What are the basic operating principles of relays in protecting electrical equipment?
Definite-Time Principle
For still greater selectivity, the principle of definite time was introduced. It prevents the abnormal currents of any value from operating the relay until a definite time has elapsed after the relay is set in operation. This principle is often combined with the inverse-time principle to give inverse-time characteristics to a certain value of current, beyond which operation can be completed only after the definite time for which the device is set has elapsed.
This combination is called the inverse definiteminimum time principle. The selection of a minimum time common to all such relays in series simplifies their coordination. The large flat portion of its characteristic, shown as curve (d) in Figure 13.3, results in only a small increase in the relay time for smaller values of fault current. Relays of this type are used in the majority of overcurrent relay applications.
source: 2010 IIEE Technical Manuals
Overcurrent Relays
Protection for electrical equipments can be found in many form. Overcurrent relays, perhaps the simplest of the many protective relays, close their contacts when the current flowing in the circuit or equipment reaches a predetermined value. The principle of operation of each of the several types is described below; the variations in their characteristics are illustrated by the curves in Figure 13.3.
Protection for electrical equipments can be found in many form. Overcurrent relays, perhaps the simplest of the many protective relays, close their contacts when the current flowing in the circuit or equipment reaches a predetermined value. The principle of operation of each of the several types is described below; the variations in their characteristics are illustrated by the curves in Figure 13.3.
Instantaneous Principle
In the basic overcurrent relay, the circuit is designed to open instantaneously, that is, as quickly as the device can be made to operate with no time delay deliberately added. As a normal result, the relay closes its contacts within one-half cycle to twenty cycles.
Although great accuracy can be obtained in the setting of the relay, the instantaneous feature often causes the settings to be made at values that are higher than desirable in order to prevent too frequent operation of the relay as a result of transient, nonpersistent conditions. The time-current characteristic of this type of relay is shown by curve (a) in Figure 13.3.
In the basic overcurrent relay, the circuit is designed to open instantaneously, that is, as quickly as the device can be made to operate with no time delay deliberately added. As a normal result, the relay closes its contacts within one-half cycle to twenty cycles.
Although great accuracy can be obtained in the setting of the relay, the instantaneous feature often causes the settings to be made at values that are higher than desirable in order to prevent too frequent operation of the relay as a result of transient, nonpersistent conditions. The time-current characteristic of this type of relay is shown by curve (a) in Figure 13.3.
Inverse-Time Principle
The inverse-time principle was developed as a refinement of the instantaneous (overcurrent) principle. With it, the time of operation of the protective device is made to vary approximately inversely with the magnitude of the current; that is, the greater the current, the shorter the time for operation.
Current settings may be made lower and sensitivity increased because the normal transient currents (small in comparison to fault currents) that may start the operation of the relay will not last long enough to complete it. Furthermore, by varying the current setting and the restraint on the movable element, the time-current characteristics of this type of relay may be varied, as indicated by curves (b) and (c) in Figure 13.3. Greater selectivity between relays and fuses in the circuit may thus be obtained, as will be discussed subsequently.
The inverse-time principle was developed as a refinement of the instantaneous (overcurrent) principle. With it, the time of operation of the protective device is made to vary approximately inversely with the magnitude of the current; that is, the greater the current, the shorter the time for operation.
Current settings may be made lower and sensitivity increased because the normal transient currents (small in comparison to fault currents) that may start the operation of the relay will not last long enough to complete it. Furthermore, by varying the current setting and the restraint on the movable element, the time-current characteristics of this type of relay may be varied, as indicated by curves (b) and (c) in Figure 13.3. Greater selectivity between relays and fuses in the circuit may thus be obtained, as will be discussed subsequently.
Definite-Time Principle
For still greater selectivity, the principle of definite time was introduced. It prevents the abnormal currents of any value from operating the relay until a definite time has elapsed after the relay is set in operation. This principle is often combined with the inverse-time principle to give inverse-time characteristics to a certain value of current, beyond which operation can be completed only after the definite time for which the device is set has elapsed.
This combination is called the inverse definiteminimum time principle. The selection of a minimum time common to all such relays in series simplifies their coordination. The large flat portion of its characteristic, shown as curve (d) in Figure 13.3, results in only a small increase in the relay time for smaller values of fault current. Relays of this type are used in the majority of overcurrent relay applications.
source: 2010 IIEE Technical Manuals
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