+ All Categories
Home > Documents > POWER SYSTEM PROTECTION - RGMCET | Group · POWER SYSTEM PROTECTION K NITEESH KUMAR RGMCET(EEE),...

POWER SYSTEM PROTECTION - RGMCET | Group · POWER SYSTEM PROTECTION K NITEESH KUMAR RGMCET(EEE),...

Date post: 24-Apr-2018
Category:
Upload: phungdan
View: 218 times
Download: 1 times
Share this document with a friend
57
POWER SYSTEM PROTECTION K NITEESH KUMAR RGMCET(EEE), NANDYAL 15/01/2017 K NITEESH KUMAR (RGMCET(EEE), NANDYAL) POWER SYSTEM PROTECTION 15/01/2017 1 / 57
Transcript

POWER SYSTEM PROTECTION

K NITEESH KUMAR

RGMCET(EEE), NANDYAL

15/01/2017

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 1 / 57

CONTENTS

CIRCUIT BREAKERS

RELAYS

TRANSFORMER PROTECTION

GENERATOR PROTECTION

FEEDER PROTECTION

OVER VOLTAGE PROTECTION

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 2 / 57

CIRCUIT BREAKER

CIRCUIT BREAKER is a over current protective device whichcan make (or) brake the circuit under normal and ub-normalconditions respectively and can be operated under no-load,fullload and fault conditions.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 3 / 57

TERMS REGARDING ARC PHENOMENON

D-8\N-SYSTEM1\SYS15-1.PM6.5

��� ����������������������

Systemvoltage

Faultcurrent

Restrikingvoltage

Arcvoltage

t

���� � �

������� �!"#$%�&"��!'%-.�!"&�'#$%�&"�)

Restriking voltage: The resultant transient voltage which appears across the breakercontacts at the instant of arc extinction is known as the restriking voltage.

Recovery voltage: The power frequency r.m.s. voltage that appears across the breakercontacts after the transient oscillations die out and final extinction of arc has resulted in allthe poles is called the recovery voltage.

Active recovery voltage: It is defined as the instantaneous recovery voltage at the instantof arc extinction.

The instantaneous recovery voltage is given by

Var = KVm sin φwhere K = 1 if the three-phase fault is also grounded and K = 1.5 if the three-phase fault isisolated.

Rate of Rise of Restriking Voltage (RRRV): As shown in Fig 15.1(a),

The average RRRV = Peak value of restriking voltage

Time taken to reach to peak value

= 2V

LCm

πRewriting the equation,

v = Vm1−���

���

costLC

The RRRV is given by

dvdt

= V

LC

t

LCm sin

This is maximum when

t

LC =

π2

or t = π2

LC

and the value is V

LCm

������ �!"#$%�&"��!'%-.�!"&�'#$%�&"�)

ARC VOLTAGERESTRIKING VOLTAGERECOVERY VOLTAGEACTIVE RECOVERY VOLTAGE

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 4 / 57

CURRENT CHOPPING

D-8\N-SYSTEM1\SYS15-1.PM6.5

���������� ��� ��2

The rate at which the restriking voltage rises is, therefore, very important in the arcextinction process because the ionization process will depend upon this rate. Therefore, it isfound that if the RRRV is smaller than the rate at which the dielectric between the contacts isdeveloped, the arc will be extinguished; otherwise there will be further restrike. This theoryhas been advocated by Dr. J. Slepian.

�� ������� ��������

When a circuit breaker is made to interrupt low inductive currents such as currents due to noload magnetising current of a transformer, it does so even before the current actually passesthrough zero value especially when the breaker exerts the same deionizing force for all currentswithin its short circuit capacity. This breaking of current before it passes through the naturalzero is termed as current chopping. This current chopping may take place even in breakerswhich produce varying degree of deionizing force. The effect of a practically instantaneouscollapse of the arc current, even of only a few amperes, is potentially very serious from thepoint of view of over-voltages which may result in the system. Referring to Fig. 15.2, the arc

Arcvoltage

Prospectivevoltage

t

Arccurrent

t

Recoveryvoltage

B C eS

L

���� ���-���!�',$11�!")

current is seen to approach zero in normal fashion initially with low arc voltage so that thereis virtually no capacitance current. At a certain arc current, because of the large deionizingforce, the current suddenly reduces to zero. The current in the arc was flowing from the sourcethrough the inductance and the circuit breaker contacts. The energy contained in theelectromagnetic field cannot become zero instantaneously. It changes into some other form of

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 5 / 57

RESISTANCE SWITCHING

D-8\N-SYSTEM1\SYS15-1.PM6.5

��3 ����������������������

C

R

L

energy. The only possibility is the conversion from electromagnetic to electrostatic form ofenergy i.e., the current is diverted to the capacitor from the arc. If ia is the instantaneousvalue of arc current where the chop takes place, the prospective value of voltage to which thecapacitor will be charged, will be

V = ia L C/where L is the series inductance and C the shunt capacitance. This voltage appears across thecircuit breaker contacts. Fortunately, the breaker gap restrikes before the voltage is allowedto reach this value (prospective voltage which normally is very high as compared to the systemvoltage). The deionizing force is still in action and the current will again be chopped. Successivechops may occur as shown in the diagram until a final chop brings the current to a zeroprematurely with no further restrike since the gap is now in an advanced stage of deionization.

Resistance Switching

As is seen in the previous section that during currentchopping very high voltages may appear across the C.B.contacts and these voltages may endanger the operationof the system. To reduce these voltages, a resistance acrossthe breaker contacts is connected as shown in Fig. 15.3.The shunt resistor performs one or more of the followingfunctions:

(i) It reduces the rate of rise of restriking voltageand thus reduces duties of the breaker.

(ii) It reduces the transient voltages during switch-ing out inductive or capacitive loads.

(iii) In a multi-break C.B. they may be used to help to distribute the transient recoveryvoltage more uniformly across the several gaps.

