ASSIGNMENT TITLE:

PROTECTIVE RELAYS

SUBMITTED BY:

UMAR RAZZAQ ( 312861 )

SUBMISSION DATE:

27-09-2013

SUBMITTED TO:

ENGR. WAQAS UMAR

PROTECTIVE RELAYS

A protective relay is a device that detects the fault and initiates the operation of the

circuit breaker to isolate the defective element from the rest of the system. The relays detect

the abnormal conditions in the electrical circuits by constantly measuring the electrical

quantities which are different under normal and fault conditions. The electrical quantities

which may change under fault conditions are voltage, current, frequency and phase angle.

Through the changes in one or more of these quantities, the faults signal their presence, type

and location to the protective relays. Having detected the fault, the relay operates to close the

trip circuit of the breaker. This results in the opening of the breaker and disconnection of the

faulty circuit.

A typical relay circuit is shown in Fig. This diagram shows one phase of 3-phase system

for simplicity. The relay circuit connections can be divided into three parts.

i. First part is the primary winding of a current transformer (C.T.) which is connected in

series with the line to be protected.

ii. Second part consists of secondary winding of C.T. and the relay operating coil.

iii. Third part is the tripping circuit which may be either A.C or D.C. It consists of a source of

supply, the trip coil of the circuit breaker and the relay stationary contacts.

When a short circuit occurs at point F on the transmission line, the current flowing in

the line increases to an enormous value. This results in a heavy current flow through the relay

coil, causing the relay to operate by closing its contacts. This in turn closes the trip circuit of the

breaker, making the circuit breaker open and isolating the faulty section from the rest of the

system. In this way, the relay ensures the safety of the circuit equipment from damage and

normal working of the healthy portion of the system.

In order to provide selectivity to the system, it is a usual practice to divide the entire

system into several protection zones. When a fault occurs in a given zone, then only the circuit

breakers within that zone will be opened. This will isolate only the faulty circuit or apparatus,

leaving the healthy circuits intact. The system can be divided into the following protection

zones:

i. Generators

ii. Low-Tension Switchgear

iii. Transformers

iv. High-Tension Switchgear

v. Transmission lines

It may be seen in Figure that there is certain amount of overlap between the adjacent

protection zones. For a failure within the region where two adjacent zones overlap, more

breakers will be opened than the minimum necessary to disconnect the faulty section. But if

there were no overlap, a failure in the region between zones would not lie in either region and,

therefore, no breaker would be opened. For this reason, a certain amount of overlap is

provided between the adjacent zones.

OVER CURRENT RELAY:-

Over Current Relay has a single I/p in the form of ac current while the o/p of relay is a

normally open contact which changes its state whenever the value of actuating current

increases from the pick up value preset according to the requirement.There are two settings,

plug settings and time settings. Time setting determine operating time of the relay and plug

settings are used to set the pick up vlue of the current.

PSM =I relay/PS

Where I relay is the current through the relay operating coil and Ps is the plug setting for

pickup value. The value of PSM tells us about the severity of current as seen by the relay. A PSM

less than one means that normal load current is flowing .At PSM greater than one the relay is

supposed to pick up. Higher values of PSM indicate how serious the fault is.

INSTANTANEOUS OVER CURRENT RELAY:-

An instantaneous relay is one in which no intentional time delay is provided. In this case,

the relay contacts are closed immediately after current in the relay coil exceeds the minimum

calibrated value. Figure shows an instantaneous solenoid type of relay. Although there will be a

short time interval between the instant of pickup and the closing of relay contacts, no

intentional time delay has been added.

The instantaneous relays have operating time less than 01 second. The instantaneous

relay is effective only where the impedance between the relay and source is small compared to

the protected section impedance. The operating time of instantaneous relay is sometimes

expressed in cycles based on the power-system frequency e.g. one-cycle would be 1/50 second

in a 50-cycle system. Such a relay has only the pickup settings and does not have any time

settings. Otherwise there is no difference in this and over current relay.

INVERSE TIME OVER CURRENT RELAY:-

An inverse-time relay is one in which the operating time is approximately inversely

proportional to the magnitude of the actuating quantity. At values of current less than pickup,

the relay never operates. At higher values, the time of operation of the relay decreases steadily

with the increase of current. The inverse-time delay can be achieved by associating mechanical

accessories with relays.

The inverse-time characteristic can also be obtained by connecting a time-limit fuse in

parallel with the trip coil terminals of the relay. The shunt path formed by time-limit fuse is of

negligible impedance as compared with the relatively high impedance of the trip coil.

Therefore, so long as the fuse remains intact, it will divert practically the whole secondary

current of CT from the trip oil. When the secondary current exceeds the current carrying

capacity of the fuse, the fuse will blow and the whole current will pass through the trip coil,

thus opening the circuit breaker. The time lag between the incidence of excess current and the

tripping of the breaker is governed by the characteristics of the fuse. Careful selection of the

fuse can give the desired inverse-time characteristics, although necessity for replacement after

operation is a disadvantage.

