Chapter No2: Role and Importance of protection schemes in power system.Contents:1.1

2.1 Introduction

2.1.1 General considerations2.1.2 Role of protection in a power station

2.2 System and substation layout

2.2.1 System layout2.2.2 Substation layout (electrical)2.2.3 Current transformer location 2.3 System earthing2.3.1 Neutral-earthing methods2.3.2 Special cases of resistance earthing

2.3 Faults

2.4.1Faults and other abnormalities2.4.2 Nature and causes of faults2.4.3 Fault statistics

2.4 Basic terms used in protection

2.5 Necessity for back-up protection

2.6 Economic considerations

2.6.1 General 2.6.2 Distribution systems2.6.3 Transmission systems

Chapter No3: Protection Principles and Components

3.1Fundamental principles

3.1.1 Methods of discrimination

3.1.2 Derivation of relaying quantities 3.1.3 Combined overcurrent and earth fault relays3.1.4 Derivation of a representative single-phase

quantity from a three-phase system 3.2 Components of protection

3.2.1 Relays3.2.2 Current transformers3.2.3 Voltage transforming devices3.2.4 Capacitor dividers3.2.5 H.F. capacitor couplers3.2.6 Line traps3.2.7 Circuit-breakers3.2.8Tripping and other auxiliary supplies3.2.9Fuses, small wiring, terminals and test links3.2.10 Pilot circuits

Chapter No4: Protective relays

4.1 Zone Protection4.2 Fault Detection4.3 Differential Protection4.4 Overcurrent Protection4.5 Distance Protection4.6 Overvoltage Protection4.7 Undervoltage Protection4.8 UnderFrequency Protection4.9 Reverse current Protection4.10 Phase unbalance Protection4.11 Reverse Phase rotation Protection4.12 Thermal Protection

Chapter No:02Role and Importance of protection schemes in

power system.

2.1 Introduction

2.1.1 General considerations:

The history of electrical-power technology throughout the world is one of steady and, in

recent years, rapid progress, which has made it possible to design and construct

economic and reliable power systems capable of satisfying the continuing growth in the

demand for electrical energy. In this, power system protection and control play a

significant part, and progress in design and development in these fields has necessarily

had to keep pace with advances in the design of primary plant, such as generators,

transformers, switchgear, overhead lines and underground cables, Indeed, progress in

the fields of protection and control is a vital prerequisite for the efficient operation and

continuing development of power supply systems as a whole, This work, in three

volumes, deals with all the relevant aspects of protection in current British practice for

generation, transmission and distribution systems, The subject matter has been divided

into a number of discrete chapters covering, as completely as is necessary for the

purpose of the work, general principles, design and performance and, by no means of

least importance, application. The purpose of the present chapter is to provide the

background knowledge necessary to a proper understanding of the aims and the role of

protection in a power system.

The word 'protection' is used here to describe the whole concept of protecting a power

system. The term 'protective gear' (or 'protective equipment') is widely used in that

sense: but here that term will be used in the narrower sense of the actual components

used in achieving the desired protection.

The function of protective equipment is not the preventive one its name would imply, in

that it takes action only after a fault has occurred: it is the ambulance at the foot of the

cliff rather than the fence at the top. Exceptions to this are the Buchholz protector, a

gas-operated device which is capable of detecting the gas accumulation produced by

an incipient fault in a power transformer, and the surge arrester which is designed to

prevent a dangerous rise of potential, usually between earth and the conductor or

terminal to which it is connected. As commonly used, 'protective gear' refers to relay

systems and does not embrace the surge arrester, the arc suppression coil and similar

preventive devices.

2.1.2 Role of protection in a power station:

We begin with this so that the subject can be seen in its proper perspective. It is fair to

say that without discriminative protection it would be impossible to operate a modern

power system. The protection is needed to remove as speedily as possible any element

of the power system in which a fault has developed. So long as the fault remains

connected, the whole system may be in jeopardy from three main effects of the fault,

namely:

(a) it is likely to cause the individual generators in a power station, or groups of

generators in different stations, to lose synchronism and fall out of step with consequent

splitting of the system;

(b) a risk of damage to the affected plant; and

(c) a risk of damage to healthy plant.

