A virtual impedance control system and method for parallel single-phase inverters

Abstract

The application discloses a kind of single-phase inverter parallel virtual impedance control system and method, inductance current sampling value is generated single-phase inverter corresponding virtual impedance voltage signal by second-order generalized integrator (SOGI), dq coordinate transformer, virtual impedance, anti-Clark transformer;Virtual impedance voltage signal is introduced into droop controller, and the voltage and angular frequency reference instruction of inverter is obtained, and the reference instruction is tracked and controlled by voltage and current double closed-loop controller, finally realizes the stable operation and power sharing of single-phase inverter parallel system.The application can effectively construct virtual impedance in single-phase inverter parallel system, improve the power sharing accuracy and stability of parallel system, and be applicable to distributed power generation, microgrid and uninterruptible power supply and other multi-machine parallel occasions.

Description

Technical Field

[0001] This invention relates to the field of parallel control technology for single-phase inverters, and particularly to a virtual impedance control system and method for parallel single-phase inverters. Background Technology

[0002] In the field of power electronics, parallel operation of single-phase inverters is an important method for constructing modular uninterruptible power supplies, distributed generation systems, and microgrids. In such systems, droop control strategies are typically introduced to achieve power sharing and stable operation among the parallel units.

[0003] However, due to slight differences in the output impedance of each inverter module, inconsistent line parameters, and load disturbances, it is difficult to achieve precise power distribution using only traditional droop control, and circulating currents are easily generated between parallel units. Circulating currents between inverters can lead to increased switching losses, accelerated aging of power devices, and affect system stability, and may even damage switching transistors, increasing the failure rate.

[0004] Introducing virtual impedance is an effective solution to suppress circulating current and improve power distribution characteristics. However, for parallel single-phase inverter systems, since the output voltage and current are single phasors and there is no naturally formed orthogonal coordinate system, the virtual impedance design method based on the rotating coordinate system in three-phase systems cannot be directly applied. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a virtual impedance control system and method for parallel single-phase inverters. It effectively constructs virtual impedance in parallel single-phase inverter systems, improves the power sharing accuracy and stability of parallel systems, and is applicable to multi-machine parallel applications such as distributed generation, microgrids, and uninterruptible power supplies.

[0006] The objective of this invention is achieved in one aspect as follows: a virtual impedance control system for parallel single-phase inverters, comprising a signal conditioning circuit, a second-order generalized integrator, a dq coordinate transformer, a virtual impedance, an inverse Clark transformer, a droop controller, a voltage and current dual closed-loop controller, and an SPWM generator.

[0007] The signal conditioning circuit is used to acquire the inductor current sampling value of the single-phase inverter in real time and send the inductor current sampling value to the second-order generalized integrator.

[0008] The second-order generalized integrator is used to generate α-axis current components and β-axis current from the sampled inductor current values, and to combine the α-axis current components and β-axis current components to form a current signal in the αβ stationary coordinate system.

[0009] The dq coordinate transformer is used to perform Park transformation on the current signal in the αβ stationary coordinate system to generate the d-axis current component and q-axis current component in the dq rotating coordinate system.

[0010] The virtual impedance is used to preset a virtual impedance, and to calculate the virtual impedance of the d-axis current component and the q-axis current component to obtain the d-axis virtual impedance voltage component and the q-axis virtual impedance voltage component.

[0011] The inverse Clark converter is used to perform an inverse coordinate transformation on the d-axis virtual impedance voltage component and the q-axis virtual impedance voltage component to generate the virtual impedance voltage signal corresponding to the single-phase inverter.

[0012] The droop controller is used to generate voltage control commands and angular frequency control commands for the inverter, and to send the voltage control commands and unit sine signals to the voltage and current dual closed-loop controller.

[0013] The voltage and current dual closed-loop controller is used to output the duty cycle modulation signal DutyCon, which is then sent to the SPWM generator.

[0014] The SPWM generator is used to generate drive signals to control the operation of the switching transistors, ultimately achieving precise power allocation and stable control of the single-phase inverter parallel system.

