Experience with high power frequency converters employing advanced GTO (IGCT) series connection

Summary form only given. Large single-phase AC systems can be found in the area of electric traction power supply. Low frequency, single-phase systems exist in several countries. Power supply from, and power exchange with the 3-phase public grid is important or even vital for those systems. The first self-commutated converters with GTOs (gate turn-off thyristors) and a fundamental frequency switching pattern in a moderate power range (8-12 MW) were built about ten years ago. The development in the following years was quite interesting. The introduction of multi-step, pulse-width modulation brought a substantial improvement of the voltage shape, and omitted the need for harmonic filters. The largest converter station built with this technology (conventional GTOs) consists of two converter groups, each rated 25 MW. It is in operation since 1994. The demand for higher ratings was met through the introduction of the next development step: series connection of GCTs (gate commutated thyristors). A converter group with a nominal rating of 100 MW is in full commercial operation since 1996. Another installation, consisting of two converter groups with a rating of 50 MW/67 MVA each, commenced commercial operation in fall of 1999. The operational experience so far has proven the viability of the concept. The extremely high reliability (no outage of power semiconductors yet) has further proven that the challenging development goals have all been successfully achieved