positive electrode M1, and the negative electrode of which is connected to the module negative electrode N1.

[0032] The medium voltage direct current energy storage system adopts a double-layer interleaved modulation method to reduce the grid current ripple and reduce the high voltage grid filter inductance. The first layer is the inter-module interleaved control, and the second layer is the inter-branch interleaved control within the module. When the medium voltage direct current energy storage system includes n modules, the reference phases of the first module to the nth module are sequentially different by T / n, and n is the switching period, so as to realize the inter-module interleaved control of the first layer, that is, the duty ratio is generated through the control loop, and the modules are turned on through interleaved control. The total number of modules is n, and the reference phase difference T / n of each module is set, so as to realize the inter-module interleaved control.

[0033] The second layer of the inter-branch interleaved control within the module is as shown in Figure 2

[0034] Figure 2 The modulation waveform in the boost discharge mode is taken as an example of the first module including 3 branches. In the energy storage unit discharge phase, the module operates in the boost mode, and power switching device Q112, power switching device Q122 and power switching device Q132 are turned on alternately for T / 3 time. The duty ratios of the middle two switching tubes of each branch are the same, that is, power switching device Q112 and power switching device Q113, power switching device Q122 and power switching device Q123, and power switching device Q132 and power switching device Q133 have the same duty ratio. Power switching device Q113, power switching device Q123 and power switching device Q133 are turned on alternately with power switching device Q112, power switching device Q122 and power switching device Q132 by T / 2, and the other switching tubes are always in the off state in the discharge phase. In the energy storage unit charging phase, the module operates in the buck mode.

[0035] ​The charging mode is opposite to the discharging mode, the power switch devices Q112, Q122, Q132, Q113, Q123 and Q133 are always turned off, and the power switch devices Q111, Q121 and Q131 are turned on staggered for T / 3 time. The duty cycles of the upper and lower switch tubes of each branch are the same, that is, the power switch devices Q111 and Q114, the power switch devices Q121 and Q124, and the power switch devices Q131 and Q134 have the same duty cycle respectively. The power switch devices Q111, Q121 and Q131 are staggered turned on with the power switch devices Q114, Q124 and Q134 respectively for T / 2.

[0036] The midpoint of the output capacitor CAP11 and the capacitor CAP12 is clamped, so that the maximum voltage borne by each switch tube of the first module is 50% of the module grid-side voltage, thereby reducing the voltage stress of the switch tube. In Chinese patent application CN115663864, the voltage stress of the module switch tube is the module grid-side voltage. Therefore, although the same switching device is used, the input voltage and the output voltage of the direct current conversion module in the application are higher than those in the prior art such as CN115663864, thereby reducing the number of modules used by the energy storage system, improving the voltage level of the energy storage side and the high-voltage side, and improving the reliability and adaptability of the system.

[0037] The medium-voltage direct-current energy storage system in the application is composed of a plurality of energy storage units and modules in series. Due to the inconsistency of the module hardware parameters and the energy storage battery parameters, the problem of inconsistent residual charge of the energy storage battery units may occur during long-term operation, which directly affects the operation performance of the system and the service life of the energy storage units. In view of this problem, the application also proposes a control strategy considering the equalization charging and discharging of the energy storage battery to avoid the problem of uneven residual charge of the energy storage battery during operation.

[0038] As shown in Figure 3 The control method of the medium-voltage direct-current energy storage system of the application adopts a double closed-loop mode, the outer loop can be an output voltage loop or an output current loop according to actual demand, and the inner loop is a battery current loop, which specifically includes the following steps:

[0039] Step 1, in the constant-voltage grid-connected mode, the grid voltage instruction V o_ref is obtained by subtracting the grid voltage instantaneous value v o from the grid voltage instruction V ref , and the energy storage system current instruction I o_ref is obtained by the first adjuster. In the constant-current grid-connected mode, the grid current instruction I oThe energy storage system current command I is obtained through the second regulator. ref Select the operating mode according to the actual operating mode required, and use the corresponding current command as the output command of the mode selector.