There is another effect, not necessarily dangerous to the system, but important from the

consumers' viewpoint, namely, a risk of synchronous motors in large industrial premises

falling out of step and tripping out, with the serious consequences that entails loss of

production and interruption of vital processes. It is the function of the protective

equipment, in association with the circuit breakers, to avert these effects. This is wholly

true of large h.v. networks, or transmission systems. In the lower-voltage distribution

systems, the primary function of protection is to maintain continuity of supply. This, in

effect, is achieved incidentally tn transmission systems if the protection operates

correctly to avert the effects mentioned above; indeed it must be so, because the

ultimate aim is to provide 100 per cent continuity of supply. Obviously this aim cannot

be achieved by the protection alone. In addition the power system and the distribution

networks must be so designed that there are duplicate or multiple outlets from power

sources to load centres (adequate generation may be taken for granted), and at least

two sources of supply (feeders) to each distributing station. There are certain

conventional ways of ensuring alternative supplies, as we shall see, but if full advantage

is to be taken of their provision (always a costly matter) the protection must be highly

selective in its functioning. For this tt must possess the quality known as discrimination,

by virtue of which it is able to select and to disconnect only the faulty element in the

power system, leaving all others in normal operation so far as that may be possible.

With a few exceptions the detection and tripping of a faulty circuit is a very simple

matter; the art and skill lie in selecting the faulty one, bearing in mind that many circuits

- generators, transformers, feeders - are usually affected, and in much the same way by

a given fault, This accounts for the multiplicity of relay types and systems in use. Other

chapters will explain their intricacies.

2.2 System and substation layout

2.2.1 System layout:Turning now to the matter of system layout, with particular reference to theimplications it has for protection, power systems, and especially distribution systems,

can in general be arranged as:

(a) radial feeders

(b) parallel feeders

Fig:2.2.1(A) Radial System

(c) ring systems

(d) combinations of (a), (b) and (c).

Arrangement (a) does not satisfy the requirements of a duplicate supply, unless there is

a source of generation at each end (Fig. 2.2.1A): nevertheless, discriminative protection

is needed to limit the extent of the dislocation of supply. Arrangement

Radial System Ring System

Fig:2.2.1(B) Typical Applications Of Parallel Feeders.

(b), two applications of which are shown in Fig. 2.2.1B, provides a satisfactory

duplicate supply. Arrangement (c) is, in effect, a logical extension of the idea of two

parallel feeders. In its simplest form (Fig. 2.2.1C) it provides a duplicate supply

to every substation, provided that the ring is closed. When the ring is open the system

reverts to one of two radial feeders. In the more complex form of Fig. 1.2.1D with

interconnecting (tie)lines and multiple power sources - a form suited to a transmission

system - more sophisticatedforms of protection are needed than would be acceptable

for the simple ring system if the aims of the protection, as already defined, are to be

fulfiUed. In thisform can be discerned also combinations of (a), (b) and (c).

Fig: 2.2.1 (C) Ring Main System.

Fig:2.2.1(D). Interconnected Power System.

2.2.2 Substation layout (electrical):

This topic is relevant to the subject inasmuch as the electrical connections of a

substation can affect the protection, albeit in minor and rather subtle ways.

Substations, with which can be grouped switching stations, are points in a power

system where transmission lines or distribution feeders are marshalled for purposes

of controlling load flow and general switching for maintenance purposes, and to

which supplies are taken from generating stations and transformed in voltage, if

necessary, for distribution.

Although substations differ greatly in size, construction, cost and complexity

according to voltage, location and function, the feature they all have in common is

the marshalling of all the associated circuits, through circuit-breakers, or switches,

on to busbars (see Fig. 2.2.2A). Herein lies one of the ways in which protection is

affected

Figure?

