Phase-to-phase voltage balance control method of cascaded H-bridge STATCOM based on feedforward compensation calculation

Abstract

The invention discloses a cascade H-bridge STATCOM interphase voltage balance control method based on feedforward compensation calculation: S1 obtains the positive and negative sequence output voltage and output current formulas of the cascade H-bridge STATCOM under the unbalanced grid voltage; S2 considers the influence of the negative sequence voltage and current, calculates the active power flow between the grid and the cascade H-bridge STATCOM, and divides it into an active constant and an active fluctuation; S3 performs positive and negative sequence decomposition on the obtained active fluctuation, balances the positive and negative sequence active power fluctuations respectively, and obtains the feedforward compensation of the active power; S4 extracts the zero-sequence active power according to the active power calculation formula, performs coordinate transformation, and obtains the calculation formula of the zero-sequence voltage injection amount in a two-phase stationary coordinate system; S5 collects the actual three-phase capacitor voltage, subtracts it from the set voltage reference value, outputs the active power difference after passing through a PI controller, superimposes the feedforward compensation calculated in S3, calculates the injected zero-sequence voltage value, and injects different zero-sequence voltages into the three phases of the cascade H-bridge STATCOM.

Description

Technical Field

[0001] The invention adopts a cascade H-bridge STATCOM phase-to-phase voltage balance control method based on feedforward compensation amount calculation, and belongs to the field of power electronic control. Background Art

[0002] Static VAR compensator (STATCOM) is a reactive power compensation device based on power electronic components as the main circuit, plus control circuit, drive circuit and detection circuit. Due to the voltage resistance limitation of power electronic devices, ordinary STATCOM usually needs to be connected to the system through a transformer in high voltage and high power scenarios. The addition of too many transformers will have a certain impact on the stability of the system and is not conducive to the economic operation of the system. The cascaded H-bridge static VAR compensator (CHB-STATCOM) is based on a multi-level and H-bridge series structure. It can directly compensate for the reactive power of high voltage and high power systems without a transformer, thereby improving the stability and economy of the system.

[0003] DC side voltage balance is an important issue that needs to be solved in CHB-STATCOM control. Most existing studies use three-layer voltage control to balance the DC side voltage, namely overall voltage control, phase-to-phase voltage control, and phase-to-phase voltage control. When the grid voltage is unbalanced, due to the generation of positive-sequence and negative-sequence components, the unbalanced flow of active power will lead to unbalanced three-phase voltage, affecting the reactive power compensation capability and even damaging the capacitor. Currently, there is a method of injecting negative-sequence current to balance the DC side voltage, which can effectively redistribute the active power among the three phases, but it will inevitably inject negative-sequence current into the system, which will affect the power quality of the system. The method of injecting zero-sequence voltage to balance the capacitor voltage is widely used because of its simplicity and effectiveness. However, when the grid voltage is unbalanced, the influence of positive-sequence and negative-sequence components needs to be taken into account. The feedback control consisting of the actual capacitor voltage and reference voltage of the three phases alone cannot balance the phase-to-phase voltage well.

[0004] Therefore, the present invention proposes a cascaded H-bridge STATOM phase-to-phase voltage balance control method considering the calculation of feedforward compensation. When the grid voltage is unbalanced, the feedforward compensation of active power is calculated and feedforward control is added, which can balance the phase voltage more accurately and improve the safety and stability of the system. Summary of the invention

[0005] Purpose of the invention: The purpose of the present invention is to provide a cascaded H-bridge STATCOM phase-to-phase voltage balance control method based on feedforward compensation calculation that overcomes the deficiencies in the prior art.

