Overvoltage Protection
❖ It is the most important protection scheme w. r. t. others as thyristors are very sensitive to over voltages.
❖ Maximum time thyristor failures happen due to over-voltage transients.
❖ A thyristor may be subjected to internal or external over-voltages.
❖ Internal Over-Voltages : After commutation of a thyristor reverse recovery current decays abruptly with high di/dt which causes a high reverse voltage [as, V = L(di/dt) so if di/dt is high then V will be large] that can exceed the rated break-over voltage and the device may be damaged.
❖ External Over-Voltages : These are caused due to various reasons in the supply line like lightning, surge conditions (abnormal voltage spike) etc.
❖ External over voltage may cause different types of problem in thyristor operation like increase in leakage current, permanent breakdown of junctions, unwanted turn-on of devices etc
❖ So, we must suppress the over-voltages.
Protective measures:
❖ The effect of over-voltages can be minimized by using non-linear resistors called voltage clamping devices like metal oxide varistors.
❖ At the time of normal operation, it offers high impedance and acts as it is not present in the circuit.
❖ But when the voltage exceeds the rated voltage then it serves as a low impedance path to protect SCR.
Over Current Protection
❖ Overcurrentmainly occurs due to different types of faults in the circuit
❖ Due to overcurrent i2R loss will increase and high generation of heat may take place that can exceed the permissible limit and burn the device.
Protective measures:
❖ SCR can be protected from overcurrent by using Circuit Breaker (CB) and fast acting current limiting fuses (FACLF).
❖ CBs are used for protection of thyristor against continuous overloads or against surge currents of long duration as a CB has long tripping time.
❖ But fast-acting fuses is used for protecting SCR against high surge current of very short duration.
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pdf made by Dr M Balasubbareddy