desired to have indication that a particular relay has operated, even though the circuit may not have been completed through the other contacts SEAL-IN AND HOLDING COILS AND SEAL-IN RELAYS In order to protect the contacts against damage resulting from a possible inadvertent attempt to interrupt the flow of the circuit tripcoil current, some relays are provided with a holding mechanism comprising a small coil in series with the contacts; this coil is on a small electromagnet that acts on a small armature on the moving contact assembly to hold the contacts tightly closed once they have established the flow of trip-coil current. This coil is called a"seal-in"or"holding"coil. Figure 2 shows such a structure. Other relays use a small auxiliary relay whose contacts by-pass the protective-relay contacts and seal the circuit closed while tripping current flows. This seal-in relay may also display the target. In either case, the circuit is arranged so that, once the trip-coil current starts to flow, it can b interrupted only by a circuit-breaker auxiliary switch that is connected in series with the trip-coil circuit and that opens when the breaker opens. This auxiliary switch is defined as an"a"contact. The circuits of both alternatives are shown in Fig 3 Figure 3 also shows the preferred polarity to which the circuit-breaker trip coil (or any other coil should be connected to avoid corrosion because of electrolytic action. No coil should be connected only to positive polarity for long periods of time; and, since here the circuit breaker and its auxiliary switch will be closed normally while the protective-relay contacts will be open, the trip-coil end of the circuit should be at negative polarit Circuit-breaker Circuit-breaker trip coil Protective-relay Auxiliary seal-in -relay seal-in coil Protective-reay上上 Auxiliary seal-in- relay contact Fig. 3. Alternative contact seal-in methods ADJUSTMENT OF PICKUP OR RESET Adjustment of pickup or reset is provided electrically by tapped current coils or by tapped auxiliary potential transformers or resistors; or adjustment is provided mechanically by adjustable spring tension or by varying the initial air gap of the operating element with t to its solenoid or electrol FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS
16 FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS desired to have indication that a particular relay has operated, even though the circuit may not have been completed through the other contacts. SEAL-IN AND HOLDING COILS, AND SEAL-IN RELAYS In order to protect the contacts against damage resulting from a possible inadvertent attempt to interrupt the flow of the circuit tripcoil current, some relays are provided with a holding mechanism comprising a small coil in series with the contacts; this coil is on a small electromagnet that acts on a small armature on the moving contact assembly to hold the contacts tightly closed once they have established the flow of trip-coil current. This coil is called a "seal-in" or "holding" coil. Figure 2 shows such a structure. Other relays use a small auxiliary relay whose contacts by-pass the protective-relay contacts and seal the circuit closed while tripping current flows. This seal-in relay may also display the target. In either case, the circuit is arranged so that, once the trip-coil current starts to flow, it can be interrupted only by a circuit-breaker auxiliary switch that is connected in series with the trip-coil circuit and that opens when the breaker opens. This auxiliary switch is defined as an " a " contact. The circuits of both alternatives are shown in Fig. 3. Figure 3 also shows the preferred polarity to which the circuit-breaker trip coil (or any other coil) should be connected to avoid corrosion because of electrolytic action. No coil should be connected only to positive polarity for long periods of time; and, since here the circuit breaker and its auxiliary switch will be closed normally while the protective-relay contacts will be open, the trip-coil end of the circuit should be at negative polarity. ADJUSTMENT OF PICKUP OR RESET Adjustment of pickup or reset is provided electrically by tapped current coils or by tapped auxiliary potential transformers or resistors; or adjustment is provided mechanically by adjustable spring tension or by varying the initial air gap of the operating element with respect to its solenoid or electromagnet. Fig. 3. Alternative contact seal-in methods
TIME DELAY AND ITS DEFINITIONS Some relays have adjustable time delay, and others are"instantaneous"or" high speec The term"instantaneous"means"having no intentional time delay" and is applied to relays that operate in a minimum time of approximately 0 I second. The term"high speed"connotes operation in less than approximately 0 I second and usually in 0.05 second or less. The operating time of high-speed relays is usually expressed in cycles based on the power-system frequency; for example, "one cycle" would be 60 second in a 60-cycle system. Originally, only the term " instantaneous was used, but, as relay speed was increased, the term "high speed"was felt to be necessary in order to differentiate such relays from the earlier, slower types. This book will use the term "instantaneous"for general reference to either instantaneous or high-speed relays, reserving the term"high- speed"for use only when the terminology is significant. Auxiliary seal-in Time dial relay and targe F通 Drag Fig 4 Close-up of an induction type overcurrent unit, showing the disc rotor and drag magnet Occasionally, a supplementary auxiliary relay having fixed time delay may be used when a certain delay is required that is entirely independent of the magnitude of the actuating quantity in the protective relay FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS
FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS 17 TIME DELAY AND ITS DEFINITIONS Some relays have adjustable time delay, and others are "instantaneous" or "high speed." The term "instantaneous" means "having no intentional time delay" and is applied to relays that operate in a minimum time of approximately 0.1 second. The term "high speed" connotes operation in less than approximately 0.1 second and usually in 0.05 second or less. The operating time of high-speed relays is usually expressed in cycles based on the power-system frequency; for example, "one cycle" would be 1/60 second in a 60-cycle system. Originally, only the term "instantaneous" was used, but, as relay speed was increased, the term "high speed" was felt to be necessary in order to differentiate such relays from the earlier, slower types. This book will use the term "instantaneous" for general reference to either instantaneous or high-speed relays, reserving the term "highspeed" for use only when the terminology is significant. Occasionally, a supplementary auxiliary relay having fixed time delay may be used when a certain delay is required that is entirely independent of the magnitude of the actuating quantity in the protective relay. Fig. 4. Close-up of an induction-type overcurrent unit, showing the disc rotor and drag magnet
Time delay is obtained in induction-type relays by a"drag magnet, "which is a permanent magnet arranged so that the relay rotor cuts the flux between the poles of the magnet, as shown in Fig. 4. This produces a retarding effect on motion of the rotor in either direction. In other relays, various mechanical devices have been used, including dash pots, bellows, and escapement mechanisms The terminology for expressing the shape of the curve of operating time versus the actuating quantity has also been affected by developments throughout the years Originally, only the terms"definite time"and"inverse time"were used. An inverse-time curve is one in which the operating time becomes less as the magnitude of the actuating quantity is increased, as shown in Fig. 5. The more pronounced the effect is, the more inverse is the curve said to be. Actually, all time curves are inverse to a greater or lesser legree. They are most inverse near the pickup value and become less inverse as the actuating quantity is increased. a definite-time curve would strictly be one in which the operating time was unaffected by the magnitude of the actuating quantity, but actually the gy is applied to a curve that becomes substantially definite slightly above the pickup value of the relay, as shown in Fig. 5 nverse curve Definite curve Pickup value Magnitude of actuating quantity Fig. 5. Curves of operating time versus the magnitude of the actuating quantity As a consequence of trying to give names to curves of different degrees of inverseness, we now have "inverse, ""very inverse, "and" extremely inverse. Although the terminology may be somewhat confusing, each curve has its field of usefulness, and one skilled in the use of these relays has only to compare the shapes of the curves to know which is best for a given application. This book will use the term"inverse"for general reference to any of the inverse curves, reserving the other terms for use only when the terminology is significant. Thus far, we have gained a rough picture of protective relays in general and have learned some of the language of the pr References to complete standards pertaining to circuit elements and terminology are given in the bibliography at the end of this chapter. With this preparation, we shall now consider the fundamental relay types FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS
18 FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS Time delay is obtained in induction-type relays by a "drag magnet," which is a permanent magnet arranged so that the relay rotor cuts the flux between the poles of the magnet, as shown in Fig. 4. This produces a retarding effect on motion of the rotor in either direction. In other relays, various mechanical devices have been used, including dash pots, bellows, and escapement mechanisms. The terminology for expressing the shape of the curve of operating time versus the actuating quantity has also been affected by developments throughout the years. Originally, only the terms "definite time" and "inverse time" were used. An inverse-time curve is one in which the operating time becomes less as the magnitude of the actuating quantity is increased, as shown in Fig. 5. The more pronounced the effect is, the more inverse is the curve said to be. Actually, all time curves are inverse to a greater or lesser degree. They are most inverse near the pickup value and