[0006] Technical solution: The present invention discloses a method for controlling phase-to-phase voltage balance of a cascaded H-bridge STATCOM based on feedforward compensation calculation, comprising the following steps:

[0007] S1: Obtain the positive and negative sequence output voltage and output current formula of the cascaded H-bridge STATCOM under the unbalanced grid voltage;

[0008] S2: Under the condition of unbalanced grid voltage, considering the influence of negative sequence voltage and current, the active power flow between the grid and the cascaded H-bridge STATCOM is calculated, and the three-phase active power flowing into the cascaded H-bridge STATCOM is divided into the constant active power and the fluctuating active power;

[0009] S3: Decompose the three-phase active power flowing into the cascaded H-bridge STATCOM into positive and negative sequences, balance the positive and negative sequence active power fluctuations respectively, and obtain the feedforward compensation of the active power;

[0010] S4: According to the active power in S2, the zero-sequence active power is extracted for coordinate transformation, and the calculation formula of the zero-sequence voltage injection amount is obtained in the two-phase stationary coordinate system;

[0011] S5: Detect and collect the actual three-phase capacitor voltage, make a difference with the set voltage reference value, output the active power difference after passing through the PI controller, add the feedforward compensation calculated in S3, and use it as the active power input value for zero-sequence voltage calculation. According to the actual operating conditions, inject the zero-sequence voltage calculated in S4 into the three phases of the cascaded H-bridge STATCOM to balance the three-phase capacitor voltage.

[0012] Furthermore, in step S1, when the grid voltage is unbalanced, positive sequence, negative sequence and zero sequence components will appear in the grid, and the output voltage of the cascaded H-bridge STATCOM under the unbalanced grid voltage is deduced to be:

[0013]

[0014] Where: u ca 、u cb 、u cc is the three-phase output voltage of the cascaded H-bridge STATCOM; U p , U n , U0 is the positive sequence, negative sequence and zero sequence components of the grid connection point voltage; θ p ,θ n , θ0 is the initial phase angle of the positive sequence, negative sequence and zero sequence components of the grid connection point voltage; w is the voltage angular frequency.

[0015] Since the cascaded H-bridge STATCOM adopts star connection, there is no zero-sequence current circulation loop, and the three-phase output current of the cascaded H-bridge STATCOM is:

[0016]

[0017] Where: i ca 、icb 、i cc is the three-phase output current of the cascaded H-bridge STATCOM; I p ,I n are the positive and negative sequence components of the output current; δ p , δ n It is the initial phase angle of the positive and negative sequence components of the output current.

[0018] Further, in step S2, when the grid voltage is unbalanced, the negative sequence voltage and current components U n ,I n It cannot be ignored, that is, the absorbed active power is divided into the same part of the three-phase power, the constant active component P, and the different part of the active fluctuation component △P i , specifically expressed as follows:

[0019] P i =P+ΔP i ,i=a,b,c (3)

[0020] Where: P i is the active power flowing into each phase; P is the active constant component; △P i is the active power fluctuation component of each phase; a, b, c represent phase a, phase b and phase c respectively.

[0021] According to the instantaneous power theory, the three-phase active power absorbed by the cascaded H-bridge STATCOM from the system is obtained using equations (1) and (2), where the active power constant component P and the active power fluctuation component △P in equation (3) are i Respectively expressed as:

[0022]

[0023]

[0024] Where: △P a , △P b , △P c They are the three-phase active power fluctuation components abc respectively.

[0025] Furthermore, in step S3, the active fluctuation component in equation (5) is transformed into positive and negative sequence coordinates, and is divided into positive sequence, negative sequence and zero sequence active fluctuation components, which are respectively expressed as:

[0026]

[0027]

[0028]

[0029] Where: ΔP ap, ΔP an , ΔP a0 are the positive sequence, negative sequence and zero sequence components of the active power fluctuation of phase a respectively; ΔP bp , ΔP bn , ΔP b0 are the positive sequence, negative sequence and zero sequence components of the active power fluctuation of phase b respectively; ΔP cp , ΔP cn , ΔP c0 are the positive sequence, negative sequence and zero sequence components of the active power fluctuation of phase c respectively; u dp 、u dn are respectively the positive and negative sequence components of the d-axis voltage at the grid connection point; u qp 、u qn are respectively the positive and negative sequence components of the q-axis voltage at the grid connection point; u d0 、u q0 are the d-axis and q-axis components of the zero-sequence voltage at the grid connection point respectively; i dp 、i dn are the positive and negative sequence components of the output d-axis current respectively; i qp 、i qn They are the positive and negative sequence components of the output q-axis current respectively.

