Research On Design Of Cascade Multilevel STATCOM And Modulation Methods

With the development of electric system, and the increasing of electric energyproduction, the influence of reactive power on the grid has drawn more and moreattention. Not only does the reactive power can make the loss of grid even worse, butit would also cause voltage fluctuation and flicker in the grid, leading to thethree-phase unbalance, and reducing the rate of utilization of the equipment etc. Thereare lots of traditional reactive compensation devices, but most of them cannot satisfythe demand of the compensation. As a latest dynamic reactive power compensationequipment, STATCOM has many advantages, it represents the developing direction ofthe reactive compensation devices.The object of the paper is the cascaded H-bridge STATCOM, which has advantagesof less switching devices and easy to modularize etc. Firstly, the paper introduces thefrequently-used main circuit’s topology of the reactive compensation devices, then itdraws forth to the casaded multilevel H-bridge’s topology which is widely used in thehigh voltage and large capacity system, and analyzes the operating principle ofSTATCOM to know that the essence of control of the device is controlling theamplitude of the converter’s ac output voltage and the phase angle. The paper alsoworks on the derivation of the mathematical model of the casaded H-bridgeSTATCOM in the dq0reference frame, by which can we understand the characteristicsof the reactive power compensation. After introducing its main circuit andmathematical model, the paper summarizes its basic control algorithm, including thereactive current and dc voltage control algorithm.A reactive power compensation device wants to get the optimal effect, expect forthe key factor of reliable control algorithm, another important part is the choice of themodulation method, different methods lead to different optimizations. The paperintroduces two kinds of commonly used modulation method at the first place, they areunipolar of double frequence SPWM method and space vector modulation methodrespectively. Based on the former introduction, the paper then presents twomodulations which are widely applied in the cascaded multilevel converters, they arecarrier phase-shifted modulation method and carrier disposition modulation method.The key point of the paper is on the reaserch of several kinds of optimized modulationmethod, which are based on the three harmonic injection method and zero-sequencevoltage injection method that is based on the space vector modulation method, and theformer method can increase the modulation ratio to1.15to increase the efficiency ofDC voltage, the later one can decrease the switching loss. However, the two methodsmentioned above are coming from the two-level topology, the paper then applies thesetwo methods to the cascaded multi-level converters topology through the MATLABsimulink, and gets similar results.The paper also gives a detail of the design of a10kV_±1MVar STATCOM’s prototype from three sectors, which are the main circuit’s structure, design of the maincircuit parameters and the controlling circuit. In the controlling circuit, the paperemphasizes the design of protective circuit in the single link’s controlling circuit, andPWM pulse generator based on FPGA in the central controlling circuit. In the end, thepaper gives out the single link’s experiment and its results.