become less inverse as the actuating quantity is increased. A definite-time curve would strictly be one in which the operating time was unaffected by the magnitude of the actuating quantity, but actually the terminology is applied to a curve that becomes substantially definite slightly above the pickup value of the relay, as shown in Fig. 5. As a consequence of trying to give names to curves of different degrees of inverseness, we now have "inverse," "very inverse," and "extremely inverse." Although the terminology may be somewhat confusing, each curve has its field of usefulness, and one skilled in the use of these relays has only to compare the shapes of the curves to know which is best for a given application. This book will use the term "inverse" for general reference to any of the inverse curves, reserving the other terms for use only when the terminology is significant. Thus far, we have gained a rough picture of protective relays in general and have learned some of the language of the profession. References to complete standards pertaining to circuit elements and terminology are given in the bibliography at the end of this chapter. 1 With this preparation, we shall now consider the fundamental relay types. Fig. 5. Curves of operating time versus the magnitude of the actuating quantity
SINGLE-QUANTITY RELAYS OF THE ELECTROMAGNETIC-ATTRACTION TYPE Here we shall consider plunger-type and attracted-armature-type a-c or d-c relays that are actuated from either a single current or voltage source OPERATING PRINCIPLE The electromagnetic force exerted on the moving element is proportional to the square of the flux in the air gap. If we neglect the effect of saturation, the total actuating force may KI- a force-conversion constant. I- the rms magnitude of the current in the actuating coil. K2- the restraining force(including friction) When the relay is on the verge of picking up, the net force is zero, and the operating K12=K2 KI RATIO OF RESET TO PICKUP One characteristic that affects the application of some of these relays is the relatively large difference between their pickup and reset values. As such a relay picks up, it shortens its air gap, which permits a smaller magnitude of coil current to keep the relay picked up than was required to pick it up. This effect is less pronounced in a-c than in d-c relays. By special design, the reset can be made as high as 90% to 95% of pickup for a-c relays, and 60%to 90% of pickup for d-c relays. Where the pickup is adjusted by adjusting the initial air gap, a higher pickup calibration will have a lower ratio of reset to pickup. For overcurrent applications where such relays are often used, the relay trips a circuit breaker which reduces the current to zero, and hence the reset value is of no consequence. However, if a low-reset relay is used in conjuction with other relays in such a way that a breaker is not always tripped when the low-reset relay operates, the application should be carefully tage of the pickup value, there is the possibility that an abnormal condition might cause the relay to pick up(or to reset), but that a return to normal conditions might not return the relay to its normal operating position, and undesired operation might result. FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS 19
FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS 19 SINGLE-QUANTITY RELAYS OF THE ELECTROMAGNETIC-ATTRACTION TYPE Here we shall consider plunger-type and attracted-armature-type a-c or d-c relays that are actuated from either a single current or voltage source. OPERATING PRINCIPLE The electromagnetic force exerted on the moving element is proportional to the square of the flux in the air gap. If we neglect the effect of saturation, the total actuating force may be expressed: F = K1I 2 – K2, where F = net force. K1 = a force-conversion constant. I = the rms magnitude of the current in the actuating coil. K2 = the restraining force (including friction). When the relay is on the verge of picking up, the net force is zero, and the operating characteristic is: K1I 2 = K2, or —-- K2 I = —– = constant √K1 RATIO OF RESET TO PICKUP One characteristic that affects the application of some of these relays is the relatively large difference between their pickup and reset values. As such a relay picks up, it shortens its air gap, which permits a smaller magnitude of coil current to keep the relay picked up than was required to pick it up. This effect is less pronounced in a-c than in d-c relays. By special design, the reset can be made as high as 90% to 95% of pickup for a-c relays, and 60% to 90% of pickup for d-c relays. Where the pickup is adjusted by adjusting the initial air gap, a higher pickup calibration will have a lower ratio of reset to pickup. For overcurrent applications where such relays are often used, the relay trips a circuit breaker which reduces the current to zero, and hence the reset value is of no consequence. However, if a low-reset relay is used in conjuction with other relays in such a way that a breaker is not always tripped when the low-reset relay operates, the application should be carefully examined. When the reset value is a low percentage of the pickup value, there is the possibility that an abnormal condition might cause the relay to pick up (or to reset), but that a return to normal conditions might not return the relay to its normal operating position, and undesired operation might result
