The ability of protective relaying to permit fuller use of the system capacity is forcefully illustrated by system stability. Figure 4 shows how the speed of protective relaying influences the amount of power that can be transmitted without loss of synchronism when short circuits occur. 4 More load can be carried over an existing system by speeding up the protective relaying. This has been shown to be a relatively inexpensive way to increase the transient stability limits Where stability is a problem, protective relaying can often be evaluated against the cost of constructing additional transmission lines or switching Other circumstances will be shown later in which certain types of protective-relaying equipment can permit savings in circuit breakers and transmission lines d fa fault Relay plus breaker time Fig. 4. Curves illustrating the relation between relay- plus-breaker time and the maximum amount of power that can be transmitted over one particular system without loss of synchronism when various faults occur. The quality of the protective-relaying equipment can affect engineering expense in pplying the relaying equipment itself. Equipment that can still operate properly when future changes are made in a system or its operation will save much future engineering and other related expense One should not conclude that the justifiable expense for a given protective-relaying equipment is necessarily proportional to the value or importance of the system element to be directly protected. A failure in that system element may affect the ability of the entire system to render service, and therefore that relaying equipment is actually protecting the service of the entire system. Some of the most serious shutdowns have been caused by consequential effects growing out of an original failure in relatively unimportant equipment that was not properly protected THE PHILOSOPHY OF PROTECTIVE RELAYING
THE PHILOSOPHY OF PROTECTIVE RELAYING 11 The ability of protective relaying to permit fuller use of the system capacity is forcefully illustrated by system stability. Figure 4 shows how the speed of protective relaying influences the amount of power that can be transmitted without loss of synchronism when short circuits occur.4 More load can be carried over an existing system by speeding up the protective relaying. This has been shown to be a relatively inexpensive way to increase the transient stability limit.5 Where stability is a problem, protective relaying can often be evaluated against the cost of constructing additional transmission lines or switching stations. Other circumstances will be shown later in which certain types of protective-relaying equipment can permit savings in circuit breakers and transmission lines. The quality of the protective-relaying equipment can affect engineering expense in applying the relaying equipment itself. Equipment that can still operate properly when future changes are made in a system or its operation will save much future engineering and other related expense. One should not conclude that the justifiable expense for a given protective-relaying equipment is necessarily proportional to the value or importance of the system element to be directly protected. A failure in that system element may affect the ability of the entire system to render service, and therefore that relaying equipment is actually protecting the service of the entire system. Some of the most serious shutdowns have been caused by consequential effects growing out of an original failure in relatively unimportant equipment that was not properly protected. Fig. 4. Curves illustrating the relation between relay-plus-breaker time and the maximum amount of power that can be transmitted over one particular system without loss of synchronism when various faults occur
HOW DO PROTECTIVE RELAYS OPERATE? Thus far, we have treated the relays themselves in a most impersonal manner, telling what they do without any regard to how they do it. This fascinating part of the story of protective relaying will be told in much more detail later. But, in order to round out this general consideration of relaying and to prepare for what is yet to come, some explanation is in order here All relays used for short-circuit protection, and many other types also, operate by virtue of the current and/or voltage supplied to them by current and voltage transformers connected in various combinations to the system element that is to be protected. Through individual or relative changes in these two quantities, failures signal their presence, type, and location to the protective relays. For every type and location of failure, there is some distinctive difference in these quantities, and there are various types of protective-relaying equipments available, each of which is designed to recognize a particular difference and to operate in response to it More possible differences exist in these quantities than one might suspect. Diff erences each quantity are possible in one or more of the following A. Magnitude B. Frequenc D. Duration E. Rate of change F. Direction der of cha G H Then, when both voltage and current are considered in combination, or relative to similar quantities at different locations, one can begin to realize the resources available for discriminatory purposes. It is a fortunate circumstance that, although Nature in her contrary way has imposed the burden of electric-power-system failure, she has at the same time provided us with a means for combat. 2H3}4} 8-(0 THE PHILOSOPHY OF PROTECTIVE RELAYING
