Off-grid inverter system for resisting three-phase unbalanced load and control method

Abstract

The invention discloses an off-grid inverter system for resisting a three-phase unbalanced load and a control method, and belongs to the technical field of wind power generation. The system comprises a wind driven generator, a diode rectifier, a direct current converter, a three-level three-phase four-bridge-arm inverter and the three-phase unbalanced load; the three-level three-phase four-bridge-arm inverter comprises a fourth bridge arm circuit, a T-type neutral point clamping three-level circuit and a filter circuit, and the first end of the fourth bridge arm circuit is connected with the direct-current voltage; the first end of the T-type neutral-point-clamped three-level circuit is connected with the second end of the fourth bridge arm circuit; the first end of the filter circuit is connected with the second end of the T-type neutral-point-clamped three-level circuit, and the second end of the filter circuit is connected with the three-phase unbalanced load. According to the method, the neutral-point potential can be ensured to be fast and stable when the load dynamically changes, the inter-phase coupling effect is effectively eliminated, independent adjustment of loads of all phases is achieved, the operation stability of the system under the three-phase unbalanced load is remarkably improved, and powerful support is provided for development of an off-grid wind power generation system.

Description

Technical Field

[0001] The present invention relates to the technical field of wind power generation, and in particular to an off-grid inverter system and a control method for resisting three-phase unbalanced load. Background Art

[0002] With the rapid development of renewable energy, wind power generation, as a clean energy technology, has been widely used in off-grid independent power supply systems. However, in actual operation, off-grid wind power generation systems often face the problem of unbalanced three-phase loads, resulting in asymmetric inverter output voltage, midpoint potential offset and other phenomena, which seriously affect the stability of the system and the quality of power supply. At present, for the problem of three-phase unbalanced loads, the existing solutions are mainly concentrated in grid-type inverters or balanced load scenarios. However, these methods have obvious limitations in off-grid wind power generation systems. In solving the problem of midpoint potential offset, the existing technologies mostly rely on complex software control algorithms. Although these methods can improve the problem of midpoint potential offset to a certain extent, the implementation process is complex, the amount of calculation is large, and the performance requirements of the controller are high, which increases the system cost and implementation difficulty. The software control method has a slow response speed under dynamic load changes, which is difficult to meet the requirements of off-grid wind power generation systems for real-time performance and stability. Summary of the invention

[0003] The purpose of this application is to overcome the defects of the prior art and provide an off-grid inverter system and control method that can resist three-phase unbalanced loads, so as to effectively deal with three-phase unbalanced loads in off-grid wind power generation systems, and solve the problem of midpoint potential offset through a simple and reliable hardware circuit method.

[0004] In a first aspect, the present application provides an off-grid inverter system for resisting three-phase unbalanced load, comprising: A wind turbine, a diode rectifier, a DC converter, a three-level three-phase four-bridge-arm inverter, and a three-phase unbalanced load, wherein the wind turbine is connected to a first end of the diode rectifier, a second end of the diode rectifier is connected to a first end of the DC converter, a second end of the DC converter is connected to a first end of the three-level three-phase four-bridge-arm inverter, and a second end of the three-level three-phase four-bridge-arm inverter is connected to the three-phase unbalanced load; The three-level three-phase four-bridge-arm inverter comprises: a fourth bridge-arm circuit, a first end of the fourth bridge-arm circuit being connected to a DC voltage output by the DC converter; A T-type midpoint clamped three-level circuit, wherein a first end of the T-type midpoint clamped three-level circuit is connected to a second end of the fourth bridge arm circuit, and the T-type midpoint clamped three-level circuit comprises three bridge arms; A filter circuit, wherein a first end of the filter circuit is connected to a second end of the T-type midpoint clamped three-level circuit, and a second end of the filter circuit is connected to a three-phase unbalanced load.

[0005] Optionally, the fourth bridge arm circuit includes: a first switch, wherein a first end of the first switch is connected to a first end of a DC voltage; a second switch, wherein a first end of the second switch is connected to a second end of the first switch, and a second end of the second switch is connected to a second end of a DC voltage; A first inductor, wherein a first end of the first inductor is connected to a series midpoint of a first switch and a second switch, and a second end of the first inductor is connected to a first end of a T-type midpoint clamped three-level circuit.

[0006] In a second aspect, the present application further provides a method for controlling an off-grid inverter capable of resisting three-phase unbalanced load, which controls an off-grid inverter system capable of resisting three-phase unbalanced load as described in any one of the first aspects, comprising the following steps: The three-phase signal output by the three-level three-phase four-bridge-arm inverter is separated to obtain a single-phase signal, and the single-phase signal is subjected to virtual coordinate transformation to obtain a single-phase signal after virtual coordinate transformation; the single-phase signal after virtual coordinate transformation includes a three-phase output voltage, a filter capacitor voltage, a load current, and a filter inductor current; Performing double closed-loop control on the single-phase signal after the virtual coordinate transformation; Performing a rotation coordinate transformation on the single-phase signal after the double closed-loop control to obtain a single-phase signal in the α-β coordinate system, and taking the α-axis component as the modulation wave of the corresponding single-phase signal; The modulation waves of the three single-phase signals are respectively input into the SPWM modulation module to obtain the switch drive signal; Through the fourth bridge arm control strategy, a complementary control signal of the fourth bridge arm switch is obtained to realize the control of the midpoint potential offset of the three-level three-phase four-bridge arm inverter.

[0007] Optionally, three-phase signals output by the three-level three-phase four-bridge-arm inverter are separated to obtain single-phase signals, and virtual coordinate transformation is performed on the single-phase signals to obtain single-phase signals after virtual coordinate transformation, including: Separate the three-phase signal output by the three-level three-phase four-bridge-leg inverter to obtain a single-phase signal; Constructing a virtual signal that is strictly orthogonal in phase with the single-phase signal; Taking the single-phase signal as the α-axis component in the α-β coordinate system, and taking the virtual signal as the β-axis component in the α-β coordinate system; The synchronous rotation angle is introduced to transform the α-axis component and the β-axis component in the α-β coordinate system into the d-axis component and the q-axis component in the dq coordinate system, and the single-phase signal after the virtual coordinate transformation is obtained.

[0008] Optionally, constructing a virtual signal that is strictly orthogonal in phase with the single-phase signal includes: delaying the single-phase signal by T / 4, i.e., lagging it by 90°, through an all-pass filter to obtain a virtual signal with a 90° lag.

