Three-phase four-leg inverter system for rural power grid three-phase imbalance treatment and control method

Abstract

The invention relates to a three-phase four-leg inverter system for rural power grid three-phase imbalance treatment and a control method. The output end of the photovoltaic cell is connected with the input end of the BOOST circuit, the output end of the BOOST circuit is connected with the input end of the three-phase four-bridge-arm inverter, the output end of the three-phase four-bridge-arm inverter is connected with the input end of the LC filter, the output end of the LC filter is connected with the three-phase four-wire system power grid, and the three-phase four-wire system power grid is connected with the load. The input end of the three-phase four-leg inverter is connected with the first sampling unit, the output end of the first sampling unit is connected with the first filtering unit, the output end of the LC filter is connected with the second sampling unit, the output end of the second sampling unit is connected with the second filtering unit, and the first filtering unit and the second filtering unit are connected with the input end of the DSP control chip. The output end of the DSP control chip is connected with the IGBT drive circuit, and the IGBT drive circuit is connected with an IGBT grid electrode in the three-phase four-bridge-arm inverter. According to the invention, the problem of asymmetry of voltage waveforms under asymmetric loads can be effectively solved.

Description

Technical Field

[0001] The present invention relates to a three-phase four-bridge-arm inverter system and a control method, and in particular to a three-phase four-bridge-arm inverter system and a control method for managing three-phase imbalance of a rural power grid in photovoltaic power generation. Background Art

[0002] As the world is facing serious energy crisis and environmental pollution problems, the development of clean energy is an important measure for countries to deal with environmental and energy problems. Among them, the photovoltaic industry is a vital industry for my country to lead the global new energy. Considering the vastness of rural areas and the population density far lower than that of cities, photovoltaic power stations that generate electricity on the user side in the "self-generation, self-use" mode can greatly save the cost of transmission lines and reduce transmission losses. Secondly, the power consumption capacity in rural areas is increasing year by year, and rural areas are often at the end of the public power grid, with poor power quality. The construction of distributed photovoltaic systems in rural areas can improve power security and power quality.

[0003] However, with the acceleration of urban-rural integration, the demand for agricultural electricity is growing, and the construction of rural power grids is also developing rapidly. However, compared with urban power grids, the rural power grid environment is more complex, the line length is longer, and the load changes are more complex, resulting in a common phenomenon of three-phase voltage imbalance. This imbalance will lead to unstable operation of the power system, which has a huge impact on the safety and stable operation of the rural power grid.

[0004] The main problems of the current rural power grid are three-phase imbalance and harmonic pollution. Since the rural power grid involves a wide variety of loads, which are often nonlinear loads, especially after the photovoltaic power generation system is connected to the grid, more harmonic currents will be generated, which increases the harmonic content of the power grid. At the same time, due to the long construction time of the rural power grid, many equipments have been aged, the equipment loss is serious, the line is aged, and the topology design of the equipment is unreasonable, which has led to the impedance imbalance of the power grid line, or the lack of reasonable load distribution, and the line is unbalanced, which has caused the increase of voltage fluctuations and power loss.

[0005] Three-phase imbalance is one of the main reasons for the unstable operation of rural power grids. Three-phase imbalance can cause voltage fluctuations, equipment damage, line overload and other problems, thus affecting the power quality and power supply capacity of the power grid. Therefore, it is of great significance to control the three-phase imbalance problem of rural power grids.

[0006] The three-phase four-bridge-arm inverter system is a new type of power regulation technology that can be applied to the three-phase imbalance management of rural power grids. The system adjusts the phase difference between the three-phase voltage and the three-phase current to the optimal state through power electronic devices, thereby achieving the balance of the three-phase power load. Compared with traditional management methods, this technology has the advantages of low cost, high control accuracy and fast response speed, and can achieve real-time adjustment, which is suitable for various load types.