[0040] Step 2: Calculate the remaining power of each energy storage battery unit and sum them up to obtain the total remaining power of the medium-voltage DC energy storage system. Where SOC_modij represents the remaining capacity of each energy storage battery, i is the module number, and j is the branch number. The average remaining capacity of each module is obtained by multiplying the total remaining capacity by the reciprocal of the number of modules (1 / n), where n is the number of modules. ave SOC_ ave Subtract the remaining power of module i The remaining power current command I of the module is obtained through the third_i regulator. ref_socmodi ;

[0041] Step 3: Transfer the energy storage system current command I ref Subtract the remaining power current of module i from module i using instruction I ref_modsoci Obtain the final current command I for module i. ref_modi where i is the module number;

[0042] Step 4: Transfer the module's final current command I ref_modi The remaining SOC of each branch energy storage battery in module i modi1 SOC_ modi2 ...SOC_ modim The current command I for each branch of module i is obtained through the 4_i regulator. ref_bati1 I ref_bati2 ...I ref_batim , where i is the module number and m is the number of branches within the module.

[0043] Step 5: Transfer the current command I of each branch of module i. ref_bati1 I ref_bati2 ...I ref_batim Subtract the energy storage port sampling current I of the corresponding branch respectively ref_i1 I ref_i2 ...I ref_im Through the 5th i1 Regulator, 5th i2 Regulator...5_ im The regulator obtains the duty cycle D_ of each branch of module i. i1 D_ i2 ...D_ im ;

[0044] Step 6, the reference phase of the first module to the nth module is once moved backward by T / n. The phase of each branch of the module is once moved backward by T / m based on the reference phase of the module, m is the total number of module branches, so as to realize the staggered modulation between the modules and the branches, and form the triangular waves of each branch;

[0045] Step 7, the duty cycle D_i of the i-th module is compared with the triangular wave of each branch through the i-th comparator, the i-th comparator, and the i-th comparator, respectively, to obtain the PWM signal PWM_i1, PWM_i2, and PWM_im of each branch, and control the on and off of the module switch tube. i1 i2 im

[0046] Those skilled in the art can easily understand that the above description is only a preferred embodiment of the present application, and is not used to limit the present application, and any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.​​​

Claims

1. A medium-voltage DC energy storage system, characterized in that, The system adopts a modular design, consisting of modules 1 through n. Each module has multiple energy storage ports and one grid connection port. The energy storage ports of the modules connect to multiple energy storage battery units, and the grid connection port is connected in series to the medium-voltage DC bus. The voltage of the energy storage ports is low-voltage DC. When the energy storage battery units discharge, the energy storage battery units achieve voltage boosting through the module circuit, and then achieve secondary voltage boosting through the grid connection ports of multiple modules connected in series, finally boosting to medium-voltage DC and connecting to the medium-voltage DC bus. When the energy storage battery units are charging, the voltage is stepped down from the medium-voltage DC bus to low voltage to charge the energy storage battery units. The module contains m branches, where the j-th branch is formed by power switching devices Qij1 to Qij4 connected in series, where i is the module number and j is the branch number; the upper endpoints of branches Ai1, Ai2, ..., Aim are connected to each other to form the positive output Mi of module i; the lower endpoints of branches Ei1, Ei2, ..., Eim are connected to each other to form the negative output Ni; the middle connection point Bij of the branch is connected to the positive terminal of the energy storage battery unit BATij through inductor Lij, and the middle connection point Dij is connected to the negative terminal of the energy storage battery unit BATij; the middle connection points Cij are connected to each other and connected to the midpoint Oi of the output capacitor; capacitors CAPi1 and CAPi2 are connected in series, with their positive terminals connected to the positive terminal Mi of the module and their negative terminals connected to the negative terminal Ni of the module.