[0009] Optionally, dual closed-loop control is performed on the single-phase signal after the virtual coordinate transformation, including: The filter capacitor voltage after the virtual coordinate transformation is input as a feedforward quantity into the current inner loop control loop to obtain a current inner loop reference value; The load current after the virtual coordinate transformation is added as a feedforward quantity into the voltage outer loop control loop to obtain the voltage outer loop reference value.

[0010] Optionally, the rotation coordinate transformation expression is: in, is the α-axis component, is the β-axis component, is the d-axis component, is the q-axis component, and θ is the synchronous rotation angular velocity.

[0011] Optionally, a complementary control signal of a fourth bridge arm switch is obtained through a fourth bridge arm control strategy to realize control of a three-level three-phase four-bridge arm inverter, including: Detecting a first capacitor voltage and a second capacitor voltage, taking a difference between the first capacitor voltage and the second capacitor voltage and passing the difference through a PI controller to obtain a set value of a first inductor current; Detecting a first inductor current, performing a subtraction between the detected first inductor current and a set value of the first inductor current and then passing the subtraction through a PI controller to obtain a fourth bridge arm modulation wave; A triangular wave is generated by a triangular wave generator, and the fourth bridge arm modulation wave is compared with the triangular wave to obtain complementary control signals of the first switch and the second switch, thereby realizing control of the midpoint potential offset of the three-level three-phase four-bridge arm inverter.

[0012] Optionally, the current setting value of the first inductor is expressed as: in, is the current setting value of the first inductor, , are the proportional and integral parameters of the PI controller respectively, s is the Laplace operator, is the first capacitor voltage, is the second capacitor voltage.

[0013] Optionally, the fourth bridge arm modulation wave expression is: in, is the modulation wave of the fourth bridge arm, , are the proportional and integral parameters of the PI controller respectively, is the current of the first inductor, s is the Laplace operator, is the set value of the first inductor current.

[0014] The present application provides an off-grid inverter system and control method that is resistant to three-phase unbalanced loads. Through the hardware circuit design of the fourth bridge arm, the midpoint potential is directly controlled, which can avoid the dependence of traditional software control methods on high-performance controllers, simplify the control logic and improve the response speed; using virtual coordinate transformation, the dq-axis components of the single-phase signal are independently separated and adjusted, which can reduce the algorithm complexity and the controller calculation burden; through the three-phase independent control method, an independent control loop is designed for each phase voltage, which can eliminate the phase-to-phase coupling effect, realize independent adjustment of each phase load, and significantly improve the system's operating stability under three-phase unbalanced loads; through the coordinated optimization of hardware and software, the complexity and manufacturing cost of the system are reduced, while the power quality and operating efficiency are improved, which can provide strong support for the development of off-grid wind power generation systems.

[0015] In order to make the above features and advantages of the invention more obvious and easy to understand, embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the related technologies, the drawings required for use in the embodiments or the related technical descriptions are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0017] Figure 1 This is a schematic diagram of the off-grid wind power generation system structure.

[0018] Figure 2 A schematic structural diagram of an off-grid inverter system for resisting three-phase unbalanced load provided in one embodiment of the present application.

[0019] Figure 3 This is a flow chart of an off-grid inverter control method for resisting three-phase unbalanced load provided in another embodiment of the present application.

[0020] Figure 4This is a block diagram of a virtual coordinate transformation control method for an off-grid inverter capable of resisting three-phase unbalanced load provided in another embodiment of the present application.

[0021] Figure 5 This is a flow chart of step S4 in the off-grid inverter control method for resisting three-phase unbalanced load provided in another embodiment of the present application.

[0022] Figure 6 This is a double closed-loop control block diagram of an off-grid inverter control method for resisting three-phase unbalanced load provided in another embodiment of the present application.

[0023] Figure 7 This is a flow chart of step S5 in the off-grid inverter control method for resisting three-phase unbalanced load provided in another embodiment of the present application.

[0024] Figure 8 This is a block diagram of the potential balance control of the fourth bridge arm midpoint in the off-grid inverter control method for resisting three-phase unbalanced load provided in another embodiment of the present application.

[0025] Fig. 9 A three-phase output voltage waveform diagram of a three-phase three-phase four-leg inverter under a three-phase unbalanced load in an off-grid inverter control method for resisting three-phase unbalanced load provided in another embodiment of the present application.

[0026] Fig.10 This is a waveform diagram of the midpoint potential offset of a three-level three-phase four-leg inverter under a three-phase unbalanced load in an off-grid inverter control method for resisting a three-phase unbalanced load provided in another embodiment of the present application.

[0027] Fig.11 This is a waveform diagram of three-phase phase voltage output by a three-level three-phase four-leg inverter with a single-phase load in an off-grid inverter control method for resisting three-phase unbalanced load provided in another embodiment of the present application.

[0028] Fig.12 This is a waveform diagram of the midpoint potential offset of a three-level three-phase four-leg inverter with a single-phase load in an off-grid inverter control method for resisting three-phase unbalanced load provided in another embodiment of the present application. DETAILED DESCRIPTION

[0029] In order to make the purpose and technical solution of the embodiment of the present application clearer, the technical solution of the embodiment of the present application will be clearly and completely described in conjunction with the drawings of the embodiment of the present application. Obviously, the described embodiment is a part of the embodiment of the present application, not all of the embodiments. Based on the described embodiment of the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0030] As an example, see Figure 1 The present application provides an off-grid inverter system for resisting three-phase unbalanced load, which may include: a wind turbine 1, a diode rectifier 2, a DC converter 3, a three-level three-phase four-bridge-arm inverter 4, and a three-phase unbalanced load 5. The wind turbine 1 is connected to a first end of the diode rectifier 2, a second end of the diode rectifier 2 is connected to a first end of the DC converter 3, a second end of the DC converter 3 is connected to a first end of the three-level three-phase four-bridge-arm inverter 4, and a second end of the three-level three-phase four-bridge-arm inverter 4 is connected to the three-phase unbalanced load 5.

[0031] In one embodiment, see Figure 2 The three-level three-phase four-bridge-arm inverter 4 may include: a T-type midpoint clamp three-level circuit 41, a fourth bridge-arm circuit 42, and a filter circuit 43. The first end of the fourth bridge-arm circuit 42 is connected to the DC voltage V output by the DC converter 3. dc The second end of the fourth bridge arm circuit 42 is connected to the first end of the T-type midpoint clamped three-level circuit 41, the second end of the T-type midpoint clamped three-level circuit 41 is connected to the first end of the filter circuit 43, and the second end of the filter circuit 43 is connected to the three-phase unbalanced load 5.