To reduce the transient recovery voltage requires a considerably lower value of resistorwhereas for voltage equalisation a resistor of relatively high ohmic value will be required. Inthis case it is required that its resistance be low compared with the reactance of the capacitance,shunting the breaks at the frequency of the recovery transient. It is often necessary tocompromise and make one resistor do more than one of these jobs Critical restriking voltagedamping is obtained if

R = 0.5LC

Example 15.1: In a system of 132 kV, the line to ground capacitance is 0.01 µF and theinductance is 5 henries. Determine the voltage appearing across the pole of a C.B. if amagnetising current of 5 amps (instantaneous value) is interrupted. Determine also the valueof resistance to be used across the contacts to eliminate the restriking voltage.

Solution: This is a case of conversion of electromagnetic energy into electrostatic energyand hence the voltage appearing across breaker contacts is nothing but the voltage across thecapacitor which is given by

���� ��������&!'�

�+��',�!")

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 6 / 57

TYPES OF CIRCUIT BREAKERS

Based on Operating voltage.

Based on Location.

Based on external design.

Based on ARC Quenching medium.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 7 / 57

Based on ARC Quenching medium.

OIL CIRCUIT BREAKERS

AIR BLAST CIRCUIT BREAKERS

SF6 CIRCUIT BREAKER

VACCUM CIRCUIT BREAKER

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 8 / 57

OIL CIRCUIT BREAKERS

PLANE-BREAK OIL CIRCUIT BREAKERS

BULK OIL CIRCUIT BREAKERS

MINIMUM OIL CIRCUIT BREAKER

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 9 / 57

MINIMUM OIL CIRCUIT BREAKER

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 10 / 57

OPERATION

Arc will initiate due to FIELD EMISSION process and itmaintain due to THERMAL IONIC process

Oil will heat and initiates the chemical decomposition (5000k).

Gas bubbles will form (1000 times than the oil)which containhydrogen..... which causes turbulance.

Hydrogen is having high heat conductivity property.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 11 / 57

ADVANTAGES

Low Quantity oil is enough.

Size is less.

Cost is less.

Oil have high dielectric strength(130 kv/sq.cm) compare toair(30 kv/sq.cm) and SF6(80 kv/sq.cm)

maintainance is also low

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 12 / 57

DISADVANTAGES

Inflammable and may cause fire hazards.

Because of the production of carbon particles in the oil due toheating,periodical replacement of OIL is required.

There is a possibility of explosion.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 13 / 57

APPLICATIONS

Used in the range of 132KV TO 275KV.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 14 / 57

AIR-BLAST CIRCUIT BREAKERS

RADIAL-BLAST CIRCUIT BREAKER

CROSS-BLAST CIRCUIT BREAKER

AXIAL-BLAST CIRCUIT BREAKER

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 15 / 57

AXIAL AIR-BLAST C.B.(ABCB)

D-8\N-SYSTEM1\SYS15-1.PM6.5

��� ����������������������

�� ��� ����� ������� ��������

The most common method of arc control in air circuit breakers is that of subjecting the arc tohigh pressure air blast. There are two types of air blast circuit breakers: (i) Axial blast types,and (ii) Cross blast types.

The designations refer to the direction of the air blast in relation to the arc.

Axial Blast Circuit Breaker

The fixed and moving contacts are held in closed position by spring pressure (Fig. 15.9). Thebreaker reservoir tank is connected to the arc chamber when a tripping impulse opens the airvalve. The air entering the arc chamber exerts pressure on the moving contacts which moveswhen the air pressure exceeds the spring force. The air moves with sonic velocity near thenozzle and the arc is subjected to high pressure and there is considerable heat loss due toforced convection. With this the diameter of the arc is reduced and the core temperature isvery high. The temperature gradients set up within the arc are very steep which results ingreater heat losses.

Air reservoir

Arcingchamber

PistonMovingcontacts

Springclosing

Seriesisolator

Fixedcontact

Air valve openedby tripping impulse

���� ���7�&%(%&��&��'��'-��(��& ��)

When the current passes through zero, the air blast is more effective because the residualcolumn is very narrow and the high rate of heat loss becomes increasingly effective. It isknown that with a given arc length and heat loss per unit surface area, the total rate of heatloss is proportional to the arc diameter, whereas the total energy content of the arc is roughlyproportional to the square of the diameter. The narrower the residual column, the more effectiveare the heat losses in reducing the temperature and conductivity. Such conditions may allowthe column to recover dielectric strength very rapidly at current zeros.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 16 / 57

OPERATION

Arc will initiate due to FIELD EMISSION process and it maintaindue to THERMAL IONIC process. (IMPULSE SINGLE)

High pressure air(10-20 kg/sq.cm) will send towards the arc sothat contacts got separated and arc quenching will takes placewith in short time.

Again contacts comes to closed position. (AUTO-RECLOSER)

Series isolator will provide more clearence.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 17 / 57

ADVANTAGES

LESS RISK OF FIRE HAZARDS.

SPEED OF OPERATION.

MAINTAINANCE IS VERY LESS.

A.B.C.B. PROVIDES MORE CLEARENCE.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 18 / 57

DISADVANTAGES

DIFFICULT TO MAINTAIN CONSTANT AIR PRESSURE.

LESS RELIABLE FOR LOW CURRENT OPERATIONS.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 19 / 57

APPLICATIONS

132KV TO 275KV (BOTH OIL AND AIR CB’S ARE USED).

ABOVE 400KV (AIR C.B USED).

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 20 / 57

Properties of SF6 Gas

High dielectric strength (80 kv/sq.cm i.e.,2.5 times of air)

Electro-negative gas.

Low thermal time constant(good coolent).

Chemically inert.

