阅读说明：本技术 一种基于氮化镓器件的光伏逆变器及其控制方法 (Photovoltaic inverter based on gallium nitride device and control method thereof ) 是由 李先允 朱晶 王书征 于 2019-10-09 设计创作，主要内容包括：本发明公开了一种基于氮化镓器件的光伏逆变器及其控制方法,包括用于将光伏电池产生的低压直流电转变为高压直流电的第一级变换器,以及用于将所述高压直流电转换成正弦波的第二级变换器；包括一个防反二极管、低压直流滤波电容、低压全桥逆变电路、谐振电路、高频变压器、倍压整流电路、高压直流滤波电容、高压全桥逆变电路和输出滤波器；整机的各个部分经优化设计后具有高电压增益、低损耗、高功率密度的优点。(The invention discloses a photovoltaic inverter based on a gallium nitride device and a control method thereof, wherein the photovoltaic inverter comprises a first-stage converter and a second-stage converter, wherein the first-stage converter is used for converting low-voltage direct current generated by a photovoltaic cell into high-voltage direct current; the high-voltage direct current power supply comprises an anti-reverse diode, a low-voltage direct current filter capacitor, a low-voltage full-bridge inverter circuit, a resonant circuit, a high-frequency transformer, a voltage-multiplying rectifying circuit, a high-voltage direct current filter capacitor, a high-voltage full-bridge inverter circuit and an output filter; after optimized design, each part of the whole machine has the advantages of high voltage gain, low loss and high power density.)

1. A photovoltaic inverter based on gallium nitride devices is characterized by comprising a first-stage converter for converting low-voltage direct current generated by a photovoltaic cell into high-voltage direct current and a second-stage converter for converting the high-voltage direct current into sine waves;

the first-stage converter comprises a low-voltage direct-current filter capacitor C₁Low-voltage full-bridge inverter circuit, resonant circuit and high-frequency transformer T₁And a voltage doubling rectifying circuit, the low-voltage DC filter capacitor C₁Connected to both ends of the photovoltaic cell; the DC side of the low-voltage full-bridge inverter circuit is connected with the low-voltage DC filter capacitor, and the AC side of the low-voltage full-bridge inverter circuit is connected with the resonant circuit and the high-frequency transformer T₁Is connected to the primary winding of the high-frequency transformer T₁The secondary winding combination is connected with the input end of the voltage doubling rectifying circuit;

the second stage converter comprises a high-voltage filter capacitor C₄And the output end of the voltage doubling rectifying circuit and the high-voltage direct current filter capacitor C₄And the direct current side of the high-voltage full-bridge inverter circuit is connected with the power grid.

2. The photovoltaic inverter according to claim 1, further comprising an anti-reverse diode, wherein an anode of the anti-reverse diode is connected to a positive terminal of the photovoltaic cell, and a cathode thereof and a negative terminal of the photovoltaic cell are respectively connected to the low-voltage direct-current filter capacitor C₁At both ends of the same.

3. The photovoltaic inverter of claim 1, wherein the resonant circuit comprises a resonant capacitor C_rAnd a resonant inductor L_r。

4. The pv inverter of claim 1, wherein an output filter is further connected between the ac side of the full-bridge inverter circuit and the grid.

5. Photovoltaic inverter according to claim 1, characterized in that the high-frequency transformer T₁Is a step-up transformer.

6. The photovoltaic inverter of claim 1, wherein the low-voltage full-bridge inverter circuit comprises four gallium nitride switches.

7. Photovoltaic inverter according to claim 1, characterized in that the high-voltage filter capacitor C₄Is a thin film capacitor.

8. The photovoltaic inverter of claim 1, wherein the full bridge inverter circuit comprises a low frequency leg comprising two silicon switches and a high frequency leg comprising two gallium nitride switches.

9. The control method of a photovoltaic inverter according to any one of claims 1 to 8, characterized in that the method comprises:

acquiring a predicted value of the voltage of the high-voltage direct-current bus in the next period according to the acquired instantaneous value of the voltage of the high-voltage direct-current bus in the current period;

obtaining a voltage gain ratio K of the first-stage converter in the next period according to the predicted value of the high-voltage direct-current bus in the next period; obtaining an equivalent load resistance R of a first-stage converter;

acquiring the switching frequency f of the next period of the first-stage converter according to the voltage gain ratio K of the next period and the equivalent load resistor R₁；

According to the switching frequency f₁And acquiring a driving signal of a switch of the low-voltage full-bridge inverter circuit.

