Research On Analysis And Control Strategy Of Circulating Current Of Parallel Three-phase PWM Inverters

Inverter is the key part of new energy power generation system,paralleling inverter modules is an effective way to increase the system capacity.However,the problem of circulating current arising from paralleling the modules has a bad effect on the system.The object of the study is the three-phase PWM parallel inverters system sharing the same DC bus and paralleling directly in the ac side,the problem of circulating current is studied in this paper,and the study is mainly aiming at two parallel inverters system.Circulating current of three-phase parallel system with asynchronous carriers is firstly modeled in this paper.The asynchronous carriers of parallel inverters and modulation signal is analyzed when SPWM is used.According to the relation of position between modulation signal and the intersecting point of parallel carriers,it is divided into two conditions to model the circulating current of parallel system,the accordingly circulating current model is obtained under the two conditions.According to the model of circulating current,it is analyzed that the variation tendency of circulating current components when the carrier phase difference gets bigger,the high frequency circulating current gets bigger along with the carrier phase difference.Open-loop scanning method is used in this paper to verify the characteristics of circulating current.Shifting the carrier phase difference with constant speed to 180 degrees is the scanning progress,during which the change of circulating current is obtained.The method of restraining high frequency circulating current by adjusting carrier phase difference is proposed in this paper.The accordingly carrier phase shifting amount is obtained by the carrier phase regulator,and then it is given to one module to adjust its carrier phase,in which way the circulating current is restrained.The proposed method can restrain the high frequency circulating current effectively,and it applies to the situation when SPWM or SVPWM is used,it also has a good performance under the situation that parallel modules are unbalanced.Compared to the system with single LCL filter,a new resonance peak will occur in the system with parallel LCL filters.As there exists circulating current path in the parallel system,the circulating current resonance will occur.The resonance of circulating current of three-phase parallel inverters based on LCL filter is analyzed in this paper.By developing the equivalent circuit of circulating current,the transfer function from output voltage of inverter to circulating current is obtained.The circulating current resonance of parallel system is analyzed according to the results of frequency-domain analysis.From the theoretical analysis,resonance of circulating current is relevant to inner LCL filter,and it can be divided into two parts according to whether it is relevant to outer grid impedance.The resonance of circulating current that has nothing to do with the outer grid impedance is mainly studied in this paper.Active damping method is adopted on one module of parallel system,the resonance of the two modules can be restrained at the same time,which shows that the restrained resonance exists in the parallel system in the form of circulating current.On the parallel inverters experiment platform in the lab,the validity of the theoretical analysis and the effectiveness of the proposed method is verified by experimental results.