The Study Of SVG For Unbalanced System Of Distribution Network

Static Var Generator(SVG)is an important reactive power compensation equipment in flexible AC power transmission,which can solve the problem of reactive power compensation and common connection point voltage compensation. It is widely applied in transmission and distribution systems. As the complexity of the transmission and distribution network increases, the phenomenon of unbalanced system is also growing.The imbalance of the system endangers the electrical equipment, the negative sequence component caused by which also fluctuates the grid voltage.Therefore, the study of the common connection point in unbalanced compensation system becomes extremely important.The reactive power theory and working principle of SVG are introduced in this paper. The main structure and its role of the system are described.Then the importance of static var compensation for modern industry is analyzed in this paper.The principle of compensation unbalanced system is deduced according to the exchange mechanism in SVG、load and power line in this paper. By analyzing and comparing of various detection methods in unbalanced system, the applicable detection method for this subject are choosed.The positive and negative sequence current separation and detection link in the original detection method are added in this paper.The trapper which used to filt out second harmonic component of DC side capacitor voltage is added,which makes the detection results more accurate.Combining the symmetrical components method, the mathematical model of the unbalanced system of SVG is established on the basis of mathematical model of the balance system in this paper. Compensation control strategy based on positive and negative dq coordinate system are proposed in this paper with two group controllers; one group is positive-sequence controller based on dq the coordinate system and the other is negative-sequence controller based on the dq coordinate system. The simulation experiment verified that it can eliminate the negative sequence grid current which caused harm to the system by this control strategy.Finally, simulation model in the condition of unbalanced load and three-phase short circuit are builded in this paper.The simulation results verify that the compensation control strategy proposed in this paper will well meet the requirements of the system, and it not only will reduce the unbalance of three-phase grid voltage, but also inhibit the fluctuation and distortion of common node voltage. Simulation result also shows that the control strategy can make SVG response fast, control high accuracy and have well robustness.