10. The method of controlling a photovoltaic inverter according to claim 9, further comprising:

reference value U for acquiring voltage instantaneous value of low-voltage direct-current bus_dcref；

According to the reference value U_dcrefObtaining the amplitude I of the reference value of the output current of the photovoltaic inverter_acref；

According to instantaneous value u of network voltage_acAcquiring a phase factor sin theta of the power grid voltage;

according to the phase factor sin theta of the power grid voltage and the amplitude I of the reference value of the output current of the photovoltaic inverter_acrefObtaining a reference value i of the output current of the photovoltaic inverter_acref；

According to the reference value i of the output current_acrefObtaining the modulation ratio d of the output voltage of the photovoltaic inverter;

according to the modulation ratio d, driving signals of two switches in the high-voltage full-bridge inverter circuit are obtained;

according to instantaneous value u of network voltage_acAnd acquiring driving signals of the other two switches of the high-voltage full-bridge inverter circuit.

Technical Field

The invention relates to the technical field of photovoltaic power generation, in particular to a photovoltaic inverter based on a gallium nitride device and a control method thereof.

Background

The photovoltaic power generation technology is a new energy technology for converting light energy into electric energy. The photovoltaic inverter converts direct current output by the photovoltaic cell panel into alternating current and sends the electric energy to an alternating current power grid. According to the capacity, the photovoltaic inverter can be divided into a centralized photovoltaic inverter, a string photovoltaic inverter, a micro photovoltaic inverter and the like. Wherein the micro-inverter is receiving attention due to its advantages in terms of maximum power tracking efficiency, flexibility, reliability, etc. The research and development of the micro grid-connected photovoltaic inverter with high efficiency and high power density has huge market value and good development prospect. The working frequency of the circuit is improved, so that technicians can use elements such as an inductor, a transformer and the like with smaller volume in the power electronic converter, thereby reducing the volume of the whole machine and improving the power density; and thus the trend is toward faster semiconductor switching devices.

At present, silicon-based semiconductor devices are mainly adopted by the micro photovoltaic inverter, however, the performance of the silicon-based semiconductor devices gradually approaches the theoretical limit of silicon materials, the updating speed is continuously reduced, and the performance of the inverter is difficult to further improve. How to provide an inverter to improve the performance of the whole machine, optimize the circuit topology, improve the efficiency and reduce the cost.

The basic requirement of the photovoltaic inverter is long-time stable grid-connected operation, and the service life of the micro photovoltaic inverter is generally required to reach 20-25 years. The electrolytic capacitor in the main loop is the bottleneck of the service life of all power electronic converters, and the use of the electrolytic capacitor must be reduced or avoided in the design of the service life, so that the power conversion technology without the electrolytic capacitor needs to be researched and developed.

Disclosure of Invention

The invention aims to provide a photovoltaic inverter based on a gallium nitride device and a control method thereof, so as to solve one of the defects caused by the prior art.

In order to achieve the purpose, the invention is realized by adopting the following technical scheme:

a photovoltaic inverter based on gallium nitride devices comprises a first-stage converter for converting low-voltage direct current generated by a photovoltaic cell into high-voltage direct current, and a second-stage converter for converting the high-voltage direct current into sine waves;

the first-stage converter comprises a low-voltage direct-current filter capacitor C₁Low voltage full bridge inverter circuitOscillation circuit and high-frequency transformer T₁And a voltage doubling rectifying circuit, the low-voltage DC filter capacitor C₁Connected to both ends of the photovoltaic cell; the DC side of the low-voltage full-bridge inverter circuit is connected with the low-voltage DC filter capacitor, and the AC side of the low-voltage full-bridge inverter circuit is connected with the resonant circuit and the high-frequency transformer T₁Is connected to the primary winding of the high-frequency transformer T₁The secondary winding combination is connected with the input end of the voltage doubling rectifying circuit;

the second stage converter comprises a high-voltage filter capacitor C₄And the output end of the voltage doubling rectifying circuit and the high-voltage direct current filter capacitor C₄And the direct current side of the high-voltage full-bridge inverter circuit is connected with the power grid.

Further, the photovoltaic module also comprises an anti-reverse diode, wherein the anode of the anti-reverse diode is connected with the positive end of the photovoltaic cell, and the cathode of the anti-reverse diode and the negative end of the photovoltaic cell are respectively connected to the low-voltage direct-current filter capacitor C₁At both ends of the same.