[0032] As an example, the fourth bridge arm circuit 42 includes: a switch T1, a switch T2, and an inductor L. The first end of the switch T1 is connected to a DC voltage V dc The first end of the switch T1 is connected to the first end of the switch T2, and the second end of the switch T2 is connected to the DC voltage V dc The second end of the inductor L is connected to the series midpoint of the switch T1 and the switch T2, the second end of the inductor L is connected to the first end of the T-type midpoint clamping three-level circuit 41, and the output current of the inductor L is .

[0033] As an example, the T-type midpoint clamp three-level circuit 41 includes: a capacitor C1, a capacitor C2, an A-phase bridge arm, a B-phase bridge arm, and a C-phase bridge arm, each phase bridge arm includes four switches, and the bridge arm side voltages of the three bridge arms are respectively , , Specifically, the first end of the capacitor C1 is connected to the first end of the switch T1, the second end of the capacitor C1 is connected to the first end of the capacitor C2, the second end of the capacitor C2 is connected to the second end of the switch T2, and the voltage across the capacitor C1 is , the voltage across capacitor C2 is The A-phase bridge arm includes a switch T3, a switch T4, a switch T9, and a switch T10, wherein a first end of the switch T3 is connected to a second end of the inductor L, a second end of the switch T3 is connected to a second end of the switch T4, a first end of the switch T4 is connected to a second end of the switch T9, a first end of the switch T9 is connected to a first end of the capacitor C1, a first end of the switch T10 is connected to a second end of the switch T9, and a second end of the switch T10 is connected to a second end of the capacitor C2; the B-phase bridge arm includes a switch T5, a switch T6, a switch T11, and a switch T12, wherein a first end of the switch T5 is connected to a second end of the inductor L, a second end of the switch T5 is connected to a second end of the switch T6, and a first end of the switch T6 is connected to a second end of the capacitor C1. The second end of the switch T11 is connected, the first end of the switch T11 is connected to the first end of the capacitor C1, the first end of the switch T12 is connected to the second end of the switch T11, and the second end of the switch T12 is connected to the second end of the capacitor C2; the C-phase bridge arm includes a switch T7, a switch T8, a switch T13, and a switch T14, wherein the first end of the switch T7 is connected to the second end of the inductor L, the second end of the switch T7 is connected to the second end of the switch T8, the first end of the switch T8 is connected to the second end of the switch T13, the first end of the switch T13 is connected to the first end of the capacitor C1, the first end of the switch T14 is connected to the second end of the switch T13, and the second end of the switch T14 is connected to the second end of the capacitor C2.

[0034] As an example, the fourth bridge arm circuit 42 realizes the midpoint potential balance by alternately turning on the switch T1 and the switch T2. Specifically, when the switch T1 is turned on, the switch T2 is in the off state, and the DC side capacitor C1, the switch T1 and the inductor L form a current loop. In this process, the energy stored in the capacitor C1 is released through the inductor L, causing the voltage of the capacitor C1 to gradually decrease; at the same time, the inductor L absorbs energy and its current Then it gradually increased.

[0035] Furthermore, when switch T2 is turned on, switch T1 is turned off, and the current loop is switched to the one consisting of the DC side capacitor C2, switch T2, and inductor L. Due to the freewheeling characteristics of the inductor, the energy stored in the inductor L is transferred to the capacitor C2, causing the voltage of the capacitor C2 to gradually increase; at the same time, the inductor L releases energy, and its current Gradually decrease.

[0036] As an example, from the perspective of energy transfer, inductor L can be used as a medium for energy exchange, and its core function is to promote the dynamic transfer of energy between capacitor C1 and capacitor C2. Through a reasonable control strategy, adjusting the conduction timing and duty cycle of switch T1 and switch T2, the energy exchange between capacitor C1 and capacitor C2 can reach a dynamic equilibrium state, that is, the voltages of the two tend to be equal, achieving precise control of the midpoint potential, and effectively suppressing the deviation of the midpoint potential, thereby improving the stability and reliability of the system.

[0037] As an example, the filter circuit 43 includes: an A-phase sub-filter circuit, a B-phase sub-filter circuit, and a C-phase sub-filter circuit. Specifically, the A-phase sub-filter circuit includes a resistor ,inductance ,capacitance , where the resistance The first end of the resistor is connected to the midpoint of the A-phase bridge arm, that is, the midpoint of the series connection of switch T4 and switch T9. The second end of the inductor is connected The first end of the inductor The second end of the capacitor is connected to the three-phase unbalanced load 5. The first end of the inductor is connected The second end of the capacitor The second end of the inductor is grounded. The output A phase inductor current is , flows into the capacitor The capacitor current of phase A is ,capacitance The output capacitor voltage of phase A is , the output A phase load current is The B-phase sub-filter circuit includes resistors ,inductance ,capacitance , where the resistance The first end of the resistor is connected to the midpoint of the B-phase bridge arm, that is, the midpoint of the series connection of switch T6 and switch T11. The second end of the inductor is connected The first end of the inductor The second end of the capacitor is connected to the three-phase unbalanced load 5. The first end of the inductor is connected The second end of the capacitor The second end of the inductor is grounded. The output B phase inductor current is , flows into the capacitor The B phase capacitor current is ,capacitance The output B phase capacitor voltage is , the output B phase load current is The C-phase sub-filter circuit includes resistors ,inductance ,capacitance , where the resistance The first end of the resistor is connected to the midpoint of the C-phase bridge arm, that is, the midpoint of the series connection of switch T8 and switch T13. The second end of the inductor is connected The first end of the inductor The second end of the capacitor is connected to the three-phase unbalanced load 5. The first end of the inductor is connected The second end of the capacitor The second end of the inductor is grounded. The output C phase inductor current is , flows into the capacitor The C phase capacitor current is ,capacitance The output C phase capacitor voltage is , the output C phase load current is .

[0038] As an example, the wind turbine generator 1 may be a permanent magnet synchronous generator; the diode rectifier 2 may include six high-power diodes to form a three-phase full-bridge topology structure.

[0039] As an example, the DC converter 3 may be a Boost DC converter; the DC converter 3 includes: an inductor L1, a diode D, a switch T15, and a capacitor C. The first end of the inductor L1 is connected to the first end of the diode rectifier 2, the second end of the inductor L1 is connected to the first end of the switch T15, the second end of the switch T15 is connected to the second end of the diode rectifier 2, the first end of the diode D is connected to the second end of the inductor L1, the second end of the diode D is connected to the first end of the capacitor C, the second end of the capacitor C is connected to the second end of the diode rectifier 2, and the voltage across the capacitor C is , the DC voltage V is connected in parallel across the capacitor C dc .

