Modelling and Simulation of 6 Pulse GTO Thyristor Converter

Abstract

Conventional thyristor (known as SCR) has being used in various applications such as in power converter of high voltage direct current (hvdc) transmission system and as a dc speed controller for dc motor application (F. Schettler et al., 2000). Conventional thyristor can only be turned on with two conditions, that is when the device is in forward blocking state or when positive gate current is applied at the gate. This thyristor cannot be turned off by applying negative gate current. It can only be turned off if anode current goes negative (reverse). This happens when negative portion of the sine wave occurs (natural commutation). Another method of turning off is known as “forced commutation” where anode current is “diverted” to another circuitry. Alternatively, we have Gate Turn-Off (GTO) thyristor to overcome the disadvantage of conventional thyristor. GTO thyristor behave like conventional thyristor, but can be turned off using gate signal. It needs very large reverse gate current (normally 1/5 of anode current) to turn-off. Since a GTO thyristor converter is self-commutated, it can be used to supply power to a week ac system, and even to a “load-only” system. At the same time, it is able to control reactive power from lead to lag to keep an ac bus voltage constant. In this paper, we create a model of GTO thyristor with its operation circuit and gate circuit using OrCAD PSPICE simulator and make an analysis of their switching characteristics at different anode currents. Then, this GTO thyristor model is use and implement in the 6 pulse converter circuit. Finally, we compare the simulation results of dc output voltage with mathematical calculation results.