Author: C. Russell Mason
Category: Technology & Engineering
We usually think of an electric power system in terms of its more impressive partsÐthe big generating stations, transformers, high-voltage lines, etc. While these are some of the basic elements, there are many other necessary and fascinating components. Protective relaying is one of these. The role of protective relaying in electric-power-system design and operation is explained by a brief examination of the over-all background. There are three aspects of a power system that will serve the purposes of this examination. These aspects are as follows: A. Normal operation B. Prevention of electrical failure. C. Mitigation of the effects of electrical failure. The term Ònormal operationÓ assumes no failures of equipment, no mistakes of personnel, nor Òacts of God.Ó It involves the minimum requirements for supplying the existing load and a certain amount of anticipated future load. Some of the considerations are: A. Choice between hydro, steam, or other sources of power. B. Location of generating stations. C. Transmission of power to the load. D. Study of the load characteristics and planning for its future growth. E. Metering F. Voltage and frequency regulation. G. System operation. E. Normal maintenance. The provisions for normal operation involve the major expense for equipment and operation, but a system designed according to this aspect alone could not possibly meet present-day requirements. Electrical equipment failures would cause intolerable outages. There must be additional provisions to minimize damage to equipment and interruptions to the service when failures occur. Two recourses are open: (1) to incorporate features of design aimed at preventing failures, and (2) to include provisions for mitigating the effects of failure when it occurs. Modern 2 THE PHILOSOPHY OF PROTECTIVE RELAYING power-system design employs varying degrees of both recourses, as dictated by the economics of any particular situation. Notable advances continue to be made toward greater reliability. But also, increasingly greater reliance is being placed on electric power. Consequently, even though the probability of failure is decreased, the tolerance of the possible harm to the service is also decreased. But it is futile-or at least not economically justifiable-to try to prevent failures completely. Sooner or later the law of diminishing returns makes itself felt. Where this occurs will vary between systems and between parts of a system, but, when this point is reached, further expenditure for failure prevention is discouraged. It is much more profitable, then, to let some failures occur and to provide for mitigating their effects. 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.1 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. 3. Fuses. C. Features that mitigate the loss of the faulty element. 1. Alternate circuits. 2. Reserve generator and transformer capacity. 3. Automatic reclosing. THE PHILOSOPHY OF PROTECTIVE RELAYING 3 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 Òprotect,Ó 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.2 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. THE FUNCTION OF PROTECTIVE RELAYING The function of protective relaying is to cause the prompt removal from service of any element of a power system when it suffers a short circuit, or when it starts to operate in any abnormal manner that might cause damage or otherwise interfere with the effective operation of the rest of the system. The relaying equipment is aided in this task by circuit breakers that are capable of disconnecting the faulty element when they are called upon to do so by the relaying equipment. Circuit breakers are generally located so that each generator, transformer, bus, transmission line, etc., can be completely disconnected from the rest of the system. These circuit breakers must have sufficient capacity so that they can carry momentarily the maximum short-circuit current that can flow through them, and then interrupt this current; they must also withstand closing in on such a short circuit and then interrupting it according to certain prescribed standards.3 Fusing is employed where protective relays and circuit breakers are not economically justifiable. Although the principal function of protective relaying is to mitigate the effects of short circuits, other abnormal operating conditions arise that also require the services of protective relaying. This is particularly true of generators and motors. A secondary function of protective relaying is to provide indication of the location and type of failure. Such data not only assist in expediting repair but also, by comparison with 4 THE PHILOSOPHY OF PROTECTIVE RELAYING human observation and automatic oscillograph records, they provide means for analyzing the effectiveness of the fault-prevention and mitigation features including the protective relaying itself. FUNDAMENTAL PRINCIPLES OF PROTECTIVE RELAYING Let us consider for the moment only the relaying equipment for the protection against short circuits. There are two groups of such equipmentÐone which we shall call ÒprimaryÓ relaying, and the other Òback-upÓ relaying. Primary relaying is the first line of defense, whereas back-up relaying functions only when primary relaying fails.