[0015] Furthermore, the input terminal of the signal conditioning circuit is connected to a single-phase inverter circuit, and the output terminal of the signal conditioning circuit is connected to the input terminal of the second-order generalized integrator, the input terminal of the droop controller, and the input terminal of the voltage-current dual closed-loop controller.

[0016] Furthermore, the output of the second-order generalized integrator is connected to the input of the dq coordinate transformer; the output of the dq coordinate transformer is connected to the input of the virtual impedance; the output of the virtual impedance is connected to the input of the inverse Clark transformer; the output of the inverse Clark transformer is connected to the input of the droop controller; the output of the droop controller is connected to the input of the voltage-current dual closed-loop controller; the output of the voltage-current dual closed-loop controller is connected to the input of the SPWM generator; and the output signal of the SPWM generator controls the operation of the switching transistors of the single-phase inverter circuit.

[0017] Another aspect of the objective of this invention is achieved as follows: a virtual impedance control method for parallel single-phase inverters, comprising the following steps:

[0018] 1) The inductor current of the single-phase inverter circuit is acquired in real time through the signal conditioning circuit, and the inductor current sample value is output and sent to the second-order generalized integrator.

[0019] 2) Two signals are output through a second-order generalized integrator. One output is used as the α-axis current component, and the other generates the β-axis current component, which is orthogonal to it. The α-axis current component and the β-axis current component constitute the current signal in the αβ stationary coordinate system.

[0020] 3) The current signal in the αβ stationary coordinate system is transformed by the dq coordinate transformer to generate the d-axis current component and q-axis current component in the dq rotating coordinate system.

[0021] 4) Multiply the d-axis current component and q-axis current component by the preset virtual impedance using the virtual impedance to obtain the d-axis virtual impedance voltage component and the q-axis virtual impedance voltage component.

[0022] 5) The d-axis virtual impedance voltage component and the q-axis virtual impedance voltage component are inversely transformed by the inverse Clark transformer to generate the virtual impedance voltage signal corresponding to the single-phase inverter, thus completing the construction of the virtual impedance voltage.

[0023] 6) Introduce the virtual impedance voltage signal into the droop controller to obtain the inverter's voltage control command and angular frequency control command;

[0024] 7) The voltage control command is tracked and controlled by a dual closed-loop voltage and current controller, which ultimately realizes the stable operation and power distribution of the single-phase inverter parallel system.

[0025] Furthermore, step 2) specifically includes: the second-order generalized integrator is a closed-loop control structure containing two integrators. In the second-order generalized integrator, the center angular frequency ω and the damping coefficient k are set. The input signal in the forward channel is adjusted by the damping coefficient k and then enters the integrator. After that, it forms a closed-loop control through the feedback channel and outputs the α component. The quadrature signal is obtained by multiplying the α component by the center angular frequency ω and then integrating it again, which is the vertical component β with a lag of 90°.

[0026] Furthermore, the angular frequency control command mentioned in step 6) has two outputs. One output is sent to the second-order generalized integrator as the center frequency reference of its internal orthogonal signal generator; the other output generates a synchronous rotation angle θ after integration, and the unit sine signal sin(θ) is calculated based on this angle.

[0027] Compared with existing technologies, the advantages of this invention are as follows: This invention employs a virtual impedance construction method based on a second-order generalized integrator to construct orthogonal signals, enabling accurate virtual impedance calculation in a rotating coordinate system for single-phase inverter parallel systems. By generating orthogonal β-axis components from the inductor current through a second-order generalized integrator, followed by dq coordinate transformation and impedance calculation, the constructed virtual impedance effectively simulates the required impedance characteristics, thereby significantly suppressing circulating currents between parallel units, improving power sharing accuracy, and enhancing system stability. This invention improves the current sharing performance and operational reliability of single-phase inverter parallel systems, contributing to the stable and efficient operation of modular power supplies, distributed microgrids, and other systems, thereby improving power quality and system redundancy. The method of this invention is implemented using digital control, with a clear algorithm structure, requiring no additional hardware circuitry, consuming minimal program resources, and offering high cost-effectiveness, making it suitable for large-scale engineering applications.