[0040] As an example, the wind turbine 1 generates three-phase AC power, which is converted into DC power by the diode rectifier 2, and then adjusted to a DC voltage level matching the energy storage system by the DC converter 3, thereby providing a stable DC source for the subsequent inverter link. Since the DC bus voltage is a constant value, the output power of the wind turbine 1 can be flexibly adjusted by adjusting the conduction time of the switch device in the DC converter 3, thereby maximizing the efficiency of wind energy utilization.

[0041] As an example, the diode rectifier 2 is used to rectify the three-phase AC output by the wind turbine 1 to obtain a DC voltage, and the DC converter 3 is used to keep the DC voltage obtained by the wind turbine 1 through the diode rectifier 2 constant. The three-level three-phase four-bridge-arm inverter 4 is used to perform virtual coordinate transformation on the output voltage and current to perform three-phase independent control, while ensuring the elimination of interphase coupling effects, achieving independent regulation of each phase load, and improving the operating stability and power quality of the off-grid wind power generation system under unbalanced load conditions, and realizing the control of the midpoint potential, so as to avoid the dependence of traditional software control methods on high-performance controllers, simplify the control logic and improve the response speed, ensure that the midpoint potential is quickly stabilized when the load changes dynamically, and further improve the system reliability.

[0042] In the above-mentioned off-grid inverter system for resisting three-phase unbalanced load, by setting the fourth bridge arm circuit 42 and the T-type midpoint clamping three-level circuit 41 to work together, the three-phase unbalanced load problem can be effectively dealt with. The switch T1 and the switch T2 in the fourth bridge arm circuit 42 are alternately turned on, which can achieve precise control of the midpoint potential, effectively suppress the deviation of the midpoint potential, and further improve the stability and reliability of the system.

[0043] In another embodiment, see Figure 3 The present application also provides an off-grid inverter control method for resisting three-phase unbalanced load. The off-grid inverter control method for resisting three-phase unbalanced load may include the following steps: step S1 to step S5.

[0044] Step S1: Separate the three-phase signal output by the three-level three-phase four-bridge arm inverter to obtain a single-phase signal, and perform virtual coordinate transformation on the single-phase signal to obtain a single-phase signal after virtual coordinate transformation; the single-phase signal after virtual coordinate transformation includes a three-phase output voltage, a filter capacitor voltage, a load current, and a filter inductor current.

[0045] Step S2: performing double closed-loop control on the single-phase signal after the virtual coordinate transformation.

[0046] Step S3: Perform a rotation coordinate transformation on the single-phase signal after the double closed-loop control to obtain a single-phase signal in an α-β coordinate system, and take the α-axis component as the modulation wave corresponding to the single-phase signal.

[0047] Step S4: input the modulation waves of the three single-phase signals into the SPWM modulation module respectively to obtain the switch driving signal.

[0048] Step S5: Obtaining a complementary control signal of the fourth bridge arm switch through the fourth bridge arm control strategy to achieve control of the midpoint potential offset of the three-level three-phase four-bridge arm inverter.

[0049] In the off-grid inverter control method for resisting three-phase unbalanced load of the present application, the three-phase signal output by the three-level three-phase four-bridge-arm inverter is separated into a single-phase signal, and virtual coordinate transformation, double closed-loop control and rotation coordinate transformation are performed, and finally a modulation wave of the single-phase signal in the α-β coordinate system is obtained; the modulation wave is input into the SPWM modulation module to obtain the switch drive signal, and combined with the fourth bridge arm control strategy, it can effectively cope with the three-phase unbalanced load, improve the output power quality of the inverter, enhance the stability and reliability of the system, reduce the harmonic content, and ensure the normal and stable operation of electrical equipment.

[0050] In step S1, see Figure 3In step S1, the three-phase signal output by the three-level three-phase four-bridge-arm inverter is separated to obtain a single-phase signal, and the single-phase signal is subjected to virtual coordinate transformation to obtain a single-phase signal after virtual coordinate transformation; the single-phase signal after virtual coordinate transformation includes a filter capacitor voltage, a load current, and a filter inductor current.

[0051] As an example, the three-phase AC power output by the wind turbine 1 is processed by the diode rectifier 2. When the three-phase AC voltage output by the wind turbine 1 is applied to the input end of the diode rectifier 2, the diode is turned on under forward bias and turned off under reverse bias. Through this periodic on and off switching, the input sinusoidal AC power is converted into DC power.

[0052] Furthermore, the DC converter 3 outputs a DC side capacitor voltage Closed-loop control is performed. Specifically, the DC side capacitor voltage With the given DC link capacitor voltage reference value The difference between the two is input to the PI controller to generate a PWM modulation wave. The expression is: in, is the switching modulation signal of the DC converter, , are the proportional and integral parameters of the PI controller respectively.

[0053] Furthermore, the duty cycle generated by the PWM modulation wave is used to control the on and off of the switch in the DC converter to keep the DC voltage constant. The three-level three-phase four-bridge-leg inverter continuously outputs a three-phase symmetrical voltage with stable amplitude and frequency.

[0054] As an example, the output voltage under balanced load conditions can be converted into a DC quantity in a dq synchronous rotating coordinate system after Clark and Park coordinate transformation. This feature allows the system to use traditional PI control strategies to accurately track the reference signal and ensure that the steady-state error is zero. When the load is unbalanced, the output voltage and current will contain positive sequence, negative sequence, and zero sequence components at the same time, and the system needs to uniformly adjust them. The expression is as follows: Among them, e a 、e b 、e c Represents the voltage components of phase A, phase B, and phase C respectively, and E mRepresents the voltage amplitude, and the superscripts P, N, and 0 represent the positive sequence, negative sequence, and zero sequence components, respectively. This method has problems such as complex algorithm, large amount of calculation, and slow dynamic response. Especially when the load changes suddenly, it is difficult to quickly track the load change, resulting in output voltage asymmetry and current distortion. In a three-phase power system, the voltage and current of each phase show obvious time-varying characteristics and symmetrical distribution characteristics in the stationary abc coordinate system. Specifically, the voltage and current of each phase not only change periodically with time, but also, under ideal conditions, the three phases maintain a strict symmetrical relationship with equal amplitude and a phase difference of 120°. By applying Clark transform and Park transform, these time-varying AC quantities can be converted from the abc coordinate system to the synchronously rotating dq coordinate system. In this new coordinate system, the AC quantity that originally changes sinusoidally with time is converted into a DC quantity synchronized with the fundamental frequency of the power grid: Among them, u d 、u q They represent the voltage DC components obtained by Park transformation under the condition of constant amplitude, i d 、i q They represent the corresponding DC components of the current through Park transformation under the condition of unchanged amplitude, , , is the three-phase voltage on the bridge arm side of the three bridge arms, , , is the three-phase current corresponding to the bridge arm side of the three bridge arms, is the synchronous rotation angular velocity. This transformation converts the time variable in the three-phase AC system into a DC quantity in the synchronous rotation coordinate system, thereby simplifying the design of the control system. For the converted DC quantity, a PI controller can be used to achieve precise tracking control, which can not only ensure the dynamic response performance of the system, but also effectively eliminate steady-state errors.

