Method for realizing neutral-point voltage balance control of three-level energy storage converter

Abstract

The invention relates to a method for realizing neutral-point voltage balance control of a three-level energy storage converter, which is applied to a soft start transient process, grid-connected and off-grid four-quadrant stable operation and off-grid no-load operation of an ANPC three-level energy storage converter. According to the neutral-point voltage balance control method provided by the invention, the energy storage converter equipment can realize neutral-point voltage balance by loading a zero-sequence compensation amount calculated through three-phase virtual current into a modulation signal in a soft start process and in an off-grid no-load mode; balanced fluctuation of the neutral-point voltage can be kept through judgment conditions of start and stop of the neutral-point voltage. The method not only has the characteristic of simple implementation of a traditional carrier modulation neutral-point voltage balance algorithm, but also has the characteristic of good real-time performance of space vector modulation.

Description

Technical Field

[0001] The present invention belongs to the technical field of power grid control, and in particular, relates to a method for realizing neutral point voltage balance control of a three-level energy storage converter. Background Art

[0002] The active clamped three-level converter (ANPC) is derived from the traditional three-level neutral point clamped converter (NPC) by replacing the diodes with active switching devices. It not only inherits the advantages of NPC such as high voltage level, low voltage withstand level of power devices, and low harmonic content of the converter output voltage, but also has a low-inductance commutation path comparable to that of a two-level converter and redundant zero-level output states for balancing the device loss distribution. However, its inherent neutral point voltage imbalance problem restricts its wide application in industrial fields such as photovoltaics, energy storage, and electric vehicles.

[0003] The existing neutral point voltage balance control methods are as follows:

[0004] (1) Using two independent DC power supplies, but this will increase the system cost.

[0005] (2) Injecting or extracting current from the power converter to the neutral point, which increases the system cost and control difficulty.

[0006] (3) Optimizing the space vector modulation and SPWM modulation algorithms. These two algorithms have certain control effects. However, the former is computationally complex and occupies a large amount of CPU resources, especially not suitable for energy storage converter devices with low switching frequencies. The implementation method of the latter is relatively simple compared to the former. However, it requires sampling too many electrical parameters for calculating the zero-sequence voltage, has poor real-time performance, and will cause regulation errors. The application scope of these two algorithms is mainly when the energy storage converter is working in a stable state and cannot be applied to the transient process of soft start, especially in the case of small current distortion. Summary of the Invention

[0007] The purpose of the present invention is to overcome the deficiencies of the prior art and propose a method for realizing neutral point voltage balance control of a three-level energy storage converter. According to the fact that in the PWM period of the nearest three-vector modulation, the output level of one phase must remain unchanged, and from the equivalent relationship between space vector modulation and carrier modulation, it can be known that the modulation signal is clamped to a fixed value by adding a zero-sequence compensation amount within one switching period. Based on this feature, it is possible to clamp the three-phase current to the midpoint of the DC bus capacitor, realize the charge and discharge control of the DC bus capacitor, and thus control the neutral point voltage balance.

[0008] The present invention solves its technical problems by adopting the following technical solutions:

[0009] A method for realizing neutral point voltage balance control of a three-level energy storage converter includes the following steps:

[0010] Step 1: Collect the three-phase voltages at the grid connection point of the energy storage converter. Select the method of collecting the three-phase currents on the bridge arm side according to whether the AC-side circuit breaker of the energy storage converter is closed. When the AC-side circuit breaker is closed (the operating condition is four-quadrant operation of grid-connected and islanding), the three-phase currents on the bridge arm side are obtained through Hall sampling;

[0011] Step 2: Determine the sector of the space vector diagram where the reference vector synthesized from the three-phase voltage signals in the grid data information falls, and determine the sector number N;

[0012] Step 3: After determining the sector N, judge which phase current among the three-phase currents helps to balance the neutral point voltage;

[0013] Step 4: Determine the action behavior of the three-phase modulation signals according to the phase current judged in Step 3;

