Over-voltage protection: Over-voltage protection to the thyristor is provided using: i) R-C Snubber circuit ii) Non-linear resistor “Thyrector‟
A) Snubber Circuit: It is basically a series r-C circuit which is connected across the device to be protected. The snubber circuit helps to minimize the effects of internal or external overvoltages by following ways: i) Internal over-voltages are generated by interruption of reverse recovery current IRR during turn-off of device. With snubber circuit, the IRR is diverted through R-C after SCR has blocked the reverse current. If LS, R, and c are suitably chosen, the voltage across thyristor can be limited to suitable safe value. ii) At the time of external over-voltages, the capacitor charges through R at slow rate, hence does not allow the voltage to change at high rate. Thus transient voltage spikes are damped and also rate of rise of forward voltage dv/dt across SCR is reduced. iii) The resistance R of snubber circuit causes power loss at the time of energy transfer oscillations between snubber circuit capacitance C and stray circuit inductance, therefore the oscillations are damped and over-voltage transients are reduced. iv) The snubber circuit can be connected directly across secondary winding of transformer to suppress the over-voltages caused by switching on or off the primary winding.
B) Non-linear resistor (Thyrector): Thyrector is a non-linear resistor device having v-I characteristic as shown in the figure. Under normal working voltage it offers very high resistance and draws very small leakage current. However, under over-voltage conditions, its resistance is reduced, it draws heavy current and maintains safe voltage across it. Thus its resistance depends upon voltage across it. This device is therefore used as surge diverter. At the time of overvoltage surge, it conducts heavy current, causing virtual short-circuit. The increased current produces more drop in source impedance and line impedance, by which the SCR volatge is maintained to safe low value. After the surge energy is dissipated in thyrector, the thyrectyior returns to its high resistance state. Thyrector is normally placed across the supply terminals.