High molecular weight.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 21 / 57

SF6 CIRCUIT BREAKER������������� )*+

The Gas System: The closed circuit gas system used in the SF6 C.Bs. is shown inFig. 15.12(b). Since the gas pressure is very high, lot of care is to be taken to prevent gas

Moving contactcross bar Current transfer

fingersMoving contact

orifice

Gas flow

Interrupterchamber

Fixed contact

,�-�.��!!%/��!�����

Filter

LP alarm

HP system LP system Low tempalarm

LP alarm

HP alarm

LP lock out

Compressor

Filter

Relief valve

Service connection

External highpressure reservoir

Heater

,�-����� ����0����

����������1������.'�+23�'4��5��&!��'�!(

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 22 / 57

OPERATION

Arc will initiate due to FIELD EMISSION process and itmaintain due to THERMAL IONIC process (IMPULSE SINGLE)

High pressure SF6 gas(14 kg/sq.cm) will send towards the arc sothat separation of contacts and arc quenching will takes placewith short time gap.

Physical dimensions of the arc will change.(area and length)

Heat dissipates with a faster rate so that arc will interrupt.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 23 / 57

ADVANTAGES

COMPACT IN SIZE.

MOISTER AND NOISE LESS OPERATION.

NO CARBON PARTICLE IS FORMED DURING ARCING.THEREFORE, THERE IS NO REDUCTION IN THEDIELECTRIC STRENGTH OF THE GAS.

NON-FLAMABLE.

SPEED OF OPERATION.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 24 / 57

DISADVANTAGES

ARCED SF6 IS POISONOUS.

CONVERTS IN TO LIQUID BELOW 16 DEGREES.

UNECONOMICAL.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 25 / 57

APPLICATIONS

Operating range is 3.3 kv to 800 kv.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 26 / 57

VACCUM C.B

������������� ���

moves through a short distance of 5 to 10 mm depending upon the operating voltage. Themetallic bellows made of stainless steel is used to move the lower contact. The design of thebellows is very important as the life of the vacuum breaker depends upon the ability of thispart to perform repeated operations satisfactorily. The periphery of the end-cap is sealed tothe envelope and the fixed contact stem is an integral part of one end-cap. One end of the fixedas well as moving contact is brought out of the chamber for external connections.

Fixed contact

End cap

Ceramic envelope

Contact tip

Sputte shield

Bellows

Moving contact

������������������������ !���"#���$��%%����!�%���&!��'�!(

The lower end of the breaker is fixed to a spring-operated or solenoid operated mechanismso that the metallic bellows inside the chamber are moved downward and upwards duringopening and closing operation respectively. It is to be noted that the operating mechanismshould provide sufficient pressure for a good connection between the contacts and should avoidany bouncing action.

Application of Vacuum Breakers

Because of the short gap and excellent recovery characteristics of vacuum breakers, they canbe used where the switching frequency is high and required to be reliable. For low faultinterrupting capacities the cost is low as compared to other interrupting devices. The vacuumswitches can be used for capacitor switching which is a very difficult task using oil C.Bs. Theycan be used along with static overcurrent relays and given an overall clearance time of lessthan 40 m-sec on phase-to-phase faults. There are many applications where a simple load-break switch is not enough and at the same time the devices used should not be costly. Theyinclude reactor switching, transformer switching, line dropping, capacitor bank switching.These applications give a fast RRRV and vacuum breakers are the best solutions. Wherevoltages are high and the current to be interrupted is low, these breakers have definiteadvantages over the air or oil C.Bs. As the maintenance required is the least, these breakers

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 27 / 57

CONSTRUCTION

Ceramic envelope is made up of glass.(initial color-silvery mirror,then milky white)

Sputte shield(stainless steel) contains moving and fixedcontacts(made of bismuth and zinc-move up to 5 to 10 mm.).Prevent metal vapour deposition on envelope.

Bellows are made of stainless steel to move the contacts.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 28 / 57

OPERATION

Huge amount of heat will form at the instant of separation.

Cathode spots will explode depends on current density and arcwill form.

Maintain arc stability.

Plasma and vapour will deposit on the CB contacts.

Recovery the vaccum by vaccum arc recovery phenomenon

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 29 / 57

ADVANTAGES

DIELECTRIC STRENGTH OF VACCUM IS 1000 TIMES OFTHE AIR.

EFFECTIVE AND RELIABLE.

LESS COST FOR LOW VOLTAGES.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 30 / 57

DISADVANTAGES

MAINTAINING OF VACCUM IS DIFFICULT.

UNECONOMICAL FOR HIGH VOLTAGES

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 31 / 57

APPLICATIONS

11KV TO 33KV

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 32 / 57

BASIC REQUIREMENTS OF RELAY

SELECTIVITY.

SENSITIVITY.

RELIABILITY.

STABILITY.

SPEED OF OPERATION.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 33 / 57

ELECTRO-MECHANICAL RELAYS

ATTRACTED ARMATURE TYPE

BALANCED BEAM TYPE

INDUCTION DISC TYPE

INDUCTION CUP TYPE

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 34 / 57

INDUCTION DISC TYPE

D-8\N-SYSTEM1\SYS14-1.PM6.5

��- � ���� ��������������

face of each pole at the air gap. The disc is normally made of aluminium so as to have lowinertia and, therefore, requires less deflecting torque for its motion. Sometimes, instead ofshading ring, shading coils are used which can be short circuited by the contact of some otherrelay. Unless the contacts of the other relay are closed, the shading coil remains open andhence no torque can be developed. Such torque control is employed where directional featureis required which will be described later.

Shading ring

Shading ring

Disc

Directionof force

To actuatingforce

��.� � �(�%�����!'�!��+

� ���������������������������������������

It is well known that for producing torque, two fluxes displaced in space and time phase arerequired. Let these fluxes be

φ1 = φm sin ωt

φ2 = φ′m sin (ωt + θ)

Flux φ1 is produced by the shaded pole and φ2 by theunshaded. The shaded pole flux lags that by the unshaded pole byangle θ. The two fluxes φ1 and φ2 will induce voltages e1 and e2respectively in the disc due to induction. These voltages willcirculate eddy currents in the disc of the relay. Assuming the discto be non-inductive, these currents will be in phase with theirrespective voltages. The vector diagram (Fig. 14.3) shows the phaserelations between various quantities.

e1 ∝ ddtφ1

∝ φmω cos ωt

and e2 ∝ φ′m ω cos (ωt + θ)The eddy current i1 ∝ e1.