[0007] Therefore, in order to solve the problem of unbalanced three-phase load in rural distribution networks, while building distributed photovoltaic power sources for grid-connected power generation, it is necessary to dynamically adapt to the three-phase unbalanced load and control the photovoltaic output power to compensate for the unbalanced load. This can improve the current situation of unbalanced three-phase load in rural distribution networks, improve the new energy absorption capacity and the flexible and economical operation of photovoltaic systems, reduce line losses in rural distribution networks, and achieve the goal of reducing energy consumption while improving the power quality of distribution networks. This is of great significance for the research on the management of the problem of unbalanced three-phase load in rural power grids.

[0008] The basic principle of the three-phase four-bridge inverter is to control the size and shape of the output voltage by controlling the on and off time of the switch tube. If the on time of each switch tube is evenly distributed, the output voltage is a sine wave, but due to the influence of the motor load on the output voltage, the output voltage needs to be controlled by PWM modulation technology. PWM control technology can adjust the output voltage by adjusting the on and off time of the switch tube according to the given adjustment signal to keep it stable.

[0009] SPWM (Sinusoidal Pulse Width Modulation) and SVPWM (Space Vector Pulse Width Modulation) are two widely used control methods. They have different degrees of influence on the output waveform quality, conversion efficiency, electromagnetic compatibility and other aspects of the inverter. Summary of the invention

[0010] In order to solve the above technical problems, the present invention provides a three-phase four-bridge arm inverter system and control method for managing three-phase imbalance in rural power grids, with the aim of effectively solving the problem of asymmetric voltage waveform under asymmetric load by controlling each phase separately.

[0011] In order to achieve the above-mentioned purpose, the three-phase four-bridge-arm inverter system for three-phase imbalance control of the rural power grid of the present invention comprises the following: the output end of the photovoltaic cell is connected to the input end of the BOOST boost circuit, the output end of the BOOST boost circuit is connected to the input end of the three-phase four-bridge-arm inverter, the output end of the three-phase four-bridge-arm inverter is connected to the input end of the LC filter, the output end of the LC filter is connected to the three-phase four-wire power grid, the three-phase four-wire power grid is connected to the load, the input end of the three-phase four-bridge-arm inverter is connected to the first sampling unit, the output end of the first sampling unit is connected to the first filtering unit, the output end of the LC filter is connected to the second sampling unit, the output end of the second sampling unit is connected to the second filtering unit, the first filtering unit and the second filtering unit are connected to the input end of the DSP control chip, the output end of the DSP control chip is connected to the IGBT drive circuit, and the IGBT drive circuit is connected to the IGBT gate in the three-phase four-bridge-arm inverter.

[0012] The positive electrode of the photovoltaic cell is connected to the first inductor and the diode of the BOOST boost circuit in sequence, the positive electrode of the diode is connected to one end of the switch, the other end of the switch is connected to the negative electrode of the photovoltaic cell, the negative electrode of the diode is connected to one end of the first capacitor, and the other end of the first capacitor is connected to the negative electrode of the photovoltaic cell.

[0013] The three-phase four-bridge-arm inverter comprises IGBTQ1 to IGBTQ8, wherein the gates of IGBTQ1 to IGBTQ8 are connected to the output end of the IGBT driving circuit, the collectors of IGBTQ1, Q3, Q5 and Q7 of the three-phase four-bridge-arm inverter are connected to the cathode of the diode of the BOOST boost circuit, the emitters of IGBTQ2, Q4, Q6 and Q8 of the three-phase four-bridge-arm inverter are connected to the cathode of the photovoltaic cell, the emitter of IGBTQ1 of the three-phase four-bridge-arm inverter is connected to the collector of IGBTQ2, the emitter of IGBTQ3 of the three-phase four-bridge-arm inverter is connected to the collector of IGBTQ4, the emitter of IGBTQ5 of the three-phase four-bridge-arm inverter is connected to the collector of IGBTQ6, and the emitter of IGBTQ7 of the three-phase four-bridge-arm inverter is connected to the collector of IGBTQ8.

