Research On Control Strategies Of Cascaded STATCOM In Unbalanced System

The inductive-resistive load and unbalanced impulse load used in daily life andindustrial production put distribution system to the test of power quality issues, such usreactive power pollution, voltage flicker and three-phase unbalanced. As the latesttechnology of reactive power compensator, STATCOM represents the developing directionof FACTS, can solve these power quality problems.This article describes a variety of STATCOM main circuit structuires, and comparedfrom many aspects. Cascade multi-level structure embodies many advantages: good outputharmonic characteristic, modular, single-phase independent, high reliability. Therefore,determination has been made: the selection of the H bridge chain structure. And on thisbasis, the basic operation principle of STATCOM was analyzed, prepared for the nextresearch.In order to study the control strategy well, mathematical models of cascadedSTATCOM are derived in ABC coordinate system. The expression of reactive power, activecurrent and reactive current absorbed by STATCOM and the output voltage of STATCOMwere obtained by analyzing the mathematic model. The influence on output performance ofSTATCOM is analyzed under unbalanced voltage of power system. Under unbalancedvoltage, two times frequency wave occurs in voltage of capacitor at DC side of STATCOM,which result in negative sequence and3tines harmonics in AC voltage and current of theconverter.To balanced the DC capacitor voltage, the paper analysis the mechanism of cascadedSTATCOM DC capacitor voltage unbalanced. a control strategy based on changing theactive power absorbed by chain link inverters is presented. The control strategy ofSTATCOM under the balance condition is discussed, which include voltage and currentloop control. This control strategy achieve the function of STCTCOM supporting thevoltage of the point of common coupling. Simulation results show the good performance.In order to compensate the imbalanced load, the positive and negative sequencecalculation method of STATCOM is analyzed. A calculation algorithm of positive and negative sequence compoment, which called delaying method, is detailed in the paper. Onthis basis, a method of current detection is discussed, which is fundamental for thethree-phase power fundamental frequency positive sequence voltage synchronous referencecoordinates. The detection method is used in unbalanced compensation control strategy.Simulation indicated the correctness of the control strategy.,...