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 two-level three-leg voltage-source converter (VSC) topology. Reactive-power control is achieved by phase-shift-angle control, and VSC harmonics are eliminated by selective harmonic elimination method (SHEM). VSC has been designed at the highest low-voltage level of 1 kV and connected to a medium-voltage (MV) bus through a low-pass input filter and Delta/Y-connected MV/1-kV coupling transformer. At the MV side of D-STATCOM, line-current harmonics are minimized to comply with the IEEE Std. 519-1992 for the weakest supply conditions by applying 8-angle TLN2 elimination technique. This necessitates switching the water-cooled high-voltage insulated-gate bipolar transistor (HV-IGBT) modules at 850 Hz, thus eliminating 5th, 7th, 11th, 13th, 17th, 19th, 23rd, and 25th voltage harmonics at the input of VSC. By carefully designing the laminated bus system and selecting minimum stray-inductance dc-link capacitors directly mountable on the laminated bus, stray inductance of the 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 for resistor-capacitor-diode (RCD) clamping snubbers. The performance of SHEM, together with the 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 the other known control techniques.