[0014] The midpoint of the bridge arm where the emitter of IGBTQ1 and the collector of IGBTQ2 of the three-phase four-bridge-arm inverter are connected is connected to the three-phase four-wire power grid A through the second inductor of the LC filter; the midpoint of the bridge arm where the emitter of IGBTQ3 and the collector of IGBTQ4 of the three-phase four-bridge-arm inverter are connected is connected to the three-phase four-wire power grid B through the third inductor of the LC filter; the midpoint of the bridge arm where the emitter of IGBTQ5 and the collector of IGBTQ6 of the three-phase four-bridge-arm inverter are connected is connected to the three-phase four-wire power grid C through the fourth inductor of the LC filter; the midpoint of the bridge arm where the emitter of IGBTQ7 and the collector of IGBTQ8 of the three-phase four-bridge-arm inverter are connected is connected to the three-phase four-wire power grid N through the fifth inductor of the LC filter.

[0015] The second inductor output end is connected to one end of the second capacitor, the other end of the second capacitor is connected to the fifth inductor output end, the third inductor output end is connected to one end of the third capacitor, the other end of the third capacitor is connected to the fifth inductor output end, the fourth inductor output end is connected to one end of the fourth capacitor, and the other end of the fourth capacitor is connected to the fifth inductor output end.

[0016] The first sampling unit detects the output voltage signal of the BOOST voltage-boosting circuit in real time.

[0017] The second sampling unit collects the three-phase output voltage V at the output end of the LC filter. abcf And the three-phase output current i labc .

[0018] A control method for a three-phase four-bridge-arm inverter system for three-phase unbalance management of a rural power grid comprises the following steps:

[0019] Step 1: The first sampling unit detects the output voltage signal of the BOOST boost circuit in real time;

[0020] Step 2: The voltage signal collected by the first sampling unit is extracted through the first filtering unit to obtain the output voltage V dc , and the output voltage V dc Input to DSP control chip;

[0021] Step 3: The second sampling unit collects the three-phase output voltage signal and the three-phase output current signal at the output end of the LC filter, and extracts the three-phase output voltage V abcf And the three-phase output current i labc ;

[0022] Step 4: Convert the three-phase output voltage V extracted in step 3 abcf , through PR control in the DSP control chip, the three-phase reference voltage V af *、V bf * and V cf *, through the three-phase reference voltage V af *、V bf * and V cf * Calculate the offset voltage V between the fourth bridge arm and the midpoint of the DC bus nf *, and then synthesize the three-phase modulation voltage with the reference voltage, and the offset voltage V nf * As the fourth phase modulation voltage, it is then compared with the triangular carrier to obtain the driving signal required by the eight IGBTs of the three-phase four-bridge-arm inverter; the DSP control chip sends the driving signal to the IGBT drive circuit, and the IGBT drive circuit controls the on-off time of the eight IGBTs of the three-phase four-bridge-arm inverter;

[0023] Where: Offset voltage V nf*Based on the following conditions:

[0024]

[0025] Right now

[0026]

[0027] in,

[0028] V min =min(V af *,V bf *,V cf *), V mid = mid(V af *,V bf *,V cf *),

[0029] V max =max(V af *,V bf *,V cf *), V af *V bf *V cf * is the three-phase reference voltage

[0030] The corresponding three-phase modulation voltage is:

[0031] V an *=V af *+V nf *

[0032] V bn *=V bf *+V nf *

[0033] V cn *=V cf *+V nf *

[0034] Among them, V nf * is the offset voltage, V af *V bf *V cf * is the three-phase reference voltage, then the conduction time of the IGBT in the three-phase four-leg inverter is:

[0035]

[0036]

[0037]

[0038]

[0039] Where T a is the conduction time of phase a of the three-phase four-leg inverter, T b is the conduction time of phase b of the three-phase four-bridge-arm inverter, T c is the conduction time of phase c of the three-phase four-leg inverter, T f is the conduction time of phase f of the three-phase four-leg inverter, T s is the sampling period, V an *V bn *V cn * is the three-phase modulation voltage, V dc is the output voltage;

[0040] The conduction time of phases a, b, and c of the three-phase four-leg inverter is controlled by adjusting the conduction time of eight IGBTs in the three-phase four-leg inverter.

