Improvement Of Injected Current Quality For Voltage-Controlled Grid-Connected Inverter

Distributed power generation system based on renewable energy is one promising approach for alleviating energy crisis and environmental pollution, and it has been drawing more and more attentions. Grid-connected inverter is the interface between the distributed generation unit and the power grid. For the voltage-controlled grid-connected inverter, the dead-time of the drive signals of the power switches is the main factor of causing the distortion of the output voltage and injected current. This paper is dedicated to the improvement of injected current quality for voltage-controlled inverter with digital control.This paper firstly investigates the impact of the dead-time of the drive signals on the output voltage and injected current of grid-connected inverter, which is called the dead-time effect hereinafter. It is revealed that the harmonic components in the output voltage resulted by the dead-time effect is concentrated at low and odd times of fundamental frequency, which cannot be filtered by output filter of the grid-connected inverter. Besides, due to small transmission line impedance, even a small output voltage harmonic will lead to serious distortion of injected current.In order to suppress the distortion in the output voltage and injected current caused by dead-time effect, the output voltage closed-loop control can be adopted. This paper investigates the frequency responses of the loop gain of the voltage-controlled inverter with digital control. To avoid the resonance caused by the output LC filter, which maybe leads to system instability, the capacitor-current-feedback active damping is adopted to damp the resonance. And in order to increase the loop gain at low frequency, a second order regulator is adopted as the voltage controller. Based on the second order regulator, according to the system specifications, including the injected current quality enhancement, steady-state error, phase margin and gain margin, the complete satisfactory regions of the controller parameters can be obtained, and then the controller parameters can be easily picked out. Based on this design method, the trial-and-error procedures are avoided.In order to enhance the injected grid current quality further, this paper proposes the method of introducing a virtual impedance to be series with the output impedance of the grid-connected inverter. The introduced virtual impedance could increase the magnitude of output impedance at low frequencies, and thus reducing the harmonics in the injected current caused by the dead-time effect. Besides, it is pointed out that the maganitude of the virtual series impedance should be small enough at high-frequency range to avoid sacrificing the dynamic performance. According to the requirements of injected grid current quality, this paper proposes a design method of the virtual series impedance.A 6 kVA single-phase voltage-controlled inverter has been built in the lab. The experimental results are given to verify the effectiveness of the theoretical analysis and the proposed controller design method.