[0014] Step 5: Superimpose the compensation amount of the modulation signal corresponding to the action behavior obtained in Step 4 on the three-phase modulation signals, and then compare the three-phase modulation signals with the carrier wave to generate PWM waves to control the balance of the neutral point voltage;

[0015] Step 6: When the AC-side circuit breaker is disconnected (the operating condition is soft start and islanding) or in islanding no-load operation, calculate the three-phase currents on the bridge arm side (i.e., virtual currents) from the filter capacitor voltages, and then substitute the obtained three-phase currents on the bridge arm side into Steps 2, 3, 4, and 5 in sequence. Finally, calculate the compensation amount of the three-phase modulation signals when the current operating condition is that the AC-side circuit breaker is disconnected, superimpose this compensation amount on the three-phase modulation signals, and finally compare the three-phase modulation signals with the carrier wave to generate PWM waves to control the balance of the neutral point voltage under this operating condition

[0016] Moreover, the specific implementation method of Step 1 is as follows: First, sample the three-phase voltages u abc of the grid connection point respectively. When the AC-side circuit breaker is closed, the three-phase currents i abc on the bridge arm side are obtained through Hall sampling, the DC bus voltage u dc , the voltage u down of the capacitor under the DC bus, and perform per-unit processing on the three-phase voltages and three-phase currents, which are respectively denoted as u a , u b , u c , i a , i b , i c . Calculate the deviation value △u of the two DC bus capacitors according to the DC bus voltage and the voltage of the capacitor under the DC bus: △u = 2u down - u dc . When the deviation value △u is greater than the limit value, activate the neutral point voltage balance control module.

[0017] Moreover, the specific implementation method of step 2 is as follows: The three-level space vector sectors are respectively sector 1 (u a >u b >u c ), sector 2 (u b >u a >u c ), sector 3 (u b >u c >u a ), sector 4 (u c >u b >u a ), sector 5 (u c >u a >u b ), sector 6 (u a >u c >u b ). The midpoint voltage balance control module judges the magnitude relationship between the three-phase voltage signals to determine the sector number N.

[0018] Moreover, the specific implementation method of step 3 is as follows: According to the product △ui abc of the deviation value △u of the DC bus capacitor and the arm-side current i a , △ui b , △ui c whether it is greater than zero. When the product value is greater than zero, it is recorded as 1, indicating that the phase current helps to balance the midpoint voltage. When the product value is less than zero, it is recorded as 0, indicating that the phase current cannot be used to balance the midpoint voltage.

[0019] Moreover, the specific implementation method of step 4 is as follows: In one switching period, the three-phase modulation signals are recorded as u max , u mid , u min from large to small in turn. And the current of the phase corresponding to u max is recorded as i maxphase , the current of the phase corresponding to u min is recorded as i midphase , the current of the phase corresponding to u min is recorded as i minphase . In each sector, there are four control behaviors of the modulation signal as follows: When △ui maxphase <0, △ui minphase <0, the action behavior of the modulation signal is that the modulation signal of the u mid phase is clamped to 0, and the compensation amount of the modulation signal is u off = -u mid ; When △ui maxphase <0, △ui minphase >0, the action behavior of the modulation signal is that the modulation signal of the u max phase is clamped to 1, and the compensation amount of the modulation signal is uoff = 1 - u max ; When △ui maxphase > 0, △ui minphase < 0, the modulation signal action behavior is that u min phase modulation signal is clamped to -1, and the compensation amount of the modulation signal is u off = -1 - u max ; When △ui maxphase > 0, △ui minphase > 0 and u mid > 0, the modulation signal action behavior is u max phase modulation signal is clamped to 1, and the compensation amount of the modulation signal u off = 1 - u max ; When △ui maxphase > 0, △ui minphase > 0 and u mid ≤ 0, the modulation signal action behavior is u min phase modulation signal is clamped to -1, and the compensation amount of the modulation signal u o ff = -1 - u min .

[0020] Moreover, the specific implementation method of step 5 is as follows:

[0021]

[0022] where u a , u b , u c are three-phase modulation signals, u off is the zero-sequence compensation amount of the modulation signal, u a ' , u b ' , u c ' are three-phase modulation signals after superimposing the zero-sequence compensation amount.