2. The medium-voltage DC energy storage system according to claim 1, characterized in that, The medium-voltage DC energy storage system adopts a two-layer interleaved modulation method. The first layer is interleaved control between modules, and the second layer is interleaved control between multiple branches within a module.

3. A medium-voltage DC energy storage system according to claim 2, characterized in that, When a medium-voltage DC energy storage system contains n modules, the reference phases of the first to the nth modules are sequentially different by T / n, where T is the switching period of each switching device in the module, thereby realizing inter-module interleaving control in the first layer.

4. A medium-voltage DC energy storage system according to claim 2, characterized in that, The second layer involves interleaving control between multiple branches within a module. The phase of each branch in the module is shifted sequentially by T / m from the module's reference phase, where m is the total number of branches in the module, thereby achieving interleaving modulation between modules and between branches.

5. A medium-voltage DC energy storage system according to claim 1, characterized in that, By clamping the midpoints of output capacitors CAPi1 and CAPi2, the maximum voltage that each switch in the module can withstand is 50% of the grid-connected voltage of the module, where i is the module number.

6. A control method for a medium-voltage DC energy storage system according to any one of claims 1-5, characterized in that, A dual closed-loop approach is adopted, with the outer loop being either an output voltage loop or an output current loop depending on actual needs, and the inner loop being a battery current loop. The approach includes the following steps: Step 1: In constant voltage grid-connected mode, the grid-connected voltage command V o_ref Subtract the instantaneous value of the grid-connected voltage v o The energy storage system current command I is obtained through the first regulator. ref In constant current grid-connected mode, the grid-connected current command I o_ref Subtract the instantaneous value of the grid-connected voltage i o The energy storage system current command I is obtained through the second regulator. ref Select the operating mode according to the actual operating mode required, and use the corresponding current command as the output command of the mode selector. Step 2: Calculate the remaining power of each energy storage battery unit and sum them up to obtain the total remaining power of the medium-voltage DC energy storage system. Where SOC_modij represents the remaining capacity of the energy storage battery unit, i is the module number, j is the branch number, and the total remaining capacity is multiplied by the reciprocal of the number of modules (1 / n), where n is the number of modules, to obtain the average remaining capacity of the modules, SOC_. ave ;SOC_ ave Subtract the remaining power of module i The remaining power and current command I of module i is obtained through the third regulator. ref_socmodi ; Step 3: Transfer the energy storage system current command I ref Subtract the remaining power current of module i from module i using instruction I ref_modsoci Obtain the final current command I for module i. ref_modi where i is the module number; Step 4: Transfer the module's final current command I ref_modi The remaining SOC of each branch energy storage battery unit in module i modi1 SOC_ modi2 ...SOC_ modim The current command I for each branch of module i is obtained through the 4_i regulator. ref_bati1 I ref_bati2 ...I ref_batim , where i is the module number and m is the number of branches within the module; Step 5: Transfer the current command I of each branch of module i. ref_bati1 I ref_bati2 ...I ref_batim Subtract the energy storage port sampling current I of the corresponding branch respectively ref_i1 I ref_i2 ...I ref_im Through the 5th i1 Regulator, 5th i2 Regulator...5_ im The regulator obtains the duty cycle D_ of each branch of module i. i1 D_ i2 ...D_ im ; Step 6: The reference phase of modules 1 to n is shifted backward by T / n, and the phase of each branch of the module is shifted backward by T / m based on the reference phase of the module, where m is the total number of branches of the module, thereby realizing inter-module and inter-branch intermodulation to form triangular waves of each branch; Step 7, Duty cycle D_ of each branch of module i i1 D_ i2 ...D_ im The triangular waves of each branch are compared with the i1th comparator, i2th comparator, ..., imth comparator to obtain the PWM signals PWM_i1, PWM_i2, ..., PWM_im of each branch, and the switching transistors of the control module are turned on and off.

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