Control method of cascade H-bridge photovoltaic grid-connected inverter under unbalanced working condition of power grid

Abstract

The invention relates to the technical field of electric energy quality control, in particular to a control method of a cascade H-bridge photovoltaic grid-connected inverter under the unbalanced working condition of a power grid, which comprises the following steps of: S1, establishing a direct current side total voltage control method to obtain a reference value of an active current component required to be injected into the power grid by the inverter; s2, establishing a current positive and negative sequence inner loop control method, and tracking the positive and negative sequence current instructions to obtain positive and negative sequence voltage modulation signals; s3, generating a negative sequence current feed-forward item based on a negative sequence current injection method, and merging the feed-forward item into calculation of a direct current side voltage control output value to complete inter-phase power balance control; and S4, carrying out carrier phase-shifting modulation based on the phase-shifting angle which changes in real time, and completing in-phase power balance control. According to the invention, the problem of power imbalance of the cascaded H-bridge photovoltaic grid-connected inverter under the unbalanced working condition of the power grid can be well improved, and the inverter is ensured to safely and stably provide three-phase balanced current.

Description

Technical Field

[0001] The present invention relates to the technical field of power quality control, and in particular to a control method of a cascaded H-bridge photovoltaic grid-connected inverter under an unbalanced power grid condition. Background Art

[0002] The cascaded H-bridge photovoltaic grid-connected inverter adopts a modular design. The DC side of each module can be connected to the photovoltaic panel and realize independent MPPT control. It can output high-quality grid-connected current and has the advantages of small switching stress, small output voltage harmonic content, small output filter size, easy packaging and expansion, etc. It can effectively improve the efficiency of photovoltaic inverters. Therefore, it is increasingly widely used in medium and high voltage frequency conversion speed regulation, flexible AC transmission and high voltage DC transmission and other occasions.

[0003] However, in actual operation, due to the shading and aging of some photovoltaic panels on the DC side, the power of each module of the cascaded H-bridge inverter is inconsistent, resulting in inductor current distortion, which may cause oscillation, overcurrent and inverter shutdown in severe cases, and even cause system crash. At the same time, uneven light intensity and ambient temperature will also cause inconsistent output power of each module of the photovoltaic power station. The input of the cascaded H-bridge inverter of different phases will have a relatively large difference, and the three-phase grid-connected current will become asymmetric. As a result, it is impossible to meet the grid connection requirements of the same frequency and phase of the three-phase grid-connected current and grid-connected voltage. Therefore, it is necessary to study the power balance control method under the unbalanced grid condition to ensure that even if the power generation of the photovoltaic array of each phase and each module within the phase is inconsistent, the inverter can still provide the grid with three-phase balanced current to achieve safe and stable operation of the system. Summary of the invention

[0004] The present invention provides a control method for a cascaded H-bridge photovoltaic grid-connected inverter under an unbalanced power grid condition, which can effectively improve the power imbalance problem of the cascaded H-bridge photovoltaic grid-connected inverter under an unbalanced power grid condition and ensure that the inverter safely and stably provides three-phase balanced current.

[0005] The present invention provides the following technical solutions:

[0006] A control method for a cascaded H-bridge photovoltaic grid-connected inverter under an unbalanced power grid condition comprises the following steps:

[0007] S1. Establish a total voltage control method on the DC side to obtain a reference value of the active current component that the inverter needs to inject into the grid;

[0008] S2. Establish a current positive and negative sequence inner loop control method to track the positive and negative sequence current instructions to obtain positive and negative sequence voltage modulation signals;

[0009] S3, generating a negative-sequence current feedforward term based on a negative-sequence current injection method, incorporating the feedforward term into the calculation of the DC side voltage control output value, and completing the inter-phase power balance control;

[0010] S4. Carrier phase shift modulation is performed based on the real-time changing phase shift angle to complete the intra-phase power balance control.

[0011] Further, the S1 includes:

[0012] S11, summing the capacitor voltages of all modules on the DC side;

[0013] S12, the capacitor voltage and the set DC bus total voltage U * dscum By comparison, after the PI controller, the reference value i of the active current component that the inverter needs to inject into the grid is obtained. d ;

[0014] S13, the reference value i d The active current component that needs to be injected into each phase is obtained by converting it to the three-phase stationary coordinate system.