[0023] Moreover, the specific implementation method of step 6 is as follows: when the AC side circuit breaker is disconnected (the working condition is soft start) or the energy storage converter is operating in off-grid no-load, the grid-connected voltage at this time is the voltage u c of the filter capacitor in the LCL filter, and the current on the bridge arm side is equal to the current i c of the filter capacitor. Since the output current on the bridge arm side is severely distorted, it will cause misoperation of the neutral point voltage algorithm module. From the phasor relationship formula of the capacitor voltage and current, it can be known that the capacitor current leads the voltage by 90°, that is, the capacitor current crosses the zero point 5 ms earlier than the voltage, or it can also be regarded as the capacitor current lags the voltage by 15 ms. Therefore, the sampled grid-connected point voltage is delayed by 15 ms and then multiplied by 2πfC to obtain the capacitor current:

[0024]

[0025] In the formula and are respectively the phasor representations of the capacitive current and voltage, f is the power frequency, and C is the filtering capacitor of the LCL filter. Then, the obtained three-phase currents are successively substituted into Steps 2 to 5. Finally, the compensation amount of the three-phase modulation signal when the AC side circuit breaker is disconnected under the current working condition is calculated, and this compensation amount is superimposed on the three-phase modulation signal to generate a new three-phase modulation signal. The new three-phase modulation signal is compared with the carrier wave to generate a PWM wave for controlling the neutral point voltage balance.

[0026] The advantages and positive effects of the present invention are as follows:

[0027] The present invention is applied to the neutral point voltage balance control method during the open-loop soft start transient process, grid-connected and off-grid four-quadrant stable operation, and off-grid no-load operation of an ANPC three-level energy storage converter. According to the proposed neutral point voltage balance control method, the zero-sequence compensation amount calculated by the three-phase virtual current during the soft start process and off-grid no-load mode of the energy storage converter device is loaded into the modulation signal to achieve neutral point voltage balance. During the stable operation of the grid-connected and off-grid four quadrants, the neutral point voltage balance fluctuation can be maintained through the start-stop judgment condition of the neutral point voltage. This method not only has the simple implementation characteristic of the neutral point voltage balance calculation of traditional carrier modulation but also has the good real-time characteristic of space vector modulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is the circuit diagram of the energy storage converter with a three-level ANPC topology according to the present invention;

[0029] Figure 2 is the space vector diagram of the three levels according to the present invention;

[0030] Figure 3 is the schematic diagram of the equivalent relationship between the nearest three vectors and carrier modulation according to the present invention;

[0031] Figure 4 is the flow chart of the zero-sequence compensation amount calculation and superposition according to the present invention;

[0032] Figure 5 is the schematic diagram of the sector judgment where the reference vector is located according to the present invention;

[0033] Figure 6 is the schematic diagram of the selection signal of the zero-sequence compensation amount according to the present invention;

[0034] Figure 7 is the calculation block diagram of the zero-sequence compensation amount according to the present invention;

[0035] Figure 8Schematic diagram of virtual current calculation on the arm side and neutral point voltage balance control during the open-loop soft start process of the present invention;

[0036] Figure 9 Schematic diagram of the neutral point voltage waveform and output current THD in the grid-connected mode of the present invention;

[0037] Among them, (a) is the neutral point voltage waveform in the grid-connected mode, and (b) is the output current THD;

[0038] Figure 10 Schematic diagram of the actual current and virtual current on the arm side in the off-grid no-load mode of the present invention

[0039] Among them, the actual current of phase A is the black curve, and the virtual current of phase A is the white curve;

[0040] Figure 11 Schematic diagram of the neutral point voltage and output voltage THD in the off-grid no-load mode of the present invention

[0041] Among them, (a) is the neutral point voltage waveform in the off-grid no-load mode, and (b) is the output voltage THD in the off-grid no-load mode. Detailed implementation manners

[0042] The present invention will be further described in detail below with reference to the accompanying drawings.