Assuming same resistance to flow of eddy current,

i2 ∝ e2

i.e., i1 ∝ φmω cos ωt

and i2 ∝ φ′m ω cos (ωt + θ)

���������.����� ��"���

$�������� !'�������%�&+

�2

�1�

i2i1

e1

e2

��.� � �(�%��

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 35 / 57

INDUCTION CUP TYPE

D-8\N-SYSTEM1\SYS14-1.PM6.5

��0 � ���� ��������������

0 10 100

1

10

Multiple of plug setting

Ope

ratin

gtim

ein

sec.

(a)

(b)

(c)

(d)

Induction Cup Relays (Fig. 14.5): This relay hasfour or more electromagnets. A stationary iron core isplaced between these electromagnets. The rotor is ahollow cylindrical cup which is free to rotate in the gapbetween the electromagnets and the stationary iron core.When the electromagnets are energized, they inducevoltages in the rotor cup and hence the eddy currents.The eddy currents due to one flux interact with the fluxdue to the other pole; thereby a torque is produced similarto the induction disc type of relay.

The induction cup type of relays are more sensitivethan the induction disc type of relays and are used inhigh speed relay applications.

The ratio of reset to pick up is inherently high in case of induction relays as compared toattracted armature relays as their operation does not involve any change in the air gap of themagnetic circuit as it is in the case of latter. The ratio lies between 95% and 100%. This is notperfectly 100% because of the friction and imperfect compensation of the control spring torque.

�� �������������������

Depending upon the time of operation the relays are categorized as: (i) Instantaneous over-current relay, (ii) Inverse time-current relay, (iii) Inverse definite minimum time (IDMT) over-current relay, (iv) Very inverse relay, and (v) Extremely inverse relay.

(i) Instantaneous over-current relay is one in which no intentional time delay is providedfor the operation. The time of operation of such relays is approximately 0.1 sec. This character-istic can be achieved with the help of hinged armature relays. The instantaneous relay is moreeffective where the impedance Zs between the sourceand the relay is small compared with the impedanceZl of the section to be protected.

(ii) Inverse time-current relay is one in whichthe operating time reduces as the actuating quantityincreases in magnitude. The more pronounced theeffect is the more inverse the characteristic is said tobe. In fact, all time current curves are inverse to agreater or lesser degree. They are normally more in-verse near the pick up value of the actuating quan-tity and become less inverse as it is increased. Thischaracteristic can be obtained with induction type ofrelays by using a suitable core which does not satu-rate for a large value of fault current. If the satura-tion occurs at a very early stage, the time of opera-tion remains same over the working range. The char-acteristic is shown by curve (a) in Fig. 14.6 and isknown as definite time characteristic.

� !'�����'!(����!'�!��+

��������.���'�������'���$�#����!�

�#��/'!������ ��%�&�1� 2�3� �$������ ����4

2�3�5��4�2�3�#��&���#����4��� �2�3

�6�����%&� ��#����+

Stationarycore

Cup

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 36 / 57

DISTANCE RELAYS

IMPEDANCE RELAY

REACTANCE RELAY

ADMITTANCE (OR) MHO RELAY

ANGLE-IMPEDANCE RELAY

QUADRILATERAL RELAY

ELLIPTICAL RELAY

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 37 / 57

IMPEDANCE RELAY CHARACTERISTICS

D-8\N-SYSTEM1\SYS14-1.PM6.5

����������� ��� �7,

or Z < KK

1

2

or Z < constant (design impedance)

This means that the impedance relay will operate only if the impedance seen by therelay is less than a prespecified value (design impedance). At threshold condition,

Z = KK

1

2

(14.7)

The operating characteristic of an impedance relay on V-I diagram is shown in Fig. 14.13.

The initial bend in the characteristic is due to the presence of spring torque.

Normally, the operating characteristics of distance relays are shown on an impedancediagram or R-X diagram. This characteristic for an impedance diagram is shown in Fig. 14.14.

V

I

No operation

Operation

X

–X

–R R

Nooperation

Operation

Z

�(������"�'.���'�������'��$��� �(������"�'.���'�������'��$���

��(� ��'����%�&�������� ��"���+ ��(� ��'����%�&������ ��"���+

This is clear from the characteristic that if the impedance as seen by the relay lieswithin the circle the relay will operate; otherwise, it will not. The position of one value of Z isshown in the figure with angle θ with the +R-axis. This means that the current lags the voltageby angle θ. In case the two were in phase, the Z vector would have coincided with +R-axis. Incase the current was lagging the voltage by 180°, the Z vector would coincide with – R-axis. Itis to be noted here that – R-axis does not mean here negative resistance axis but the one asexplained. When I lags behind V, the Z vector lies in the upper semi-circle and Z lies in thelower when I leads the voltage. Since the operation of the relay is independent of the phaserelation between V and I, the operating characteristic is a circle and hence it is a non-directionalrelay.

The impedance relays normally used are high speed relays. These relays may use abalance beam structure or an induction cup structure.

The directional property to the impedance relay can be given by using the impedancerelay along with a directional unit as is done in case of a simple overcurrent relay to work as adirectional over current relay. This means the impedance unit will operate only when the

D-8\N-SYSTEM1\SYS14-1.PM6.5

����������� ��� �7,

or Z < KK

1

2

or Z < constant (design impedance)

This means that the impedance relay will operate only if the impedance seen by therelay is less than a prespecified value (design impedance). At threshold condition,

Z = KK

1

2

(14.7)

The operating characteristic of an impedance relay on V-I diagram is shown in Fig. 14.13.

The initial bend in the characteristic is due to the presence of spring torque.

Normally, the operating characteristics of distance relays are shown on an impedancediagram or R-X diagram. This characteristic for an impedance diagram is shown in Fig. 14.14.