2.2.3 Current transformer location: 2.3 System earthing:

2.3.1 Neutral-earthing methods:

2.3.2 Special cases of resistance earthing:

2.3 Faults

2.4.1Faults and other abnormalities:

2.4.2 Nature and causes of faults:

2.4.3 Fault statistics:

2.4 Basic terms used in protection

2.5 Necessity for back-up protection

2.6 Economic considerations

2.6.1 General:

2.6.2 Distribution systems:

2.6.3 Transmission systems:

Chapter No3: Protection Principles and Components

3.1Fundamental principles

3.1.1 Methods of discrimination:

3.1.2 Derivation of relaying quantities: 3.1.3 Combined overcurrent and earth fault relays:

3.1.4 Derivation of a representative single-phase quantity from a three-phase system:

3.2 Components of protection

3.2.1 Relays:

3.2.2 Current transformers:

3.2.3 Voltage transforming devices:

3.2.4 Capacitor dividers:

3.2.5 H.F. capacitor couplers:

3.2.6 Line traps:

3.2.7 Circuit-breakers:

3.2.8Tripping and other auxiliary supplies:

3.2.9Fuses, small wiring, terminals and test links:

3.2.10 Pilot circuits:

Chapter No4:Protective relays

Introduction To Protective Relays:

A particular type of equipment applied to electric power systems to detect abnormal and

intolerable conditions and to initiate appropriate corrective actions. These devices

include lightning arresters, surge protectors, fuses, and relays with associated circuit

breakers, reclosers, and so forth.

a protective relay is a device designed to trip a circuit breaker when a fault is detected.

The first protective relays were electromagnetic devices, relying on coils operating on

moving parts to provide detection of abnormal operating conditions such as over-

current, over-voltage, reverse power flow, over- and under- frequency. Microprocessor-

based digital protection now emulate the original devices, as well as providing types of

protection and supervision impractical with electromechanical relays. In many cases a

single microprocessor relay provides functions that would take two or more

electromechanical devices. By combining several functions in one case, numerical

relays also save capital cost and maintenance cost over electromechanical relays.

However, due to their very long life span, tens of thousands of these "silent sentinels"

are still protecting transmission lines and electrical apparatus all over the world. An

important transmission line or generator unit will have cubicles dedicated to protection,

with many individual electromechanical devices, or one or two microprocessor relays.

4.1 Zone Protection:

To limit the extent of the power system that is disconnected when a fault occurs,

protection is arranged in zones. Ideally, the zones of protection should overlap, so that

no part of the power system is left unprotected.

For practical physical and economic reasons, this ideal is not always achieved,

accommodation for current transformers being in some cases available only on one side

of the circuit breakers. This leaves a section between the current transformers and the

circuit breaker A that is not completely protected against faults. In Figure 2 b) a fault at

F would cause the busbar protection to operate and open the circuit breaker but the

fault may continue to be fed through the feeder. The feeder protection, if of the unit type,

would not operate, since the fault is outside its zone. This problem is dealt with by

intertripping or some form of zone extension, to ensure that the remote end of the

feeder is tripped also. The point of connection of the protection with the power system

usually defines the zone and corresponds to the location of the current transformers.

Unit type protection will result in the boundary being a clearly defined closed loop.

Figure 3 illustrates a typical arrangement of overlapping zones.

Alternatively, the zone may be unrestricted; the start will be defined but the extent (or

‘reach’) will depend on measurement of the system quantities and will therefore be

subject to variation, owing to changes in system conditions and measurement errors.

Fig: 1 Fig: 2 (a&b)

Fig: 3

4.2 Fault Detection

Fault detection is accomplished by a number of techniques. Some of the common

methods are the detection of changes in electric current or voltage levels, power

direction, ratio of voltage to current, temperature, and comparison of the electrical

quantities flowing into a protected area with the quantities flowing out. The last-

mentioned is known as differential protection

4.3 Differential Protection

The relays used in power system protection are of different types. Among them

differential relay is very commonly used relay for protecting transformers and

generators from localised faults.

Differential relays are very sensitive to the faults occurred within the zone of protection

but they are least sensitive to the faults that occur outside the protected zone. Most of

the relays operate when any quantity exceeds beyond a pre-determined value for

example over current relay operates when current through it exceeds pre-determined

value. But the principle of differential relay is somewhat different. It operates depending

upon the difference between two or more similar electrical quantities.

Definition of Differential Relay

The differential relay is one that operates when there is a difference between two or

more similar electrical quantities exceeds a pre-determined value. In differential relay

scheme circuit, there are two currents come from two parts of an electrical power circuit.