[0028] The disclosed method for constructing virtual impedance for parallel single-phase inverters can be applied not only to single-phase parallel systems such as photovoltaic energy storage, but also, based on the core idea of ​​constructing orthogonal signals using a second-order generalized integrator, to other fields requiring single-phase signal processing, such as single-phase phase-locked loops. The advantages of this invention will be further explained in subsequent descriptions. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0030] Figure 1 This is a schematic diagram of the control strategy for parallel connection of the single-phase H6 bridge inverter circuit of the present invention.

[0031] Figure 2 This is a schematic diagram of the second-order generalized integral of the present invention.

[0032] Figure 3 The following is a simulation waveform diagram of an embodiment of the present invention.

[0033] Figure 1Symbol names in the code: 1 Single-phase H6 bridge inverter circuit one, 2 Single-phase H6 bridge inverter circuit two, 3 Signal conditioning circuit one, 4 Signal conditioning circuit two, 5 Second-order generalized integrator one, 6 Second-order generalized integrator two, 7 dq coordinate transformer one, 8 dq coordinate transformer two, 9 Virtual impedance one, 10 Virtual impedance two, 11 Inverse Clark transformer one, 12 Inverse Clark transformer two, 13 Droop controller one, 14 Droop controller two, 15 Voltage and current dual closed-loop controller one, 16 Voltage and current dual closed-loop controller two, 17 SPWM generator one, 18 SPWM generator two, 19 Controllable bidirectional switch.

[0034]

[0035] Figure 2 Symbol names in:

[0036] Detailed Implementation

[0037] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] A virtual impedance control system for a single-phase inverter in parallel includes a signal conditioning circuit, a second-order generalized integrator, a dq coordinate transformer, a virtual impedance, an inverse Clark transformer, a droop controller, a voltage and current dual closed-loop controller, and an SPWM generator.

[0039] The signal conditioning circuit is used to acquire the inductor current sampling value of the single-phase inverter output inductor current in real time, and send the inductor current sampling value to the second-order generalized integrator;

[0040] The second-order generalized integrator is used to generate α-axis current components and β-axis current from the sampled inductor current values, and to combine the α-axis current components and β-axis current components to form a current signal in the αβ stationary coordinate system.

[0041] The dq coordinate transformer is used to perform Park transformation on the current signal in the αβ stationary coordinate system to generate the d-axis current component and q-axis current component in the dq rotating coordinate system.

[0042] Virtual impedance is used to preset a virtual impedance and to calculate the virtual impedance voltage components of the d-axis and q-axis current components.

[0043] The inverse Clark converter is used to perform an inverse coordinate transformation on the d-axis virtual impedance voltage component and the q-axis virtual impedance voltage component to generate the virtual impedance voltage signal corresponding to the single-phase inverter.

[0044] The droop controller is used to generate voltage control commands and angular frequency control commands for the inverter, and sends the voltage control commands and unit sine signals to the voltage and current dual closed-loop controller.

[0045] The voltage and current dual closed-loop controller is used to output the duty cycle modulation signal DutyCon, which is then sent to the SPWM generator.

[0046] The SPWM generator is used to generate drive signals to control the operation of the switching transistors, ultimately achieving precise power distribution and stable control of a single-phase inverter parallel system.

[0047] The input terminal of the signal conditioning circuit is connected to the single-phase inverter circuit, and the output terminal of the signal conditioning circuit is connected to the input terminal of the second-order generalized integrator, the input terminal of the droop controller, and the input terminal of the voltage and current dual closed-loop controller.

[0048] The output of the second-order generalized integrator is connected to the input of the dq coordinate transformer; the output of the dq coordinate transformer is connected to the input of the virtual impedance; the output of the virtual impedance is connected to the input of the inverse Clark transformer; the output of the inverse Clark transformer is connected to the input of the droop controller; the output of the droop controller is connected to the input of the voltage-current dual closed-loop controller; the output of the voltage-current dual closed-loop controller is connected to the input of the SPWM generator; the output signal of the SPWM generator controls the operation of the switching transistors of the single-phase inverter circuit.