12 THE PHILOSOPHY OF PROTECTIVE RELAYING HOW DO PROTECTIVE RELAYS OPERATE? Thus far, we have treated the relays themselves in a most impersonal manner, telling what they do without any regard to how they do it. This fascinating part of the story of protective relaying will be told in much more detail later. But, in order to round out this general consideration of relaying and to prepare for what is yet to come, some explanation is in order here. All relays used for short-circuit protection, and many other types also, operate by virtue of the current and/or voltage supplied to them by current and voltage transformers connected in various combinations to the system element that is to be protected. Through individual or relative changes in these two quantities, failures signal their presence, type, and location to the protective relays. For every type and location of failure, there is some distinctive difference in these quantities, and there are various types of protective-relaying equipments available, each of which is designed to recognize a particular difference and to operate in response to it.6 More possible differences exist in these quantities than one might suspect. Differences in each quantity are possible in one or more of the following: A. Magnitude. B. Frequency. C. Phase angle. D. Duration. E. Rate of change. F. Direction or order of change. G. Harmonics or wave shape. Then, when both voltage and current are considered in combination, or relative to similar quantities at different locations, one can begin to realize the resources available for discriminatory purposes. It is a fortunate circumstance that, although Nature in her contrary way has imposed the burden of electric-power-system failure, she has at the same time provided us with a means for combat. Fig. 5. Illustration for Problem 2
PROBLEMS Compare protective relaying with insurance 2. The portion of a power system shown by the one-line diagram of Fig. 5, with generating sources back of all three ends, has conventional primary and back-up relaying. In each of the listed cases, a short circuit has occurred and certain circuit breakers have tripped as stated. Assume that the tripping of these breakers was correct under the circumstances. Where was the short circuit? Was there any failure of the protective relaying, including breakers, and if so, what failed? Assume only one failure at a time. Draw a sketch showing the overlapping of primary protective zones and the exact ocations of the various fault Breakers Tripped 3,4,5,6 ,4,5,6 4,5,7,8 BIBLIOGRAPHY 1."Power System Fault Control, " AlEE Committee Report, AlEE Trans., 70(1951) PP.410-417 2."Protective Relay modernization Program Releases Latent Transmission Capacity, by M.F. Hebb, ]r, andJ. T. Logan, AIEE District Conference Paper 55-354 Plan System and Relaying Together, "Elec. Word, July 25, 1955, p. 86 3. "Standards for Power Circuit Breakers. Publ. SG4- 1954. National Electrical Manufacturers Association. 155 East 44th St. New York IN. Y Interrupting Rating Factors for Reclosing Service on Power Circuit Breakers, Publ C37.7-1952, American Standards Association, Inc, 70 East 45th St. New York 17, N. Y 4. Power System Stability, Vol. Il, by S. B Crary, John Wiley Sons, New York, 1947 5. Costs Study of 69- to 345-Kv Overhead Power-Transmission Systems, "by J G. Holm, AlEE Trans,63(1944),pp.4064 6. A Condensation of the Theory of Relays, "by A. R van C. Warrington, Gen. Elec. Rev 43,No.9(Sept,1940),PP.370373 Principles and Practices of Relaying in the United States, "by E. L. Harder and W.E. Marter, AIEE Trans., 67, Part II(1948), Pp. 1005-1022. Discussions, pp. 1022-1023 Principles of high-Speed Relaying, by w. A. Lewis ngineer 1943 THE PHILOSOPHY OF PROTECTIVE RELAYING 13
THE PHILOSOPHY OF PROTECTIVE RELAYING 13 PROBLEMS 1. Compare protective relaying with insurance. 2. The portion of a power system shown by the one-line diagram of Fig. 5, with generating sources back of all three ends, has conventional primary and back-up relaying. In each of the listed cases, a short circuit has occurred and certain circuit breakers have tripped as stated. Assume that the tripping of these breakers was correct under the circumstances. Where was the short circuit? Was there any failure of the protective relaying, including breakers, and if so, what failed? Assume only one failure at a time. Draw a sketch showing the overlapping of primary protective zones and the exact locations of the various faults. Case Breakers Tripped a 4, 5, 8 b 3, 7, 8 c 3, 4, 5, 6 d 1, 4, 5, 6 e 4, 5, 7, 8 f 4, 5, 6 BIBLIOGRAPHY 1. “Power System Fault Control,” AIEE Committee Report, AIEE Trans., 70 (1951), pp. 410-417. 2. “Protective Relay Modernization Program Releases Latent Transmission Capacity,” by M. F. Hebb, Jr., and J. T. Logan, AIEE District Conference Paper 55-354. “Plan System and Relaying Together,” Elec. World, July 25, 1955, p. 86. 