Study On Control Scheme Of Three-phase Unbalanced Load In Rural Distribution Area

With the rapid development of economy,the quality of rural life is improved obviously,more and more high power loads switch in ordinary families,making the problem of the imbalance of three-phase load in rural electric power distribution area become one of the main problems that influences the safe and stable operation of power system.The imbalance of three-phase load causes a series of damage,such as the increasing loss of distribution transformer and line,the declining of transfer efficiency of electric energy,the increasing temperature caused by increasing zero sequence current of distribution transformer,and it is hard to guarantee a good power quality.In view of the unbalanced three-phase load,now,mainly have three kinds of solutions.The first one is by the way of reactive compensation in the low side of distribution transformer.It's defect is only to a certain extent to their own problems,but will not be able to solve the problems on the power line.The second one,mainly for large load,is to asymmetric transfers of power load.It has a high cost,low reliability and difficult to control which needs to increase the parallel compensation device.The third one is artificial commutation.Due to the randomness and uncertainly of power load,artificial cannot rely on online real-time adjustment according to actual load imbalance,inevitably affect the reliability of the user,and pose a safety hazard to a certain extent.In view of the advantages and disadvantages of the above scheme,this paper presents a three-phase unbalanced control scheme based on phase-changing switch.This scheme can realize automatically charged phase-changing in client load,reduce the degree of three-phase imbalance in rural electric power distribution area.This control scheme mainly includes automatically phase-changing switch and distribution control terminal.Automatically phase-changing switch is the executor of the strategy of phase-changing,moreover,distribution control terminal is the centre of control scheme.Firstly,ensuring the best combining scheme of automatically phase-changing switch through analyzing composition and structure of switch unit,the switching process is analyzed in detail,and the optimal control strategy is worked out by comparing five kinds of control strategy;and then study the function of distribution control terminal,the monitoring calculation theory of three-phase unbalance current on the three phase four wire system is discussed in detail,GRPS wireless and PLC are combined to transmit signal between distribution control terminal and automatically phase-changing switch;finally the hardware and software of the control scheme is studied.In this paper,we establish a mathematical model of single objective that can make phase sequence adjustment after the neutral line current with minimum,and a multi-objective mathematical model that can achieve the aim that minimum degree of three phase imbalance in rural electric power distribution area and minimum time of switching of automatically phase-changing switch during changing phase.According to wiring installation parameters of user load in the practical engineering,using PSCAD/EMTDC simulation software to build optimization models of three-phase unbalanced load with 220V voltage system which simulated the unbalanced three-phase load,the optimal phase sequence adjustment strategy is simulated and analyzed,and processing data and analyzing waveform is accomplished with the help of MATLAB software,validate the adjustment ability of three-phase current unbalance degree,the simulation results show that the model is in line with reality,and the proposed optimal phase sequence adjustment strategy is effective.