| There are many unbalanced inductive loads in power distribution system.The reactive power and negative sequence current not only increase line loss,but also cause the malfunction of relay protection devices starting with negative sequence current,which threatens the safe operation of power system.Therefore,reactive power compensation and load imbalance suppression by shunt compensation devices to improve power quality has been widely studied.Dynamic Capacitor(D-CAP)is a new type of power quality control device with low cost and excellent performance which can make a good development in the reactive power compensation field.Currently,D-CAP has successfully realized dynamic reactive power compensation and harmonic suppression,but the negative sequence current compensation has not been studied yet.In this paper,the comprehensive compensation of D-CAP for reactive power and negative sequence current under unbalanced inductive load are studied.Firstly,basic structure and reactive power compensation principle of D-CAP are analyzed.Then the relationship between power factor correction and imbalance suppression in three-phase three-wire system is studied,and the feasibility of using D-CAP to compensate negative sequence current is analyzed.The equivalent compensation capacitance expression of delta-connected D-CAP to compensate unbalanced load is deduced.Based on the equivalent compensation capacitance expression,the ability of delta-connected D-CAP to compensate negative sequence current is determined.Then,for the load whose negative sequence current exceeds the compensation ability of delta-connected D-CAP,an optimal compensation method is proposed to make delta-connected D-CAP compensate the negative sequence current of unbalanced load as much as possible.For star-connected D-CAP,the neutral point potential drift will occur compensating unbalanced load,and the rated voltage at both ends of D-CAP is usually 1.1-1.3 times of the grid voltage,so its compensation ability will be limited by the rated voltage.In this paper,the negative sequence current compensation capability of star-connected D-CAP is analyzed,and the negative sequence current limit is imposed so that D-CAP can compensate the negative sequence current as much as possible without exceeding its rated voltage.Finally,the compensation characteristics of delta-and star-connected D-CAP are compared and analyzed.The correctness of theoretical analysis and the effectiveness of control strategy are verified by MATLAB/Simulink and the experimental prototype of 33kVA/220 V three-phase Buck-type D-CAP. |
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