Further, the resonant circuit comprises a resonant capacitor C_rAnd a resonant inductor L_r。

Further, an output filter is connected between the alternating current side of the full-bridge inverter circuit and the power grid.

Further, the high frequency transformer T₁Is a step-up transformer.

Further, the low-voltage full-bridge inverter circuit comprises four gallium nitride switches.

Further, the high-voltage filter capacitor C₄Is a thin film capacitor.

Further, the full-bridge inverter circuit comprises a low-frequency bridge arm and a high-frequency bridge arm, the low-frequency bridge arm comprises two silicon switches, and the high-frequency bridge arm comprises two gallium nitride switches.

The invention also provides a control method of the photovoltaic inverter based on the gallium nitride device, which comprises the following steps:

acquiring a predicted value of the voltage of the high-voltage direct-current bus in the next period according to the acquired instantaneous value of the voltage of the high-voltage direct-current bus in the current period;

obtaining a voltage gain ratio K of the first-stage converter in the next period according to the predicted value of the high-voltage direct-current bus in the next period;

obtaining an equivalent load resistance R of a first-stage converter;

acquiring the switching frequency f of the next period of the first-stage converter according to the voltage gain ratio K of the next period and the equivalent load resistor R₁；

According to the switching frequency f₁And acquiring a driving signal of a switch of the low-voltage full-bridge inverter circuit.

Further, the method further comprises:

reference value U for acquiring voltage instantaneous value of low-voltage direct-current bus_dcref；

According to the reference value U_dcrefObtaining the amplitude I of the reference value of the output current of the photovoltaic inverter_acref；

According to instantaneous value u of network voltage_acAcquiring a phase factor sin theta of the power grid voltage;

according to the phase factor sin theta of the power grid voltage and the amplitude I of the reference value of the output current of the photovoltaic inverter_acrefObtaining a reference value i of the output current of the photovoltaic inverter_acref；

According to the reference value i of the output current_acrefObtaining the modulation ratio d of the output voltage of the photovoltaic inverter;

according to the modulation ratio d, driving signals of two switches in the high-voltage full-bridge inverter circuit are obtained;

according to instantaneous value u of network voltage_acAnd acquiring driving signals of the other two switches of the high-voltage full-bridge inverter circuit.

According to the technical scheme, the embodiment of the invention at least has the following effects:

1. the LLC resonant soft switching circuit usually used for the voltage reduction circuit is designed to be in a voltage boosting mode and is combined with the voltage doubling rectifying circuit, so that the effect of further improving the voltage boosting ratio is achieved, and the LLC resonant soft switching circuit is particularly suitable for application occasions where the voltage of a photovoltaic battery end in a micro photovoltaic inverter is low and grid-connected inversion needs high direct-current bus voltage; simultaneously owing to adopt novel gallium nitride switching element, make the circuit can work in the high frequency, LLC resonance has realized zero voltage and has switched on (ZVS) in addition for the circuit response is fast, and the ripple of voltage, electric current reduces, so the volume of circuit major components such as inductance, electric capacity, transformer can both reduce, thereby improves power density by a wide margin, reduces the loss.

2. The invention uses the silicon switch controlled by power frequency and the gallium nitride switch modulated by high frequency respectively at two bridge arms of the full bridge inverter circuit, gives full play to the advantages of two switch devices, not only meets the requirements of grid-connected current waveform quality, but also reduces the loss and the cost as much as possible.

3. The invention provides a power decoupling control method adopting power feedforward and voltage feedback control, which predicts the fluctuation of the voltage of a high-voltage direct-current bus by introducing power feedforward and adjusts the working frequency to control the voltage gain K of a first-stage DC-DC converter to fluctuate along with alternating-current power according to the principle of an LLC resonant soft switching circuit; because the output voltage of the first-stage DC-DC converter actively adapts to the voltage fluctuation caused by power fluctuation on the high-voltage direct-current bus, the voltage of the low-voltage direct-current bus can be kept basically stable. As a result, on one hand, the photovoltaic cell can maintain the maximum generated power due to the small voltage ripple of the low-voltage direct-current bus, and the low-voltage filter capacitor C is reduced₁The capacity of (a); on the other hand, the control method allows the high-voltage direct-current bus to fluctuate in a larger amplitude, and does not need to adopt a high-voltage electrolytic capacitor with larger capacity but shorter service life to control voltage ripples, so that a thin-film capacitor with small capacity and high reliability can be used, and the service life of the whole machine is greatly prolonged.

