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

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 GTO, 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 PWM current source inverter. This paper describes 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