Modelling, analysis and control of static VAR compensator (SVC) using three-level inverter

Abstract

A static var compensator (SVC) using three-level GTO voltage source inverter (VSI) is presented for high-power/high-voltage applications. The three-level inverter has the advantages that the blocking voltage of each switching device is one half of DC-link voltage whereas full DC-link voltage for two-level inverter, and the harmonic contents of three-level inverter output voltage are far less than those of two-level one at the same switching frequency. By introducing three-level inverter, the presented SVC system is operated at a very low switching frequency $(f_{sw}=180Hz)$ without excessive harmonic contents and can be coupled directly to 3.3kV AC mains using 4500V GTO available at present. In addition, in order to design a controller having good performances, the SVC system is completely modelled by using the circuit DQ-transformation and thus a general and simple instructive equivalent circuit is obtained; the inductor set becomes a second order gyrator-coupled system and the switch set becomes a transformer as well. The analysis of the equivalent circuit gives all system behaviors. Based on the DC analysis, the linked reactor L and the DC capacitor C are designed so as to meet the requirements of the SVC system. Also, on the basis of the AC analysis, two control methods to compensate the required reactive power are presented. To compensate the required reactive power, there are two different methods of adjusting the inverter output voltage. One is to change the DC-side capacitor voltage with a fixed modulation index (MI) of the switching function by controlling the phase angle of inverter output voltage with respect to the source voltage. The other is to change MI of the switching function while the DC-side capacitor voltage is kept constant by some means. In the former case, from the transfer function of the open loop system a controller having fast dynamic response is designed and verified by the experimental results. In the latter case, the plan...