[0030] According to equations (6) to (8), it can be seen that the zero-sequence voltage will not affect the size of the three-phase total active power, but will only affect the distribution of the overall active power in the three phases. The three-phase capacitor voltage can be balanced by adjusting the three-phase distribution of the zero-sequence voltage. According to equation (5), equation (3) can be expressed as:

[0031] P i =P+ΔP ip +ΔP in +ΔP i0 ,i=a,b,c (9)

[0032] Where: ΔP ip , ΔP in , ΔP i0 It is the positive-sequence, negative-sequence and zero-sequence active power fluctuation components of each phase.

[0033] The PI controller is used to adjust the error between the three-phase actual capacitor voltage and the set voltage reference value. On the basis of forming feedback control, the three-phase positive sequence and negative sequence active power fluctuations are used as feedforward compensation to improve the accuracy of three-phase active power distribution. The feedforward compensation of active power can be obtained by solving equations (6) to (9) simultaneously:

[0034]

[0035] Where ΔP aF , ΔP bF , ΔPcF They are the active power feedforward compensation amounts of the three phases abc respectively.

[0036] Further, in step S4, according to formula (5), the zero-sequence component of the three-phase active power of abc is subjected to Clark transformation to obtain the zero-sequence component of the active power in the two-phase stationary coordinate system:

[0037]

[0038] Where ΔP α0 , ΔP β0 is the zero-sequence active power component in the two-phase stationary coordinate system; ΔP a0 , ΔP b0 , ΔP c0 are the zero-sequence active power components in the three-phase stationary coordinate system; δ0 is the initial phase angle of the zero-sequence component of the output current; T 3s / 2s Represents Clark transform.

[0039] The zero-sequence voltage U0 is expressed as:

[0040]

[0041] Where: U x0 , U y0 They represent the x-axis and y-axis components of the zero-sequence voltage respectively.

[0042] According to formula (12), U0 can be expressed as U x0 and U y0 , and replace U0 in equation (11). Then, by performing positive and negative sequence rotation coordinate transformation on equation (11), the calculation formula for zero sequence voltage can be obtained as follows:

[0043]

[0044] Further, in step S5, according to formula (13), the calculation of the three-phase injected zero-sequence voltage requires the zero-sequence active power as a known quantity, which can be derived from formula (9):

[0045] P i0 =P i -P-(ΔP ip +ΔP in )=P iFB -P iF ,i=a,b,c (14)

[0046] Where: P i0 is the zero-sequence component of active power flowing into each phase; P i is the active power flowing into each phase; P is the active constant component; P iFBP is the active power difference generated by the difference between the capacitor voltage actually collected and the reference voltage after passing through the PI controller in the feedback control; iF is the feedforward compensation of active power; ΔP ip , ΔP in It is the positive-sequence and negative-sequence active power fluctuation component of each phase.

[0047] After collecting the voltage value of the three-phase capacitor of the cascaded H-bridge STATCOM, it is subtracted from the set voltage reference value and then outputted as the active power difference P through the PI controller. iFB , added to the active feedforward compensation calculated in S3, and then the reference value of zero-sequence active power P is obtained according to formula (14): i0 , the zero-sequence voltage value to be injected is calculated by equation (11) and equation (13), and the zero-sequence voltage calculated in S4 is injected into the three-phase abc to balance the three-phase distribution of the active power of the cascaded H-bridge STATCOM and the three-phase capacitor voltage.

