Research On The Control Technology Of Cascade STATCOM

With the fast development of electrical power system, a large number of non-linear, shock devices with low power factor are facing to serious challenge in the power grid. Therefore, it is very important to sustain voltage stability、reduce harmonic pollution and improve the power factor in the electrical system. To address the issue, cascade STATCOM was employed for the advantage of improving the power factor of FACTS.Here, simulation models were built and employed in various control strategies with the help of MATLAB / simulink software. Four aspects of work were done in the dissertation.Firstly, based on the basic principle of STATCOM reactive power compensation and instantaneous reactive power theory, several methods were detected and compared, and a notch filter was found to be able to accurately achieve fundamental positive and negative sequence current separation by providing precise reference value of dq transformation for unbalanced system control.Secondly, the "Y" cascade multi-level inverter was chosen as the main circuit of STATCOM in several multi-level topological structures, for the advantages of extensive modules and low level of harmonic contents. And, the cascaded H-bridge number and the device parameters were analyzed quantitatively, and detail arguments were determined finally based on a specific example. At last, the technology of CPS-SPWM was employed.Thirdly, in the reactive power compensation and H-bridge DC voltage balance, the methods of main floor DC side balance and direct current decoupling were proposed, The methods could provide the first stage output instruction, then, through the second layer and the primary control of DC voltage balance, a fine-tuning voltage which correct the first stage output instruction was obtained. With improved CPS-SPWM, the need of reactive power compensation and a good solution to the DC voltage balance were fit.Improved CPS-SPWM used the carrier-inverting mode, which a compared wave between improved modulation wave and carrier wave was utilized to control the switching device state of each H-bridge, which is equivalent to increase the switching frequency. The method has the advantage of increasing the output pulse frequency of the voltage, guarding the safe operation of STATCOM and effectively reducing the higher-order harmonics.Fourthly, Unbalanced load is the most common imbalance system, Compensation Method for balance system is not suitable for imbalance system, it can’t be completed to provide reactive power compensation to improve the power factor. Thesis put forth a positive and negative control superposition of two subsystems. Simulation analysis confirmed: This control method can play better when the load is unbalanced reactive power compensation performance, effectively reducing the current imbalance and improve power quality.