[0049] A virtual impedance control method for parallel single-phase inverters includes the following steps:

[0050] 1) The inductor current of the single-phase inverter circuit is acquired in real time through the signal conditioning circuit, and the inductor current sample value is output and sent to the second-order generalized integrator.

[0051] 2) Two signals are output through a second-order generalized integrator. One output is used as the α-axis current component, and the other generates a β-axis current component that is orthogonal to it. The α-axis current component and the β-axis current component constitute the current signal in the αβ stationary coordinate system. The second-order generalized integrator is a closed-loop control structure containing two integrators. The center angular frequency ω and the damping coefficient k are set in the second-order generalized integrator. The input signal in the forward channel is adjusted by the damping coefficient k and then enters the integrator. After that, it forms a closed-loop control through the feedback channel and outputs the α component. The orthogonal signal is obtained by multiplying the α component by the center angular frequency ω and then integrating it again, which is the vertical component β with a 90° lag.

[0052] 3) The current signal in the αβ stationary coordinate system is transformed by the dq coordinate transformer to generate the d-axis current component and q-axis current component in the dq rotating coordinate system.

[0053] 4) Multiply the d-axis current component and q-axis current component by the preset virtual impedance using the virtual impedance to obtain the d-axis virtual impedance voltage component and the q-axis virtual impedance voltage component.

[0054] 5) The d-axis virtual impedance voltage component and the q-axis virtual impedance voltage component are inversely transformed by the inverse Clark transformer to generate the virtual impedance voltage signal corresponding to the single-phase inverter, thus completing the construction of the virtual impedance voltage.

[0055] 6) The virtual impedance voltage signal is introduced into the droop controller to obtain the inverter's voltage control command and angular frequency control command. The angular frequency control command has two outputs. One output is sent to the second-order generalized integrator as the center frequency reference of its internal quadrature signal generator. The other output generates a synchronous rotation angle θ after integration, and the unit sine signal sin(θ) is calculated based on this angle.

[0056] 7) The voltage control command is tracked and controlled by a dual closed-loop voltage and current controller, which ultimately realizes the stable operation and power distribution of the single-phase inverter parallel system.

[0057] like Figure 1 As shown, the parallel control system based on the single-phase H6 bridge inverter circuit obtains the inductor current iinv1 and output voltage uo1 of the single-phase H6 bridge inverter circuit 1 in real time. After passing through the signal conditioning circuit 3, the corresponding inductor current sampling value Iinv1 and output voltage sampling value Uo1 are obtained.

[0058] The inductor current sample value Iinv1 is sent to the second-order generalized integrator (SOGI1) 5. The second-order generalized integrator 5 outputs two signals: one output is used as the α-axis current component Iα1, and the other generates the β-axis current component Iβ1, which is orthogonal to it. The α-axis current component Iα1 and the β-axis current component Iβ1 are sent to the dq coordinate transformer 7. The dq coordinate transformer 7 outputs the d-axis current component Id1 and the q-axis current component Iq1. The d-axis current component Id1 and the q-axis current component Iq1 are sent to the virtual impedance 9. The virtual impedance 9 outputs the d-axis virtual impedance voltage Uvd1 and the q-axis virtual impedance voltage Uvq1. The d-axis virtual impedance voltage Uvd1 and the q-axis virtual impedance voltage Uvq1 are sent to the inverse Clark converter 11. The inverse Clark converter 11 outputs the virtual impedance voltage signal Uv_comp1 corresponding to the single-phase inverter.

[0059] The inductor current sampling value Iinv1, the output voltage sampling value Uo1, the given voltage reference value Uref, and the rated angular frequency ωn are sent to the droop controller 13. The droop controller 13 outputs the final voltage control command Ucmd1 and the final angular frequency control command ω1. The angular frequency control command ω1 has two outputs. One output is sent to the second-order generalized integrator 5 as the center frequency reference of its internal quadrature signal generator. The other output is integrated to generate a synchronous rotation angle θ1, and the unit sine signal sin(θ1) is calculated based on this angle.