[0043] A method for realizing neutral point voltage balance control of a three-level energy storage converter includes the following steps:

[0044] Step 1: Collect and calculate grid data information.

[0045] As Figure 1 shown, first sample the three-phase voltages u abc at the grid connection point, the three-phase currents i abc on the arm side, the DC bus voltage u dc , the voltage u down of the capacitor under the DC bus respectively, and perform per-unit processing on the three-phase voltages and three-phase currents, which are respectively denoted as u a , u b , u c , i a , i b , i c . Calculate the deviation value △u of the two DC bus capacitors according to the DC bus voltage and the voltage of the capacitor under the DC bus, △u = 2u down -u dc . When the deviation value △u is greater than the limit value, activate the neutral point voltage balance control module.

[0046] Step 2: Determine the sector of the space vector diagram where the reference vector synthesized from the three-phase voltage signals in the grid data information falls, and determine the sector number N.

[0047] As shown Figure 2 in the figure, the three-level space vector sectors are sector 1 (u a > u b > u c ), sector 2 (u b > u a > u c ), sector 3 (u b > u c > u a ), sector 4 (u c > u b > u a ), sector 5 (u c > u a > u b ), sector 6 (u a > u c > u b ). Determine the magnitude relationship between the three-phase voltage signals to determine the sector number N.

[0048] Step 3: After determining sector N, determine which phase current in the three-phase current helps to balance the neutral point voltage.

[0049] According to the product △ui abc of the deviation value △u of the DC bus capacitor and the arm-side current i a , △ui b , △ui c whether it is greater than zero. When the product value is greater than zero, it is recorded as 1, indicating that this phase current helps to balance the neutral point voltage. When the product value is less than zero, it is recorded as 0, indicating that this phase current cannot be used to balance the neutral point voltage.

[0050] Step 4: Determine the action behavior of the three-phase modulation signals according to the phase current judged in Step 3.

[0051] As known from Figure 3 , according to the equivalent relationship between carrier modulation and the nearest three-vector modulation, selecting the corresponding switch sequence to balance the neutral point voltage is equivalent to clamping the modulation signal of the corresponding phase to a fixed value (1, 0, -1). In a switching period, the three-phase modulation signals are recorded as u max , u mid , u min from large to small in turn, and the current of the corresponding phase of u max is recorded as i maxphase , the current of the corresponding phase of u min is recorded as i midphase , the current of the corresponding phase of u min is recorded as i minphase . In each sector, the modulation signal has four control behaviors as follows: When △ui maxphase < 0, △ui minphase< 0, the action behavior of the modulation signal is u mid The phase modulation signal is clamped to 0, and the compensation amount of the modulation signal is u off = -u mid ; When △ui maxphase < 0, △ui minphase > 0, the action behavior of the modulation signal is u max The phase modulation signal is clamped to 1, and the compensation amount of the modulation signal is u off = 1 - u max ; When △ui maxphase > 0, △ui minphase < 0, the action behavior of the modulation signal is u min The phase modulation signal is clamped to -1, and the compensation amount of the modulation signal is u off = -1 - u max ; When △ui maxphase > 0, △ui minphase > 0 and u mid > 0, the action behavior of the modulation signal is u max The phase modulation signal is clamped to 1, and the compensation amount of the modulation signal u off = 1 - u max ; When △ui maxphase > 0, △ui minphase > 0 and u mid ≤ 0, the action behavior of the modulation signal is u min The phase modulation signal is clamped to -1, and the compensation amount of the modulation signal u off = -1 - u min .

[0052] Step 5: Superimpose the compensation amount of the modulation signal corresponding to the action behavior of the modulation signal obtained in Step 4 (i.e., the zero-sequence compensation amount) onto the three-phase modulation signals.

[0053]

[0054] In the formula, u a , u b , u c are the three-phase modulation signals, u off is the zero-sequence compensation amount of the modulation signal, u a ' , u b ' , u c ' are the three-phase modulation signals after superimposing the zero-sequence compensation amount.