[0015] Further, the S2 includes:

[0016] S21, construct positive sequence inner loop control;

[0017] S22, construct negative sequence inner loop control;

[0018] S23. The output value of the total voltage control on the DC side is used as the reference positive-sequence current command. In combination with the grid voltage unbalance parameter and the instantaneous power mathematical model, the positive and negative-sequence current command values ​​under different control objectives are obtained. The positive and negative-sequence current commands are tracked respectively to obtain the positive and negative-sequence voltage modulation signals, and the two are synthesized into an SPWM modulation signal.

[0019] Further, the S3 includes:

[0020] S31, adding the capacitor voltages of each module collected in each phase, respectively calculating the three-phase DC voltage values, comparing them with the given values, and obtaining the required active power fluctuation after passing through the PI controller;

[0021] S32. The required interphase compensation current is calculated through the active power fluctuation in the dq rotating coordinate system, and converted to the abc three-phase stationary coordinate system, and the three-phase current fluctuation is output and superimposed on the current loop control system.

[0022] Further, in S32, a constraint relationship of three-phase current fluctuation is established, by adjusting the d-axis component i of the negative sequence current. nd To adjust the active power of phase A, the q-axis component i of the negative sequence current is adjusted nqTo adjust the active power difference between phase C and phase B.

[0023] Furthermore, the d-axis component i of the negative sequence current nd and the q-axis component i nq The calculations are as follows:

[0024]

[0025] In the formula, and are the positive sequence d-axis voltage and q-axis voltage of the power grid respectively, and are the negative sequence d-axis voltage and q-axis voltage of the power grid respectively, i pq It is the positive sequence reactive current of the power grid.

[0026] Further, the S4 includes:

[0027] S41, respectively collecting the output voltage U of the kth H-bridge module in each phase k and DC side voltage U dck , calculate the duty cycle D of the kth H-bridge module k ;

[0028] S42. Will U k , U dck and D k Input it into the phase shift angle calculation module, use the solve function to perform calculations, and output the carrier phase shift angles of the five H-bridge modules in real time;

[0029] S43, using the carrier phase shift angle as the phase difference value of the k-th column triangular carrier signal, thereby achieving control of the intra-phase power balance of the cascaded H-bridge photovoltaic grid-connected inverter.

[0030] Furthermore, it also includes:

[0031] S5. A photovoltaic grid-connected inverter system simulation model is built in the Matlab / Simulink environment for verification. The photovoltaic grid-connected inverter system simulation model is a star structure, the number of H-bridge cascades is 5, the simulation adopts a variable step size, and the solver algorithm is ode23tb.

[0032] The principles and advantages of the present invention are: establishing a positive and negative sequence inner loop control of current, obtaining a positive sequence current command value based on a reference value of the total voltage control on the DC side, including a feedforward negative sequence current signal for phase-to-phase power balance control in the negative sequence current command, and calculating the carrier phase shift angle of each H-bridge module in the cascade system in real time, and realizing intra-phase power balance control by carrier phase shift modulation through phase shift angle variation.

[0033] Compared with the traditional power control method, the present invention has the following advantages:

[0034] 1) This method can reduce the fluctuation of DC bus voltage during the regulation of the cascade system under unbalanced grid conditions and improve the dynamic performance of the system;

[0035] 2) Realize separate control of active power and reactive power;

[0036] 3) It can achieve the effect of equivalent frequency doubling switching frequency, improve the output power quality, reduce the difficulty of filter design, and reasonably solve the power imbalance problem between modules caused by the influence of the natural environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is an overall block diagram of a control method for a cascaded H-bridge photovoltaic grid-connected inverter under an unbalanced grid condition;

[0038] Figure 2 It is the block diagram of the total voltage control strategy on the DC side;

[0039] Figure 3 This is the control block diagram of the current positive and negative sequence inner loop control strategy;

[0040] Figure 4 It is the control block diagram of the inter-phase power balance control strategy;

[0041] Figure 5 It is the control block diagram of the intra-phase power balance control strategy of phase A;

[0042] Figure 6 It is a schematic diagram of simulation experiment results under the condition of power grid power balance;

[0043] Figure 7 It is a schematic diagram of simulation experiment results using traditional control methods under unbalanced power conditions of the power grid;