[0060] The final voltage control command Ucmd1 and the unit sine signal sin(θ1) are sent to the voltage-current dual closed-loop controller 15. In the voltage loop, the final voltage control command Ucmd1 is subtracted from the output voltage sample value Uo1 to obtain the output voltage error signal err_Vo1. The output voltage error signal err_Vo1 is sent to the voltage loop PI regulator, which outputs the current reference signal Iinv_ref1. In the current loop, the current reference signal Iinv_ref1 and the inductor current sample value I... The difference between inv1 and the inductor current is used to obtain the inductor current error signal err_Iinv1. The inductor current error signal err_Iinv1 is sent to the current loop PI regulator. The current loop PI regulator outputs the duty cycle modulation signal DutyCon1 based on voltage and current dual closed-loop control. The duty cycle modulation signal DutyCon1 of voltage and current dual closed-loop control is sent to SPWM generator-17. SPWM generator-17 generates drive signals (1A1, 2A1, 1B1, 2B1) to control the switching transistor's turn-on and turn-off.

[0061] For another single-phase inverter in the parallel control system, the virtual impedance construction and control process is completely consistent with that of the single-phase inverter mentioned above.

[0062] The input terminal of signal conditioning circuit 24 is connected to single-phase H6 bridge inverter circuit 2, and the output terminal of signal conditioning circuit 24 is connected to the input terminal of second-order generalized integrator 2 (SOGI2) 6, the input terminal of droop controller 2 14, and the input terminal of voltage and current dual closed-loop controller 2 16.

[0063] The output of the second-order generalized integrator 26 is connected to the input of the dq coordinate transformer 28; the output of the dq coordinate transformer 28 is connected to the input of the virtual impedance 210; the output of the virtual impedance 210 is connected to the input of the inverse Clark transformer 212; the output of the inverse Clark transformer 212 is connected to the input of the droop controller 214; the output of the droop controller 214 is connected to the input of the voltage-current dual closed-loop controller 216; the output of the voltage-current dual closed-loop controller 216 is connected to the input of the SPWM generator 218; the SPWM generator 218 outputs a drive signal to control the operation of the switching transistor.

[0064] The parallel control system based on the single-phase H6 bridge inverter circuit obtains the inductor current iinv2 and output voltage uo2 of the single-phase H6 bridge inverter circuit 2 in real time. After passing through the signal conditioning circuit 4, the corresponding inductor current sampling value Iinv2 and output voltage sampling value Uo2 are obtained.

[0065] The inductor current sample value Iinv2 is sent to the second-order generalized integrator 6. The second-order generalized integrator 6 outputs two signals: one output is used as the α-axis current component Iα2, and the other generates the β-axis current component Iβ2, which is orthogonal to it. The α-axis current component Iα2 and the β-axis current component Iβ2 are sent to the dq coordinate transformer 8. The dq coordinate transformer 8 outputs the d-axis current component Id2 and the q-axis current component Iq2. The d-axis current component Id2 and the q-axis current component Iq2 are sent to the virtual impedance 10. The virtual impedance 10 outputs the d-axis virtual impedance voltage Uvd2 and the q-axis virtual impedance voltage Uvq2. The d-axis virtual impedance voltage Uvd2 and the q-axis virtual impedance voltage Uvq2 are sent to the inverse Clark converter 12. The inverse Clark converter 12 outputs the virtual impedance voltage signal Uv_comp2 corresponding to the single-phase inverter.

[0066] The inductor current sampling value Iinv2, the output voltage sampling value Uo2, the given voltage reference value Uref, and the rated angular frequency ωn are sent to the droop controller 14. The droop controller 14 outputs the final voltage control command Ucmd2 and the final angular frequency control command ω2. The angular frequency control command ω2 has two outputs. One output is sent to the second-order generalized integrator 6 as the center frequency reference of its internal quadrature signal generator. The other output is integrated to generate a synchronous rotation angle θ2, and the unit sine signal sin(θ2) is calculated based on this angle.