[0055] The obtained three-phase modulation signals u a ' , u b ' , u c' Compare with the carrier wave to generate a PWM wave for controlling the neutral point voltage balance.

[0056] Step 6: When the AC side circuit breaker is disconnected (the working condition is the soft start process) or the grid-connected operation is no-load, calculate the three-phase current on the arm side of the energy storage converter (i.e., the virtual current) from the filter capacitor voltage, and then substitute the obtained three-phase current on the arm side into Steps 2, 3, 4, and 5 in sequence. Finally, calculate the compensation amount of the three-phase modulation signal under the current working condition that the AC side circuit breaker is disconnected, add this compensation amount to the three-phase modulation signal, and finally compare the three-phase modulation signal with the carrier wave to generate a PWM wave for controlling the neutral point voltage balance under this working condition;

[0057] When the AC side circuit breaker is disconnected (the working condition is soft start) or the energy storage converter operates in grid-connected no-load, the grid connection point voltage at this time is the voltage u of the filter capacitor in the LCL filter c , and the current on the arm side is equal to the current i of the filter capacitor c , due to the serious distortion of the output current on the arm side, it will cause misoperation of the neutral point voltage algorithm module. From the phasor relationship between the capacitor voltage and current, it can be known that the capacitor current leads the voltage by 90°, that is, the capacitor current crosses the zero point 5 ms earlier than the voltage, or it can also be regarded as the capacitor current lags the voltage by 15 ms (the power frequency period is 20 ms). Therefore, the sampled grid connection point voltage is delayed by 15 ms and then multiplied by 2πfC to obtain the capacitor current:

[0058]

[0059] In the formula and are the phasor representations of the capacitor current and voltage respectively, f is the power frequency, and C is the filter capacitor of the LCL filter.

[0060] This current is also the three-phase current on the arm side, and then substitute the obtained three-phase current on the arm side into Steps 2, 3, 4, and 5 in sequence. Finally, calculate the compensation amount of the three-phase modulation signal under the current working condition that the AC side circuit breaker is disconnected, add this compensation amount to the three-phase modulation signal to generate a new three-phase modulation signal, and compare the new three-phase modulation signal with the carrier wave to generate a PWM wave for controlling the neutral point voltage balance.

[0061] Embodiment 1

[0062] The three-level neutral point voltage balance control method proposed by the present invention is based on the equivalent relationship between the nearest three vectors and carrier modulation, and the characteristic that the output level of one phase must remain unchanged within a switching period in the nearest three-vector modulation. By superimposing a suitable signal on the modulation signal to be equivalent to the nearest three-vector modulation, this method of the present invention can clamp the current that helps balance the neutral point voltage to the midpoint of the DC bus capacitor, realize the charging and discharging of the upper and lower capacitors of the DC bus, and then control the neutral point voltage balance. When the deviation value △u of the DC bus capacitor voltage exceeds the set threshold, the neutral point voltage balance control module is activated. According to the voltage at the grid connection point and the sampling of the output current on the bridge arm side, the zero-sequence compensation amount is calculated, and then superimposed on the three-phase modulation signal to generate a new three-phase modulation signal, which is compared with the carrier to generate a PWM wave. The specific flow chart is as Figure 4 shown.

[0063] 1. When the energy storage converter device operates in the steady state of grid connection and off-grid, when the deviation value of the DC bus capacitor voltage exceeds the set threshold, the neutral point voltage balance control module is activated. According to the voltage at the grid connection point, the sector in which the reference vector synthesized by the three-phase voltage falls is judged. The specific block diagram is as Figure 5 shown. When u a >u b >u c , the reference vector falls in sector 1, and the sector judgment signal output is 1; when u b >u a >u c , the reference vector falls in sector 2, and the sector judgment signal output is 2; when u b >u c >u a , the reference vector falls in sector 3, and the sector judgment signal output is 3; when u c >u b >u a , the reference vector falls in sector 4, and the sector judgment signal output is 4; when u c >u a >u b , the reference vector falls in sector 5, and the sector judgment signal output is 5; when u a >u c >u b , the reference vector falls in sector 6, and the sector judgment signal output is 6.