[0048] Beneficial effects: The present invention discloses a cascaded H-bridge STATCOM phase-to-phase voltage balance control method based on feedforward compensation calculation. On the basis of injecting zero-sequence voltage to balance the three-phase capacitor voltage, feedforward compensation control is added. The feedforward compensation amount of active power under grid voltage imbalance is calculated and used as a part of zero-sequence voltage calculation, which can improve the balancing effect of the three-phase voltage and is beneficial to the stability and safety of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is a star connection topology of cascaded H-bridge STATCOM;

[0050] Figure 2 It is a structural block diagram of a power generation system applied to an offshore wind farm in a specific implementation manner of the present invention;

[0051] Figure 3 It is a control diagram for realizing the reactive power compensation function of the cascaded H-bridge STATCOM in a specific embodiment of the present invention;

[0052] Figure 4 This is a phase-to-phase voltage balance control diagram of the cascaded H-bridge STATCOM in a specific implementation manner of the present invention;

[0053] Figure 5 This is a reactive power waveform diagram on the 35KV busbar of a wind farm in a specific implementation manner of the present invention;

[0054] Figure 6 It is a three-phase capacitor voltage waveform diagram without feedforward compensation in a specific implementation mode of the present invention;

[0055] Figure 7This is a three-phase capacitor voltage waveform diagram under the control method of the technical solution of the present invention in a specific implementation manner of the present invention. DETAILED DESCRIPTION

[0056] This specific implementation discloses a cascaded H-bridge STATCOM phase-to-phase voltage balance control method based on feedforward compensation calculation. The technical solution of the present invention is further explained below through embodiments and in conjunction with the accompanying drawings.

[0057] The interphase voltage balance control method of the cascaded H-bridge STATCOM based on feedforward compensation calculation proposed in the present invention can be applied to Figure 2 In the offshore wind farm power generation system grid-connected environment shown in the figure, G1 and G2 represent a combination of 8 permanent magnet synchronous generators (PMSGs), and G3-G6 represent a combination of 4 PMSGs, that is, the offshore wind farm in the embodiment is actually composed of 36 PMSG units.

[0058] After the step-up transformer, a cascaded H-bridge STATCOM is connected to the 35KV bus for reactive power compensation. The control method is as follows: Figure 3 As shown, the positive and negative sequences of the grid voltage and current are decomposed respectively, and feedback control is performed with the detected reference current. The errors between the actual current value and the reference value are controlled within the allowable range under the positive sequence coordinate and the negative sequence coordinate respectively, so as to achieve the control purpose of dynamic reactive power compensation.

[0059] The phase-to-phase voltage balance control adopts the cascaded H-bridge STATCOM phase-to-phase voltage balance control method based on feedforward compensation calculation proposed by the present invention. The control block diagram is as follows: Figure 4 As shown, the specific implementation steps include:

[0060] S1: Obtain the positive and negative sequence output voltage and output current formula of the cascaded H-bridge STATCOM under the unbalanced grid voltage;

[0061] S2: Under the condition of unbalanced grid voltage, considering the influence of negative sequence voltage and current, the active power flow between the grid and the cascaded H-bridge STATCOM is calculated, and the three-phase active power flowing into the cascaded H-bridge STATCOM is divided into the constant active power and the fluctuating active power;

[0062] S3: Decompose the three-phase active power flowing into the cascaded H-bridge STATCOM into positive and negative sequences, balance the positive and negative sequence active power fluctuations respectively, and obtain the feedforward compensation of the active power;

[0063] S4: According to the active power in S2, the zero-sequence active power is extracted for coordinate transformation, and the calculation formula of the zero-sequence voltage injection amount is obtained in the two-phase stationary coordinate system;

[0064] S5: Detect and collect the actual three-phase capacitor voltage, make a difference with the set voltage reference value, output the active power difference after passing through the PI controller, add the feedforward compensation calculated in S3, and use it as the active power input value for zero-sequence voltage calculation. According to the actual operating conditions, inject the zero-sequence voltage calculated in S4 into the three phases of the cascaded H-bridge STATCOM to balance the three-phase capacitor voltage.