[0067] The final voltage control command Ucmd2 and the unit sine signal sin(θ2) are sent to the voltage-current dual closed-loop controller 16. In the voltage loop, the final voltage control command Ucmd2 is subtracted from the output voltage sample value Uo2 to obtain the output voltage error signal err_Vo2. The output voltage error signal err_Vo2 is sent to the voltage loop PI regulator, which outputs the current reference signal Iinv_ref2. In the current loop, the current reference signal Iinv_ref2 and the inductor current sample value I... The difference between inv2 and the inductor current is used to obtain the inductor current error signal err_Iinv2. The inductor current error signal err_Iinv2 is sent to the current loop PI regulator. The current loop PI regulator outputs the duty cycle modulation signal DutyCon2 based on voltage and current dual closed-loop control. The duty cycle modulation signal DutyCon2 of voltage and current dual closed-loop control is sent to SPWM generator 18. SPWM generator 18 generates drive signals (1A2, 2A2, 1B2, 2B2) to control the switching transistor's turn-on and turn-off.

[0068] A second-order generalized integrator constructs orthogonal components for the single-phase current signal, enabling precise calculation of the virtual impedance in the dq rotating coordinate system. An inverse Clark transformer then generates the virtual impedance voltage signal Uv_comp corresponding to the single-phase inverter. This virtual impedance voltage signal Uv_comp is introduced into the droop control loop, equivalent to connecting a controllable impedance in series at the inverter output, thereby actively adjusting the output characteristics of each parallel unit to achieve power sharing and circulating current suppression. The control flow of single-phase H6 bridge inverter circuit 1 is completely symmetrical with that of single-phase H6 bridge inverter circuit 2, together forming a complete parallel system control strategy.

[0069] The second-order generalized integrator 1.5 and the second-order generalized integrator 2.6 generate α-axis and β-axis current components. The method for generating the αβ two-phase stationary coordinate system current is as follows: Figure 2 Its core lies in the fact that the second-order generalized integrator is implemented by a closed-loop control structure containing two integration stages. In the second-order generalized integrator, the center angular frequency ω and the damping coefficient k are set. The input signal in the forward channel is adjusted by the damping coefficient k and then enters the integrator. It then forms a closed-loop control through the feedback channel and outputs the α component. The quadrature signal is obtained by multiplying the α component by the center angular frequency ω and then integrating it again to obtain the vertical component β that lags by exactly 90°.

[0070] Based on the aforementioned method for constructing virtual impedance in parallel single-phase inverters, a virtual impedance voltage signal is generated through a second-order generalized integrator, dq coordinate transformation, virtual impedance, and inverse Clark transformation, and then introduced into a droop control loop. This effectively reshapes the equivalent output impedance of each inverter unit, thereby significantly improving the power sharing accuracy of the parallel system. Simultaneously, this voltage actively compensates for and suppresses circulating current components caused by inconsistent line parameters and module differences, ensuring the stable and reliable operation of the system.

[0071] A specific embodiment of the present invention is as follows:

[0072] like Figure 1 As shown, single-phase H6 bridge inverter circuit 1 and single-phase H6 bridge inverter circuit 2 are connected in parallel. Their filter inductance is L1=L2=L3=L4=0.65mH, filter capacitor is C1=C2=10.92µF, and bus capacitor is C... dc1 =C dc2 =1.88mF, the bus voltage V in the experiment bus2 =V bus2 =400V, line impedance Zline1=(0.04+j*ω*400*10 -6 )Ω, line impedance Zline2=(0.05+j*ω*500*10 -6 Ω, switching frequency fs =20KHz, interrupt period T s =200µs, common load R1=6.6125Ω, R2=6.6125Ω.

