The type of electrical failure that causes greatest concern is the short circuit, or ÒfaultÓ as it is usually called, but there are other abnormal operating conditions peculiar to certain elements of the system that also require attention. Some of the features of design and operation aimed at preventing electrical failure are:
A. Provision of adequate insulation.
B. Coordination of insulation strength with the capabilities of lightning arresters.
C. Use of overhead ground wires and low tower-footing resistance.
D. Design for mechanical strength to reduce exposure, and to minimize the likelihood of failure causable by animals, birds, insects, dirt, sleet, etc.
E. Proper operation and maintenance practices.
Some of the features of design and operation for mitigating the effects of failure are:
A. Features that mitigate the immediate effects of an electrical failure.
1. Design to limit the magnitude of short-circuit current.
a. By avoiding too large concentrations of generating capacity.
b. By using current-limiting impedance.
2. Design to withstand mechanical stresses and heating owing to short-circuit currents.
3. Time-delay undervoltage devices on circuit breakers to prevent dropping loads during momentary voltage dips.
4. Ground-fault neutralizers (Petersen coils).
B. Features for promptly disconnecting the faulty element.
1. Protective relaying.
2. Circuit breakers with sufficient interrupting capacity.
C. Features that mitigate the loss of the faulty element.
1. Alternate circuits.
2. Reserve generator and transformer capacity.
3. Automatic reclosing.
D. Features that operate throughout the period from the inception of the fault until after its removal, to maintain voltage and stability.
1. Automatic voltage regulation.
2. Stability characteristics of generators.
E. Means for observing the electiveness of the foregoing features.
1. Automatic oscillographs.
2. Efficient human observation and record keeping.
F. Frequent surveys as system changes or additions are made, to be sure that the foregoing features are still adequate. Thus, protective relaying is one of several features of system design concerned with minimizing damage to equipment and interruptions to service when electrical failures occur. When we say that relays “protect” we mean that, together with other equipment, the relays help to minimize damage and improve service. It will be evident that all the mitigation features are dependent on one another for successfully minimizing the effects of failure. Therefore, the capabilities and the application requirements of protective-relaying equipments should be considered concurrently with the other features.This statement is emphasized because there is sometimes a tendency to think of the protective-relaying equipment after all other design considerations are irrevocably settled. Within economic limits, an electric power system should be designed so that it can be adequately protected.