TENDENCY TOWARD VIBRATION Unless the pole pieces of such relays have"shading rings" to split the air-gap flux into two out-of-phase components, such relays are not suitable for continuous operation on alternating current in the picked-up position. This is because there would be excessive vibration that would produce objectionable noise and would cause excessive wear. This tendency to vibrate is related to the fact that a-c relays have higher reset than d-c relays; an a-c relay without shading rings has a tendency to reset every half cycle when the flux passes DIRECTIONAL CONTROL Relays of this group are used mostly when"directional"operation is not required. More will be said later about"directional control"of relays; suffice it to say here that plunger or attracted-armature relays do not lend themselves to directional control nearly as well as induction-type relays, which will be considered later EFFECT OF TRANSIENTS Because these relays operate so quickly and with almost equal current facility on either alternating current or direct current, they are affected by transients, and particularly by d-c offset in a-c waves. This tendency must be taken into consideration when the proper adjustment for any application is being determined. Even though the steady-state value of an offset wave is less than the relay's pickup value, the relay may pick up during such a transient, depending on the amount of offset, its time constant, and the operating speed of the relay. This tendency is called"overreach"for reasons that will be given later TIME CHARACTERISTICS This type of relay is inherently fast and is used generally where time delay is not required Time delay can be obtained, as previously stated, by delaying mechanisms such as bellows, dash pots, or escapements. Very short time delays are obtainable in d-c relays by encircling the magnetic circuit with a low-resistance ring, or"slug"as it is sometimes called. This ring delays changes in flux, and it can be positioned either to have more effect on air increase if time-delay pickup is desired, or to have more effect on air-gap-flux decrease if time-delay reset is required DIRECTIONAL RELAYS OF THE ELECTROMAGNETIC ATTRACTION TYPE Directional relays of the electromagnetic-attraction type are actuated by d-c or by rectified a-c quantities. The most common use of such relays is for protection of d-c circuits whe the actuating quantity is obtained either from a shunt or directly from the circuit. FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS
20 FUNDAMENTAL RELAY-OPERATING PRINCIPLES AND CHARACTERISTICS TENDENCY TOWARD VIBRATION Unless the pole pieces of such relays have "shading rings" to split the air-gap flux into two out-of-phase components, such relays are not suitable for continuous operation on alternating current in the picked-up position. This is because there would be excessive vibration that would produce objectionable noise and would cause excessive wear. This tendency to vibrate is related to the fact that a-c relays have higher reset than d-c relays; an a-c relay without shading rings has a tendency to reset every half cycle when the flux passes through zero. DIRECTIONAL CONTROL Relays of this group are used mostly when "directional" operation is not required. More will be said later about "directional control" of relays; suffice it to say here that plunger or attracted-armature relays do not lend themselves to directional control nearly as well as induction-type relays, which will be considered later. EFFECT OF TRANSIENTS Because these relays operate so quickly and with almost equal current facility on either alternating current or direct current, they are affected by transients, and particularly by d-c offset in a-c waves. This tendency must be taken into consideration when the proper adjustment for any application is being determined. Even though the steady-state value of an offset wave is less than the relay's pickup value, the relay may pick up during such a transient, depending on the amount of offset, its time constant, and the operating speed of the relay. This tendency is called "overreach" for reasons that will be given later. TIME CHARACTERISTICS This type of relay is inherently fast and is used generally where time delay is not required. Time delay can be obtained, as previously stated, by delaying mechanisms such as bellows, dash pots, or escapements. Very short time delays are obtainable in d-c relays by encircling the magnetic circuit with a low-resistance ring, or "slug" as it is sometimes called. This ring delays changes in flux, and it can be positioned either to have more effect on air increase if time-delay pickup is desired, or to have more effect on air-gap-flux decrease if time-delay reset is required. DIRECTIONAL RELAYS OF THE ELECTROMAGNETICATTRACTION TYPE Directional relays of the electromagnetic-attraction type are actuated by d-c or by rectified a-c quantities. The most common use of such relays is for protection of d-c circuits where the actuating quantity is obtained either from a shunt or directly from the circuit