Over current fault conditions, over current protection
Introduction of overcurrent fault conditions
Most thyrister circuit are fast operating and can be controlled in such way as to prvent the load current from rising too high to cause circuit damage where the current can rise out of control to many times the normal rated value,and steps have to be taken to limit these conditions and to protect the other circuit components against the effects of the high current..
Cause of overcurrent fault condition
An overcurrent can occur due to any of the following caues:-
(i) Output short-circuit.
(ii) Internal faults thyristor circuit.
(iii) Inversion failure in naturaly commutated circuits.
(iv) Commutation failure in a forced commutation circuits,and
(v) Short circuit between one of the phases of mains and the bridge.
O ver current protection
As the thyristors have a restricted over current capacity, special fast-acting fuses and circuit breakers are usually provided for overcurrent protection. As the fault current increases, the fuse opens and clears the fault current in few milliseconds.
Necessary of overcurrent protection
(1) It must carry continuously the device rated current.
(2) Its thermal storage capacity must be less than that of device being protected, that is I2t let through value of the fuse before the fault-current is cleared must be less than the rated I2t of the device to be protected.
(3) The fuse voltage during arcing must be high enough to force the current down and dissipate the circuit the circuit energy.
(4) After breaking the current , the fuse must be able to withstand any restiking voltage which appears across it.