These two currents meet at a junction point where a relay coil is connected. According

to Kirchhoff Current Law, the resultant current flowing through the relay coil is nothing

but summation of two currents, coming from two different parts of the electrical power

circuit. If the polarity and amplitude of both currents are so adjusted that the phasor sum

of these two currents, is zero at normal operating condition. Thereby there will be no

current flowing through the relay coil at normal operating conditions. But due to any

abnormality in the power circuit, if this balance is broken, that means the phasor sum of

these two currents no longer remains zero and there will be non-zero current flowing

through the relay coil thereby relay being operated.

In current differential scheme, there are two sets of current transformer each connected

to either side of the equipment protected by differential relay. The ratio of the current

transformers are so chosen, the secondary currents of both current transformers

matches each other in magnitude. The polarity of current transformers are such that the

secondary currents of these CTs opposes each other. From the circuit is clear that only

if any non-zero difference is created between this to secondary currents, then only this

differential current will flow through the operating coil of the relay. If this difference is

more than the pick-up value of the relay, it will operate to open the circuit breakers to

isolate the protected equipment from the system. The relaying element used in

differential relay is attracted armature type instantaneously relay since differential

scheme is only adapted for clearing the fault inside the protected equipment in other

words differential relay should clear only internal fault of the equipment hence the

protected equipment should be isolated as soon as any fault occurred inside the

equipment itself. They need not be any time delay for coordination with other relays in

the system.

Types of Differential Relay

There are mainly two types of differential relay depending upon the principle of

operation. 1. Current Balance Differential Relay 2. Voltage Balance Differential Relay

In current differential relay two current transformers are fitted on the either side of the

equipment to be protected. The secondary circuits of CTs are connected in series in

such a way that the carry secondary CT current in same direction. The operating coil of

the relaying element is connected across the CT’s secondary circuit. Under normal

operating conditions, the protected equipment (either power transformer or alternator)

carries normal current. In this situation, say the secondary current of CT1 is I1 and

secondary current of CT2 is I2. It is also clear from the circuit that the current passing

through the relay coil is nothing but I1-I2. As we said earlier, the current transformer’s

ratio and polarity are so chosen, I1 = I2, hence there will be no current flowing through

the relay coil. Now if any fault occurs external to the zone covered by the CTs, faulty

current passes through primary of the both current transformers and thereby secondary

currents of both current transformers remain same as in the case of normal operating

conditions. Therefore at that situation the relay will not be operated. But if any ground fault

occurred inside the protected equipment as shown, two secondary currents will be no

longer equal. At that case the differential relay is being operated to isolate the faulty

equipment (transformer or alternator) from the system.

Principally this type of relay systems suffers from some disadvantages

1. There may be a probability of mismatching in cable impedance from CT secondary

to the remote relay panel.

2. These pilot cables’ capacitance causes incorrect operation of the relay when large

through fault occurs external to the equipment.

3. Accurate matching of characteristics of current transformer cannot be achieved

hence there may be spill current flowing through the relay in normal operating

conditions.

Percentage Differential Relay

This is designed to response to the differential current in the term of its fractional

relation to the current flowing through the protected section. In this type of relay, there

are restraining coils in addition to the operating coil of the relay. The restraining coils

produce torque opposite to the operating torque. Under normal and through fault

conditions, restraining torque is greater than operating torque. Thereby relay remains

inactive. When internal fault occurs, the operating force exceeds the bias force and

hence the relay is operated. This bias force can be adjusted by varying the number of

turns on the restraining coils. As shown in the figure below, if I1 is the secondary current

of CT1 and I2 is the secondary current of CT2 then current through the operating coil is I1

- I2 and current through the restraining coil is (I1+ I2)/2. In normal and through fault

condition, torque produced by restraining coils due to current (I1+ I2)/2 is greater than

torque produced by operating coil due to current I1- I2 but in internal faulty condition

these become opposite. And the bias setting is defined as the ratio of (I1- I2) to (I1+ I2)/2

It is clear from the above explanation, greater the current flowing through the restraining

coils, higher the value of the current required for operating coil to be operated. The relay

is called percentage relay because the operating current required to trip can be

expressed as a percentage of through current.