Drawings

FIG. 1 is a circuit block diagram of an inverter according to an embodiment of the present invention;

FIG. 2 is a schematic diagram of power and voltage ripple analysis at various locations of an inverter in accordance with an embodiment of the present invention;

FIG. 3 is a flow chart of a first stage DC-DC circuit control method in accordance with an embodiment of the present invention;

FIG. 4 is a block diagram of a second stage DC-AC circuit control method in accordance with an embodiment of the present invention;

FIG. 5 is a flow chart of a perturbation observation method employed in an embodiment of the present invention;

FIG. 6 shows K-F in an embodiment of the present invention_xSchematic representation of the relationship.

Detailed Description

In order to make the technical means, the creation characteristics, the achievement purposes and the effects of the invention easy to understand, the invention is further described with the specific embodiments.

A two-stage soft-switching micro photovoltaic inverter which is composed of gallium nitride devices and has high efficiency, high power density and high reliability; the invention optimally designs the first-stage soft switching DC-DC converter working at high switching frequency by utilizing the advantage of high switching speed of the gallium nitride device so as to reduce the volume and improve the efficiency of the whole machine. In the second-stage DC-AC converter, a single-phase full-bridge inverter circuit with a low-frequency bridge arm and a high-frequency bridge arm mixed is adopted, wherein the low-frequency bridge arm adopts a conventional silicon-based semiconductor switching device, and the high-frequency bridge arm adopts a gallium nitride switching device, so that the cost is reduced while the performance index is met; and finally, high-efficiency power conversion from the photovoltaic cell panel to the single-phase alternating current power grid is realized.

As shown in fig. 1, the photovoltaic inverter based on the gan device provided by the invention comprises an anti-reverse diode and a low-voltage dc filter capacitor C₁Low-voltage full-bridge inverter circuit, resonant circuit and high-frequency transformer T₁Voltage doubling rectifying circuit and high-voltage direct current filter capacitor C₄The high-voltage full-bridge inverter circuit and the output filter; the anode of the anti-reverse diode is connected with the positive terminal of the photovoltaic cell, and the cathode of the anti-reverse diode and the negative terminal of the photovoltaic cell are respectively connected with the low-voltage direct-current filter capacitor C₁Both ends of (a); the four switches of the low-voltage full-bridge inverter circuit adopt gallium nitride switches Q₁～Q₄The DC side of the filter capacitor and a low-voltage DC filter capacitor C₁Connected to the AC side of the transformer, a resonant circuit and a high-frequency transformer T₁The primary winding of the transformer is connected; the resonant circuit comprises a resonant capacitor C_rAnd a resonant inductor L_r(ii) a High-frequency transformer T₁Respectively, of the secondary windingTwo rectifier diodes D connected to a voltage doubler rectifier circuit₁、D₂And two capacitors C₂、C₃To (c) to (d); output end of voltage doubling rectifying circuit and high-voltage direct current filter capacitor C₄The direct current side of the high-voltage full-bridge inverter circuit is connected with the direct current side of the high-voltage full-bridge inverter circuit; the output filter comprises an inductor L_fCapacitor C_fInductance L_fConnected in series between the AC side of the high-voltage full-bridge inverter circuit and the power grid, and a capacitor C_fConnected in parallel to the two terminals of the single-phase network.

The first-stage soft switching DC-DC converter in the photovoltaic inverter consists of a low-voltage direct-current filter capacitor C₁Low-voltage full-bridge inverter circuit, resonant circuit and high-frequency transformer T₁And a voltage-doubling rectifying circuit. The low-voltage direct current output by the photovoltaic cell is converted into high-voltage direct current, and the input and the output of the whole machine are electrically isolated. The low-voltage full-bridge inverter circuit has small voltage and current stress, adopts a low-voltage gallium nitride switch, and has Q₁、Q₄And Q₂、Q₃Dividing the signal into two groups, and respectively applying two high-frequency square wave control signals with opposite phases and proper dead time; implementing Q by a resonant circuit₁～Q₄Zero voltage turn-on (ZVS) effectively reduces switching losses. By magnetic integration techniques, using high-frequency transformers T₁The primary side leakage inductance replaces the resonance inductance L_rThe volume of the whole machine is reduced; high-frequency transformer T₁The turn ratio of the primary side to the secondary side is 1: n, and the rectified direct-current voltage can be doubled by the voltage doubling rectifying circuit, so that the transformer T₁The number of turns of the secondary side can be properly reduced, and the transformer magnetic core with smaller volume is favorably used.