IDMT (INVERSE DEFINITE MINIMUM TIME OVER CURRENT RELAY):-

In this type of relay, there is a definite time elapse between the instant of pickup and

the closing of relay contacts. This particular time setting is independent of the amount of

current through the relay coil; being the same for all values of current in excess of the pickup

value. Practically all inverse-time relays are also provided with definite minimum time feature

in order that the relay may never become instantaneous in its action for very long overloads.

The characteristic is inverse in the initial part which tends to a definite minimum

operating time as the current becomes very high. The reason for the operating time, becoming

definite minimum at higher values of current.

PROTECTION OF THREE PHASE FEEDER:-

DIFFERENTIAL PROTECTION:-

Differential protection is based on the fact that any fault within an electrical equipment

would cause the current entering it to be different from that leaving it .Thus we can compare

the two currents whether in magnitude or in phase or both and issue a trip signal if the

difference exceeds the predetermined set value .This method of detecting faults is very

attractive when both ends of apparatus are physically located near each other. A typical

situation, where this is true is in the case of a Transformer or generator or a bus bar.

BUCHOLZ RELAY:-

Buchholz Relay is a safety device mounted on some oil-filled Power Transformers and

reactors, equipped with an external overhead oil reservoir called a conservator. The Buchholz

Relay is used as a protective device sensitive to the effects of dielectric failure inside the

equipment in case of large internal faults.

When a large internal fault such as a winding to core fault occurs on the transformer

winding, there is severe heating of the oil. This causes gases to be liberated from the oil around

350 degree Celsius. This causes a buildup of oil pressure causing oil to rush into the

conservator. A vane is placed in the path of surge of oil between the transformer and the

conservator. A set of contacts operated by this vane is used as trip contacts of Bucholz relay.

This output from Bucholz relay is used to trip transformer.

Bucholz relay also has another set of contacts operated by a float. These contacts stay

open when the transformer tank is filled with oil but in case of leakage of oil or decomposition

of oil the floats sinks causing the contacts to close. This loss of oil results in rise of temperature

of transformer but does not initiate immediate tripping. So these contacts are always

connected to an alarm which alerts the operator.

RELAY TIMING:-

An important characteristic of a relay is its time of operation. By the time of operation

is meant length of the time from the instant when the actuating element is energized to the

instant when the relay contacts are closed. Sometimes it is desirable and necessary to control

the operating time of a relay. For this purpose, mechanical accessories are used with relays.

IMPORTANT TERMS:-

It is desirable to define and explain some important terms much used in connection with

relays.

PICK-UP CURRENT:-

It is the minimum current in the relay coil at which the relay starts to operate. So long as

the current in the relay is less than the pick-up value, the relay does not operate and the

breaker controlled by it remains in the closed position. However, when the relay coil current is

equal to or greater than the pickup value, the relay operates to energize the trip coil which

opens the circuit breaker.

CURRENT SETTING:-

It is often desirable to adjust the pick-up current to any required value. This is known as

current setting and is usually achieved by the use of tapings on the relay operating coil. The

taps are brought out to a plug bridge as shown in Figure. The plug bridge permits to alter the

number of turns on the relay coil. This changes the torque on the disc and hence the time of

operation of the relay. The values assigned to each tap are expressed in terms of percentage

full-load rating of C.T. with which the relay is associated and represents the value above which

the disc commences to rotate and finally closes the trip circuit.

Pick-up current = Rated secondary current of C.T. x Current setting

For example, suppose that an overcurrent relay having current setting of 125% is

connected to a supply circuit through a current transformer of 400/5. The rated secondary

current of C.T. is 5 amperes. Therefore, the pick-up value will be 25% more than 5 A i.e. 5 x 125

= 625 A. It means that with above current setting, the relay will actually operate for a relay coil

current equal to or greater than 625 A. The current plug settings usually range from 50% to

200% in steps of 25% for overcurrent relays and 10% to 70% in steps of 10% for earth leakage

relays. The desired current setting is obtained by inserting a plug between the jaws of a bridge

type socket at the tap value required.

PLUG-SETTING MULTIPLIER (P.S.M.):-

It is the ratio of fault current in relay coil to the pick-up current i.e.

P.S.M. = Fault current in relay coil

Pick - up current

= ____________Fault current in relay coil_________

Rated secondary current of CT x Current setting

For example, suppose that a relay is connected to a 400/5 current transformer and set at 150%.

With a primary fault current of 2400 A, the plug-setting multiplier can be calculated as under:

Pick-up value = Rated secondary current of CT x Current setting

= 5 x 15 = 75 A

Fault current in relay coil = _2400 x 5_

400

= 30 A

P.S.M. = 30/75 = 4

TIME-SETTING MULTIPLIER:-

A relay is generally provided with control to adjust the time of operation. This

adjustment is known as time-setting multiplier. The time-setting dial is calibrated from 0 to 1 in

steps of 0.05 sec. These figures are multipliers to be used to convert the time derived from

time/P.S.M. curve into the actual operating time. Thus if the time setting is 01 and the time

obtained from the time/P.S.M. curve is 3 seconds, then actual relay operating time = 3 x 01 =

03 second. For instance, in an induction relay, the time of operation is controlled by adjusting

the amount of travel of the disc from its reset position to its pickup position. This is achieved by

the adjustment of the position of a movable backstop which controls the travel of the disc and

thereby varies the time in which the relay will close its contacts for given values of fault current.