[0073] In the PSIM simulation circuit, the inductor current sampling value Iinv and the output voltage sampling value Uo are sent to the DLL control module. In the DLL control module, the β-axis component of the inductor current is constructed through a second-order generalized integrator. Then, the dq coordinate transformation, virtual impedance voltage calculation, virtual impedance voltage inverse Clark transformation, droop control, and dual closed-loop control are performed to output the modulation wave signal. The modulation wave signal is sent to the SPWM generator to generate the drive signal to control the switching of the switching transistors in the single-phase H6 bridge inverter circuit.

[0074] like Figure 3 The waveforms are those of two single-phase H6 bridge inverters connected in parallel. The waveforms demonstrate the feasibility of the virtual impedance construction method described in this patent. A sudden load increase from 8kW to 16kW was applied at 0.4 seconds. Taking into account line impedance imbalances, virtual impedance, droop control, and dual closed-loop control were incorporated into the program control. Through the virtual impedance construction and droop control methods of this invention, stable, high-precision current sharing, and effective circulating current suppression experimental waveforms were obtained under load step and parameter asymmetry conditions. P_avg is the active power of single-phase H6 bridge inverter circuit 1, and P_avg2 is the active power of single-phase H6 bridge inverter circuit 2; Q_avg is the reactive power of single-phase H6 bridge inverter circuit 1, and Q_avg2 is the reactive power of single-phase H6 bridge inverter circuit 2; Iinv1 is the inductor current of single-phase H6 bridge inverter circuit 1, and Iinv2 is the inductor current of single-phase H6 bridge inverter circuit 2; Uo1 is the output voltage of single-phase H6 bridge inverter circuit 1, and Uo2 is the output voltage of single-phase H6 bridge inverter circuit 2.

[0075] from Figure 3 Simulation waveforms show that under two typical operating conditions—load step change and inverter parameter asymmetry—the output current of the two parallel inverters can achieve fast and accurate current sharing, and the system circulating current is effectively suppressed to a negligible level. During dynamic processes, the current response is rapid and without overshoot, while in steady state, the current waveform is smooth and stable. Therefore, the virtual impedance construction method of this invention achieves high-precision current sharing control, strong circulating current suppression capability, and excellent dynamic and steady-state performance for the parallel inverter system.

[0076] The virtual impedance construction method proposed in this invention is applicable not only to resistive loads, but also to nonlinear and unbalanced load scenarios. Under various operating conditions such as load abrupt changes, power distribution, and differences in parallel unit parameters, it can effectively improve the current sharing accuracy of the system and significantly suppress circulating current, thus ensuring the stable and reliable operation of the parallel system.

[0077] The control system and method of this invention are based on algorithms and mature signal processing modules such as second-order generalized integrators, dq coordinate transformation, and inverse Clark transformation. They are easy to implement in modular programming in digital signal processors (DSPs) or microcontrollers (MCUs), without the need to add additional physical sensors or hardware circuits in the power loop. They have the advantages of low implementation cost, high reliability, and ease of engineering application.

[0078] The method of this invention is not only applicable to the two single-phase inverters in parallel system, but its core concept of virtual impedance construction and circulating current suppression can be further extended to multiple inverters in parallel or three-phase inverters in parallel system, providing an effective control solution for the stable parallel operation of large-scale distributed generation units and microgrids.

[0079] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A virtual impedance control system for parallel single-phase inverters, characterized in that, Includes signal conditioning circuit, second-order generalized integrator, dq coordinate transformer, virtual impedance, inverse Clark transformer, droop controller, voltage and current dual closed-loop controller, and SPWM generator; The signal conditioning circuit is used to acquire the inductor current sampling value of the single-phase inverter in real time and send the inductor current sampling value to the second-order generalized integrator. The second-order generalized integrator is used to generate α-axis current components and β-axis current from the sampled inductor current values, and to combine the α-axis current components and β-axis current components to form a current signal in the αβ stationary coordinate system. The dq coordinate transformer is used to perform Park transformation on the current signal in the αβ stationary coordinate system to generate the d-axis current component and q-axis current component in the dq rotating coordinate system. The virtual impedance is used to preset a virtual impedance, and to calculate the virtual impedance of the d-axis current component and the q-axis current component to obtain the d-axis virtual impedance voltage component and the q-axis virtual impedance voltage component. The inverse Clark converter is used to perform an inverse coordinate transformation on the d-axis virtual impedance voltage component and the q-axis virtual impedance voltage component to generate the virtual impedance voltage signal corresponding to the single-phase inverter. The droop controller is used to generate voltage control commands and angular frequency control commands for the inverter, and to send the voltage control commands and unit sine signals to the voltage and current dual closed-loop controller. The voltage and current dual closed-loop controller is used to output the duty cycle modulation signal DutyCon, which is then sent to the SPWM generator. The SPWM generator is used to generate drive signals to control the operation of the switching transistors, ultimately achieving precise power allocation and stable control of the single-phase inverter parallel system.

