Multi-level converters for static VAr compensation

There has been much interest in the use of two level voltage source convertors as advanced static VAr compensators (ASVC). It has been shown that there are two basic problems that must be overcome before this type of ASVC can be put into practical use. Firstly, it is necessary to connect a number of gate turn off (GTO) thyristors in series to form the high voltage switches in the ASVC convertor in order to achieve the required power rating. All the GTO thyristors in the series string must be switched together. This is very difficult to achieve and so far, only a few series-connected GTO thyristors have been successfully switched in commercial equipment. Even then, the losses are significantly greater compared with those in a series string of thyristors used in conventional SVCs. Secondly, the ASVC has a poorer harmonic performance compared with the TCR based SVC. Both problems can be overcome by adopting a multi-level topology. The authors discuss the principles of operation and circuit topology of a 3-phase multi-level ASVC. The factors to consider in determining the number of levels are discussed. Multi-bridge arrangements and balancing of DC capacitor voltages are also discussed