3. “Standards for Power Circuit Breakers,” Publ. SG4-1954, National Electrical Manufacturers Association, 155 East 44th St., New York 17, N. Y. “Interrupting Rating Factors for Reclosing Service on Power Circuit Breakers,” Publ. C37.7-1952, American Standards Association, Inc., 70 East 45th St., New York 17, N. Y. 4. Power System Stability, Vol. II, by S. B. Crary, John Wiley & Sons, New York, 1947. 5. “Costs Study of 69- to 345-Kv Overhead Power-Transmission Systems,” by J. G. Holm, AIEE Trans., 63 (1944), pp. 406- 422. 6. “A Condensation of the Theory of Relays,” by A. R. van C. Warrington, Gen. Elec. Rev., 43, No. 9 (Sept., 1940), pp. 370-373. “Principles and Practices of Relaying in the United States,” by E. L. Harder and W. E. Marter, AIEE Trans., 67, Part II (1948), pp. 1005-1022. Discussions, pp. 1022-1023. “Principles of High-Speed Relaying,” by W. A. Lewis, Westinghouse Engineer, 3 (Aug., 1943), pp. 131-134
FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS Protective relays are the"tools"of the protection engineer. As in any craft, an intimate knowledge of the characteristics and capabilities of the available tools is essential to their most effective use. Therefore, we shall spend some time learning about these tools without too much regard to their eventual use. GENERAL CONSIDERATIONS All the relays that we shall consider operate in response to one or more electrical quantitic either to close or to open contacts. We shall not bother with the details of actua mechanical construction except where it may be necessary for a clear understanding of the operation. One of the things that tend to dismay the novice is the great variation in appearance and types of relays, but actually there are surprisingly few fundamental differences. Our attention will be directed to the response of the few basic types to the electrical quantities that actuate them. OPERATING PRINCIPLES There are really only two fundamentally different operating principles: (1) electro- magnetic attraction, and(2)electromagnetic induction Electromagnetic attraction relays operate by virtue of a plunger being drawn into a solenoid, or an armature being attracted to the poles of an electromagnet. Such relays may be actuated by d-c or by a-c quantities Electromagnetic-induction relays use the principle of the induction motor whereby torque is developed by induction in a rotor; this operating principle applies only to relays actuated by alternating current, and in dealing with those relays we shall call them simply induction-type"relays DEFINITIONS OF OPERATION Mechanical movement of the operating mechanism is imparted to a contact structure to close or to open contacts. When we say that a relay"operates, we mean that it either closes or opens its contacts-whichever is the required action under the circumstances. Most relays have a"control spring or are restrained by gravity, so that they assume a given position when completely de-energized; a contact that is closed under this condition called a "closed ct. and one that n is called and "open"contact. This is standardized nomenclature, but it can be quite confusing and awkward to use. A much etter nomenclature in rather extensive use is the designation "a"for an"open"contact, 14 FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS
14 FUNDAMENTAL REL FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS TING PRINCIPLES AND CHARACTERISTICS 2 FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS Protective relays are the "tools" of the protection engineer. As in any craft, an intimate knowledge of the characteristics and capabilities of the available tools is essential to their most effective use. Therefore, we shall spend some time learning about these tools without too much regard to their eventual use. GENERAL CONSIDERATIONS All the relays that we shall consider operate in response to one or more electrical quantities either to close or to open contacts. We shall not bother with the details of actual mechanical construction except where it may be necessary for a clear understanding of the operation. One of the things that tend to dismay the novice is the great variation in appearance and types of relays, but actually there are surprisingly few fundamental differences. Our attention will be directed to the response of the few basic types to the electrical quantities that actuate them. OPERATING PRINCIPLES There are really only two fundamentally different operating principles: (1) electromagnetic attraction, and (2) electromagnetic induction. Electromagnetic attraction relays operate by virtue of a plunger being drawn into a solenoid, or an armature being attracted to the poles of an electromagnet. Such relays may be actuated by d-c or by a-c quantities. Electromagnetic-induction relays use the principle of the induction motor whereby torque is developed by induction in a rotor; this operating principle applies only to relays actuated by alternating current, and in dealing with those relays we shall call them simply "induction-type" relays. DEFINITIONS OF OPERATION Mechanical movement of the operating mechanism is imparted to a contact structure to close or to open contacts. When we say that a relay "operates," we mean that it either closes or opens its contacts-whichever is the required action under the circumstances. Most relays have a "control spring," or are restrained by gravity, so that they assume a given position when completely de-energized; a contact that is closed under this condition is called a "closed" contact, and one that is open is called and "open" contact. This is standardized nomenclature, but it can be quite confusing and awkward to use. A much better nomenclature in rather extensive use is the designation “a” for an "open" contact