[0064] 2. After determining the sector where the reference vector is located, according to the product △ui abc of the deviation value △u of the DC bus capacitor and the current i a on the bridge arm side, △ui b , △ui cIs it greater than zero? When the product value is greater than zero, it is recorded as 1, indicating that the phase current helps to balance the neutral point voltage. When the product value is less than zero, it is recorded as 0, indicating that the phase current cannot be used to balance the neutral point voltage. Therefore, in a sector, the selection signal n of the zero-sequence compensation amount is expressed as n = 2x + y, where the value ranges of x and y are {0, 1}. Therefore, the value range of n is {0, 1, 2, 3}. The specific calculation process is as Figure 6 shown.

[0065] 3. Determination of zero-sequence compensation amount

[0066] Within one switching period, the three-phase modulation signals are recorded in descending order as u max , u mid , u min , while the current of the corresponding phase of u max is recorded as i maxphase , the current of the corresponding phase of u min is recorded as i midphase , and the current of the corresponding phase of u min is recorded as i minphase . There are four control behaviors of the modulation signal in each sector as follows: When △ui maxphase <0, △ui minphase <0, the action behavior of the modulation signal is that the modulation signal of phase u mid is clamped to 0, and the compensation amount of the modulation signal is u off =-u mid ; When △ui maxphase <0, △ui minphase >0, the action behavior of the modulation signal is that the modulation signal of phase u max is clamped to 1, and the compensation amount of the modulation signal is u off =1 - u max ; When △ui maxphase >0, △ui minphase <0, the action behavior of the modulation signal is that the modulation signal of phase u min is clamped to -1, and the compensation amount of the modulation signal is u off =-1 - u max ; When △ui maxphase >0, △ui minphase >0 and u mid >0, the action behavior of the modulation signal is that the modulation signal of phase u max is clamped to 1, and the compensation amount of the modulation signal u off =1 - u max ; When △ui maxphase >0, △ui minphase >0 and u mid ≤0, the action behavior of the modulation signal is that the modulation signal of phase u min is clamped to -1, and the compensation amount of the modulation signal u off=-1 - u min 。 Figure 7 The calculation block diagram of the zero - sequence compensation amount is given, and the compensation amounts u of the six sectors of the three - level space vector are as off shown in Table 1.

[0067] Table 1 Zero - sequence compensation amounts for different sectors

[0068]

[0069]

[0070] 4. Superimpose the zero - sequence compensation amount on the three - phase modulation signals

[0071]

[0072] where u a , u b , u c are the three - phase modulation signals, u off is the zero - sequence compensation amount of the modulation signal, u a ' , u b ' , u c ' are the three - phase modulation signals after superimposing the zero - sequence compensation amount, and the specific process is as Figure 4 shown.

[0073] 5. Virtual currents on the bridge - arm side during the open - loop soft - start and off - grid no - load processes of the energy storage converter

[0074] During the soft - start and off - grid no - load processes, the grid - connection point voltage at this time is the voltage u of the filter capacitor in the LCL filter c , and the current on the bridge - arm side is equal to the current i of the filter capacitor c . Since the output current distortion on the bridge - arm side is serious, it will cause misoperation of the mid - point voltage algorithm module. From the phasor relationship between the capacitor voltage and current, the capacitor current leads the voltage by 90°, that is, the capacitor current crosses the zero - point 5 ms earlier than the voltage, and it also lags the voltage by 15 ms (the power - frequency period is 20 ms). Therefore, the sampled grid - connection point voltage is delayed by 15 ms and then multiplied by 2πfC to obtain the capacitor current. The specific block Figure 8 is shown.