CT Ratio and Connection for Differential Relay

This simple thumb rule is that the current transformers on any star winding should be

connected in delta and the current transformers on any delta winding should be

connected in star. This is so done to eliminate zero sequence current in the relay circuit.

If the CTs are connected in star, the CT ratio will be In/1 or 5 A CTs to be connected in

delta, the CT ratio will be In/0.5775 or 5X0.5775 A

Voltage Balance Differential Relay

In this arrangement the current transformer are connected either side of the equipment

in such a manner that EMF induced in the secondary of both current transformers will

oppose each other. That means the secondary of the current transformers from both

sides of the equipment are connected in series with opposite polarity. The differential

relay coil is inserted somewhere in the loop created by series connection of secondary

of current transformers as shown in the figure. In normal operating conditions and also

in through fault conditions, the EMFs induced in both of the CT secondary are equal and

opposite of each other and hence there would be no current flowing through the relay

coil. But as soon as any internal fault occurs in the equipment under protection, these

EMFs are no longer balanced hence current starts flowing through the relay coil thereby

trips circuit breaker.

There are some disadvantages in the voltage balance differential relay such as A multy

tap transformer construction is required to accurate balance between current

transformer pairs. The system is suitable for protection of cables of relatively short

length otherwise capacitance of pilot wires disturbs the performance. On long cables the

charging current will be sufficient to operate the relay even if a perfect balance of

current transformer achieved.

These disadvantages can be eliminated from the system by introducing Translay

system which is nothing but modified balance voltage differential relay system. Translay

scheme is mainly applied for differential protection of feeders. Here, two sets of current

transformers are connected either end of the feeder. Secondary of each current

transformer is fitted with individual double winding induction type relay. The secondary

of each current transformer feeds primary circuit of double winding induction type relay.

The secondary circuit of each relay is connected in series to form a closed loop by

means of pilot wires. The connection should be such that, the induced voltage in

secondary coil of one relay will oppose same of other. The compensating device

neutralises the effect of pilot wires capacitance currents and effect of inherent lack of

balance between the two current transformers. Under normal conditions and through

fault conditions, the current at two ends of the feeder is same thereby the current

induced in the CT’s secondary would also be equal. Due to these equal currents in the

CT’s secondary, the primary of each relay induce same EMF. Consequently, the EMF

induced in the secondaries of the relays are also same but the coils are so connected,

these EMFs are in opposite direction. As a result, no current will flow through the pilot

loop and thereby no operating torque is produced either of the relays. But if any fault

occurs in the feeder within the zone in between current transformers, the current leaving

the feeder will be different from the current entering into the feeder. Consequently, there

will be no equality between the currents in both CT secondaries. These unequal

secondary CT currents will produce unbalanced secondary induced voltage in both of

the relays. Therefore, current starts circulating in the pilot loop and hence torque is

produced in both of the relays. As the direction of secondary current is opposite into

relays, therefore, the torque in one relay will tend to close the trip contacts and at the

same time torque produced in other relay will tend to hold the movement of the trip

contacts in normal un-operated position. The operating torque depends upon the

position and nature of faults in the protected zone of feeder. The faulty portion of the

feeder is separated from healthy portion when at least one element of either relay

operates.

This can be noted that in Translay protection scheme, a closed copper ring is fitted with

the Central limb of primary core of the relay. These rings are utilised to neutralise the

effect of pilot capacity currents. Capacity currents lead the voltage impressed of the pilot

by 90° and when they flow in low inductive operating winding, produce flux that also

leads the pilot voltage by 90°. Since the pilot voltage is that induced in the secondary

coils of the relay, it lags by a substantial angle behind the flux in the field magnetic air

gap. The closed copper rings are so adjusted that the angle is approximately 90°. In this

way fluxes acting on the disk are in phase and hence no torque is exerted in the relay

disc.

4.4 Over-current Protection:

    Introduction:

As the fault impedance is less than load impedance, the fault current is more than

load current. If a short circuit occurs the circuit impedance is reduced to a low value and

therefore a fault is accompanied by large current.