[0055] Based on the above principle, the three-phase signal output by the three-level three-phase four-bridge-arm inverter is separated to obtain a single-phase signal, and the single-phase signal is subjected to virtual coordinate transformation respectively, so as to establish a mathematical model under an independent single-phase dq rotating coordinate system. In the present application, the three-phase signal output by the three-level three-phase four-bridge-arm inverter in step S1 includes: three-phase output voltage, filter capacitor voltage, load current, and filter inductor current.

[0056] Specifically, step S1 may include the following steps: separating the three-phase signal output by the three-level three-phase four-bridge-arm inverter to obtain a single-phase signal; constructing a virtual signal that is strictly orthogonal in phase with the single-phase signal; using the single-phase signal as the α-axis component in the α-β coordinate system, and using the virtual signal as the β-axis component in the α-β coordinate system; introducing a synchronous rotation angle, transforming the α-axis component and the β-axis component in the α-β coordinate system into the d-axis component and the q-axis component in the dq coordinate system, and obtaining the single-phase signal after the virtual coordinate transformation.

[0057] As an example, a single-phase signal includes: A phase output voltage u a , A phase filter capacitor voltage , A phase load current , A phase inductor current ;B phase output voltage u b , B phase filter capacitor voltage , B phase load current , B phase inductor current ;C phase output voltage u c , C phase filter capacitor voltage , C phase load current , C phase load current .

[0058] As an example, the single-phase signal after the virtual coordinate transformation includes: A-phase output voltage d-axis component , A phase output voltage q-axis component , A phase filter capacitor voltage d-axis component , q-axis component of the A-phase filter capacitor voltage , A phase load current d-axis component , q-axis component of phase A load current , A phase inductor current d-axis component , q-axis component of phase A inductor current ; B phase output voltage d-axis component , B phase output voltage q-axis component , B phase filter capacitor voltage d-axis component , q-axis component of the B-phase filter capacitor voltage , B phase load current d-axis component , q-axis component of phase B load current , B phase inductor current d-axis component , q-axis component of the B-phase inductor current ; C phase output voltage d-axis component , C phase output voltage q-axis component , C phase filter capacitor voltage d-axis component , q-axis component of C-phase filter capacitor voltage , C phase load current d-axis component , q-axis component of C-phase load current , C phase inductor current d-axis component , C phase inductor current q-axis component .

[0059] The following specifically introduces step S1 by taking the A phase signal as an example.

[0060] As an example, see Figure 4 , constructing a virtual AC quantity that is strictly orthogonal to the original A-phase signal in phase, which can not only maintain the amplitude characteristics of the original signal, but also ensure that the phase difference between the virtual signal and the original signal is constant at 90°.

[0061] Specifically, the all-pass filter is used to lag the A-phase signal by 90° (i.e., delay T / 4) to obtain a 90° lagged A-phase virtual signal. The transfer function of the all-pass filter in the s domain is The expression is: Among them, f is the fundamental frequency, which is 50Hz, and s is the Laplace operator.

[0062] As an example, assume that the output voltage of phase A of the three-level three-phase four-bridge-arm inverter is u a , the expression is: in, is the voltage amplitude, is the angular frequency, is the phase angle, For time.

[0063] As an example, the A phase output voltage is filtered through an all-pass filter. Delay T / 4, that is, 90° lag, to obtain a 90° lagging virtual output voltage of phase A : in, is the voltage amplitude, is the angular frequency, is the phase angle, For time.

[0064] Furthermore, the original A phase output voltage As the α-axis component in the α-β coordinate system , the virtual output voltage of phase A after 90° lag Considered as the β-axis component , introduce θ as the synchronous rotation angular velocity, and convert the α-axis component and β-axis component in the α-β coordinate system Convert to the dq coordinate system and obtain the voltage signals in the synchronous rotating coordinate system and the stationary coordinate system: in, is the d-axis voltage component, is the q-axis voltage component, is the α-axis voltage component, is the β-axis voltage component, and θ is the synchronous rotation angular velocity.

[0065] Similarly, the current signal in the synchronous rotating coordinate system and the stationary coordinate system can be obtained as: in, is the d-axis voltage component, is the q-axis voltage component, is the α-axis voltage component, is the β-axis voltage component, and θ is the synchronous rotation angular velocity.

[0066] Similarly, the B-phase voltage signal, B-phase current signal, C-phase voltage signal, and C-phase current signal in the dq coordinate system can be obtained. The virtual coordinate transformation not only simplifies the control design of the single-phase system, but also facilitates the dynamic performance analysis and optimization of the system. By converting AC quantities into DC quantities, various control strategies can be applied more effectively to achieve precise regulation and stable operation of the system.

[0067] In step S2, see Figure 3 In step S2, double closed-loop control is performed on the single-phase signal after the virtual coordinate transformation.

[0068] As an example, see Figure 5 , step S2 may include the following steps: step S21~step S22.

[0069] Step S21: Input the filter capacitor voltage after the virtual coordinate transformation as the feedforward quantity into the current inner loop control loop to obtain the current inner loop reference value and perform the filter inductor current , Decoupling control.

[0070] Step S22: adding the load current after the virtual coordinate transformation as a feedforward quantity into the voltage outer loop control loop to obtain a voltage outer loop reference value, and performing decoupling control on the filter capacitor voltage after the virtual coordinate transformation.