[0044] Figure 8 It is a schematic diagram of simulation experiment results of using the control method of the present invention under the condition of unbalanced power in the power grid. DETAILED DESCRIPTION

[0045] The following is further described in detail through specific implementation methods:

[0046] Embodiment 1

[0047] The control method of cascaded H-bridge photovoltaic grid-connected inverter under unbalanced grid conditions is shown in the overall control block diagram. Figure 1 As shown, the following steps are included:

[0048] S1. DC side total voltage control method

[0049] The control block diagram of the DC side total voltage control strategy is as follows: Figure 2 As shown, the purpose is to control the capacitor voltage and voltage of all sub-modules of the three-phase to be constant, including the following sub-steps:

[0050] S11, summing the capacitor voltages of all modules on the DC side;

[0051] S12, the sum of the capacitor voltage and the set value of the DC bus total voltage U * dscum By comparison, after the PI controller, the reference value i of the active current component that the inverter needs to inject into the grid is obtained. d ;

[0052] S13, the reference value i d The active current component that needs to be injected into each phase is obtained by converting it to the three-phase stationary coordinate system.

[0053] S2. Current positive and negative sequence inner loop control method

[0054] The control block diagram of the current positive and negative sequence inner loop control strategy is as follows Figure 3 As shown, the following steps are included:

[0055] S21. Construct positive sequence inner loop control.

[0056] When the grid voltage is balanced, there is only positive sequence current. The mathematical model of the cascaded H-bridge inverter under the positive sequence working environment is shown in formula (1):

[0057]

[0058] In the formula, i pd and i pq is the positive sequence dq axis current signal in the input current loop; u pd and u pq It is the positive sequence dq axis modulation voltage signal output by the current loop; and is the positive sequence dq axis voltage of the power grid.

[0059] As can be seen from equation (1), the control part of the inverter includes the control of both active power and reactive power. Controlling active power can ensure the stability of the DC side capacitor voltage and realize the premise of normal operation of the entire system. On the basis of effective control of active power, controlling reactive power can perform smooth dynamic reactive power compensation on the power grid. In the positive sequence current closed-loop control, due to the active current component i pd and reactive current component i pq There is coupling, and decoupling control is required to achieve separate control of active power and reactive power.

[0060] S22. Construct negative sequence inner loop control.

[0061] When the three-phase voltage of the power grid is unbalanced, there are both positive-sequence current and negative-sequence current components. In order to solve the control problem of negative-sequence current, a negative-sequence current loop is added to the positive-sequence current loop to form a positive-negative sequence dual synchronous rotating coordinate system current control loop strategy.

[0062] The mathematical model of the negative sequence component in the negative sequence rotating coordinate system is shown in formula (2):

[0063]

[0064] In the formula, i nd and i nq is the negative sequence dq axis current signal in the input current loop; u nd and u nq It is the negative sequence dq axis modulation voltage signal output by the current loop; and is the negative sequence dq axis voltage of the power grid.

[0065] S23. The output value of the total voltage control on the DC side is used as the reference positive-sequence current command. In combination with the grid voltage unbalance parameter and the instantaneous power mathematical model, the positive and negative-sequence current command values ​​under different control objectives are obtained. The positive and negative-sequence current commands are tracked respectively to obtain the positive and negative-sequence voltage modulation signals, and the two are synthesized into an SPWM modulation signal.

[0066] S3, inter-phase power balance control

[0067] The negative sequence current generated by the system compensating for the unbalanced load and the zero sequence voltage generated by the system neutral point voltage drift will cause the problem of phase power imbalance. In order to ensure the safe operation of the equipment and achieve accurate current control, the phase voltage of the capacitor needs to be balanced and controlled.

[0068] The present invention adopts a phase-to-phase power balance control strategy based on negative sequence current injection method. The control block diagram is as follows: Figure 4 As shown, the following steps are included:

[0069] S31. Add the capacitor voltages of each module collected in each phase, calculate the three-phase DC voltage values ​​respectively, compare them with the given values, and obtain the required injected active power fluctuations through the PI controller.

[0070] S32. The required interphase compensation current is calculated through the active power fluctuation in the dq rotating coordinate system, and converted to the abc three-phase stationary coordinate system, and the three-phase current fluctuation is output and superimposed on the current loop control system.