[0075] Embodiment 2

[0076] Meanwhile, in order to verify the feasibility and effectiveness of the proposed method, the present invention conducts simulation verification in matlab Simulink. The parameters of the simulation system are shown in Table 2. In order to make the capacitor voltages unbalanced, the initial value of the upper capacitor voltage on the DC side is set to 800 V and the initial value of the lower capacitor voltage is set to 400 V in this simulation. The grid - connection simulation results are asFigure 9 As shown. It can be seen from the figure that the method proposed by the present invention can quickly adjust the voltage of the DC bus capacitor, control the deviation value of the DC bus capacitor voltage within 50V, and the THD of the output current is 1.72%, meeting the national standard requirements (less than 3%). From Figure 10 it can be seen that the actual current (black curve) on the bridge arm side has large harmonics and serious distortion, which will cause the midpoint voltage balance module to malfunction, while the virtual current can maintain a sine curve without distortion and can participate in the midpoint voltage balance control. Figure 11 This is the simulation result of using virtual current for midpoint voltage balance control in the off-grid no-load mode. The deviation value of the DC bus capacitor voltage is within 10V, and the THD of the output voltage is 1.72%, meeting the national standard requirements (less than 5%).

[0077] Table 2 Simulation system parameters

[0078]

[0079] It should be emphasized that the embodiments described in the present invention are illustrative rather than restrictive. Therefore, the present invention includes but is not limited to the embodiments described in the specific embodiments. Any other embodiments obtained by those skilled in the art according to the technical solutions of the present invention also fall within the scope of protection of the present invention.

Claims

1. A method for realizing midpoint voltage balance control of a three-level energy storage converter, characterized in that: The following steps are involved: Step 1: Collect the three-phase voltage of the grid connection point of the energy storage converter, and select the method of collecting the three-phase current on the bridge arm side according to whether the AC side circuit breaker of the energy storage converter is closed. When the AC side circuit breaker is closed, the three-phase current on the bridge arm side is obtained by Hall sampling for the four-quadrant operation of the grid and the off-grid operation; Step 2: determine the sector of the space vector diagram where the reference vector synthesized by the three-phase voltage signal in the power grid data information falls, and determine the sector number N; Step 3: After determining sector N, determine which phase of the three-phase current is helpful to balance the midpoint voltage according to the three-phase current calculated in step 1 or step 6; Step 4: Determine the action behavior of the three-phase modulation signal according to the phase current determined in step 3; Step 5: superimpose the compensation amount of the three-phase modulation signal corresponding to the action behavior obtained in step 4 onto the three-phase modulation signal, and then compare the three-phase modulation signal with the carrier to generate a PWM wave to control the midpoint voltage balance; Step 6. When the AC side circuit breaker is disconnected or the system is off-grid and running without load, the three-phase current on the bridge arm side of the energy storage converter is calculated from the voltage of the filter capacitor, and then the obtained three-phase current on the bridge arm side is substituted into steps 2, 3, 4, and 5 in sequence, and finally the compensation amount of the three-phase modulation signal under the current working condition is calculated, and the compensation amount is superimposed on the three-phase modulation signal to generate a new three-phase modulation signal. Finally, the new three-phase modulation signal is compared with the carrier to generate a PWM wave to control the midpoint voltage balance under the working condition.

2. A method for realizing midpoint voltage balance control of a three-level energy storage converter according to claim 1, characterized in that: The specific implementation method of step 1 is: respectively sampling the three-phase voltage u of the grid connection point abc When the AC circuit breaker is closed, the three-phase current i on the bridge arm side is obtained by Hall sampling. abc , DC bus voltage u dc , capacitor voltage u under DC bus down , and the three-phase voltage and three-phase current are normalized and recorded as u a ,u b ,u c ,i a ,i b ,i c , according to the DC bus voltage and the capacitor voltage under the DC bus, the deviation value of the two DC bus capacitors is calculated as △u=2u down -u dc When the deviation value △u is greater than the limit value, the midpoint voltage balance control module is activated.

3. The method for realizing midpoint voltage balance control of a three-level energy storage converter according to claim 1, characterized in that: The specific implementation method of step 2 is: divide the three-level space vector sectors into sectors 1, 2, 3, 4, 5 and 6 respectively, the midpoint voltage balance control module determines the magnitude relationship between the three-phase voltage signals, determines the sector number N, and the u of sector 1 a >u b >u c , u of sector 2 b >u a >u c , u in sector 3 b >u c >u a , u in sector 4 c >u b >u a , u in sector 5 c >u a >u b , u in sector 6 a >u c >u b .

