Chain STATCOM Control Of Asymmetric Load Compensation System Research

Static synchronous compensator(STATCOM) is a core technology for flexible AC transmission system(FACTS). In this paper, the key technologies of cascade STATCOM are studied. The problems of asymmetrical load compensation are researched.The topology and working principle of chain STATCOM is introduced.Then mathematical model of chain STATCOM is established in the two-phase synchronous coordinates by the coordinate transformation.Step wave modulation strategy is used and the basic principle of the modulation strategy is analyzed for cascade STATCOM. The phenomenon of capacitor voltage imbalance is analyzed and the mathematical expression of capacitor voltage is given.The law of capacitor voltage imbalance is got.Combined with stepped-wave modulation strategy, the method based on selecting conduction angles is proposed to concrol the capacitor voltage. The principle of this control strategy and mode judging method is introduced.A solution that is suitable for asymmetrical load compensation in chain STATCOM is used.The solution is combined ip-iq method with the split-phase admittance detection algorithm of symmetrical components. Finally the control system can be designed as a second critical damping ratio system. The nonlinear systems caused by equivalent series resistance of STATCOM make it difficult that design the control system.The part of nonlinear systems was separated and use output linearization method to transform the nonlinear system into linear system. Combine the load admittance feedforward method with bus admittance feedback method to improve the response speed and the steady precision of the control system.The model has been built in the SIMULINK and verify the effectiveness of the modulation strategy and the algorithm of the capacitor voltage balancing.During this research project, simulation tests are performed in order to analyzeproblems modulation strategy, control of capacitor voltage balancing and controlmethod of unbalanced load compensation and so on. Meanwhile cascade STATCOMexperimental system of 380V/5A is designed.