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| RELAY PANEL IN SUBSTATION |
The capital investment involved in a power system for the generation, transmission and distribution of electrical power is so great that proper precautions must be taken to ensure that the equipment not only operates as nearly as possible to peak efficiencies but also that it is protected from accidents. The purpose of the protective relays and protective relaying systems is to operate the correct circuit breakers so as to disconnect only the faulty equipment from the system as quickly as possible, thus minimising the trouble and damage caused by faults when they do occur.
The modern power system is very complex and even though protective equipment forms 4 to 5% of the total cost involved in the system, they play a very important role in the system design for good quality of reliable supply. The most severe electrical failures in a power system are shunt faults which are characterized by an increase in system current, reduction in voltage, power factor and frequency. The protective relays do not eliminate the possibility of faults on the system, rather their action starts only after the fault has occurred on the system. It would be ideal if protection could anticipate and prevent faults but this is impossible except where the original cause of a fault creates some effects which can operate a protective relay. So far only one type of relay falls into this category, this is the gas detector relay (Buchholz relay) used to protect transformers which operate when the oil level in the conservator pipe of a transformer is lowered by the accumulation of gas caused by a poor connection or by an incipient breakdown of insulation (slowly developing fault).
There are two groups of relaying equipment for protecting any equipment:
Backup protection relaying works only when the primary relaying equipment fails which means back-up relaying is inherently slow in action. Primary relaying may fail because of failure of any of the following:
(i) Protective relays (moving mechanism etc.).
(ii) Circuit breaker.
(iii) D.C. tripping voltage supply.
(iv) Current or voltage supply to the relays.
Since it is required that backup relays should operate in case primary relays fail, the backup relays should not have anything common with primary relays. Hitherto, the practice has been to locate the backup relays at a different station. A second job of the backup relays is to act as primary protection in case the primary protection equipment is taken out for repair and maintenance.
Definitions
Relay: A relay is an automatic device which senses an abnormal condition in an electric circuit and closes its contacts. These contacts, in turn, close the circuit breaker trip coil circuit,
thereby it opens the circuit breaker and the faulty part of the electric circuit is disconnected
from the rest of the healthy circuit.
Pick up Level: The value of the actuating quantity (current or voltage) which is on the threshold (border) above which the relay operates.
Reset Level: The value of current or voltage below which a relay opens its contacts and comes to original position.
Operating Time: The time which elapses between the instant when the actuating quantity exceeds the pick-up value to the instant when the relay contacts close.
Reset Time: The time which elapses between the instant when the actuating quantity becomes less than the reset value to the instant when the relay contact returns to its normal position.
Primary Relays: The relays which are connected directly to the circuit to be protected.
Secondary Relays: The relays which are connected to the circuit to be protected through current and potential transformers.
Auxiliary Relays: Relays which operate in response to the opening or closing of its operating circuit to assist another relay in the performance of its function. This relay may be instantaneous or may have a time delay.
Reach: A distance relay operates whenever the impedance seen by the relay is less than a prespecified value. This impedance or the corresponding distance is known as the reach of the realy.
Underreach: The tendency of the relay to restrain at the set value or the impedance or impedance lower than the set value is known as under reach.
Overreach: The tendency of the relay to operate at impedances larger than its setting is known as overreach.
Characteristics of Relay
A protective relay is required to satisfy four basic functional characteristics:
(i) Reliability,
(ii) Selectivity,
(iii) Speed, and
(iv) Sensitivity.
Reliability: The relay should be reliable is a basic requirement. It must operate when it is required. There are various components which go into operation before a relay operates. Therefore, every component and circuit which is involved in the operation of the relay plays an important role; for example, lack of suitable current and voltage transformers may result in unreliable operation. Since the protective relays remain idle most of the time on the power system, proper maintenance will play a vital role in improving the reliable operation of the relay. Inherent reliability is a matter of design based on long experience. This can be achieved partly by:
• simplicity and robustness in construction,
• high contact pressure,
• dust free enclosures,
• good contact material,
• good workmanship, and
• careful maintenance.
Selectivity: It is the basic requirement of the relay in which it should be possible to select which part of the system is faulty and which is not and should isolate the faulty part of the system from the healthy one. Selectivity is achieved in two ways:
(i) unit system of protection,
and (ii) non-unit system of protection.
Unit system of protection means the one in which the protection responds only to faults within its own zone and does not make note of the conditions elsewhere, e.g., the differential protection of transformers and generators. Here the protection scheme will work only if the fault is in the transformer or the generator respectively. Non-unit system of protection is one in which the selectivity is obtained by grading the time or current settings of the relays at different locations, all of which may respond to a given fault.
Speed: A protective relay must operate at the required speed. It should neither be too slow which may result in damage to the equipment nor should it be too fast which may result in undesired operation during transient faults. The shorter the time for which a fault is allowed to persist on the system, the more load can be transferred between given points on the power system without loss of synchronism. Fig. shows the curves which represent the power that can be transmitted as a function of fault clearing time for various types of faults. It can be seen from the curves that the severest fault is the 3-phase fault and the least severe is the L-G fault in terms of transmission of power.
Sensitivity: A relay should be sufficiently sensitive so that it operates reliably when required under the actual conditions in the system which produce the least tendency for operation. It is normally expressed in terms of minimum volt-amperes required for the relay operation.