V

I

No operation

Operation

X

–X

–R R

Nooperation

Operation

Z

�(������"�'.���'�������'��$��� �(������"�'.���'�������'��$���

��(� ��'����%�&�������� ��"���+ ��(� ��'����%�&������ ��"���+

This is clear from the characteristic that if the impedance as seen by the relay lieswithin the circle the relay will operate; otherwise, it will not. The position of one value of Z isshown in the figure with angle θ with the +R-axis. This means that the current lags the voltageby angle θ. In case the two were in phase, the Z vector would have coincided with +R-axis. Incase the current was lagging the voltage by 180°, the Z vector would coincide with – R-axis. Itis to be noted here that – R-axis does not mean here negative resistance axis but the one asexplained. When I lags behind V, the Z vector lies in the upper semi-circle and Z lies in thelower when I leads the voltage. Since the operation of the relay is independent of the phaserelation between V and I, the operating characteristic is a circle and hence it is a non-directionalrelay.

The impedance relays normally used are high speed relays. These relays may use abalance beam structure or an induction cup structure.

The directional property to the impedance relay can be given by using the impedancerelay along with a directional unit as is done in case of a simple overcurrent relay to work as adirectional over current relay. This means the impedance unit will operate only when the

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 38 / 57

REACTANCE RELAY

D-8\N-SYSTEM1\SYS14-1.PM6.5

����������� ��� �7�

near unity power factor condition. Under the condition of high power factor or leading powerfactor, the impedance seen by the relay is a very low or even negative reactance. The relay thatis used to give directional feature to the reactance relay, is known as mho relay or admittancerelay which is dealt in the next section.

The mho relay when used alongwith the reactance relay is known as starting relay orstarting unit.

The structures used for the reactance relay are

1. Induction cup.

2. Double-induction loop structure.

A typical reactance relay using induction cup structure is shown in Fig. 14.17.

It is a four-pole structure. This has operating, polarising and restraining coils. Theoperating torque is produced by the interaction of fluxes due to the windings carrying currentcoils, i.e., interaction of fluxes of poles 1, 2 and 3 and the restraining torque is developed due tothe interaction of fluxes due to the poles 1, 3 and 4. The operating torque will be proportionalto I2 and the restraining torque proportional to VI cos (θ – 90°). The desired maximum torqueangle is obtained with the help of R-C circuits as shown in Fig. 14.17.

2 4

1

3Polarising

Operating

Restraining

Polarising

VI

�'.�����'� ��"�����$������'���'����%�&+

The mho relay: In this relay the operating torque is obtained by the V-I element andrestraining torque due to the voltage element. This means a mho relay is a voltage restraineddirectional relay. From the universal torque equation

T = K3VI cos (θ – τ) – K2V2 (14.10)

For the relay to operate

K3VI cos (θ – τ) > K2V2

orVVI

KK

23

2< cos (θ – τ)

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 39 / 57

REACTANCE RELAY CHARACTERISTICS

D-8\N-SYSTEM1\SYS14-1.PM6.5

�7- � ���� ��������������

KK

1

3

R

Z

–R

Operation

No operationX

Operation

Nooperation

–R R

Z

directional unit has operated. The characteristic of such acombination will be as shown in Fig. 14.15.

From the characteristic it is clear that if the impedancevector as seen by the relay lies in a zone indicated by thethick line (intersection of straight line and circle) the relaywill operate, otherwise, it will not.

Reactance relay: In this relay the operating torque isobtained by current and the restraining torque due to acurrent-voltage directional element. This means, a reactancerelay is an over-current relay with directional restraint. Thedirectional element is so designed that its maximum torqueangle is 90°, i.e., τ = 90° in the universal torque equation.

T = K1I2 – K3VI cos (θ – τ)

= K1I2 – K3VI cos (θ – 90°)

= K1I2 – K3VI sin θ (14.8)

For the operation of the relay,

K1I2 > K3VI sin θ

orVI

I

KK2

1

3sin θ <

or Z sin θ < KK

1

3

X < KK

1

3(14.9)

This means for the operation of the relay thereactance seen by the relay should be smaller thanthe reactance for which the relay has been designed.The characteristic will be as shown in Fig. 14.16.

This means if the impedance vector head lieson the parallel lines (R-axis and the operatingcharacteristic) this will have a constant X component.The important point about this characteristic is thatthe resistance component of the impedance has noeffect on the operation of the relay. It responds onlyto the reactance component of the impedance. Therelay will operate for all impedances whose heads lie below the operating characteristic whetherbelow or above the R-axis.

This relay as can be seen from the characteristic, is a non-directional relay. This will notbe able to discriminate when used on transmission lines, whether the fault has taken place inthe section where the relay is located or it has taken place in the adjoining section. It is notpossible to use a directional unit of the type used alongwith impedance relay because in thatcase the relay will operate even under normal load conditions if the system is operating at or

���������(������"�'.���'���/

����'��$������(� ��'����%�&����.

���'�����%� !���+

.���'�������'��$��

���'���'�� ��%�&+

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 40 / 57

ADMITTANCE (OR) MHO RELAY

D-8\N-SYSTEM1\SYS14-1.PM6.5

�70 � ���� ��������������

or Z < KK

3

2 cos (θ – τ) (14.11)

This characteristic, when drawn on an admittance diagram is a straight line passingthrough the origin and if drawn on an impedance diagram it is a circle passing through theorigin as shown in Fig. 14.18.

The relay operates when the impedance seen by the relay falls within this circle. Therelay is inherently directional so that it needs only one pair of contacts which makes it fasttripping for fault clearance and reduces the VA burden on the current transformers.

90°Z

� KK

3

1

Z

�KK

3

1

A

B RC

������!��.��'.���'�������'+ ������#��$$�'���$���'���������'�+

The impedance angle of the protected line is normally 60° to 70° which is shown by theline AB in Fig. 14.19.

2

1

3Polarising

Restraining

Polarising

VI

4

Operating

�'.�����'� ��"�����$����.����%�&+

The arc resistance R is represented by BC. By making τ, the maximum torque angle,equal to or a little less lagging than θ, the circle is made to fit very closely round the fault areaso that the relay is an accurate measuring device and does not operate during power swingswhich may occur on long or heavily loaded lines.