[0065] In step S1, when the grid voltage is unbalanced, positive sequence, negative sequence and zero sequence components will appear in the grid. The output voltage of the cascaded H-bridge STATCOM under the unbalanced grid voltage is deduced to be:

[0066]

[0067] Where: u ca 、u cb 、u cc is the three-phase output voltage of the cascaded H-bridge STATCOM; U p , U n , U0 is the positive sequence, negative sequence and zero sequence components of the grid connection point voltage; θ p ,θ n , θ0 is the initial phase angle of the positive sequence, negative sequence and zero sequence components of the grid connection point voltage; w is the voltage angular frequency.

[0068] Since the cascaded H-bridge STATCOM adopts star connection, there is no zero-sequence current circulation loop, and the three-phase output current of the cascaded H-bridge STATCOM is:

[0069]

[0070] Where: i ca 、i cb 、i cc is the three-phase output current of the cascaded H-bridge STATCOM; I p ,I n are the positive and negative sequence components of the output current; δ p , δ n It is the initial phase angle of the positive and negative sequence components of the output current.

[0071] In step S2, when the grid voltage is unbalanced, the negative sequence voltage and current components U n ,I n It cannot be ignored, that is, the absorbed active power is divided into the same part of the three-phase power, the constant active component P, and the different part of the active fluctuation component △P i , specifically expressed as follows:

[0072] P i =P+ΔP i ,i=a,b,c (3)

[0073] Where: P i is the active power flowing into each phase; P is the active constant component; △P i is the active power fluctuation component of each phase; a, b, c represent phase a, phase b and phase c respectively.

[0074] According to the instantaneous power theory, the three-phase active power absorbed by the cascaded H-bridge STATCOM from the system is obtained using equations (1) and (2), where the active power constant component P and the active power fluctuation component △P in equation (3) are i Respectively expressed as:

[0075]

[0076]

[0077] Where: △P a , △P b , △P c They are the three-phase active power fluctuation components abc respectively.

[0078] In step S3, the active fluctuation component in equation (5) is transformed into positive and negative sequence coordinates, and is divided into positive sequence, negative sequence and zero sequence active fluctuation components, which are respectively expressed as:

[0079]

[0080]

[0081]

[0082] Where: ΔP ap , ΔP an , ΔP a0 are the positive sequence, negative sequence and zero sequence components of the active power fluctuation of phase a respectively; ΔP bp , ΔP bn , ΔP b0 are the positive sequence, negative sequence and zero sequence components of the active power fluctuation of phase b respectively; ΔP cp , ΔP cn , ΔP c0 are the positive sequence, negative sequence and zero sequence components of the active power fluctuation of phase c respectively; u dp 、u dn are respectively the positive and negative sequence components of the d-axis voltage at the grid connection point; u qp 、u qn are respectively the positive and negative sequence components of the q-axis voltage at the grid connection point; u d0 、u q0 are the d-axis and q-axis components of the zero-sequence voltage at the grid connection point respectively; i dp 、i dnare the positive and negative sequence components of the output d-axis current respectively; i qp 、i qn They are the positive and negative sequence components of the output q-axis current respectively.

[0083] According to equations (6) to (8), it can be seen that the zero-sequence voltage will not affect the size of the three-phase total active power, but will only affect the distribution of the overall active power in the three phases. The three-phase capacitor voltage can be balanced by adjusting the three-phase distribution of the zero-sequence voltage. According to equation (5), equation (3) can be expressed as:

[0084] P i =P+ΔP ip +ΔP in +ΔP i0 ,i=a,b,c (9)

[0085] Where: ΔP ip , ΔP in , ΔP i0 It is the positive-sequence, negative-sequence and zero-sequence active power fluctuation components of each phase.

