Study On The Control Method Of Two-Stage Inverter

The instantaneous output power of the two-stage inverter will pulsate at twice the frequency of the output voltage,which will cause the second harmonic current to flow into the front-end DC-DC converter and the DC power supply of the input side.The existence of this second harmonic current will increase the loss of the front-end circuit,and will also increase the required capacity of the DC power supply of the input side,which will reduce the overall efficiency of the inverter.In order to improve the operating conditions of the DC power supplies such as solar cells,fuel cells,and uninterruptible power supplies,to extend their service life and improve the efficiency of the system,it is necessary to suppress the second harmonic current flowing into the front-end circuit.Firstly,for the front-end stage of Boost circuit,the AC small signal model of its control system is derived in this paper.Then the mechanism of the generation of second harmonic current and its harm to the system are analyzed.After that,the control method of suppressing the second harmonic current flowing into the front-end stage of DC-DC converter has been derived from the perspective of introducing a virtual impedance.Subsequently,this paper proposes a single voltage-loop-closed control method only using inductor current feedback scheme,which can effectively suppress the second harmonic current flowing into the front-end circuit and effectively improve the dynamic characteristics of the intermediate DC bus voltage during load mutation.This control method only requires two sensors,which measure the output voltage and the inductor current of the Boost converter,to achieve the desired control effect through digital control.Since the intermediate bus capacitance of industrial two-stage inverters generally has a large value,this control method is very suitable for industrial applications.Then,for the downstream inverter circuit,the mathematical model of its control system is deduced,then the PWM control method of the fixed power frequency working bridge arm and high frequency working bridge arm in unipolar modulation is selected by analyzing in this paper.Based on the research of the outer voltage loop using a quasiPR controller,the dynamic performance of the two control schemes is explored.Through simulation and experiment comparison,the high gain of the quasi-PR controller to the power frequency sinusoidal signal is studied.While the AC reference input signal has no static tracking error,it can also reduce the cutoff frequency of system to a certain extent and reduce the noise.In the end,by the two-stage inverter experiment platform constructed in this paper the feasibility and control effect of the control scheme proposed in this paper are verified respectively.