Fig.1 The main circuit configuration of the +/-80MVA GCT STATCOM

The main circuit configuration of the +/-80MVA GCT STATCOM

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A multistage voltage-source three-level converter configuration using multistage-transformer is applied to reduce voltage harmonics along with harmonic elimination pulse-width modulation (PWM) for AC filter-less design.

Each stage consists of three single phase converters with DC capacitors rated at 6kV of DC voltage. The converters incorporate the largest capacity GCT thyristor rated at 6kV, 6kA using snubberless design to achieve higher efficiency and more compactness. The discharge circuits (OV-GCTs in Fig. 1) are also connected to the DC circuit for over-voltage protection. The OV-GCT is comprised of the GCT and a resistance to discharge the DC capacitors rapidly for continuous operation during AC system faults.

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