Protective Relaying (Part4)
The boundary between the normal and faulty conditions is noc crisp. There are certain operating conditions inherent to the operation of the aower system which are definitely not normal, but these are not electrical faults either. Some examples are the magnetizing inrush current of a transformer, starting current of an induction motor, and the conditions during power swing.
1.1 Should Protective Relays Trip During Abnormal Operating Conditions?
How the protective system should respond to the abnormal operating conditions needs careful consideration. It may or may not be required to take cognizance of the abnormal operating condition. Some examples of abnormal operating conditions are starting currents of motors, inrush currents of transformers and stable power swings. Magnitudewise, these currents may qualify as faults, but there is no need to provide protection from them. Thus, the protective system.must be able to discriminate between the normal operating conditions, abnormal operating conditions, and fault.
1.2 Can Protective Relays Prevent Faults?
It can be seen from the above discussion that protective relays cannot prevent faults. To a certain extent, faults can be prevented by using the properly designed and maintained equipment. However, it is not possible to totally prevent the occurrence of faults.
1.3 What are Protective Relays Supposed to Do?
The protective relays are supposed to detect the fault with the help of current and voltage transformers, and selectively remove only the faulty part from the rest of the system by tripping an appropriate number of circuit breakers. This, the relay has to do with utmost sensitivity, selectivity and speed. In a power system, faults are not an everyday occurrence. A typical relay, therefore, spends all of its life monitoring the power system. It must, therefore, be ready all the time in anticipation of a fault. It is said that a relay operates far more number of times during testing and maintenance than during actual fault! Thus, relaying is like an insurance against damage due to faults.
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