Basic Principle of Operation of Protective System – Each relay in a protection scheme performs a certain function and responds in a given manner to a certain type of change in the circuit quantities. For example one type of relay may operate when the current increases above a certain magnitude, while another may compare current and voltage and operate when the ratio V/1 is less than a given value. The first relay is known as an over current relay while the latter an under-impedance relay. Similarly various combinations of these electrical quantities could be worked out according to the requirements at a particular situation, because for every type and location of failure there is some distinctive difference in these quantities, and there are various types of protective relaying equipment available, each of which is designed to recognize a particular difference and to operate in response to it.
We are quite familiar in day-to-day life that there is an economic limit to the amount that can be spent on different types of insurances in order to safeguard life and property. Similarly in a power system there is an economic limit to the amount that can be spent on the protection of the system. Usually this is a very complex affair since the probability of failure or fault is a function of component, location, time, etc. All these factors can lead to different alternatives for the same problem; and a choice has to be made keeping in view the economic justifiably. The cost of protection is linked with cost of the plant to be protected and increases with cost of the plant. Usually the protective gear should not cost more than 5% of the total cost. However, when the apparatus to be protected is of paramount importance like the generator or the main transmission line, economic considerations are often subordinated to reliability. Table 1.3 shows the average costs in units per circuit.
We define below some of the important terms used in the study of protective relays and switch gear.
Unit or Element. A self-contained relay unit which in conjunction with one or more other relay units performs a complex relay function, e.g. a directional unit combined with an overcurrent unit gives a directional overcurrent relay.
Energizing Quantity. The electrical quantity, i.e. current or voltage either alone or in combination with other electrical quantities required for the functioning of the relay.
Characteristic Quantity. The quantity to which the relay is designed to respond, e.g. current in an overcurrent relay, impedance in an impedance relay, phase angle in a directional relay, etc. Some relays have a calibrated response to one or more quantities, such quantities are called characteristic quantities.
Setting. The actual value of the energizing or characteristic quantity at which the relay is designed to operate under given conditions.
Power Consumption (Burden). The power consumed by the circuits of the relay at the rated current or voltage. It is expressed in volt-amperes for a.c. and watts for d.c. to the on position. The value of the characteristic quantity above which this change occurs is known as pickup value.
Dropout or Reset. A relay is said to dropout when it moves from the on position to the off position. The value of the characteristic quantity below which this change occurs is known as dropout or reset value.
Operating Time. The time which elapses between the instant of application of a characteristic quantity equal to the pickup value and the instant when the relay operates its contacts.
Resetting Time. The time which the operated relay takes to come back to its initial position as a result of a specified sudden change of the characteristic quantity, the time being measured from the instant at which the change occurs.
Overshoot Time. The time during which stored operating energy is dissipated after the characteristic quantity has been suddenly restored from a specified value to the value which it had at the initial position of the relay.
Characteristic Angle. The phase angle at which the performance of the relay is declared.
Characteristics (of a Relay in Steady State). The locus of the pickup or reset when drawn on a graph. In some relays the two curves are coincident and become the locus of balance or zero torque.
Reinforcing Relay. Relay which is energized by the contacts of the main relay and with its contacts in parallel with those of the main relay relieves them of their current carrying duty. The seal-in contacts are usually of higher current rating than those of the main relay.
Seal-in-Relay. Similar to the reinforcing relay described above except connected to stay until its coil circuit is interrupted by a switch on the circuit breaker.
Primary Relays. Those which are connected directly in the protected circuit.
Secondary Relays. Those connected to the protected circuit through current and potential transformers of its operating circuit to assist another relay in the performance of its function. The auxiliary relay may be instantaneous or may have a time lag and may operate within large limits of the characteristic quantity.
Backup Relay. A relay which operates usually after a slight time delay if the normal relay does not operate to trip its circuit breaker. A backup relay acts as a second line of defense.
Consistency. Accuracy with which the relay can repeat its electrical or time characteristics.
Flag or Target. A device used for indicating the operation of a relay, it is usually spring or gravity operated.
Reach. Remote limit of the zone of protection provided by the relay used mostly in connection with distance relays to indicate how far along a line the tripping zone of the relay extends.
Overreach or Underreach. Errors in relay measurement resulting in wrong operation or failure to operate respectively.
Blocking. Preventing the protective relay from tripping either due to its own characteristic or due to an additional relay.