[0071] The solution process of negative sequence current is as follows:

[0072] The three-phase output voltage of the cascaded H-bridge photovoltaic grid-connected inverter is shown in formula (3):

[0073]

[0074] In the formula, u aN 、u bN 、u cN is the three-phase output voltage of the cascaded H-bridge photovoltaic grid-connected inverter, U P and U n are the amplitudes of the positive and negative sequence voltages respectively; θn is the phase angle of the negative sequence voltage.

[0075] The three-phase output current of the cascaded H-bridge photovoltaic grid-connected inverter is shown in formula (4):

[0076]

[0077] Where ia, ib, ic are the three-phase output currents of the cascaded H-bridge photovoltaic grid-connected inverter, I P is the amplitude of the positive sequence current; I n is the magnitude of the negative sequence current produced by the unbalanced load; and are the phase angles of the positive sequence current and the negative sequence current respectively.

[0078] According to equations (3) and (4), the average active power within a power frequency cycle can be obtained, as shown in equation (5):

[0079]

[0080] Among them, the average active power under the action of positive sequence voltage and negative sequence current is shown in formula (6):

[0081]

[0082] According to formula (6), we can get:

[0083]

[0084] The transformation formula for converting the negative sequence current to the dq rotating coordinate system is:

[0085]

[0086]

[0087] Substituting equation (8) into equation (7), we can get the relationship between negative sequence current and active power:

[0088] P′ a =U P i nd / 2 (9)

[0089]

[0090] From formula (9), we can know that adjusting the negative sequence current component i on the d axis nd The active power of phase A can be adjusted; the component i of the negative sequence current on the q axis can be adjusted nq The active power difference between phase C and phase B can be adjusted; a +P′ b +P′ c =0, by controlling the component of the negative sequence current on the dq axis, the phase power can be controlled to achieve the purpose of balancing the capacitor voltage.

[0091] In the negative sequence rotating coordinate system, the negative sequence voltage is shown as formula (10):

[0092] u nd =U n cosθ n =U P I n cosθ n / I P (10)

[0093] u nq =-U n sinθ n =-U P I n sinθ n / I P

[0094] When the inverter is inductively operated, the positive sequence current lags the positive sequence voltage by 90°, and the phase between the negative sequence current and the negative sequence voltage satisfies Substituting formula (8) into formula (10), we can obtain:

[0095] i nd =u nq I P / U P (11)

[0096] i nq =–u nd I P / U P

[0097] When the inverter operates capacitively, the positive sequence current leads the positive sequence voltage by 90°, and the phase between the negative sequence current and the negative sequence voltage satisfies Substituting formula (8) into formula (10), we can obtain:

[0098] i nd =-u nq I P / U P (12)

[0099] inq =u nd I P / U P

[0100] Since the inverter is inductively operated, the positive sequence reactive current i pq =-I P , when the inverter is capacitively operating, the positive sequence reactive current i pq =I P , so the negative sequence current under any operating conditions is as shown in formula (13):

[0101] i nd =-u nq i pq / U P (13)

[0102] i nq =u nd i pq / U P

[0103] In steady state, the positive sequence voltage between the grid and the inverter output satisfies:

[0104]

[0105] The negative sequence voltage between the power grid and the inverter output satisfies:

[0106]

[0107] Ignore system loss, that is, i pd ≈0, combining equations (13) to (15), we can obtain:

[0108]

[0109] In the formula, and are the positive sequence d-axis voltage and q-axis voltage of the power grid respectively, and are the negative sequence d-axis voltage and q-axis voltage of the power grid respectively, i pq It is the positive sequence reactive current of the power grid.

[0110] According to formula (16), the negative-sequence current feedforward term is added to the instruction for generating the negative-sequence current and incorporated into the output value of the DC side total voltage control to jointly participate in the calculation of the negative-sequence current, so as to reduce the fluctuation of the DC bus voltage during the regulation of the cascade system under the unbalanced grid condition and improve the dynamic performance of the system.

[0111] S4, Intra-phase power balance control

[0112] Since the power generation state of photovoltaic modules is greatly affected by the natural environment, the power imbalance within the phase of the photovoltaic panel on the DC side of the cascaded H-bridge is relatively serious. This leads to the inability of traditional carrier phase-shift modulation to achieve the effect of equivalent doubled switching frequency, which reduces the output power quality and increases the difficulty of filter design. Therefore, reasonably solving the problem of power imbalance between modules is the key to the stable operation of the cascaded H-bridge photovoltaic grid-connected inverter.