A typical mho relay using induction cup structure is shown in Fig. 14.20.

The operating torque is produced by the inter-action of fluxes due to the poles 1, 2 and 3and the restraining torque due to the poles 1, 3 and 4.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 41 / 57

STATIC RELAY ADVANTAGES

The power consumption is low and hence provides less burdenon the CTs and PTs as compared to the conventionalelectromechanical relays.

The relays are fast in operation.

No moving parts, hence friction or contact troubles are absentand as a result minimum maintenance is required.

The relays have greater sensitivity as amplification of signals canbe obtained very easily.

The relay has a high reset to pick up ratio and the reset is veryquick.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 42 / 57

STATIC RELAY DISADVANTAGES

The characteristics vary with temperature and ageing.

The reliability of the scheme depends upon a large number ofsmall components and their electrical connections.

The relays have low short time overload capacity compared withelectromechanical relays.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 43 / 57

CARRIER CURRENT PROTECTION

D-8\N-SYSTEM1\SYS14-3.PM6.5

�������������� '))

and amplifier with an output usually of about 15 to 20 watts at a frequency between 50 and500 kHz. Below 50 kHz the size and cost of the coupling components would be too high; above500 kHz the line losses and hence the signal attenuation would be too great on long lines.15 watts output has been considered to be sufficient from loss point of view for lines of length100 miles. Carrier current can be used only on overhead lines because the capacitance of acable would attenuate the carrier signals to ineffectual values.

C.B.

Currenttrans-former

Line trap

Sequencenetworks

Low passfilter

Transmitter

Oscillator

ModulatorLine

amplifier

ReceiverReceiverStarting

relay

Output andtrippling relay

Mixer andoutput

Attenua-tor

Receiverbandpassfilter

Trans-mitterbandpassfilter

Directionalfilters Coupling

filter

Couplingcapacitor

�����������7� �9���+��, ��:���,�!������!������������� ,����� !����,�"

Phase Comparison Scheme

Phase comparison relaying blocks the operation of the relay at both ends of the line wheneverthe carrier current signals are displaced in time so that there is little or no time interval whena signal is not being transmitted from one end or the other. Tripping of the relays will occurwhen the signals at the two ends are concurrent and there is time between the consecutiveconcurrent signals when no signal is being transmitted from both the ends (when feeder is fedfrom both the ends). To achieve phase comparison on these lines, the line current transformersare so connected that their secondary currents are 180° out of phase when current is flowing inthe feeder under both normal and/or external fault condition. When an internal fault takesplace, the current at one of the ends reverses and thus the two currents are in phase (when fedfrom both ends) and, therefore, there is time when no signal is being received and the relayoperates. In case the feeder is fed from one end, for an internal fault the current at one of the

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 44 / 57

D-8\N-SYSTEM1\SYS14-3.PM6.5

'). ����������$�� � ��%

ends reduces to zero and hence again there is time when no signal is received and the relaywill operate. This is illustrated diagrammatically in Fig. 14.60.

1

2

3

4

5

6

End A End BExternal fault

End A End BInternal fault

���!����� �������������� ,����� !����,�

)"���,��5�����!�-." �� !���5�����!�-�"���!�,�������+!��-

'"�����;����+!��-2" ����5����;����+!��-�!�3"������"

The operation of this scheme is explained with the help of a block diagram (Fig. 14.59)as follows:

The block diagram shows the equipments required at end A of the line. Similar equipmentis connected at end B of the same line. The line current transformers are connected as summationtransformer; thus 3-phase currents are reduced to a single phase quantity and is fed to asequence network which is sensitive only to negative sequence currents. The output from thesequence network is fed into the starting equipment which operates in two stages known aslow set and high set. The differential between the settings of the two relays is such that, on theincidence of a fault, the low set relays at both the terminals operate at a lower current thanany of the high set relays. The low set relays start the comparison (phase) process and the highset relays control the tripping circuit.

The contacts of the low set relay allow the 50 Hz output from the sequence network to befed into the transmitter through a low-pass filter. This 50 Hz input to the transmitter modulatesthe high frequency input from the oscillator. The output from the modulator is partly fed to thelocal mixer circuit and partly is amplified through an amplifier and fed to the line through thetransmitter band pass filter and the coupling equipment. The transmitted signal enters thereceiver circuit through the receiver band pass filter at end B after passing through the couplingequipment at that end. It is then attenuated and passed into the mixer circuit.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 45 / 57

TRANSFORMER PROTECTION

D-8\N-SYSTEM1\SYS14-3.PM6.5

�������������� '()

The average current through the restraining coil = 340 5

400×

= 4.25 amps.

With 10% slope the operating current will be

0.1 × restraining current + 0.2 = 0.1 × 4.25 + 0.2 = 0.625 amp.

Since the current through the operating coil is 0.5 amp, therefore the relay will notoperate.

�������� �� ������� �����������

Transformers are normally protected against short circuits and over-heating. For short circuitsnormally percentage differential protection is recommended for transformers rated for morethan 1 MVA. For low rating overcurrent relaying is used.

The primary and secondary currents of a transformer are normally different from eachother and are related by their turns ratio. These currents are displaced in phase from eachother by 30° if the windings are star-delta connected. The differential protection scheme isconsidered to be suitable if it satisfies the two conditions: (i) The relays must not operateunder normal load conditions and for through fault (external fault) conditions; and (ii) it mustoperate for severe enough internal fault conditions. In fact, these are the tests that any goodprotection scheme must satisfy. For differential protection, the vector difference of two currentsis fed to the operating coil of the relay. This means for an external fault the line currents of thetwo CTs should be equal in magnitude and should be in phase opposition so that the differencecurrent is zero.

The CTs on the star side of the power transformer are connected in delta, and on thedelta side, they are connected in star as the line currents of star-delta power transformer willbe displaced in phase by 30°. It is required that this phase displacement must be nullified byconnecting the CTs in that fashion.