[0086] The PI controller is used to adjust the error between the three-phase actual capacitor voltage and the set voltage reference value. On the basis of forming feedback control, the three-phase positive sequence and negative sequence active power fluctuations are used as feedforward compensation to improve the accuracy of three-phase active power distribution. The feedforward compensation of active power can be obtained by solving equations (6) to (9) simultaneously:

[0087]

[0088] Where ΔP aF , ΔP bF , ΔP cF They are the active power feedforward compensation amounts of the three phases abc respectively.

[0089] In step S4, according to formula (5), the zero-sequence component of the three-phase active power of abc is transformed by Clark transformation to obtain the zero-sequence component of the active power in the two-phase stationary coordinate system:

[0090]

[0091] Where ΔP α0 , ΔP β0 is the zero-sequence active power component in the two-phase stationary coordinate system; ΔP a0 , ΔP b0 , ΔP c0 are the zero-sequence active power components in the three-phase stationary coordinate system; δ0 is the initial phase angle of the zero-sequence component of the output current; T 3s / 2s Represents Clark transform.

[0092] The zero-sequence voltage U0 is expressed as:

[0093]

[0094] Where: U x0 , U y0 They represent the x-axis and y-axis components of the zero-sequence voltage respectively;

[0095] According to formula (12), U0 can be expressed as U x0 and U y0 , and replace U0 in equation (11). Then, by performing positive and negative sequence rotation coordinate transformation on equation (11), the calculation formula for zero sequence voltage can be obtained as follows:

[0096]

[0097] In step S5, according to formula (13), the calculation of the three-phase injected zero-sequence voltage requires the zero-sequence active power as a known quantity, which can be derived from formula (9):

[0098] P i0 =P i -P-(ΔP ip +ΔP in )=P iFB -P iF ,i=a,b,c (14)

[0099] Where: P i0 is the zero-sequence component of active power flowing into each phase; P i is the active power flowing into each phase; P is the active constant component; P iFB P is the active power difference generated by the difference between the capacitor voltage actually collected and the reference voltage after passing through the PI controller in the feedback control; iF is the feedforward compensation of active power; ΔP ip , ΔP in It is the positive-sequence and negative-sequence active power fluctuation component of each phase.

[0100] After collecting the voltage value of the three-phase capacitor of the cascaded H-bridge STATCOM, it is subtracted from the set voltage reference value and then outputted as the active power difference P through the PI controller. iFB , added to the active feedforward compensation calculated in S3, and then the reference value of zero-sequence active power P is obtained according to formula (14): i0 , the zero-sequence voltage value to be injected is calculated by equation (11) and equation (13), and the zero-sequence voltage calculated in S4 is injected into the three-phase abc to balance the three-phase distribution of the active power of the cascaded H-bridge STATCOM and the three-phase capacitor voltage.

[0101] To verify the effectiveness of the cascaded H-bridge STATCOM phase voltage balance control method based on feedforward compensation calculation of the present invention, please refer to Figure 5-Figure 7 In the embodiment, a single-phase asymmetric fault occurs in the power grid at 2s, and the steady-state operation is restored at 2.5s, so as to simulate the voltage imbalance of the power grid. Figure 5 The waveform diagram of the reactive power on the 35KV bus before and after compensation shows that when there is no reactive power compensation device to compensate the system, the wind farm will absorb about 7Mvar of reactive power from the grid, reducing the power factor and of the system. After adding the cascaded H-bridge STATCOM, the reactive power on the bus can be brought close to 0 in both steady state and unbalanced grid voltage, improving the power factor of the system and realizing the reactive power compensation function of the cascaded H-bridge STATCOM. Figure 6 The waveform diagram of the three-phase capacitor voltage under the common phase-to-phase control strategy without considering the feedforward compensation amount is shown in Figure 1. Figure 7 The three-phase capacitor voltage waveform is shown after the technical solution proposed by the present invention is adopted. Figure 6 and Figure 7 It can be seen that under the condition of unbalanced grid voltage, the common phase-to-phase voltage balance control strategy cannot balance the three-phase voltage well, and the voltages of each phase will have a large difference. The cascaded H-bridge STATCOM phase-to-phase voltage balance control method based on feedforward compensation calculation can balance the three-phase capacitor voltage well, significantly reduce the difference of the three-phase voltage, and can keep the fluctuation within a very small range, which illustrates the effectiveness of the present invention.