[0071] As an example, the d-axis components of the filter capacitor voltage and inductor current are , The expression in the complex frequency domain is: in, is the d-axis component of the filter capacitor voltage, is the q-axis component of the filter capacitor voltage, is the d-axis component of the voltage on the bridge arm side of the three-level three-phase four-bridge-arm inverter, is the d-axis component of the inductor current, is the d-axis component of the load current, is the q-axis component of the inductor current, is the filter capacitor value, is the filter inductance value, is the equivalent resistance of the filter inductor, is the d-axis fundamental angular frequency, and s is the Laplace operator.

[0072] Similarly, the q-axis components of the filter capacitor voltage and inductor current can be obtained: , The expression in the complex frequency domain is: in, is the d-axis filter capacitor voltage, is the q-axis component of the filter capacitor voltage, is the q-axis component of the voltage on the bridge arm side of the three-level three-phase four-bridge-arm inverter, is the d-axis component of the inductor current, is the q-axis component of the load current, is the q-axis component of the inductor current, is the filter capacitor value, is the filter inductance value, is the equivalent resistance of the filter inductor, is the d-axis fundamental angular frequency, and s is the Laplace operator.

[0073] As an example, from the expressions of the d-axis component in the complex frequency domain and the q-axis component in the complex frequency domain, it can be seen that after the rotation transformation, the system forms a significant cross-coupling relationship between the d-axis and the q-axis. Specifically, the output inductor current , Not only affected by the voltage on the bridge arm side of the three-level three-phase four-bridge-arm inverter , Direct regulation is also affected by the coupling voltage introduced by the rotating coordinate system , And the filter capacitor voltage , At the same time, the filter capacitor voltage , In the controlled inductor current , At the same time, it is also affected by the coupled current component , and load current , This complex multivariable coupling relationship makes it impossible to completely decouple the control loops of the d-axis and q-axis. Appropriate decoupling compensation strategies must be introduced to eliminate inter-axis interference to ensure that each axis can be accurately and independently controlled.

[0074] As an example, the double closed-loop control block diagram is as follows Figure 6 As shown, the following takes signal A as an example to specifically introduce steps S21-S22.

[0075] Specifically, in step S21, the d-axis component of the A-phase filter capacitor voltage after the virtual coordinate transformation is , q-axis component of the A-phase filter capacitor voltage As a feedforward quantity, it is introduced into the current inner loop control loop to obtain the current inner loop reference value, so as to realize the d-axis component of the filter inductor current. , q-axis component of filter inductor current The decoupling control expression is: in, is the given reference value d-axis component of the inductor current, is the given reference value q-axis component of the inductor current, , They correspond to the proportional and integral coefficients of the current inner loop PI regulator, is the d-axis component of the filter capacitor voltage, is the q-axis component of the filter capacitor voltage, is the d-axis component of the voltage on the bridge arm side of the three-level three-phase four-bridge-arm inverter, is the q-axis component of the voltage on the bridge arm side of the three-level three-phase four-bridge-arm inverter, is the d-axis component of the inductor current, is the d-axis component of the load current, is the q-axis component of the inductor current, is the d-axis fundamental angular frequency, is the filter inductance value.

[0076] As an example, the proportional and integral coefficients of the current inner loop PI regulator are , It can be calculated by the following expression: in, is the filter inductance value, is the equivalent resistance of the filter inductor, is the switching cycle, .

[0077] Further, in step S22, the d-axis component of the A-phase load current after the virtual coordinate transformation is , q-axis component of phase A load current As a feedforward quantity, it is added into the voltage outer loop control loop to obtain the voltage outer loop reference value, so as to realize the d-axis component of the filter capacitor voltage. , q-axis component of filter capacitor voltage The decoupling control expression is: in, is the given reference value d-axis component of the capacitor voltage, is the q-axis component of the capacitor voltage given reference value, , They correspond to the proportional and integral coefficients of the voltage outer loop PI regulator, is the d-axis component of the filter capacitor voltage, is the q-axis component of the filter capacitor voltage, is the d-axis component of the inductor current, is the d-axis component of the load current, is the q-axis component of the load current, is the q-axis component of the inductor current, is the d-axis fundamental angular frequency, is the filter capacitor value.

[0078] As an example, the proportional and integral coefficients of the voltage outer loop PI regulator are , It can be calculated by the following expression: in, is the filter inductance value, is the filter capacitor value, , They correspond to the proportional and integral coefficients of the current inner loop PI regulator, , .

[0079] As an example, by adopting a feedforward decoupling control strategy and introducing corresponding compensation terms in the control loop, the mutual interference between the d-axis and the q-axis can be effectively eliminated, and ultimately the independent and precise control of the single-phase output variable in the dq coordinate system can be achieved. This dual closed-loop control method can effectively offset the cross-coupling terms introduced by the rotating coordinate system, which can not only improve the dynamic response performance of the system, but also enhance the system's anti-interference ability, providing a reliable guarantee for achieving high-quality inverter output.

[0080] In step S3, see Figure 3 In step S3, the single-phase signal after the double closed-loop control is subjected to a rotation coordinate transformation to obtain a single-phase signal in an α-β coordinate system, and the α-axis component is taken as a modulation wave corresponding to the single-phase signal.

[0081] As an example, the modulation wave of the single-phase signal includes: a modulation wave of an A-phase signal, a modulation wave of a B-phase signal, and a modulation wave of a C-phase signal.

[0082] Specifically, taking the A-phase signal as an example, the d-axis component of the A-phase voltage signal on the bridge arm side of the three-level three-phase four-bridge-arm inverter after double closed-loop control is , A phase voltage q-axis component After the rotation coordinate transformation, the voltage component of phase A in the α-β coordinate system is obtained , , the expression is as follows: in, is the α-axis voltage component, is the β-axis voltage component, is the d-axis component of phase A voltage The matrix of is the q-axis component of phase A voltage , θ is the synchronous rotation angular velocity.

[0083] Furthermore, take the α-axis voltage component As the modulation wave of the A phase signal .

[0084] As an example, the modulation wave of the B phase signal can be obtained by following steps S1 to S3. , and the modulation wave of the C phase signal .

[0085] In step S4, see Figure 3 In step S4, the modulation waves of the three single-phase signals are respectively input into the SPWM modulation module to obtain the switch drive signal.

[0086] Specifically, the modulation wave of the three-phase signal , , The SPWM modulation module generates a high-frequency triangular carrier signal with a fixed frequency and amplitude. , , The inverter is used to compare the high-frequency triangular carrier signal with the high-frequency triangular carrier signal. When the amplitude of the modulation wave is greater than the amplitude of the high-frequency triangular carrier signal, a high-level drive signal is output; when the amplitude of the modulation wave is less than the amplitude of the high-frequency triangular carrier signal, a low-level drive signal is output. The switch is turned on and off according to the high and low level states of the drive signal. In this way, the inverter can convert the DC power supply into AC output, and the amplitude, frequency and phase of the output AC voltage can be controlled by the modulation wave.