A so-called time dial with an evenly divided scale provides this adjustment. The actual time of

operation is calculated by multiplying the time setting multiplier with the time obtained from

time/P.S.M. curve of the relay.

TIME/P.S.M. CURVE:-

Figure shows the curve between time of operation and plug setting multiplier of a

typical relay. The horizontal scale is marked in terms of plug-setting multiplier and represents

the number of times the relay current is in excess of the current setting. The vertical scale is

marked in terms of the time required for relay operation. If the P.S.M. is 10, then the time of

operation (from the curve) is 3 seconds.

The actual time of operation is obtained by multiplying this time by the time-setting multiplier.

It is evident from Figure that for lower values of overcurrent, time of operation varies inversely

with the current but as the current approaches 20 times full-load value, the operating time of

relay tends to become constant. This feature is necessary in order to ensure discrimination on

very heavy fault currents flowing through sound feeders.

CALCULATION OF RELAY OPERATING TIME:-

In order to calculate the actual relay operating time, the following things must be known:

i. Time/P.S.M. curve

ii. Current setting

iii. Time setting

iv. Fault current

v. Current transformer ratio

The procedure for calculating the actual relay operating time is as follows:

i. Convert the fault current into the relay coil current by using the current transformer

ratio.

ii. Express the relay current as a multiple of current setting i.e. calculate the P.S.M.

iii. From the Time/P.S.M. curve of the relay, read off the time of operation for the

calculated P.S.M.

iv. Determine the actual time of operation by multiplying the above time of the relay by

time setting multiplier in use.

DISTANCE OR IMPEDANCE RELAYS:-

There is a group of relays in which the operation is governed by the ratio of applied

voltage to current in the protected circuit. Such relays are called distance or impedance relays

and are used for protection of Transmission lines. In an impedance relay, the torque produced

by a current element is opposed by the torque produced by a voltage element. The relay will

operate when the ratio V/I is less than a predetermined value. Figure illustrates the basic

principle of operation of an impedance relay. The voltage element of the relay is excited

through a potential transformer (P.T.) from the line to be protected. The current element of the

relay is excited from a current transformer (C.T.) in series with the line. The portion AB of the

line is the protected zone. Under normal operating conditions, the impedance of the protected

zone is ZL. The relay is so designed that it closes its contacts whenever impedance of the

protected section falls below the pre-determined value i.e. ZL in this case.

Now suppose a fault occurs at point F1 in the protected zone. The impedance Z (= *V/I)

between the point where the relay is installed and the point of fault will be less than ZL and

hence the relay operates. Should the fault occur beyond the protected zone (say point F2), the

impedance Z will be greater than ZL and the relay does not operate.

Types:-

A distance or impedance relay is essentially an ohmmeter and operates whenever the

impedance of the protected zone falls below a pre-determined value. There are two types of

distance relays in use for the protection of power supply, namely;

(i) Definite-distance relay which operates instantaneously for fault up to a pre-determined

distance from the relay.

(ii) Time-distance relay in which the time of operation is proportional to the distance of fault

from the relay point. A fault nearer to the relay will operate it earlier than a fault farther away

from the relay.

THERMAL RELAYS:-

Thermal relays work on the principle of unequal linear expansion of two different metals

as a result of heat generated by the passage of fault current. Figure (a) shows a bimetallic relay

consisting of strips AB and CD of two different materials. Both the strips get heated up by the

same amount but are deformed by different amounts. Figure (b) shows the relay operation. The

material with higher coefficient of expansion is at the outer surface of the curve while that with

lower coefficient is at the inner surface.

Since the heating effect is proportional to the square of the current, the energy

dissipated is given by (I2R)t where t is the time for which the current flows through the relay.

As accumulation of sufficient amount of heat depends on thermal inertia of relay, which is

rather large, so these relays are used where very quick operation is not required. A typical

application of thermal relay is to provide protection against prolonged overheating of motors.

The thermal overload relay lets the motor supply overload for a preset amount of time before

tripping it off.

REVERSE POWER RELAY:-

There are situations when directional relays have to be used under non fault conditions.

Such an application of directional relays exists in reverse power protection of a turbo

alternator. Reverse power is applied when there is no collapse in system voltage. Directional

relays for short circuit protection are connected in such a way that they develop maximum

operating torque under fault conditions. Reverse power relays on the other hand are connected

in such a way that they develop maximum torque when direction of power flow reverses.

FREQUENCY RELAY:-

Frequency Relay is used for detection of increasing or decreasing frequency in AC

systems up to 400V. Frequency relays monitor overshooting and undershooting against the set

frequency thresholds in 50 Hz or 60 Hz systems. They mostly have the following properties:

i. It is an electronic measuring relay.

ii. It is used for monitoring frequency.

iii. No additional supply voltage is required for frequency relay.

iv. It is connected with an alarm relay which in turn gives signal to alarm LED.

Title (2)Relays (2)