The second-stage soft switching DC-AC converter in the photovoltaic inverter consists of a high-voltage direct current filter capacitor C₄The high-voltage full-bridge inverter circuit and the output filter. The high-voltage direct current output by the first-stage soft switching DC-DC converter is converted into sine wave through SPWM modulation control and is output to a single-phase alternating current power grid. The high-voltage direct current filter capacitor C has small current and small required capacitor capacity₄Using thin-film capacitors of small capacityBy combining a proper control technology, the double-frequency power fluctuation caused by single-phase inversion can be absorbed, and the fluctuation is prevented from being transmitted to a low-voltage direct-current bus of the first-stage DC-DC converter, so that a large-capacity electrolytic capacitor (usually thousands to tens of thousands of uF) is avoided from being adopted at a low-voltage side, and the service life of the whole machine is effectively prolonged. The high-voltage full-bridge inverter circuit adopts switches with higher voltage resistance, wherein two switches Q of a left bridge arm₅、Q₆Is a silicon switch, adopts power frequency control to achieve the effect of reducing the switching loss of an inverter circuit, and two switches Q of a right bridge arm₇、Q₈For the GaN switch, high frequency modulation is used, which can increase the equivalent switching frequency, thereby reducing the volume of the output filter.

Specifically, the terminal voltage of the photovoltaic cell connected with the common micro photovoltaic inverter is about 30-45V, so that the low-voltage direct-current capacitor C₁The four switches of the low-voltage full-bridge inverter circuit can adopt 100V gallium nitride MOSFET switches, and Q is₁、Q₄And Q₂、Q₃Dividing the signals into two groups, and respectively applying two paths of high-frequency square wave control signals with opposite phases and dead time of 0.1-0.2 microseconds, wherein the frequency of the control signals can reach more than 1MHz and is far higher than the switching frequency of a silicon switch; implementing Q by a resonant circuit₁～Q₄Zero voltage turn-on (ZVS) effectively reducing switching losses; by magnetic integration techniques, using high-frequency transformers T₁The primary side leakage inductance replaces the resonance inductance L_rAnd the volume of the whole machine is reduced.

If the rated voltage of a single-phase power grid connected with the micro photovoltaic inverter is 220V, the voltage of the high-voltage direct-current bus needs to reach 320-380V. High-voltage direct current filter capacitor C due to large high-voltage direct current voltage fluctuation caused by single-phase inversion₄A high voltage thin film capacitor with a withstand voltage of 650V may be selected. The high-voltage full-bridge inverter circuit adopts a 600V or 650V-resistant switch, wherein two switches Q of a left bridge arm₅、Q₆Is a silicon MOSFET switch, adopts power frequency control to achieve the effect of reducing the switching loss of an inverter circuit, and two switches Q of a right bridge arm₇、Q₈For the MOSFET switch of gallium nitride material, high-frequency SPWM modulation with carrier frequency of thousands of hertz is adoptedHigh equivalent switching frequency, thereby reducing the size of the output filter.

The high-frequency transformer T has large direct-current voltage difference between the low-voltage side and the high-voltage side₁The transformer is designed as a step-up transformer, and the turn ratio of the primary side to the secondary side is 1: n. Further preferably, a voltage doubler rectifier circuit is used at the transformer output to double the voltage, so that the transformer T₁The number of turns of the secondary side can be properly reduced, and the transformer magnetic core with smaller volume is favorably used. High-frequency transformer T₁And the function of realizing the input/output electrical isolation of the whole machine is also realized.

When the power is constant, the current of the high-voltage direct-current bus is small, and the required capacitance is small, so that the high-voltage direct-current filter capacitor C₄By adopting a small-capacity thin film capacitor and combining the power decoupling control method adopting power feedforward and voltage feedback control, the double-frequency power fluctuation caused by single-phase inversion can be absorbed at the high-voltage side, and the fluctuation is prevented from being transmitted to the low-voltage direct-current bus, so that the large-capacity electrolytic capacitor (usually thousands to tens of thousands uF) at the low-voltage side is avoided, and the service life of the whole machine is effectively prolonged.

The invention also discloses a control method of the photovoltaic inverter based on the gallium nitride device, which adopts a power decoupling control method of power feedforward and voltage feedback control and comprises the steps of generating a driving signal of a switch of a low-voltage full-bridge inverter circuit in a first-stage DC-DC circuit; and generating a driving signal of a switch of a high-voltage full-bridge inverter circuit in the second-stage DC-AC circuit.

