Research On Control Strategy For Parallel Three-phase Inverter

With the rapid development of renewable energy,micro-grid technology and distributed generation technology become increasingly important.The parallel inverters can improve the reliability and redundancy of the system,increase the power system's flexibility,reduce the system's size and weight,and reduce the production and maintenance costs.So they are considered as one of an important direction in the field of the power electronics technology.However,in the parallel system,one of the main problems is the circulating current.The circulating current can lead to current waveform distortion and overall system performance degradation,may even cause equipment damage or system operation failure.Therefore,the circulation current suppression strategy is needed to study to keep the system stable.In addition,there are many unbalanced loads in the actual micro-grid,which can lead to unbalanced threephase voltage,resulting in energy loss and affecting the stability of the system.Therefore,when designing the control strategy,it is necessary to take into account the influences of the unbalanced loads.In this paper,the control strategies of the parallel inverters system are studied.The main works are summarized as following:(1)The topology of the parallel inverters system is analyzed,and the mathematical model of it in the three-phase stationary coordinate frame is established.Also,the model is converted to the two-phase stationary coordinate frame and the two-phase rotation coordinate frame,laying the foundation of the control strategies.By analyzing the equivalent circuit model of two parallel inverters,the mechanism of the circulating current is described and the circuits of the circulating current are reduced.(2)The control method of the voltage outer loop and current inner loop dual-loop PI control is introduced in detail.Aiming at the circulating current in the parallel inverter system,an improved fuzzy PI control is proposed.The control strategy is based on the traditional dualloop PI control,a zero-sequence current controller is added in the inner current loop controller to suppress the circulating current.At the same time,a fuzzy logic controller is added in the voltage outer loop to improve the anti-jamming capability of the system.The model of parallel inverters system is built in the MATLAB/Simulink,and the effectiveness of the proposed fuzzy PI control strategy is verified.(3)The control strategy need to be further improved because the unbalanced loads in the actual power grid,an improved dual-loop control strategy is proposed.Because the PI and PR controllers have some limitations,the quasi-PR controller is used instead of the PI controller in the current inner loop of the improved fuzzy PI control,and the basis of setting the control parameters is described in detail.The simulation results in the MATLAB/Simulink show that although the improved fuzzy PI control has some control effects,the three-phase unbalance voltage still exists,the total harmonic distortion is still high;by using the improved dual-loop control strategy,the three-phase output voltage is basically balanced,the total harmonic distortion decreases a lot,and the circulating current has been effectively suppressed,the control effect is better.