Research Of Three Phase Grid Connected Inverter Based On Peak Current Mode

Today people face two serious problems:energy shortage and environmental pollution. Under this background, developing new energy sources is imminent. In all kinds of new energy, the solar energy has its own advantage, so it has got a wide range of development. In the photovoltaic inverter system, the topology structure of grid connected inverter and PWM control method is two key issues. This paper chooses the three level inverter of T topology structure as study object, peak current mode as control strategy.In the Grid-connected system, the instability of grid voltage will influence the current, leading to current fluctuation and quality droping. In the stand-alone system, the load sometimes changes suddenly, leading to current changing violently. In high power system, there may be many inverters. There is mutual interference between inverters and even resonance. In view of the above problems, the voltage mode control method cannot obtain satisfactory control effect. So this paper adopts current mode control method, which includes average current control and peak current control method. Under the circumstances of load changing and grid voltage suddenly, peak current mode has more advantage. The current cannot change substantially and maintain stable. So this paper adopts peak current control mode as PWM modulation strategy.The peak current control method has many advantages:fast dynamic response, has ability of over current automatic protection. In the photovoltaic inverter system, however, few people have studied this issue. So this paper’s applying this kind of method to the control of inverter can be called an attempt and innovation. Through theory analysis, computer simulation and experiment, this paper proves that the inverter can output stable current based on peak current mode. Even when the load is short, the current waveform is still a stable sinusoidal signal. The inverter is equivalent to the AC constant current source, proving that the method is workable.This paper takes T type three-level-inverter as research object, with peak current control as the main method, conducts in-depth study on this basis. The main content of this paper include:Firstly, the theory of peak current control is studied. This paper introduces the basic principle of peak current control method, analyzes the stability of this method, and introduces the principle of the slope compensation.Secondly, the neutral point potential balance is researched and controlled. This paper uses control method of neutral point potential hysteresis loop. This method is independent from specific PWM strategy, so it can combine with peak current control method well and the control effect is satisfactory.Moreover, the control of reactive power is studied. This paper controls the reactive power by controlling the reactive current so that the inverter could run in different power state.Then, the resonant peak of LCL filter is analyzed. The passive damping method and active damping method have been studied separately. On the basic of peak current control, this paper carried out the simulation of the system in active damping method, and the correctness of the control method is verified.Finally, the experimental platform is built and some related experiments are completed, including SPPL, peak current control, neutral point potential control and grid-connected running.