Over-current protection is that protection in which the relay picks up when the

magnitude of current exceeds the pickup level.

 The basic element in Over-current protection is an Over-current relay.

The Over-current relays are connected to the system, normally by means of CT's.

 

Over-current relaying has following types:

1.      High speed Over-current protection.

2.      Definite time Over-current protection.

3.      Inverse minimum time Over-current protection.

4.      Directional Over-current protection (of above types).

 

Over-current protection includes the protection from overloads. This is most widely

used protection. Overloading of a machine or equipment generally) means the machine

is taking more current than its rated current. Hence with overloading, there is an

associated temperature rise. The permissible temperature rise has a limit based on

insulation class and material problems. 

Over-current protection of overloads is generally provided by thermal relays.

 

Over-current protection includes short-circuit protection. Short circuits a be phase

faults, earth faults or winding faults. Short-circuit currents are generally several times (5

to 20) full load current. Hence fast fault clearance is always desirable on short-circuits.

 

When a machine is protected by differential protection, the over-current is provided

in addition as a back-up and in some cases to protect the machine from sustained

through fault.

Several protective devices are used for over-current protection these include:

1.      Fuses

2.      Circuit-breakers fitted with overloaded coils or tripped by over-current relays.

3.      Series connected trip coils operating switching devices.

4.      Over-current relays in conjunction with current transformers.

 The primary requirements of over-current protection are:

The protection should not operate for starting currents, permissible over-

current, and current surges. To achieve this, the time delay is provided (in

case of inverse relays). If time delay cannot be permitted, high-set

instantaneous relaying is used.

The protection should be coordinated with neighboring over-current

protections so as to discriminate.

         Applications of Over-current Protection

Over-current protection has a wide range of applications. It can be applied where there

is an abrupt difference between fault current within the protected section and that

outside the protected section and these magnitudes are almost constant.

 

The over-current protection is provided for the following

 Motor Protection:

Over-current protection is the basic type of protection used against overloads and short-

circuits in stator windings of motors. Inverse time and instantaneous phase and ground

over-current relays can be employed for motors above 1200 H.P. For small/medium

size motors where cost of CT's and protective relays is not economically justified,

thermal relays and HRC fuses are employed, thermal relays used for overload

protection and HRC fuses for short-circuit protection.

Transformer Protection:

Transformers are provided with over-current protection against faults, only, when the

cost of differential relaying cannot be justified. However, over-current relays are

provided in addition to differential relays to take care of through faults. Temperature

indicators and alarms are always provided for large transformers.

Small transformers below 500 kVA installed in distribution system are generally

protected by drop-out fuses, as the cost of relays plus circuit-breakers is not generally

justified Line Protection.

The lines (feeders) can be protected by

(1)               Instantaneous over-current relays.

(2)               Inverse time over-current relays.

(3)               Directional over-current relay.

Lines can be protected by impedance or carrier current protection also.

 

Protection of Utility Equipment:

The furnaces, industrial installations commercial, industrial and domestic equipment are

all provided with over-current protection.

 

4.5 Distance Protection:

There is one type of relay which functions depending upon the distance of fault in

the line. More specifically, the relay operates depending upon the impedance between

the point of fault and the point where relay is installed. These relays are known as

distance relay or impedance relay.

Working Principle of Distance or Impedance Relay:

The working principle of distance relay or impedance relay is very simple. There is

one voltage element from potential transformer and an current element fed from current

transformer of the system. The deflecting torque is produced by secondary current of

CT and restoring torque is produced by voltage of potential transformer. In normal

operating condition, restoring torque is more than deflecting torque. Hence relay will not

operate. But in faulty condition, the current becomes quite large whereas voltage

becomes less. Consequently, deflecting torque becomes more than restoring torque

and dynamic parts of the relay starts moving which ultimately close the No contact of

relay. Hence clearly operation or working principle of distance relay, depends upon the

ratio of system voltage and current. As the ratio of voltage to current is nothing but

impedance a distance relay is also known as impedance relay.

