VSC BASED D-STATCOM WITH SELECTIVE HARMONIC ELIMINATION

This paper describes the design, implementation and performance of a medium-size distribution type static synchronous compensator (D-STATCOM) with the simplest 2-level, 3-leg VSC topology. Reactive power control is achieved by phase-shift-angle control, and voltage source converter (VSC) harmonics arc eliminated by selective harmonic elimination method (SHEM) technique. VSC has been designed at the highest low-voltage level of 1 kV, and connected to medium-voltage (MV) bus through a low-pass input filter and AN connected MV/1 kV coupling transformer. At the medium-voltage side of D-STATCOM, line current harmonics are minimized to comply with IEEE Std. 519-1992 for the weakest supply conditions by applying 8-angle TLN2 elimination technique. This makes necessary switching the water-cooled high voltage (HV) IGBT modules at 850 Hz, thus eliminating 5(th), 7(th), 11(th), 13(th), 17(th), 19(th), 23(rd) and 25(th) voltage harmonics at the input of VSC. By carefully designing the laminated bus system, and selecting minimum stray inductance de link capacitors directly mountable on the laminated bus, stray inductance of commutation path is brought to a nearly absolute minimum of 60 nH, thus maximizing the utilization of wire bond, single-side cooled HV IGBTs, and eliminating the need to RCD clamping snubbers, The performance of SHEM together phase-shift-angle control has been tested in the field on a 0-1780 kVAr capacitive, 6.3 kV VSC based D-STATCOM (-750/+900 kVAr VSC) prototype. Field test results show that SHEM together with phase-shift-angle control leads to optimum switching frequency, and device utilization for HV IGBTs and high system performance at the expense of slower response as compared to other known control techniques.