Burman Managers Engineers and Scientist
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Sunday 15 March 2015
FUNDAMENTALS OF PROTECTIVE RELAYING
Sunday, February 9, 2014
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.
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