The operation of such relay depends upon the predetermined value of voltage to current

ratio. This ratio is nothing but impedance. The relay will only operate when this voltage

to current ratio becomes less than its predetermined value. Hence, it can be said that

the relay will only operate when the impedance of the line becomes less than

predetermined impedance (voltage / current). As the impedance of a transmission line is

directly proportional to its length, it can easily be concluded that a distance relay can

only operate if fault is occurred within a predetermined distance or length of line.

Types of Distance or Impedance Relay:

There are mainly two types of distance relay-

1. Definite distance relay.

2. Time distance relay.

4.6 Overvoltage Protection:

Lightning in the area near the power lines can cause very short-time

overvoltages in the system and possible breakdown of the insulation. Protection for

these surges consists of lightning arrestors connected between the lines and ground.

Normally the insulation through these arresters prevents current flow, but they

momentarily pass current during the high-voltage transient to limit overvoltage.

Overvoltage protection is seldom applied elsewhere except at the generators, where it

is part of the voltage regulator and control system. In the distribution system,

overvoltage relays are used to control taps of tap-changing transformers or to switch

shunt capacitors on and off the circuits.

4.7 Undervoltage Protection:

This must be provided on circuits supplying power to motor loads. Low-voltage

conditions cause motor to draw excessive currents, which can damage the motors. If a

low-voltage condition develops while the motor is working, the relay senses this

condition and removes the motor from services.

Under-voltage relays can also be used effectively prior to starting large induction

or synchronous motors. These types of motors will not reach their rated speeds if

started under a low-voltage condition. Relays can be applied to measure terminal

voltage, and if it is below a pre-determined value the relay prevents starting of the

motor.

4.8 UnderFrequency Protection:

A loss or deficiency in the generation supply. The

transmission lines, or other components of the system, resulting

primarily from faults, can leave the system with an excess of load.

Solid-state and digital-type under-frequency relays are connected

at various points in the system to detect this resulting decline in

the normal system frequency. They operate at disconnect loads

or to separate the system into areas so that the available

generation equals the load until a balance is re-established.

4.9 Reverse current Protection:

This is provided when a change in the normal direction of current indicates an abnormal

condition in the system. In an a.c circuit, reverse current implies a phase shift of the

current of nearly 180°  from normal. This is actually a change in direction of power flow and

can be directed by a.c directional relays.

A common application of reverse-current protection is shown in fig: 4.9 . in this example,

a utility supplies power to an industrial plant having some generation of its own. Under

normal conditions, current flows from the utility to the plant (fig: 4.9a). in the event of a

fault occurring on the utility feeder (fig: 4.9b) the current reverses direction and flow

from the plant to the fault location. The relay operates and trips the circuit breaker ,

isolating the plant from the utility, thus preventing an excessive burden on the plant

generator. Usually in these cases, the plant generator can not carry the plant load, so

that under-frequency relays are used to shed non-critical load. When the utility tie is

restored, the shed loads then can be reconnected for full plant service.

Fig:4.9 reverse current protection. (a) Normal Load Conditions. (b) Internal Fault

Condition; Relay Trip Circuit Breaker Under Reverse-Current Condition.

4.10 Phase unbalance Protection:

This protection is used on feeders supplying motors where there is a possibility of

one phase opening as a result of a fuse failure or a connector failure. One type of relay

compares the current in one phase against the currents in the other phases. When the

unbalance becomes too great, the relay operates. Another type monitors the three-

phase bus voltages for unbalance. Reverse phases will operate this relay.

4.11 Reverse Phase rotation Protection:

Where direction of rotation is important, electric motors must be protected

against phase reversal. A reverse-phase-rotation relay is applied to sense the phase

rotation. This relay is a miniature three-phase motor with the same desired direction of

rotation as the motor it is protecting. If the direction of rotation is correct, the relay will let

the motor start. If incorrect, the sensing relay will prevent the motor starter from

operating.

4.12 Thermal Protection:Motors and generators are particularly subject to overheating due to overloading and

mechanical friction. Excessive temperatures leads to deterioration of insulation and

increased losses within the machine. Temperature-sensitive elements, located inside

the machine, from part of a bridge circuit used to supply current to a relay. When a

predetermined temperature is reached, the relay operates, initiating opening of a circuit

breaker or sounding of an alarm.