[0087] In step S5, refer to Figure 3 In step S5, a complementary control signal of the fourth bridge arm switch is obtained through the fourth bridge arm control strategy to realize control of the three-level three-phase four-bridge arm inverter.

[0088] As an example, see Figure 7 , step S5 may include the following steps: step S51 to step S53.

[0089] Step S51: Detecting capacitor voltage and The difference between the two voltage values ​​is obtained by the PI controller to obtain the set value of the fourth bridge arm inductor current. .

[0090] Step S52: Detect the fourth bridge arm inductor current , the detected fourth bridge arm inductor current The setting value of the fourth bridge arm inductor current After the difference is made, the modulation wave of the fourth bridge arm is obtained through the PI controller.

[0091] Step S53: Generate a triangular wave through a triangular wave generator, compare the fourth bridge arm modulated wave with the triangular wave, and obtain a complementary control signal of the fourth bridge arm switch.

[0092] As an example, Figure 8 The fourth bridge arm midpoint potential balance control block diagram is shown in step S51. In step S52, the voltage of capacitor C1 is detected. and the voltage across capacitor C2 , the voltage and voltage Make a difference and change the voltage and voltage The difference is input into the PI controller to calculate the set value of the fourth bridge arm inductor current , to reduce the difference between the two capacitor voltages, the expression is as follows: in, , are the proportional and integral parameters of the PI controller respectively, and s is the Laplace operator.

[0093] Further, in step S52, the fourth bridge arm inductor current is detected , the detected current of the fourth bridge arm inductor L The setting value of the fourth bridge arm inductor current Compare the two and input the difference into the PI controller to obtain the modulation wave of the fourth bridge arm , to further refine the modulation signal and ensure that the fourth bridge arm inductor current To more precisely track its set value, the expression is as follows: in, , are the proportional and integral parameters of the PI controller respectively, and s is the Laplace operator.

[0094] Further, in step S53, a triangular wave with a set frequency is generated by a triangular wave generator, and the refined fourth bridge arm modulation wave Comparing with the triangular wave, the complementary control signals of switch T1 and switch T2 are obtained, and the on and off states of the switch tube are accurately controlled to realize the fourth bridge arm inductor current The fourth bridge arm control strategy can effectively manage the energy flow between the fourth bridge arm capacitors to ensure the voltage and voltage Maintaining the desired balance state can improve the stability of the system and optimize the efficiency of electrical energy utilization, which is crucial to the performance of power electronic equipment.

[0095] In one example, the present application also builds a simulation model of an off-grid wind power generation system inverter that is resistant to three-phase unbalanced load in Matlab / Simulink simulation software. Fig. 9 The three-phase output voltage waveform of the three-level three-phase four-bridge-arm inverter under the condition of three-phase unbalanced load. Fig.10 This is a waveform diagram of the midpoint potential deviation of a three-level three-phase four-bridge inverter with a three-phase unbalanced load. The three-phase unbalanced load and the single-phase unbalanced load are respectively put into operation at 0.5s. Fig. 9 The three-phase output voltage waveform of the inverter under three-phase unbalanced load conditions is shown. Fig.10The dynamic offset characteristics of the midpoint potential when the load is unbalanced are presented. It can be clearly observed that after adopting the control method proposed in this application, the dynamic response performance of the off-grid wind power generation system under load mutation conditions has been significantly improved. The three-level three-phase four-bridge-arm inverter can quickly suppress the fluctuation of the midpoint potential, so that it can quickly restore the stable state during the dynamic change of the load, thereby effectively improving the operational reliability of the off-grid wind power generation system. Even under severe three-phase load imbalance conditions, the three-level three-phase four-bridge-arm inverter can still continuously output a three-phase symmetrical voltage with stable amplitude and frequency.

[0096] In another example, the present application also simulates the most serious three-phase unbalanced load operating condition, that is, the single-phase load (the other two-phase loads are both disconnected) operating condition in Matlab / Simulink simulation software. Fig.11 The waveform diagram of the midpoint potential deviation of the three-level three-phase four-bridge-leg inverter under the condition of three-phase unbalanced load is shown in Figure 2. Fig.12 The waveform diagram of the midpoint potential deviation of a three-level three-phase four-bridge-arm inverter with a single-phase load. It can be seen that even in the face of the most severe three-phase unbalanced load condition, the wind power generation system inverter can still ensure that the midpoint potential is quickly stabilized when the load changes dynamically and can continuously output a three-phase symmetrical voltage with stable amplitude and frequency after the control method proposed in this application.

[0097] In the off-grid inverter control method for resisting three-phase unbalanced load of the present application, the dq-axis components of the single-phase signal are independently separated and adjusted through virtual coordinate transformation, which can avoid the redundant calculations caused by coordinate transformation in the traditional three-phase unbalanced load problem and reduce the algorithm complexity and controller calculation burden; through the voltage and current dual closed-loop control method of three-phase independent control, an independent control loop is designed for each phase, the phase coupling effect is eliminated, and the independent adjustment of each phase load is realized, which can significantly improve the dynamic response capability of the system under load mutation conditions; through the fourth bridge arm control strategy, the midpoint potential is directly controlled to ensure that the midpoint potential is fast and stable when the load changes dynamically, further improving the system reliability; through the coordinated optimization of hardware and software, the complexity and manufacturing cost of the system are reduced, while the power quality and operating efficiency are improved, filling the deficiencies of the existing technology in off-grid wind power generation systems; it can make full use of wind resources, reduce dependence on traditional energy, and thus reduce energy costs.

[0098] It should be understood that, although the steps in the flowchart of the accompanying drawings are displayed in sequence as indicated by the arrows, these steps are not necessarily executed in sequence in the order indicated by the arrows. Unless otherwise specified herein, there is no strict order restriction on the execution of these steps, and these steps can be executed in other orders. Moreover, at least a portion of the steps in the accompanying drawings may include multiple sub-steps or multiple stages, and these sub-steps or stages are not necessarily executed at the same time, but can be executed at different times, and the execution order of these sub-steps or stages is not necessarily sequential, but can be executed in turn or alternately with other steps or at least a portion of the sub-steps or stages of other steps.