Claims

1. A cascaded H-bridge STATCOM capacitor voltage balance control method based on feedforward compensation calculation, characterized in that: The method comprises the following steps: S1: Obtain the positive and negative sequence output voltage and output current formula of the cascaded H-bridge STATCOM under the unbalanced grid voltage; S2: Under the condition of unbalanced grid voltage, considering the influence of negative sequence voltage and current, the active power flow between the grid and the cascaded H-bridge STATCOM is calculated, and the three-phase active power flowing into the cascaded H-bridge STATCOM is divided into the constant active power and the fluctuating active power; S3: Decompose the three-phase active power flowing into the cascaded H-bridge STATCOM into positive and negative sequences, balance the positive and negative sequence active power fluctuations respectively, and obtain the feedforward compensation of the active power; S4: According to the active power in S2, the zero-sequence active power is extracted for coordinate transformation, and the calculation formula of the zero-sequence voltage injection amount is obtained in the two-phase stationary coordinate system; S5: Detect and collect the actual three-phase capacitor voltage, make a difference with the set voltage reference value, output the active power difference after passing through the PI controller, add the feedforward compensation calculated in S3, and use it as the active power input value for zero-sequence voltage calculation. According to the actual operation situation, inject the zero-sequence voltage calculated in S4 into the three phases of the cascaded H-bridge STATCOM to balance the three-phase capacitor voltage. In step S1, when the grid voltage is unbalanced, positive sequence, negative sequence and zero sequence components will appear in the grid. The output voltage of the cascaded H-bridge STATCOM under the unbalanced grid voltage is deduced to be: Where: u ca 、u cb 、u cc is the three-phase output voltage of the cascaded H-bridge STATCOM; U p , U n , U0 is the positive sequence, negative sequence and zero sequence components of the grid connection point voltage; θ p ,θ n , θ0 is the initial phase angle of the positive, negative and zero sequence components of the grid-connected point voltage; w is the voltage angular frequency; According to the star connection of the cascaded H-bridge STATCOM, there is no zero-sequence current circulation loop, and the three-phase output current of the cascaded H-bridge STATCOM is: Where: i ca 、i cb 、i cc is the three-phase output current of the cascaded H-bridge STATCOM; I p ,I n are the positive and negative sequence components of the output current; δ p , δ n is the initial phase angle of the positive and negative sequence components of the output current; In step S2, when the grid voltage is unbalanced, the negative sequence voltage and current components U n ,I n It cannot be ignored, that is, the absorbed active power is divided into the same part of the three-phase power, the constant active component P, and the different part of the active fluctuation component △P i , specifically expressed as follows: P i =P+ΔP i , i=a,b,c (3) Where P i is the active power flowing into each phase; P is the active constant component; △P i is the active power fluctuation component of each phase; a, b, c represent phase a, phase b and phase c respectively; According to the instantaneous power theory, the three-phase active power absorbed by the cascaded H-bridge STATCOM from the system is obtained using equations (1) and (2), where the active power constant component P and the active power fluctuation component △P in equation (3) are i Respectively expressed as: Where: △P a , △P b , △P c They are the three-phase active power fluctuation components a, b and c respectively; In step S3, the active fluctuation component in equation (5) is transformed into positive and negative sequence coordinates to be divided into positive sequence, negative sequence and zero sequence active fluctuation components, which are respectively expressed as: Where: △P ap , △P an , △P a0 are the positive sequence, negative sequence and zero sequence components of the active power fluctuation of phase a respectively; △P bp , △P bn , △P b0 are the positive sequence, negative sequence and zero sequence components of the active power fluctuation of phase b respectively; △P cp , △P cn , △P c0 are the positive sequence, negative sequence and zero sequence components of the active power fluctuation of phase c respectively; u dp 、u dn are respectively the positive and negative sequence components of the d-axis voltage at the grid connection point; u qp 、u qn are respectively the positive and negative sequence components of the q-axis voltage at the grid connection point; u d0 、u q0 are the d-axis and q-axis components of the zero-sequence voltage at the grid connection point respectively; i dp 、i dn are the positive and negative sequence components of the output d-axis current respectively; i qp 、i qn They are respectively the positive-sequence and negative-sequence components of the output q-axis current; According to equations (6) to (8), it can be seen that the zero-sequence voltage does not affect the size of the total three-phase active power, but only affects the distribution of the overall active power in the three phases. The three-phase capacitor voltage is balanced by adjusting the three-phase distribution of the zero-sequence voltage. Equation (3) can be expressed as follows through equation (5): P i =P+ΔP ip +ΔP in +ΔP i0 , i=a,b,c (9) Where: △P ip , △P in , △P i0 It is the positive sequence, negative sequence and zero sequence active power fluctuation component of each phase; The PI controller is used to adjust the error between the three-phase actual capacitor voltage and the set voltage reference value. On the basis of forming feedback control, the three-phase positive sequence and negative sequence active power fluctuations are used as feedforward compensation to improve the accuracy of three-phase active power distribution. The simultaneous solution of equations (6) to (9) yields the feedforward compensation of active power: In the formula, △P aF , △P bF , △P cF They are respectively the active power feedforward compensation of the three phases abc; According to formula (5), the zero-sequence component of the three-phase active power of abc is transformed by Clark, and the zero-sequence component of the active power in the two-phase stationary coordinate system is obtained as follows: In the formula, △P α0 , △P β0 is the zero-sequence active power component in the two-phase stationary coordinate system; △P a0 , △P b0 , △P c0 are the zero-sequence active power components in the three-phase stationary coordinate system; δ0 is the initial phase angle of the zero-sequence component of the output current; T 3s / 2s represents Clark transform; The zero-sequence voltage U0 is expressed as: Where: U x0 , U y0 They represent the x-axis and y-axis components of the zero-sequence voltage respectively; According to formula (12), U0 is expressed as U x0 and U y0 , and replace U0 in equation (11). Then, by performing positive and negative sequence rotation coordinate transformation on equation (11), the calculation formula for zero sequence voltage can be obtained as follows:

