Assessment of two different STATCOM configurations for FACTS application in power systems

The static synchronous compensator (STATCOM) using GTOs (gate-turn-off thyristors) is a recent addition to the family of FACTS. Devices available from the power electronics industry and is now commercially employable for static shunt reactive power compensation in electrical power transmission and distribution systems. Two convertor configurations both based on voltage-sourced convertors are described in this paper. One is a multipulse three-phase Graetz bridge convertor with a single DC capacitor and an alternative is a multilevel convertor using separate single-phase bridges in series (i.e., chain link circuit) and employing multiple DC capacitors. The fundamental operation of both types of the STATCOM and their application to reactive power compensation line presented. A 15-link chain circuit STATCOM and a 24-pulse Graetz bridge STATCOM are digitally modelled using the PSCAD/EMTDC program in the time-domain. The waveforms under both balanced and unbalanced AC system conditions are presented so as to compare the harmonic and phase-balancing performance of the two schemes and to illustrate the potential advantages of the single-phase chain circuit over the three-phase Graetz bridge convertor.