[0099] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0100] Although the present application has been disclosed as above with the embodiments, it is not intended to limit the present application. Any person with ordinary knowledge in the technical field can make some changes and modifications without departing from the spirit and scope of the present application. Therefore, the scope of protection of the present application shall be determined by the scope of the attached patent application.

Claims

1. An off-grid inverter system for resisting three-phase unbalanced load, characterized in that: include: A wind turbine, a diode rectifier, a DC converter, a three-level three-phase four-bridge-arm inverter, and a three-phase unbalanced load, wherein the wind turbine is connected to a first end of the diode rectifier, a second end of the diode rectifier is connected to a first end of the DC converter, a second end of the DC converter is connected to a first end of the three-level three-phase four-bridge-arm inverter, and a second end of the three-level three-phase four-bridge-arm inverter is connected to the three-phase unbalanced load; The three-level three-phase four-bridge-arm inverter comprises: a fourth bridge-arm circuit, a first end of the fourth bridge-arm circuit being connected to a DC voltage output by the DC converter; A T-type midpoint clamped three-level circuit, wherein a first end of the T-type midpoint clamped three-level circuit is connected to a second end of the fourth bridge arm circuit, and the T-type midpoint clamped three-level circuit comprises three bridge arms; A filter circuit, wherein a first end of the filter circuit is connected to a second end of the T-type midpoint clamped three-level circuit, and a second end of the filter circuit is connected to a three-phase unbalanced load.

2. The off-grid inverter system for resisting three-phase unbalanced load according to claim 1, characterized in that: The fourth bridge arm circuit comprises: a first switch, wherein a first end of the first switch is connected to a first end of a DC voltage; a second switch, wherein a first end of the second switch is connected to a second end of the first switch, and a second end of the second switch is connected to a second end of a DC voltage; A first inductor, wherein a first end of the first inductor is connected to a series midpoint of a first switch and a second switch, and a second end of the first inductor is connected to a first end of a T-type midpoint clamped three-level circuit.

3. A control method for an off-grid inverter capable of resisting three-phase unbalanced load, controlling the off-grid inverter system capable of resisting three-phase unbalanced load as claimed in any one of claims 1 to 2, characterized in that: The following steps are involved: The three-phase signal output by the three-level three-phase four-bridge-arm inverter is separated to obtain a single-phase signal, and the single-phase signal is subjected to virtual coordinate transformation to obtain a single-phase signal after virtual coordinate transformation; the single-phase signal after virtual coordinate transformation includes a three-phase output voltage, a filter capacitor voltage, a load current, and a filter inductor current; Performing double closed-loop control on the single-phase signal after the virtual coordinate transformation; Performing a rotation coordinate transformation on the single-phase signal after the double closed-loop control to obtain a single-phase signal in the α-β coordinate system, and taking the α-axis component as the modulation wave of the corresponding single-phase signal; The modulation waves of the three single-phase signals are respectively input into the SPWM modulation module to obtain the switch drive signal; Through the fourth bridge arm control strategy, a complementary control signal of the fourth bridge arm switch is obtained to realize the control of the midpoint potential offset of the three-level three-phase four-bridge arm inverter.

4. The off-grid inverter control method for resisting three-phase unbalanced load according to claim 3, characterized in that: The three-phase signals output by the three-level three-phase four-bridge-arm inverter are separated to obtain single-phase signals, and the single-phase signals are respectively subjected to virtual coordinate transformation to obtain single-phase signals after virtual coordinate transformation, including: Separate the three-phase signal output by the three-level three-phase four-bridge-arm inverter to obtain a single-phase signal; Constructing a virtual signal that is strictly orthogonal in phase with the single-phase signal; Taking the single-phase signal as the α-axis component in the α-β coordinate system, and taking the virtual signal as the β-axis component in the α-β coordinate system; The synchronous rotation angle is introduced to transform the α-axis component and the β-axis component in the α-β coordinate system into the d-axis component and the q-axis component in the dq coordinate system, and the single-phase signal after the virtual coordinate transformation is obtained.

5. The off-grid inverter control method for resisting three-phase unbalanced load according to claim 4, characterized in that: Constructing a virtual signal that is strictly orthogonal to the single-phase signal in phase, including: delaying the single-phase signal by T / 4 through an all-pass filter, that is, lagging it by 90°, to obtain a virtual signal with a 90° lag.

6. The off-grid inverter control method for resisting three-phase unbalanced load according to claim 3, characterized in that: The single-phase signal after the virtual coordinate transformation is subjected to double closed-loop control, including: The filter capacitor voltage after the virtual coordinate transformation is input as a feedforward quantity into the current inner loop control loop to obtain a current inner loop reference value; The load current after the virtual coordinate transformation is added as a feedforward quantity into the voltage outer loop control loop to obtain the voltage outer loop reference value.

7. The off-grid inverter control method for resisting three-phase unbalanced load according to claim 3, characterized in that: The rotation coordinate transformation expression is: in, is the α-axis component, is the β-axis component, is the d-axis component, is the q-axis component, and θ is the synchronous rotation angular velocity.

8. The off-grid inverter control method for resisting three-phase unbalanced load according to claim 3, characterized in that: Through the fourth bridge arm control strategy, a complementary control signal of the fourth bridge arm switch is obtained to realize the control of the midpoint potential offset of the three-level three-phase four-bridge arm inverter, including: Detecting a first capacitor voltage and a second capacitor voltage, taking a difference between the first capacitor voltage and the second capacitor voltage and passing the difference through a PI controller to obtain a set value of a first inductor current; Detecting a first inductor current, performing a subtraction between the detected first inductor current and a set value of the first inductor current and then passing the subtraction through a PI controller to obtain a fourth bridge arm modulation wave; A triangular wave is generated by a triangular wave generator, and the fourth bridge arm modulation wave is compared with the triangular wave to obtain complementary control signals of the first switch and the second switch, thereby realizing control of the three-level three-phase four-bridge arm inverter.

9. The off-grid inverter control method for resisting three-phase unbalanced load according to claim 8, characterized in that: The set value expression of the first inductor current is: in, is the set value of the first inductor current, , are the proportional and integral parameters of the PI controller respectively, s is the Laplace operator, is the first capacitor voltage, is the second capacitor voltage.

10. The off-grid inverter control method for resisting three-phase unbalanced load according to claim 8, characterized in that: The modulation wave expression of the fourth bridge arm is: in, is the modulation wave of the fourth bridge arm, , are the proportional and integral parameters of the PI controller respectively, is the first inductor current, s is the Laplace operator, is the set value of the first inductor current.

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