[0041] The T a is the conduction time of IGBTQ1 and IGBTQ2; T b is the conduction time of IGBTQ3 and IGBTQ4; T c is the conduction time of IGBTQ5 and IGBTQ6; T f is the on-time of IGBTQ7 and IGBTQ8.

[0042] Advantages and effects of the present invention: The present invention uses a simpler three-phase four-bridge-arm PWM method based on a triangular carrier wave, which does not require a large amount of decoupling operations and Park transformation to determine the projection of the voltage vector on the space cube. The offset voltage is directly introduced into the four-bridge-arm system, and the three-phase reference voltage V af *、V bf * and V cf * Calculate the offset voltage V nf *, and then synthesize the three-phase modulation voltage with the reference voltage, and the offset voltage V nf *As the fourth phase modulation voltage, compared with the triangular carrier, the control signal of the device can be obtained to control the inverter. In this way, each phase can be controlled separately. This method greatly reduces the amount of calculation and the cost of the required hardware, and can effectively solve the problem of asymmetric voltage waveform under asymmetric load. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 is a block diagram of the present invention.

[0044] Figure 2 The invention discloses a connection principle diagram of a BOOST voltage-boosting circuit, a three-phase four-bridge-arm inverter and an LC filter.

[0045] Figure 3 It is the overall dynamic response diagram of the present invention.

[0046] Figure 4 It is a dynamic response diagram of the three-phase load of the present invention being cut into two-phase operation during operation.

[0047] Figure 5 It is a three-phase voltage and current waveform diagram under three-phase asymmetric load of the present invention.

[0048] Figure 6 It is a three-phase voltage and current waveform diagram of the present invention when operating under an asymmetric load with one phase missing among the three phases.

[0049] Figure 7 It is a three-phase voltage and current waveform diagram under an asymmetric linear load when the three-phase is running with two phases missing according to the present invention.

[0050] Figure 8 It is a three-phase voltage and current waveform diagram under the operation of asymmetric nonlinear load of the present invention.

[0051] In the figure: 1. Photovoltaic cell; 2. BOOST boost circuit; 3. Three-phase four-bridge-arm inverter; 4. LC filter; 5. Three-phase four-wire power grid; 6. Load; 7. First sampling unit; 8. Second sampling unit; 9. First filtering unit; 10. Second filtering unit; 11. IGBT drive circuit; 12. DSP control chip; 13. Power supply. DETAILED DESCRIPTION

[0052] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0053] The three-phase four-bridge-arm inverter system for three-phase imbalance control of the rural power grid of the present invention comprises: a photovoltaic cell 1 output end connected to a BOOST boost circuit 2 input end, a BOOST boost circuit 2 output end connected to a three-phase four-bridge-arm inverter 3 input end, a three-phase four-bridge-arm inverter 3 output end connected to an LC filter 4 input end, an LC filter 4 output end connected to a three-phase four-wire power grid 5, a three-phase four-wire power grid 5 connected to a load 6, a three-phase four-bridge-arm inverter 3 input end connected to a first sampling unit 7, an output end of the first sampling unit 7 connected to a first filtering unit 9, and an LC filter 4 connected to a load 6. The output end is connected to the second sampling unit 8, the output end of the second sampling unit 8 is connected to the second filtering unit 10, the first filtering unit 9 and the second filtering unit 10 are connected to the input end of the DSP control chip 12, the output end of the DSP control chip 12 is connected to the IGBT drive circuit 11, the IGBT drive circuit 11 is connected to the IGBT gate in the three-phase four-bridge-arm inverter 3, and the power supply 13 supplies power to the first sampling unit 7, the second sampling unit 8, the first filtering unit 9, the second filtering unit 10, the IGBT drive circuit 11 and the DSP control chip 12. The DSP control chip model is TMS320F28335.

