Research On Phase Optimization Of Source-load Access To Suppress Three-phase Imbalance

China’s low-voltage distribution network generally adopts three-wire four-wire power supply system.Due to the spatial and temporal distribution characteristics of single-phase load,there is a long-term three-phase imbalance problem in the low-voltage distribution station area.The promotion of the "Whole County Photovoltaic" plan has led to the large-scale access of distributed photovoltaics to low-voltage distribution networks,and distributed photovoltaics are usually small in capacity,mostly using single-phase access methods.The disorderly access of single-phase distributed photovoltaics tends to aggravate the three-phase imbalance of low-voltage distribution networks.Three-phase unbalance will increase the loss of distribution lines and damage distribution transformers and low-voltage distribution equipment.Therefore,this paper studies the access phase of distributed photovoltaic and load.Firstly,in order to solve the problem of three-phase imbalance caused by single-phase load,a method of load phase selection switching is proposed.Based on the idea of demand side response,the load side actively sends out the proposed input signal,and the overall structure and process of switching are analyzed.It overcomes the problems of complex commutation of traditional commutation switches and short-term power outage during switching.Through simulation verification,the load phase selection switching method can effectively reduce the three-phase unbalance.Secondly,considering the access phase difference of single-phase photovoltaic power generation,a source-load phase-selection switching method for suppressing three-phase imbalance is proposed.The distributed photovoltaic and load are used as the node injection current,and the one-day access phase of single-phase photovoltaic power generation and the access phase of single-phase load are taken as optimization variables,and the three-phase imbalance and the minimum distribution network loss are taken as the optimization objectives.The source-load phase-selection optimization control model of low-voltage distribution network is established.The NSGA-II is used to solve the model,and the optimization result of single-phase photovoltaic power generation only accessing one phase in one day is obtained,and the phase-selection switching is carried out when the photovoltaic power is not output at night.At the same time,the single-phase load is also selected.The example verifies that the source-load phase selection switching method can effectively suppress the three-phase imbalance in the low-voltage distribution station area.Finally,the commutation switch type device is used for single-phase photovoltaic power generation and single-phase load.With the minimum three-phase unbalance and network loss of the system at each moment as the goal,and the access phase of single-phase photovoltaic power generation and single-phase load as the optimization variable,the optimal control model of source-load commutation switch is established.The improved NSGA-II algorithm is used to solve the problem,and the access phase of single-phase photovoltaic and load at each moment is obtained.This method is calculated every hour,and the phase difference between photovoltaic and load is adjusted online in real time.The adjustment ability and economy are analyzed.The simulation results show that the optimal control method of source-load commutation switch can suppress the three-phase unbalance of low-voltage distribution network to the greatest extent,reduce network loss and improve voltage and power quality.