[0113] This embodiment takes a five-module cascade H-bridge photovoltaic grid-connected inverter as an example, and implements an intra-phase power balance control strategy based on carrier phase shift modulation with real-time change of phase shift angle. The control block diagram is as follows: Figure 5 As shown, the following steps are included:

[0114] S41, respectively collecting the output voltage U of the kth H-bridge module in each phase k and DC side voltage U dck , calculate the duty cycle D of the kth H-bridge module k ;

[0115] S42. Will U k , U dck and D k Input it into the phase shift angle calculation module, use the solve function to perform calculations, and output the carrier phase shift angles of the five H-bridge modules in real time;

[0116] Taking phase A as an example, the process of solving the carrier phase shift angle required in variable phase shift angle carrier phase shift modulation is as follows.

[0117] Output voltage U of the kth H-bridge module of phase A of the cascaded H-bridge photovoltaic grid-connected inverter ak (t) is shown in formula (17):

[0118]

[0119] in,

[0120]

[0121] From the expansion of the single module output voltage, the output voltage of the entire A phase when five modules are cascaded can be obtained as shown in formula (19):

[0122]

[0123] Then the i-th harmonic component is as shown in formula (20):

[0124]

[0125] When the system is balanced, each H-bridge module has the same DC side voltage and duty cycle. According to equation (20), U Hai(t)=0(1≤i<5). The system enters an unbalanced operation state. If the traditional carrier phase shift modulation strategy is used, the low-frequency harmonic components cannot be eliminated. In order to eliminate low-frequency harmonics, a variable phase shift angle carrier phase shift modulation strategy is adopted. By changing the carrier phase shift angle in real time, the cosine harmonic components of all modules are and sinusoidal harmonic components The coefficients sum to 0.

[0126] If you want to eliminate the first harmonic component U of phase A in the five-module cascade H-bridge system Ha1 (t) must satisfy the conditions shown in formula (21):

[0127]

[0128] The harmonic component elimination conditions of other phase modules can be obtained by referring to formula (21).

[0129] S43, using the carrier phase shift angle as the phase difference value of the k-th column triangular carrier signal, thereby achieving control of the intra-phase power balance of the cascaded H-bridge photovoltaic grid-connected inverter.

[0130] S5, simulation verification;

[0131] In order to verify the correctness of the control strategy proposed in the present invention and its effectiveness in the application of the cascaded H-bridge photovoltaic grid-connected inverter system, the applicant built a 0.8MW three-phase photovoltaic grid-connected inverter system simulation model in the Matlab / Simulink environment, using a star-shaped main circuit and 5 H-bridge cascades. The simulation uses a variable step size, the solver algorithm is ode23tb, and the simulation parameters are shown in Table 1.

[0132] Table 1

[0133]

[0134] The simulation conditions of the system under power imbalance are set as follows:

[0135] 0-0.5s sets the ideal three-phase balanced voltage;

[0136] Different light intensities are set for the photovoltaic panels connected to different modules on the DC side within 0.5s-1s to simulate the power imbalance conditions between phases and within phases.

[0137] The settings of the light intensity of the photovoltaic panels connected to different modules when the system is unbalanced are shown in Table 2.

[0138] Table 2W / m 2

[0139]

[0140] Figure 6The simulation results of the system under power balance are shown in Figure 2. Figure 7 and Figure 8 The simulation results of the control method of the cascaded H-bridge photovoltaic grid-connected inverter under the condition of power imbalance are shown in the figure. It can be seen from the experimental results that when the system is balanced, the traditional control method can meet the grid connection requirements; when the system is unbalanced, the traditional control method will generate a large number of harmonics, and the control method including inter-phase power balance control and intra-phase power balance control proposed by the present invention can effectively balance the capacitor voltage, suppress output current distortion, and improve power quality.

