A new current-source converter using a symmetric gate-commutated thyristor (SGCT)

In recent years, extensive semiconductor development has gone into both bipolar and MOS structures for medium-voltage (MV) applications. However, the progression of MOS structures in MV applications has been difficult and the issues of module isolation, reliability, and dv/dt and motor/bearing life continue to limit its acceptance in these applications. A more suitable device structure and the natural choice for MV applications is the bipolar thyristor structure. The device that has been in use for many years, the gate-turn-off thyristor, is being replaced by the gate-commutated thyristor (GCT). So far, the GCT has been only thought of as fulfilling the needs for the voltage-source topology. However, the symmetric GCT (SGCT) is viable and has significant advantages when implemented in a pulsewidth-modulated current-source inverter (PWM-CSI). This paper will describe the design and characteristics of an 800-A 6.5-kV SGCT and the effect of its implementation in a PWM-CSI. These effects include operation at a higher switching frequency, elimination/reduction and modification of the snubber circuitry, reduction in size of the passive components, and a major impact on the cost of the converter. The paper includes experimental results on a 4160-V 1250-hp PWM-CSI ac drive.