The first stage DC-DC circuit control algorithm flow is shown in fig. 3, and has the following steps:

step 1: collecting instantaneous value u of power grid voltage_acAnd the instantaneous value i of the output current of the photovoltaic inverter_acCalculating the AC instantaneous power p output by the photovoltaic inverter_ac＝u_ac×i_ac. Collecting low-voltage DC bus voltage instantaneous value u_dc1And a photovoltaic cell output current instantaneous value i_dc1Respectively calculating the average value U of the two values with a period of 10ms_dc1And I_dc1Calculating the output power P of the photovoltaic cell_pv＝U_dc1×I_dc1. Collecting high-voltage direct-current bus voltage instantaneous value u_dc2U is calculated according to the following formula_dc2The change amount Deltau in the next switching period_dc2Wherein T is₀For the current switching cycle of the first stage DC-DC converter:

subjecting the obtained u to_dc2And Δ u_dc2Adding to obtain a predicted value u 'of the voltage of the high-voltage direct-current bus in the next switching period'_dc2＝u_dc2+Δu_dc2Then, the voltage gain ratio K of the first-stage DC-DC converter in the next switching period is calculated according to the following formula:

wherein n is the transformation ratio of the high-frequency transformer.

Step 2: acquiring input current instantaneous value i at direct current side of high-voltage full-bridge inverter circuit_dc2Respectively calculating u obtained in step 1 by using 10ms as a period_dc2And i_dc2Average value of U_dc2And I_dc2Calculating the equivalent load resistance R of the first-stage DC-DC converter according to the following formula:

let the quality factorInductance ratio parameterLLC resonant frequencyStandardized switching frequencyWherein L is_rIs the resonant inductance of LLC, C_rIs the resonant capacitance of LLC_mFor high-frequency transformers T₁Primary side excitation inductance of f₁The switching frequency of the next period of the first stage DC-DC converter to be required. According to the principle of the LLC soft-switching resonant converter, the following formula will be given:

FIG. 6 is a K-F_xExamples of relationship curves. K-F from off-line calculation_xThe relation curve is looked up to obtain F satisfying the following conditions_xThe solution of (a):

according to F_xAnd f₁To find f₁＝F_x×f_rThe switching frequency is used as the switching frequency of the next period of the low-voltage full-bridge inverter circuit; and t of the next cycle₁＝1/f₁。

And step 3: performing voltage-frequency conversion on the switching frequency obtained in the step 2 to obtain a square wave signal with a duty ratio of 50%, inverting the square wave signal, and adding proper dead time to obtain Q₁、Q₄And Q₂、Q₃And driving signals of the two groups of switches.

The second stage DC-AC circuit control has the following steps:

step 1: according to the instantaneous value u of the low-voltage DC bus voltage_dc1And a photovoltaic cell output current instantaneous value i_dc1Calculating u by taking a perturbation and observation method as an algorithm for tracking the maximum power of the photovoltaic cell with a period of 100ms_dc1Reference value U of_dcref. The algorithm block diagram is shown in fig. 5.

Step 2: calculating the reference value U obtained in the step 1_dcrefWith instantaneous value u_dc1Using a proportional integral controller (PI) to obtain the amplitude I of the reference value of the output current of the photovoltaic inverter_acref(ii) a Controlling the power grid obtained in the step 1 by a first-stage DC-DC circuitInstantaneous value u of voltage_acObtaining a phase factor sin theta of the grid voltage through a phase-locked loop module, and further obtaining a reference value i of the output current_acref＝I_acrefX sin theta; calculating a reference value i of the output current_acrefControlling the instantaneous value i obtained in the step 1 with a first-stage DC-DC circuit_acObtaining a modulation ratio d of the output voltage of the photovoltaic inverter by using a ratio controller (P); subtracting the power frequency square wave with the amplitude of 1 from the modulation ratio d, carrying out SPWM (sinusoidal pulse Width modulation) on the obtained result and a high-frequency triangular carrier wave to obtain two switches Q of a right bridge arm of the high-voltage full-bridge inverter circuit₇、Q₈And two switches Q of the left arm₅、Q₆The switching signal of (2) is a power frequency square wave with a duty ratio of 50%. The algorithm block diagram is shown in fig. 4.

It will be appreciated by those skilled in the art that the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed above are therefore to be considered in all respects as illustrative and not restrictive. All changes which come within the scope of or equivalence to the invention are intended to be embraced therein.