[0054] The positive electrode of the photovoltaic cell 1 is connected to the first inductor L1 and the diode D1 of the BOOST boost circuit 2 in sequence, the positive electrode of the diode D1 is connected to one end of the switch S1, the other end of the switch S1 is connected to the negative electrode of the photovoltaic cell 1, the negative electrode of the diode D1 is connected to one end of the first capacitor C1, and the other end of the first capacitor C1 is connected to the negative electrode of the photovoltaic cell 1.

[0055] The three-phase four-arm inverter 3 includes IGBTQ1 to IGBTQ8, wherein the gates of IGBTQ1 to IGBTQ8 are connected to the output end of the IGBT drive circuit 11, the collectors of IGBTQ1, IGBTQ3, IGBTQ5, and IGBTQ7 of the three-phase four-arm inverter 3 are connected to the cathode of the diode D1 of the BOOST boost circuit 2, the emitters of IGBTQ2, IGBTQ4, IGBTQ6, and IGBTQ8 of the three-phase four-arm inverter 3 are connected to the cathode of the photovoltaic cell 1, the emitter of IGBTQ1 of the three-phase four-arm inverter 3 is connected to the collector of IGBTQ2, the emitter of IGBTQ3 of the three-phase four-arm inverter 3 is connected to the collector of IGBTQ4, the emitter of IGBTQ5 of the three-phase four-arm inverter 3 is connected to the collector of IGBTQ6, and the emitter of IGBTQ7 of the three-phase four-arm inverter 3 is connected to the collector of IGBTQ8.

[0056] The midpoint of the bridge arm where the emitter of IGBTQ1 and the collector of IGBTQ2 of the three-phase four-bridge-arm inverter 3 are connected is connected to the three-phase four-wire power grid A through the second inductor L2 of the LC filter 4; the midpoint of the bridge arm where the emitter of IGBTQ3 and the collector of IGBTQ4 of the three-phase four-bridge-arm inverter 3 are connected is connected to the three-phase four-wire power grid B through the third inductor L3 of the LC filter; the midpoint of the bridge arm where the emitter of IGBTQ5 and the collector of IGBTQ6 of the three-phase four-bridge-arm inverter 3 are connected is connected to the three-phase four-wire power grid C through the fourth inductor L4 of the LC filter; the midpoint of the bridge arm where the emitter of IGBTQ7 and the collector of IGBTQ8 of the three-phase four-bridge-arm inverter 3 are connected is connected to the three-phase four-wire power grid N through the fifth inductor L5 of the LC filter.

[0057] The output end of the second inductor L2 is connected to one end of the second capacitor C2, the other end of the second capacitor C2 is connected to the output end of the fifth inductor L5, the output end of the third inductor L3 is connected to one end of the third capacitor C3, the other end of the third capacitor C3 is connected to the output end of the fifth inductor L5, the output end of the fourth inductor L4 is connected to one end of the fourth capacitor C4, and the other end of the fourth capacitor C4 is connected to the output end of the fifth inductor L5.

[0058] The first sampling unit 7 detects the output voltage signal of the BOOST voltage-boosting circuit 2 in real time.

[0059] The second sampling unit 8 collects the three-phase output voltage V at the output end of the LC filter 4. abcf And the three-phase output current i labc .

[0060] A control method for a three-phase four-bridge-arm inverter system for three-phase unbalance management of a rural power grid comprises the following steps:

[0061] Step 1: The first sampling unit detects the output voltage signal of the BOOST boost circuit in real time;

[0062] Step 2: The voltage signal collected by the first sampling unit is extracted through the first filtering unit to obtain the output voltage V dc , and the output voltage V dc Input to DSP control chip;

[0063] Step 3: The second sampling unit collects the three-phase output voltage signal and the three-phase output current signal at the output end of the LC filter, and extracts the three-phase output voltage V abcf And the three-phase output current i labc ;

