FUNDAMENTALS OF PROTECTIVE RELAYING

Introduction 

Protection Relay Electrical power systems are comprises of generators, power transformers, busbar sections, circuit breakers, transmission lines, either overhead or underground cables and motors, and etc. The relays are essential components and protective relaying is major parts of the electrical systems. These are employed in generation, transmission and distribution systems of electrical power systems and networks. For large countries the transmission is basically via overhead transmission lines. However in Singapore the transmission of electricity is entirely through underground cables. This text is intended for the beginners, young graduates or school leavers who started their career in electrical engineering, facility maintenance and repair, installation, marine offshore and seafaring, oil & gas and petrochemical engineering construction and building construction (M&E or Building Services) engineering. While one must be thoroughly familiar with the local and international standards and code of practices, should also have basic electrical engineering knowledge of the systems. These are the fundamentals for the beginners

Before discussing the protective relaying systems, it is necessary to review the terminology or the definition used generally in the electrical engineering, particularly in protective relaying.

Definitions and Terminology

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 contact and comes to original position

Operating time: The time which elapses between the instant when the actuating quantity exceed the pick-up value to the instant when the relay contact closes

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 contacts close.

Primary relay: The relays which are connected directly in the circuit to be protected.

Secondary relay: The relays which are connected in the circuit to be protected through current and potential transformers.

 

Auxiliary relays: The 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 relay.

Underreach: The tendency of the relay to restrain at the set value of the impedance or impedance lower than the set value is known as under-reach.

Overderreach: The tendency of the relay to operate at impedances larger than its setting is known as over-reach.

Sensibility : The ability of the protective system to operate so as to trip only the minimum number of breakers directly supplying the defective part of the system is called sensitivity of the relaying system.

Reliability : The basic requirement of the relay is the reliability. It must operate as and when it is required.

Selectivity: It is 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 (ii) non-unit system of protection.

Discrimination: The ability of the protective relay system to distinguish between power system conditions for which it is not intended to operate. It means that the discrimination is said to be achieved when only the protective device immediately upstream of a fault operates, all other circuits remaining closed. This ensures that healthy circuits continue to operate normally. In order to check that discrimination will be achieved on a particular system it will be necessary to conduct a discrimination or co-ordination study.

SELECTIVITY

Relays have an assigned area known as the primary protection zone, but they may properly operate in response to conditions outside this zone. In these instances, they provide backup protection for the area outside their primary zone. This is designated as the backup or overreached zone. Selectivity (also known as relay coordination) is the process of applying

and setting the protective relays that overreach other relays such that they operate as fast as possible within their primary zone, but have delayed operation in their backup zone. This is necessary to permit the primary relays assigned to this backup or overreached area time to operate. Otherwise, sets of relays may operate for faults in this overreached area; the assigned primary relays for the area and the backup relays. Operation of the backup protection is incorrect and undesirable unless the primary protection of that area fails to clear the fault. Consequently, selectivity or relay coordination is important to assure maximum service continuity with minimum system dis- connection. 

SPEED

Obviously, it is desirable that the protection isolates a trouble zone as rapidly as possible. In some applications this is not difficult, but in others, particularly where selectivity is involved, faster operation can be accomplished by more complex and a higher-cost protection. Zero-time or very high speed protection, although inherently desirable, may result in an increased number of undesired operations. As a broad generality, the faster the operation, the higher the probability of incorrect operation. Time, generally of a very small amount, remains as one of the best means of distinguishing between tolerable and intolerable transients. A high-speed relay is one that operates in less than 50 msec (three cycles on a 60 Hz basis) (IEEE 100). The term instantaneous is defined to indicate that no (time) delay is purposely introduced in the action of the device (IEEE100). In practice, the terms instantaneous and high-speed are used interchangeably to describe protective relays that operate in 50 msec or less. Modern high-speed circuit breakers operate in the range of 17–50 msec (one to three cycles at 60 Hz); others operate at less than 83 msec (five cycles at 60 Hz). Thus, the total clearing time (relays plus breaker) typically ranges

from approximately 35–130 msec (two to eight cycles at 60 Hz). In the lower-voltage systems, in which time-coordination is required between protective relays, relay-operating times generally will be slower; typically on the order of 0.2–1.5 sec for the primary zone. Primary-zone

relay time longer than 1.5–2.0 sec are unusual for faults in this zone, but they are possible and do exist. Thus, speed is important, but it is not always absolutely required, nor is it always practical to obtain high speed without additional cost and complexity, which may not be justified.

Relay speed is especially important when the protected facility exists in a stability sensitive area of the power system network. Faster fault clearing reduces the amount that generators can accelerate during the fault and, therefore, improves stability margins. Early designs of microprocessor type relays were more or less slower than electromechanical or solid-state analog

designs. Modern microprocessor relay designs, however, incorporate processors and algorithms that provide operating speeds that are in the same range as that of other types of relays.

Burden: The load on current transformers (CT’s) and voltage transformers (VT’s) is commonly known as their burden. The term burden usually describes the impedance connected to the transformer secondary winding but may specify the volt-ampere (VA) delivered to the load.

Transducers

    It is desirable to discuss briefly about the transducers before starting the subject, the relay, protection relays. The suitable current and voltage transformers are required for the reliable operation of the protective relaying system. The transducers convert the high level signals to a lower levels in electrical power protection systems. The increase in current at the electrical system terminals is associated by a reduction in voltages.

      The increase in current (over current) and reduction in voltage (under voltage) caused by the fault can be used to detect that fault has occurred in the system. Based on these two types of transducers or instrument transformers as below are basically used in the protective relaying systems.

1)      Current transformers (CT)

2)      Voltage transformer (VT)

The reduction in levels of currents and voltages are necessary for the following reasons:

(a)    The lower level input to the relays ensures that the physical size of the relays small and hence reduce the cost.

(b)   The personnel who work with the relays will be working in a safe environment.

In principle the construction of these transducers are same as power transformers but specially designed to suit the application. It is necessary that a current transformer reproduce in its secondary winding a current which duplicates the primary current waveforms as close as possible. It performs this function quite well. Similarly the voltage transformer (VT) performs to produce the secondary voltage required to work.

Burden: The load on current transformers (CT’s) and voltage transformers (VT’s) is commonly known as their burden. The term burden usually describes the impedance connected to the transformer secondary winding but may specify the volt-ampere (VA) delivered to the load. For example, a transformer delivering 5 ampere to a resistive burden of 0.1 ohm may also be said to have a burden of 2.5 VA at 5 amperes.