Let us take first of all a star-star transformer (Fig. 14.46). When the star point of boththe transformers is ungrounded, a line-to-ground fault has no meaning because no fault current

i

i

ii

ii

0 0 0

I

I

*!+� �!�������&���� ���!�� �,���� ����� !- ��� �+� �����"

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 46 / 57

BUCHHOLZ RELAY

D-8\N-SYSTEM1\SYS14-3.PM6.5

'(# ����������$�� � ��%

Test-cock

Alarm

Mercuryswitch

Drain

Toconservator

Trip

Hinge

To transformertank

�����������7���� �8����5"

For a minor or incipient fault, the slow generation of gas gives rise to gas bubbles whichtry to go to the conservator but are trapped in the upper portion of the relay chamber, therebya fall in oil level takes place. This disturbs the equilibrium of the gas float. The float tilts andthe alarm circuit is closed through the mercury switch and the indication is given.

For a heavy fault, large volumes of gases are generated which cause violent displacementof the oil and impinge upon the baffle plates of the lower float and thus the balance of the lowerfloat is disturbed. The lower float is tilted and the contacts are closed which are arranged totrip the transformer.

������� �������������

It is a voltage balanced system in which the secondary CT voltages (voltages are proportionalto the CT secondary current as air-cored CTs are used) at the ends of the feeder are compared.The CTs are connected in opposition (see Fig. 14.55). Associated with the CT at each end is aninduction relay. The upper magnet system acts as a quadrature transformer and produces atthe pilot terminals a voltage which varies with the primary current. As long as the currents atthe two ends are equal, the voltages induced are also equal and hence no current flows in thepilot wires. In case the CTs are of ordinary instrument type where there is possibility ofdissymmetry in the characteristics of the CTs at the two ends, compensating devices are providedin the relay to neutralize the effect of unbalancing of the CTs. In case of a through fault or dueto asymmetry in CTs under normal conditions the current through the pilot wires is capacitiveand, therefore, the flux in the series magnet (due to capacitive current) is in phase with theleakage flux from the upper magnet thereby the net torque on the disc is zero. This is shown inthe phasor diagram (Fig. 14.56). Here V is the voltage across the CT secondary and E is theinduced voltage across the pilot wires, φv the flux in the upper magnet, φc the flux in the lowermagnet and Ic the pilot current.

Whenever an internal fault occurs, current flows through the pilot wires because eitherone of the voltages has reversed in polarity (if the feeder is fed from both the ends) or thevoltage at one end has collapsed (if the feeder is fed from one end only). The relay at an end

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 47 / 57

GENERATOR PROTECTION

STATOR PROTECTION

ROTOR PROTECTION

PRIME MOVER FAILURE PROTECTION

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 48 / 57

STATOR PROTECTION

STAR GROUNDED CONNECTION

�&4 ������������� � ��

of this, the impedance of the induction generator as seen by the relay shifts into the fourthquadrant of the R-X diagram and this impedance swings into off-set mho relay characteristicas shown in Fig. 14.43 and the relay will operate.

Stator Protection

It is the general practice to provide differential protection for generators above 10 MVA. Thisform of protection is most suited and should be used if justified economically.

If all the six terminals of a star connected 3φ generator are available, the scheme ofpercentage differential relay shown in Fig. 14.44 (a) is provided.

It can be seen that for an external fault the relay does not operate and for an internalfault it does operate.

Winding

) ���#���#�����#����#��!�!��������#��

In case the generator is delta connected, Fig. 14.44 (b) gives the scheme of percentagedifferential protection.

Winding C.B.

�����������(�) ���#���#�����#�:∆�#������!��������#��

It can be seen again that for an external line-to-line fault, the relays do not operatewhereas for an internal fault they will operate.

���#���#�����#����#��!�!��������#��

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 49 / 57

STATOR PROTECTION

DELTA CONNECTION

�&4 ������������� � ��

of this, the impedance of the induction generator as seen by the relay shifts into the fourthquadrant of the R-X diagram and this impedance swings into off-set mho relay characteristicas shown in Fig. 14.43 and the relay will operate.

Stator Protection

It is the general practice to provide differential protection for generators above 10 MVA. Thisform of protection is most suited and should be used if justified economically.

If all the six terminals of a star connected 3φ generator are available, the scheme ofpercentage differential relay shown in Fig. 14.44 (a) is provided.

It can be seen that for an external fault the relay does not operate and for an internalfault it does operate.

Winding

�����������(�) ���#���#�����#����#��!�!��������#��

In case the generator is delta connected, Fig. 14.44 (b) gives the scheme of percentagedifferential protection.

Winding C.B.

) ���#���#�����#�:∆ �#������!��������#��

It can be seen again that for an external line-to-line fault, the relays do not operatewhereas for an internal fault they will operate.

���#���#�����#�:

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 50 / 57

ROTOR PROTECTION

�������������� �&�

The loss of excitation of a generator may result in loss of synchronism and slightlyincreased generator speed since the power input to machine remains unchanged. The machine,therefore, behaves as an induction generator and draws its exciting current from the systemwhich is equal to its full load rated current. This leads to overheating of the stator winding androtor body because of currents induced in the rotor body due to slip speed. This situationshould not be allowed to continue for long and corrective measures in terms of restoration ofexcitation or disconnection of alternator, should be taken. The loss of excitation may also leadto pole slipping conditions which result in voltage reduction for outputs above half the ratedload.

Rotor ProtectionFigure 14.40 shows the modern method of protecting the rotor against earth faults or opencircuits. A small power supply is connected to the positive pole of the field circuit. A faultdetecting relay and a high resistance to limit the current are connected in series with thiscircuit. A fault at any point on the field circuit will pass a current of sufficient magnitudethrough the relay to cause operation. The earth fault relays are instantaneous and are connectedto the alarm circuit for indication as a single ground fault does not require immediate attentionto the set.