and b"for a" closed" contact. This nomenclature will be used in this book. The present standard method for showing"a"and"b contacts on connection diagrams is illustrated in Fig. 1. Even though an a "contact may be closed under normal operating hould op in Fig. 1; and similarly even though a " b contact may normally be open, it should be own close When a relay operates to open a"b"contact or to close an"a contact, we say that it"picks up,and the smallest value of the Fig. 1.Contact symbols and designations actuating quantity that will cause such operation, as the quantity is slowly increased from zero, is called the"pickup"value. When a relay operates to close a b"contact, or to move to a stop in place of a" b"contact, we say that it resets"; and the rgest value of the actuating quantity at which this occurs, as the quantity is slowly decreased from above the pickup value, is called the"reset"value. When a relay operates to open its"a "contact, but does not reset, we say that it"drops out, "and the largest value of the actuating quantity at which this occurs is called the drop-out"value OPERATION INDICATORS Generally, a protective relay is provided with an indicator that shows when the relay has operated to trip a circuit breaker. Such" operation indicators"or"targets"are distinctively colored elements that are actuated either mechanically by movement of the relay's operating mechanism, or electrically by the flow of contact current, and come into viev when the relay operates. They are arranged to be reset manually after their indication has been noted, so as to be ready for the next operation. One type of indicator is shown in Fig. 2. Electrically operated targets are generally preferred because they give definite assurance that there was a current flow in the contact circuit. Mechanically operated targets may be used when the closing of a relay contact always completes the trip circuit where tripping is not dependent on the closing of some other series contact. A mechanical target may be used with a series circuit comprising contacts of other relays when it is Seal-in coil Seal-in pole pieces Mowable b contact Stationary contacts Seal-in armature Ta Fig. 2. One type of contact mechanism showing target and seal-in elements. FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS 15
FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS 15 and “b” for a "closed" contact. This nomenclature will be used in this book. The present standard method for showing "a" and “b” contacts on connection diagrams is illustrated in Fig. 1. Even though an “a” contact may be closed under normal operating conditions, it should be shown open as in Fig. 1; and similarly, even though a “b” contact may normally be open, it should be shown closed. When a relay operates to open a “b” contact or to close an “a” contact, we say that it "picks up," and the smallest value of the actuating quantity that will cause such operation, as the quantity is slowly increased from zero, is called the "pickup" value. When a relay operates to close a “b” contact, or to move to a stop in place of a “b” contact, we say that it "resets"; and the largest value of the actuating quantity at which this occurs, as the quantity is slowly decreased from above the pickup value, is called the "reset" value. When a relay operates to open its “a” contact, but does not reset, we say that it "drops out," and the largest value of the actuating quantity at which this occurs is called the "drop-out" value. OPERATION INDICATORS Generally, a protective relay is provided with an indicator that shows when the relay has operated to trip a circuit breaker. Such "operation indicators" or "targets" are distinctively colored elements that are actuated either mechanically by movement of the relay's operating mechanism, or electrically by the flow of contact current, and come into view when the relay operates. They are arranged to be reset manually after their indication has been noted, so as to be ready for the next operation. One type of indicator is shown in Fig. 2. Electrically operated targets are generally preferred because they give definite assurance that there was a current flow in the contact circuit. Mechanically operated targets may be used when the closing of a relay contact always completes the trip circuit where tripping is not dependent on the closing of some other series contact. A mechanical target may be used with a series circuit comprising contacts of other relays when it is Fig. 1. Contact symbols and designations Fig. 2. One type of contact mechanism showing target and seal-in elements