2. The cascaded H-bridge STATCOM capacitor voltage balance control method based on feedforward compensation calculation according to claim 1 is characterized in that: In step S5, according to formula (13), the calculation of the three-phase injected zero-sequence voltage requires the zero-sequence active power as a known quantity, which can be derived from formula (9): P i0 =P i -P-(ΔP ip +ΔP in )=P iFB -P iF ,i=a,b,c (14) Where: P i0 is the zero-sequence component of active power flowing into each phase; P i is the active power flowing into each phase; P is the active constant component; P iFB P is the active power difference generated by the difference between the capacitor voltage actually collected and the reference voltage after passing through the PI controller in the feedback control; iF is the feedforward compensation of active power; △P ip , △P in It is the positive-sequence and negative-sequence active power fluctuation component of each phase; After collecting the voltage value of the three-phase capacitor of the cascaded H-bridge STATCOM, it is subtracted from the set voltage reference value and then outputted as the active power difference P through the PI controller. iFB , added to the active feedforward compensation calculated in S3, and then the reference value of zero-sequence active power P is obtained according to formula (14): i0 , the zero-sequence voltage value to be injected is calculated by equations (11) and (13), and the zero-sequence voltage calculated in S4 is injected into the three phases a, b, and c to balance the three-phase distribution of the active power of the cascaded H-bridge STATCOM and the three-phase capacitor voltage.

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