Design and Implementation Of Medium Voltage Thyristor Gate Driver for Medium Voltage Induction Motor In Soft Starter Application

Abstract

Triggering a thyristor requires meeting its gate energy specifications. The gate should be driven hard and fast to ensure complete gate turn on and thus minimize di/dt effects. Usually this means a gate current of at least three times the gate turn on current with a pulse rise of less than one microsecond and a pulse width greater than 10 microseconds. MV applications of soft starters have an additional complication of several SCRs in series in each phase. If any one SCR in a phase should commutate OFF early, the whole chain is interrupted, plus there is a very high probability that a very high voltage will be presented across that one SCR until the rest of the SCRs in that phase switch OFF. This increases the risk of SCR failure due to over voltage and care must be taken to ensure that the voltages are balanced across the SCRs during their OFF state. So most reliable method of controlling SCRs in series, is the continuous fire or Hard fire technique and according to that gate driver circuit will design. To aid the engineer during the initial design or specification stage, a survey is made of the three fundamental series SCR gate drive topologies and their most common variants, including floating bias drivers, MV pulse transformers and light triggered designs. The advantages, limitations, and inherent features of several methods are described and compared with the competing methods. The proposed medium voltage thyristor gate driver circuit is simulated in Psim 6.0 software tool. Simulation of proposed gate driver circuit is performed on medium voltage induction motor of rating 3300V, 250 KW. It's known that the maximal torque of induction motor is approximate in direct proportion to the square of the voltage of the stator. Accordingly, a closed-loop control system is designed in which the voltage is used as the feedbacks, which is a solution to the true soft starter. In this report, the control strategies adopted is described in detail. The experiments prove that the proposed solution of the gate driver has better performances so it can be used for medium voltage soft starter application.