4. The method for realizing midpoint voltage balance control of a three-level energy storage converter according to claim 1, characterized in that: The specific implementation method of step 3 is: according to the deviation value △u of the DC bus capacitor and the bridge arm side current i abc The product of △ui a , △ui b , △ui c Whether it is greater than zero, when the product value is greater than zero, it is recorded as 1, indicating that the phase current helps to balance the midpoint voltage. When the product value is less than zero, it is recorded as 0, indicating that the phase current cannot be used to balance the midpoint voltage.

5. The method for realizing midpoint voltage balance control of a three-level energy storage converter according to claim 1, characterized in that: The specific implementation method of step 4 is: within a switching cycle, the three-phase modulation signal is recorded as u from large to small. max ,u mid ,u min , and u max The current of the corresponding phase is recorded as i maxphase ,u min The current of the corresponding phase is recorded as i midphase ,u min The current of the corresponding phase is recorded as i minphase , the modulation signal has four control behaviors in each sector as follows: maxphase <0,△ui minphase <0, the modulation signal action behavior is u mid The phase modulation signal is clamped to 0, and the compensation amount of the modulation signal is u off =-u mid When △ui maxphase <0,△ui minphase >0, the modulation signal action behavior is u max The phase modulation signal is clamped to 1, and the compensation amount of the modulation signal is u off =1-u max When △ui maxphase >0,△ui minphase <0, the modulation signal action behavior time u min The phase modulation signal is clamped to -1, and the compensation amount of the modulation signal is u off =-1-u max When △ui maxphase >0,△ui minphase >0 and u mid >0, the modulation signal action behavior is u max The phase modulation signal is clamped to 1, and the compensation amount of the modulation signal u off =1-u max When △ui maxphase >0,△ui minphase >0 and u mid ≤0, the modulation signal action behavior is u min The phase modulation signal is clamped to -1, and the compensation amount of the modulation signal is u off =-1-u min .

6. A method for realizing midpoint voltage balance control of a three-level energy storage converter according to claim 1, characterized in that: The specific implementation method of step 5 is: Where u a ,u b ,u c is a three-phase modulation signal, u off is the zero-sequence compensation of the modulation signal, u a ' ,u b ' ,u c ' It is the three-phase modulation signal after superimposing the zero-sequence compensation. The obtained three-phase modulated signal u a ' ,u b ' ,u c ' The PWM wave is generated by comparing with the carrier wave to control the midpoint voltage balance.

7. A method for realizing midpoint voltage balance control of a three-level energy storage converter according to claim 1, characterized in that: The specific implementation method of step 6 is: when the AC circuit breaker is disconnected or the energy storage converter is working in the off-grid no-load process, the grid connection point voltage at this time is the voltage u of the filter capacitor in the LCL filter c , the current on the bridge arm side is equal to the current i of the filter capacitor c , due to the serious distortion of the output current on the bridge arm side, the midpoint voltage algorithm module will malfunction. From the phasor relationship between the capacitor voltage and current, it can be seen that the capacitor current leads the voltage by 90°, which means that the capacitor current crosses the zero point 5ms earlier than the voltage. It can also be regarded as the capacitor current lags the voltage by 15ms. Therefore, the grid-connected point voltage after sampling is delayed by 15ms and then multiplied by 2πfC to obtain the capacitor current: In the formula and are the phasor representations of capacitor current and voltage respectively, f is the power frequency, and C is the filter capacitor of the LCL filter; The three-phase current on the bridge arm side is obtained Substitute into steps 2 to 5 in sequence, and finally calculate the compensation amount of the three-phase modulation signal when the current working condition is that the AC side circuit breaker is disconnected, and add the compensation amount to the three-phase modulation signal to generate a new three-phase modulation signal. The new three-phase modulation signal is compared with the carrier to generate a PWM wave to control the midpoint voltage balance.

Images