[0064] Step 4: Convert the three-phase output voltage V extracted in step 3 abcf , through PR control in the DSP control chip, the three-phase reference voltage Vaf *、V bf * and V cf *, through the three-phase reference voltage V af *、V bf * and V cf * Calculate the offset voltage V between the fourth bridge arm and the midpoint of the DC bus nf *, and then synthesize the three-phase modulation voltage with the reference voltage, and the offset voltage V nf * As the fourth phase modulation voltage, it is then compared with the triangular carrier to obtain the driving signal required by the eight IGBTs of the three-phase four-bridge-arm inverter; the DSP control chip sends the driving signal to the IGBT drive circuit, and the IGBT drive circuit controls the on-off time of the eight IGBTs of the three-phase four-bridge-arm inverter;

[0065] Where: Offset voltage V nf *Based on the following conditions:

[0066]

[0067] Right now

[0068]

[0069] in,

[0070] V min =min(V af *,V bf *,V cf *), V mid = mid(V af *,V bf *,V cf *),

[0071] V max =max(V af *,V bf *,V cf *), V af *V bf *V cf * is the three-phase reference voltage

[0072] The corresponding three-phase modulation voltage is:

[0073] V an *=V af *+V nf *

[0074] V bn *=V bf *+V nf *

[0075] V cn *=Vcf *+V nf *

[0076] Among them, V nf * is the offset voltage, V af *V bf *V cf * is the three-phase reference voltage

[0077] Then the conduction time of the IGBT in the three-phase four-leg inverter is:

[0078]

[0079]

[0080]

[0081]

[0082] Where T a is the conduction time of phase a of the three-phase four-leg inverter, T b is the conduction time of phase b of the three-phase four-bridge-arm inverter, T c is the conduction time of phase c of the three-phase four-leg inverter, T f is the conduction time of phase f of the three-phase four-leg inverter, T s is the sampling period, V an *V bn *V cn * is the three-phase modulation voltage, V dc is the output voltage;

[0083] The conduction time of phases a, b, and c of the three-phase four-leg inverter is controlled by adjusting the conduction time of eight IGBTs in the three-phase four-leg inverter.

[0084] The T a is the conduction time of IGBTQ1 and IGBTQ2; T b is the conduction time of IGBTQ3 and IGBTQ4; T c is the conduction time of IGBTQ5 and IGBTQ6; T f is the on-time of IGBTQ7 and IGBTQ8.

[0085] The present invention has a good ability to carry unbalanced loads. After being connected to the power grid, it can effectively balance and compensate the three-phase unbalanced loads, and effectively solve the problem of asymmetric voltage waveforms under the condition of three-phase unbalance.

[0086] The present invention is under three-phase symmetrical load. Figure 3It can be seen that the present invention responds very quickly and reaches a steady state in about 4 to 5 fundamental wave sinusoidal cycles (about 10ms). Figure 4 The three-phase load is cut into two-phase operation during operation. The C-phase load is in a phase-off state from 0.2s to 0.3s. It can be seen that the present invention is almost unaffected and still operates in a stable state.

[0087] from Figure 5 It can be seen from the three-phase voltage and current waveforms under three-phase asymmetric loads that the three-phase loads are 20Ω, 10Ω, and 5Ω respectively. It can be seen that the present invention still operates stably, and the three-phase output voltage is basically balanced, with an unbalance degree of <2%.

[0088] Figure 6 The three-phase voltage and current waveforms are shown when the three-phase is missing one phase under asymmetric load. The three-phase loads are 20Ω, 10Ω, and phase C is missing. It can be seen that the present invention still operates stably, the three-phase output voltage is basically balanced, and the unbalance degree meets the requirements.

[0089] Figure 7 The three-phase voltage and current waveforms are shown in Figure 1. The three-phase loads are 20Ω, phase B is missing, and phase C is missing. It can be seen that the present invention still operates stably, the three-phase output voltage is basically balanced, and the unbalance degree meets the requirements.

[0090] Figure 8 The three-phase voltage and current waveforms under asymmetric nonlinear load operation are shown in Figure 1. The three-phase loads are resistive and inductive loads, which are (20Ω, 3H), (20Ω, 2H), and (20Ω, 1H). It can be seen that the present invention still operates stably, the three-phase output voltage is basically balanced, and the unbalance degree meets the requirements.

