Research On Suppression Of Second Harmonic Current In Quasi-Z-Source Photovoltaic Inverter

With the depletion of traditional fossil energy and the progress of science and technology,new energy power generation has developed rapidly.Distributed photovoltaic power generation has been widely used due to its advantages of cleanliness,high efficiency and flexibility.Distributed photovoltaic power generation mainly uses a single-phase inverter for power conversion.The quasi-Z-source inverter allows the bridge arm to go straight,and the single-stage structure can realize the boost and inverter of electric energy.The application in the distributed photovoltaic power generation system can reduce the cost of photovoltaic power generation and improve the system reliability.However,the instantaneous power of the single-phase inverter will be pulsating at twice the power frequency.There is also a double frequency power in the quasi-Z-source inverter,which will lead to the second harmonic current in the front stage and reduce the efficiency and life of the photovoltaic power generation system.Therefore,in order to improve the power conversion efficiency of the photovoltaic power generation system and prolong its service life,it is necessary to suppress the second harmonic current in the Z-source photovoltaic inverter.Firstly,the generation mechanism of second harmonic current in single-phase inverter and the working principle of quasi-Z-source inverter are analyzed.The small signal mathematical model and circuit model of quasi-Z-source inverter are established.At the same time,the second harmonic voltage and current components in inductance and capacitance components are modeled and analyzed.Secondly,a bidirectional Buck-Boost decoupling circuit is paralleled at the bus of the quasi-Z-source inverter to suppress the second harmonic current.The bidirectional Buck-Boost decoupling circuit is used to decouple the power of the front and rear stages of the quasi-Z-source inverter,so that the instantaneous power of the front stage is constant.According to the working principle of the bidirectional Buck-Boost decoupling circuit and the double frequency energy conservation,the control algorithm of the decoupling circuit is designed,and the parameters of the decoupling inductance and capacitance components in the circuit are designed.Then,from the perspective of impedance,the second harmonic current is suppressed by series virtual impedance in the inductance branch of the quasi-Z-source inverter small-signal equivalent circuit model.The control system parameters are designed according to the stability conditions of the quasi-Z-source inverter system and the second harmonic current suppression requirements.In order to improve the dynamic response of the system,the load current feedforward control on the inverter side is introduced,and the virtual impedance is combined with the band-pass filter to make the inductance branch impedance of the quasi-Z-source inverter present high impedance at twice the frequency and low impedance at other frequencies.Then a damping resistor is added to the band-pass filter to suppress the phase mutation of the system.Based on the series virtual impedance,a parallel virtual impedance control strategy is proposed,which makes the impedance of the parallel virtual impedance infinite at the double frequency,and ensures the suppression effect of the series virtual impedance on the second harmonic current.The low impedance outside the double frequency reduces the closed-loop impedance of the quasi-Z-source inverter and improves the dynamic performance of the system.Finally,the grid-connected control of the quasi-Z-source photovoltaic inverter is carried out.The grid-connected performance of the PI regulator and the quasi-PR regulator is analyzed,and the parameters of the quasi-PR regulator are designed.The proposed virtual impedance control strategy and grid-connected control strategy are verified by RT-LAB platform.The sampling circuit,control circuit and drive circuit are designed.The series virtual impedance control strategy is experimentally verified on a 300 W prototype.The experimental results verify the correctness of the theoretical analysis and the effectiveness of the control strategy.