Study On Control Strategy And Reliability Of STATCOM

The STATCOM based on cascaded H-bridge has become the hot topic for its easy expanded, high voltage withstand and modellized. The main content of this thesis researched the capacitor voltage balance control strategy of STATCOM based on cascaded H-bridge, the method of using high frequency zero-sequence voltage to decrease the double frequency fluctuation of capacitor voltage, the reliability of STATCOM based on cascaded H-bridge and its fault tolerant control.Firstly, the STATCOM based on cascaded H-bridge mathematical model is deduced in this thesis. The characteristics of switching frequency of single-polar double frequency and unipolar double frequency CPS-SPWM were analyzed to get the conclusion that the single-polar owns the better characteristics.The power decoupling control is used to control active and reactive power.This thesis analysed the reason of capacitor unbalanced voltage, introduced layer algorithm and negative-sequence current method to control active power of each H module to accomplish the goal of balancing capacitor voltage. The double frequency fluctuation of capacitor voltage was further derived respectively through mathematical deduction. This thesis introduced a method to decrease the double frequency fluctuation with a high frequency zero-sequence voltage. The control strategy with high frequency zero-sequence voltage was studied in this paper.According to the topology structure, the reliability of STATCOM with different H-bridge numbers and different redundancy module numbers were analysised in this thesis. This thesis researched the STATCOM control strategy under H-bridge fault and unbalanced grid conditions and used the fault tolerant control, negative-sequence current decoupling method and DSOGI-FLL to deal with the fault status. Through simulation and experiment, the validity of upper control strategy was verified.Based on simulation model, the main circuit was built.The control circuit with DSP+FPGA was designd, welded and debugged. All the control strategies were verified in this experimental platform. According to experiment, the reactive power was compensated by the statcom dynamically.