[0091] The above experimental results show that the two-stage photovoltaic three-phase four-bridge-arm inverter system for three-phase unbalanced management of rural power grids of the present invention can continuously output stable voltage and current waveforms, and the symmetry of the output voltage waveform is ensured by adding a PR controller in each phase. A simpler three-phase four-bridge-arm PWM method based on a triangular carrier is used, and the concept of offset voltage is introduced into the four-bridge-arm system, so that the photovoltaic three-phase four-bridge-arm inverter system can be effectively controlled, and finally the system is guaranteed to have good ability to carry unbalanced loads and nonlinear loads.

Claims

1. The three-phase four-bridge-arm inverter system for three-phase imbalance control in rural power grid is characterized by The output end of the photovoltaic cell is connected to the input end of the BOOST boost circuit, the output end of the BOOST boost circuit is connected to the input end of the three-phase four-bridge-arm inverter, the output end of the three-phase four-bridge-arm inverter is connected to the input end of the LC filter, the output end of the LC filter is connected to the three-phase four-wire power grid, the three-phase four-wire power grid is connected to the load, the input end of the three-phase four-bridge-arm inverter is connected to the first sampling unit, the output end of the first sampling unit is connected to the first filtering unit, the output end of the LC filter is connected to the second sampling unit, the output end of the second sampling unit is connected to the second filtering unit, the first filtering unit and the second filtering unit are connected to the input end of the DSP control chip, the output end of the DSP control chip is connected to the IGBT drive circuit, and the IGBT drive circuit is connected to the IGBT gate in the three-phase four-bridge-arm inverter.

2. The three-phase four-bridge-arm inverter system for three-phase imbalance control of the rural power grid according to claim 1 is characterized in that The positive electrode of the photovoltaic cell is connected to the first inductor and the diode of the BOOST boost circuit in sequence, the positive electrode of the diode is connected to one end of the switch, the other end of the switch is connected to the negative electrode of the photovoltaic cell, the negative electrode of the diode is connected to one end of the first capacitor, and the other end of the first capacitor is connected to the negative electrode of the photovoltaic cell.

3. The three-phase four-bridge-arm inverter system for three-phase imbalance control of the rural power grid according to claim 1 is characterized in that The three-phase four-bridge-arm inverter comprises IGBTQ1 to IGBTQ8, wherein the gates of IGBTQ1 to IGBTQ8 are connected to the output end of the IGBT driving circuit, the collectors of IGBTQ1, Q3, Q5 and Q7 of the three-phase four-bridge-arm inverter are connected to the cathode of the diode of the BOOST boost circuit, the emitters of IGBTQ2, Q4, Q6 and Q8 of the three-phase four-bridge-arm inverter are connected to the cathode of the photovoltaic cell, the emitter of IGBTQ1 of the three-phase four-bridge-arm inverter is connected to the collector of IGBTQ2, the emitter of IGBTQ3 of the three-phase four-bridge-arm inverter is connected to the collector of IGBTQ4, the emitter of IGBTQ5 of the three-phase four-bridge-arm inverter is connected to the collector of IGBTQ6, and the emitter of IGBTQ7 of the three-phase four-bridge-arm inverter is connected to the collector of IGBTQ8.

4. The three-phase four-bridge-arm inverter system for three-phase imbalance control of the rural power grid according to claim 3 is characterized in that The midpoint of the bridge arm where the emitter of IGBTQ1 and the collector of IGBTQ2 of the three-phase four-bridge-arm inverter are connected is connected to the three-phase four-wire power grid A through the second inductor of the LC filter; the midpoint of the bridge arm where the emitter of IGBTQ3 and the collector of IGBTQ4 of the three-phase four-bridge-arm inverter are connected is connected to the three-phase four-wire power grid B through the third inductor of the LC filter; the midpoint of the bridge arm where the emitter of IGBTQ5 and the collector of IGBTQ6 of the three-phase four-bridge-arm inverter are connected is connected to the three-phase four-wire power grid C through the fourth inductor of the LC filter; the midpoint of the bridge arm where the emitter of IGBTQ7 and the collector of IGBTQ8 of the three-phase four-bridge-arm inverter are connected is connected to the three-phase four-wire power grid N through the fifth inductor of the LC filter.