2. The virtual impedance control system for parallel single-phase inverters according to claim 1, characterized in that, The input terminal of the signal conditioning circuit is connected to a single-phase inverter circuit, and the output terminal of the signal conditioning circuit is connected to the input terminal of the second-order generalized integrator, the input terminal of the droop controller, and the input terminal of the voltage-current dual closed-loop controller.

3. A virtual impedance control system for parallel single-phase inverters according to claim 1, characterized in that, The output of the second-order generalized integrator is connected to the input of the dq coordinate transformer; the output of the dq coordinate transformer is connected to the input of the virtual impedance; the output of the virtual impedance is connected to the input of the inverse Clark transformer; the output of the inverse Clark transformer is connected to the input of the droop controller; the output of the droop controller is connected to the input of the voltage-current dual closed-loop controller; the output of the voltage-current dual closed-loop controller is connected to the input of the SPWM generator; the output signal of the SPWM generator controls the operation of the switching transistors of the single-phase inverter circuit.

4. A virtual impedance control method for parallel single-phase inverters, characterized in that, Includes the following steps: 1) The inductor current of the single-phase inverter circuit is acquired in real time through the signal conditioning circuit, and the inductor current sample value is output and sent to the second-order generalized integrator. 2) Two signals are output through a second-order generalized integrator. One output is used as the α-axis current component, and the other generates the β-axis current component, which is orthogonal to it. The α-axis current component and the β-axis current component constitute the current signal in the αβ stationary coordinate system. 3) The current signal in the αβ stationary coordinate system is transformed by the dq coordinate transformer to generate the d-axis current component and q-axis current component in the dq rotating coordinate system. 4) Multiply the d-axis current component and q-axis current component by the preset virtual impedance using the virtual impedance to obtain the d-axis virtual impedance voltage component and the q-axis virtual impedance voltage component. 5) The d-axis virtual impedance voltage component and the q-axis virtual impedance voltage component are inversely transformed by the inverse Clark transformer to generate the virtual impedance voltage signal corresponding to the single-phase inverter, thus completing the construction of the virtual impedance voltage. 6) Introduce the virtual impedance voltage signal into the droop controller to obtain the inverter's voltage control command and angular frequency control command; 7) The voltage control command is tracked and controlled by a dual closed-loop voltage and current controller, which ultimately realizes the stable operation and power distribution of the single-phase inverter parallel system.

5. The method for constructing virtual impedance in parallel single-phase inverters according to claim 4, characterized in that, Step 2) specifically includes: The second-order generalized integrator is a closed-loop control structure containing two integrators. In the second-order generalized integrator, the center angular frequency ω and the damping coefficient k are set. The input signal in the forward channel is adjusted by the damping coefficient k and then enters the integrator. After that, it forms a closed-loop control through the feedback channel and outputs the α component. The quadrature signal is obtained by multiplying the α component by the center angular frequency ω and then integrating it again, which is the vertical component β with a lag of 90°.

6. The method for constructing virtual impedance for parallel single-phase inverters according to claim 4, characterized in that, The angular frequency control command mentioned in step 6) has two outputs. One output is sent to the second-order generalized integrator as the center frequency reference of its internal quadrature signal generator; the other output generates a synchronous rotation angle θ after integration, and the unit sine signal sin(θ) is calculated based on this angle.

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