| There are growing applications of power electronics in power systems. These applications include advanced static var compensators (ASVC, STATCON or STATCOM) and active filters. ASVCs are used to improve the transient stability of the power system and control flicker in the voltage due to arc-furnace loads. Active filters are needed to neutralize the current harmonics produced by non-linear loads. However, conventional active filters and ASVCs are hampered in their application at medium voltage levels (e.g., 13.8 kV) due to the need for a line-frequency interface transformer. Alternative methods involve the use of complex control strategy or severe under-utilization of solid state devices.; This thesis proposes a per-phase, all-active approach suitable for power quality applications. It uses a hybrid combination of slow and fast switching series-connected converters. This approach results in the elimination of the bulky line-frequency transformer. It also uses simple control strategy and optimally utilizes solid state devices. This thesis explains the principle of operation of the proposed topology. It includes simulation and experimental results from a proof-of-concept laboratory prototype for an active filter application. Different control schemes like the harmonic elimination method are investigated. Improvements in the control strategy are suggested to improve the dynamic performance of the system and experimentally verified. A comparative evaluation of the proposed scheme is conducted with a conventional six-switch PWM Advanced Static Var Compensator. Finally, the thesis discusses issues like startup and protection, which are essential in order to make this scheme practicable. |
