Research On Control Strategies Of STATCOM Considering Unbalance Compensation

In this paper, comprehensive analysis has been made on the H bridge chain STATCOM (static synchronous compensator) assossiating with a real project. To compensate for system voltage imbalance, independent control is needed for three-phase STATCOM. To meet 110kV voltage level grid, you need a large capacity of high-voltage STATCOM. By comprehensive comparison of different STATCOM main circuit structures, H bridge chain structure is selected. In order to get superior control performance, you need precise mathematical model of H bridge chain STATCOM. the biggest drawback of H bridge chain structure is the DC capacitor voltage imbalance. For high voltage and large capacity STATCOM calls for a greater number of DC capacitors, the DC capacitor voltage unbalance problem becomes more difficulty.This paper aims to solve the basic problems of practical engineering, and to seek flexible, reliable, high performance product solutions. With these purposes, five aspects has been done in this paper:(1) Making a choice of STATCOM main circuit; (2) Simulation of the substation at which a STATCOM will be installed and the determination of STATCOM installation capacity; (3) To construct H chain bridge STATCOM mathematical model considering unbalance compensation; (4) Research on DC capacitor voltage unbalance control strategy; (5) Research on unbalance compensation control.This article describes a variety of STATCOM main circuit structures, and reliability, harmonics, control complexity, scope of application, flexibility, economy and other aspects are compared. H bridge chain structure embodies many advantages: high reliability, good output harmonic characteristic, single-phase independent control, modular, flexible and convenient installed. Therefore, determination has been made:the selection of the H bridge chain structure.According to actual substation operation mode and actual parameters of the station, the substation was simulated in PSCAD/EMTDC. Then, the accuracy of the simulation is verify. Then, to improve the bus voltage and to compensate unbalance, the proper STATCOM installation capacity is determined based on comprehensive utilization of the exist compensation capacitors at the station.In order to study the control strategy well, mathematical models of H bridge chain STATCOM, which are under the balance grid voltage condition, are derived in ABC coordinate system and DQO coordinate system respectively. On this basis, the chain STATCOM mathematical model, which is under unbalanced grid voltage condition, is derived. To balance the DC capacitor voltage, the paper presents a control strategy based on active power compensation. Unbalance compensation adopts control of two-coordinate system in DQ0 coordinates. Control of the two-coordinate system can not only control the positive and negative sequence components independently, but also be able to achieve their active and reactive decoupling control. One case varifiese DC capacitors voltage balance control strategy, and the other case confirms the control of two-coordinate system.