[0141] The above are only embodiments of the present invention. The invention is not limited to the fields involved in this implementation case. The common knowledge such as the known specific structure and characteristics in the scheme is not described in detail here. It should be pointed out that for those skilled in the art, several deformations and improvements can be made without departing from the structure of the present invention, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by the present invention shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A control method for a cascaded H-bridge photovoltaic grid-connected inverter under unbalanced grid conditions, characterized in that: The following steps are involved: S1. Establish a total voltage control method on the DC side to obtain a reference value of the active current component that the inverter needs to inject into the grid; S2. Establish a current positive and negative sequence inner loop control method to track the positive and negative sequence current instructions to obtain positive and negative sequence voltage modulation signals; S3, generating a negative-sequence current feedforward term based on a negative-sequence current injection method, incorporating the feedforward term into the calculation of the DC side voltage control output value, and completing the inter-phase power balance control; S4. Carrier phase shift modulation is performed based on the real-time changing phase shift angle to complete the intra-phase power balance control.

2. The control method of the cascaded H-bridge photovoltaic grid-connected inverter under the unbalanced power grid condition according to claim 1, characterized in that: The S1 includes: S11, summing the capacitor voltages of all modules on the DC side; S12, the capacitor voltage and the set DC bus total voltage U * dscum By comparison, after the PI controller, the reference value i of the active current component that the inverter needs to inject into the grid is obtained. d ; S13, the reference value i d The active current component that needs to be injected into each phase is obtained by converting it to the three-phase stationary coordinate system.

3. The control method of the cascaded H-bridge photovoltaic grid-connected inverter under the unbalanced power grid condition according to claim 2, characterized in that: The S2 includes: S21, construct positive sequence inner loop control; S22, construct negative sequence inner loop control; S23. The output value of the total voltage control on the DC side is used as the reference positive-sequence current command. In combination with the grid voltage unbalance parameter and the instantaneous power mathematical model, the positive and negative-sequence current command values ​​under different control objectives are obtained. The positive and negative-sequence current commands are tracked respectively to obtain the positive and negative-sequence voltage modulation signals, and the two are synthesized into an SPWM modulation signal.

4. The control method of the cascaded H-bridge photovoltaic grid-connected inverter under unbalanced grid conditions according to claim 3, characterized in that: The S3 includes: S31, adding the capacitor voltages of each module collected in each phase, respectively calculating the three-phase DC voltage values, comparing them with the given values, and obtaining the required active power fluctuation after passing through the PI controller; S32. The required interphase compensation current is calculated through the active power fluctuation in the dq rotating coordinate system, and converted to the abc three-phase stationary coordinate system, and the three-phase current fluctuation is output and superimposed on the current loop control system.

5. The control method of the cascaded H-bridge photovoltaic grid-connected inverter under the unbalanced power grid condition according to claim 4, characterized in that: In S32, a constraint relationship of three-phase current fluctuation is established, and the d-axis component i of the negative sequence current is adjusted. nd To adjust the active power of phase A, the q-axis component i of the negative sequence current is adjusted nq To adjust the active power difference between phase C and phase B.

6. The control method of the cascaded H-bridge photovoltaic grid-connected inverter under unbalanced grid conditions according to claim 5, characterized in that: The d-axis component of the negative sequence current i nd and the q-axis component i nq The calculations are as follows: In the formula, and are the positive sequence d-axis voltage and q-axis voltage of the power grid respectively, and are the negative sequence d-axis voltage and q-axis voltage of the power grid respectively, i pq It is the positive sequence reactive current of the power grid.

7. The control method of the cascaded H-bridge photovoltaic grid-connected inverter under unbalanced grid conditions according to claim 6, characterized in that: The S4 includes: S41, respectively collecting the output voltage U of the kth H-bridge module in each phase k and DC side voltage U dck , calculate the duty cycle D of the kth H-bridge module k ; S42. Will U k , U dck and D k Input it into the phase shift angle calculation module, use the solve function to perform calculations, and output the carrier phase shift angles of the five H-bridge modules in real time; S43, using the carrier phase shift angle as the phase difference value of the k-th column triangular carrier signal, thereby achieving control of the intra-phase power balance of the cascaded H-bridge photovoltaic grid-connected inverter.

8. The control method of the cascaded H-bridge photovoltaic grid-connected inverter under unbalanced grid conditions according to claim 1, characterized in that: Also includes: S5. A photovoltaic grid-connected inverter system simulation model is built in the Matlab / Simulink environment for verification. The photovoltaic grid-connected inverter system simulation model is a star structure, the number of H-bridge cascades is 5, the simulation adopts a variable step size, and the solver algorithm is ode23tb.

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