5. The three-phase four-bridge-arm inverter system for three-phase imbalance control of the rural power grid according to claim 4 is characterized in that The second inductor output end is connected to one end of the second capacitor, the other end of the second capacitor is connected to the fifth inductor output end, the third inductor output end is connected to one end of the third capacitor, the other end of the third capacitor is connected to the fifth inductor output end, the fourth inductor output end is connected to one end of the fourth capacitor, and the other end of the fourth capacitor is connected to the fifth inductor output end.

6. The three-phase four-bridge-arm inverter system for three-phase imbalance control of the rural power grid according to claim 1 is characterized in that The first sampling unit detects the output voltage signal of the BOOST voltage-boosting circuit in real time.

7. The three-phase four-bridge-arm inverter system for three-phase imbalance control of the rural power grid according to claim 1 is characterized in that The second sampling unit collects the three-phase output voltage V at the output end of the LC filter. abcf And the three-phase output current i labc .

8. The control method of the three-phase four-bridge-arm inverter system for three-phase imbalance control of the rural power grid according to claim 1 is characterized in that The steps include: Step 1: The first sampling unit detects the output voltage signal of the BOOST boost circuit in real time; Step 2: The voltage signal collected by the first sampling unit is extracted through the first filtering unit to obtain the output voltage V dc , and the output voltage V dc Input to the DSP control chip and compare with the set output voltage to obtain the error signal (); Step 3: The second sampling unit collects the three-phase output voltage signal and the three-phase output current signal at the output end of the LC filter, and extracts the three-phase output voltage V abcf And the three-phase output current i labc ; Step 4: Convert the three-phase output voltage V extracted in step 3 abcf , through PR control in the DSP control chip, the three-phase reference voltage V af *、V bf * and V cf *, through the three-phase reference voltage V af *、V bf * and V cf * Calculate the offset voltage V between the fourth bridge arm and the midpoint of the DC bus nf *, and then synthesize the three-phase modulation voltage with the reference voltage, and the offset voltage V nf * As the fourth phase modulation voltage, it is then compared with the triangular carrier to obtain the driving signal required by the eight IGBTs of the three-phase four-bridge-arm inverter; the DSP control chip sends the driving signal to the IGBT drive circuit, and the IGBT drive circuit controls the on-off time of the eight IGBTs of the three-phase four-bridge-arm inverter; Where: Offset voltage V nf *Based on the following conditions: Right now in, V min =min(V af *,V bf *,V cf *),V mid =mid(V af *,V bf *,V cf *), V max =max(V af *,V bf *,V cf *), V af *V bf *V cf * is the three-phase reference voltage The corresponding three-phase modulation voltage is: V an *=V af *+V nf * V bn *=V bf *+V nf * V cn *=V cf *+V nf * Among them, V nf * is the offset voltage, V af *V bf *V cf * is the three-phase reference voltage, then the conduction time of the IGBT in the three-phase four-leg inverter is: Where T a is the conduction time of phase a of the three-phase four-leg inverter, T b is the conduction time of phase b of the three-phase four-bridge-arm inverter, T c is the conduction time of phase c of the three-phase four-leg inverter, T f is the conduction time of phase f of the three-phase four-leg inverter, T s is the sampling period, V an *V bn *V cn * is the three-phase modulation voltage, V dc is the output voltage; The conduction time of phases a, b, and c of the three-phase four-leg inverter is controlled by adjusting the conduction time of eight IGBTs in the three-phase four-leg inverter.

9. The control method of the three-phase four-bridge-arm inverter system for three-phase imbalance control of the rural power grid according to claim 8 is characterized in that The T a is the conduction time of IGBTQ1 and IGBTQ2; T b is the conduction time of IGBTQ3 and IGBTQ4; T c is the conduction time of IGBTQ5 and IGBTQ6; T f is the on-time of IGBTQ7 and IGBTQ8.

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