Research On Three-phase Unbalanced Control Of Low Voltage Distribution Network

The industrial boom and the improvement of residents living quality depend on the quality of power supply.One of the most common ways to improve power quality is to control three-phase unbalance of low-voltage distribution network and compensate reactive power to improve power factor.Common methods to deal with three-phase unbalance include capacitor compensation device,intelligent commutation switch and static var compensator(DSTATCOM).Among them,capacitor compensation device has the disadvantages of high cost,large volume,and the compensation effect depends on the power factor of the power grid.Commutation switch requires very short commutation time,otherwise it will cause power grid fluctuation or even power outage.The commutation time must be very short when the commutation switch works normally.Otherwise,it will cause the fluctuation of power grid and even lead to power failure.Therefore,this paper studies the design of DSTATCOM for different scenarios based on two algorithms.It is of great significance to reduce three-phase unbalance and improve power factor.The main contents of this thesis are as follows:(1)Explain what is three-phase unbalance and give the calculation method of three-phase unbalance degree.Reactive power compensation is proposed to achieve three-phase unbalance control,and improve the power factor,which provides a theoretical basis for follow-up research.The DSTATCOM device is introduced and its design principle is expounded.The operation of DSTATCOM under three operating conditions and the advantages of DSTATCOM are described.(2)Based on DSTATCOM topological structure mathematical modeling,Clarke and Park transform are implemented.Based on the principle of symmetrical component,the positive and negative sequence of current is separated.Positive sequence control realizes reactive current compensation and DC voltage stability,while negative sequence control realizes negative sequence current compensation.The positive and negative control signals are superimposed and modulated with triangular carrier wave as the control signal of DSTATCOM.Finally,the feasibility of the scheme is verified by simulation under two working conditions: pure resistive load and resistive inductive load.(3)Linearization analysis of DSTATCOM mathematical model is carried out by using feedback precise linearization,and inner loop current control link is designed.Based on the sliding mode control algorithm(SMC),the outer loop voltage control link is designed to realize the DC side voltage stabilization.This design is based on DSC improved current positive and negative separation algorithm.The positive sequence link takes reactive current as instruction,the negative sequence link takes negative sequence current as instruction,and superimposes control signals to achieve three-phase unbalance control and power factor improvement.Finally,the feasibility of the scheme is verified by simulation from two working conditions of pure resistive load and resistive inductive load.(4)Two treatment schemes of capacitor compensation device and thyristor commutation switch are analyzed and discussed,and the characteristics of the two schemes are summarized.Finally,according to the two DSTATCOM systems designed in this paper,four schemes are compared and analyzed.