+

Relay

R

+

ExciterField

winding

�#�#�����%"������#�����#��

Unbalanced LoadingFigure 14.41 shows the protection of alternators against negative phase sequence currents.The negative sequence current segregating network is used, the output of which is proportionalto the generator negative phase sequence current and is fed into a relay with an inverse squarelaw characteristic, i.e., I2t = K or t ∝ 1/I2. The pick up and time delay adjustments are providedsuch that the relay characteristic can be chosen to match closely any machine characteristic.The relay is connected to trip the generator main breaker. Sometimes an auxiliary alarm relayis provided which gives warning when the maximum continuous permissible negative phasesequence current is exceeded. The relay normally used is an IDMT relay.

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 51 / 57

PRIME MOVER PROTECTION

�������������� �&3

X

R

Mho relay characteristic

Typical characteristicduring loss of excitation

System faultcharacteristic

C.B.

Mho relay

value of 110% of the normal value and operates instantaneously at about 130% to 150% of therated voltage. The relay unit should be compensated against the frequency and it should beenergized from a potential transformer other than the one used for the automatic voltageregulator. The operation of the relay introduces resistance in the generator or exciter fieldcircuit and if over-voltage still persists, the main generator breaker and the generator or exciterfield breaker is tripped.

Failure of Prime Mover

Whenever a prime mover fails, the generator connected to the system starts motoring; therebyit draws electrical power from the system and drives the prime mover. The power taken by thegenerator under such condition is very low being about 2% for the turbo-alternators and 10%for the engine driven sets. The power factor of the current depends upon the excitation leveland hence may be either leading or lagging. The wattmetric relay with directional characteristicis used. The relay must be associated with a time delay relay to prevent tripping due to powerswings.

Loss of Excitation

Very large alternators cannot be allowed to run asynchronously for long as the relative motionbetween the stator field and the rotor induces large currents in the rotor body and, therefore,there is high rate of heating of the rotor surfaces and the loss of excitation scheme is arrangedto trip after certain time delay. The protection scheme uses an offset mho relay operated froma.c. current and voltage at the generator terminals.The relay setting is so arranged that the relayoperates whenever the excitation goes below acertain value and the machine starts runningasynchronously. Fig. 14.42 shows the relayconnection and Fig. 14.43 shows the variouscharacteristics on R-X diagram.

It is seen that the impedance as seen by therelay during loss of excitation will swing into therelay characteristic and thus the relay will operate.The loci of impedance for system fault and for powerswings is also shown in Fig. 14.43 and it can be seenthat for these conditions the relay will not operate.

Under normal operating condition when asynchronous alternator is connected to the grid itsupplies lagging reactive power to the system inaddition to the active power and the p.f. is laggingand the impedance of the alternator as seen by therelay lies in the first quadrant of the R-X diagram.However, due to failure of excitation, thesynchronous alternator now works as an inductiongenerator and it draws lagging reactive power fromthe grid, of course it supplies active power to thegrid and hence it operates at leading p.f. As a result

������#������#�,"#�

�#,,#"�9������#��

�����������#,,#"�9������#�

�%��������,����

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 52 / 57

CHARACTERISTICS OF LOSS OF EXCITATION

�������������� �&3

X

R

Mho relay characteristic

Typical characteristicduring loss of excitation

System faultcharacteristic

C.B.

Mho relay

value of 110% of the normal value and operates instantaneously at about 130% to 150% of therated voltage. The relay unit should be compensated against the frequency and it should beenergized from a potential transformer other than the one used for the automatic voltageregulator. The operation of the relay introduces resistance in the generator or exciter fieldcircuit and if over-voltage still persists, the main generator breaker and the generator or exciterfield breaker is tripped.

Failure of Prime Mover

Whenever a prime mover fails, the generator connected to the system starts motoring; therebyit draws electrical power from the system and drives the prime mover. The power taken by thegenerator under such condition is very low being about 2% for the turbo-alternators and 10%for the engine driven sets. The power factor of the current depends upon the excitation leveland hence may be either leading or lagging. The wattmetric relay with directional characteristicis used. The relay must be associated with a time delay relay to prevent tripping due to powerswings.

Loss of Excitation

Very large alternators cannot be allowed to run asynchronously for long as the relative motionbetween the stator field and the rotor induces large currents in the rotor body and, therefore,there is high rate of heating of the rotor surfaces and the loss of excitation scheme is arrangedto trip after certain time delay. The protection scheme uses an offset mho relay operated froma.c. current and voltage at the generator terminals.The relay setting is so arranged that the relayoperates whenever the excitation goes below acertain value and the machine starts runningasynchronously. Fig. 14.42 shows the relayconnection and Fig. 14.43 shows the variouscharacteristics on R-X diagram.

It is seen that the impedance as seen by therelay during loss of excitation will swing into therelay characteristic and thus the relay will operate.The loci of impedance for system fault and for powerswings is also shown in Fig. 14.43 and it can be seenthat for these conditions the relay will not operate.

Under normal operating condition when asynchronous alternator is connected to the grid itsupplies lagging reactive power to the system inaddition to the active power and the p.f. is laggingand the impedance of the alternator as seen by therelay lies in the first quadrant of the R-X diagram.However, due to failure of excitation, thesynchronous alternator now works as an inductiongenerator and it draws lagging reactive power fromthe grid, of course it supplies active power to thegrid and hence it operates at leading p.f. As a result

�����������������#������#�,"#�

�#,,#"�9������#��

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 53 / 57

FEEDER PROTECTION

PROTECTION OF PARALLEL FEEDER

PROTECTION OF RING MAIN FEEDER

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 54 / 57

PARALLEL FEEDER PROTECTION��4 ������������� � ��

feeder 1 corresponds to load current and after some time the non-directional relay in feeder 2will operate, thereby isolating feeder 2 from the source.

Load

1

2

��#�����#�#"�������� "��!��,�

Protection of Ring Mains

As shown in Fig. 14.32, four substations are inter-connected and fed through one source. Therelays at A and B are non-directional relays. The coordination can be achieved by opening thering at A and considering the system as a radial feeder connected to one source (Fig. 14.32(b)).

The relays used are directional overcurrent relays with the relay near end A havingminimum time of operation. Next open the ring at B as shown in Fig. 14.32(c).

The total protection scheme consists of six directional overcurrent relays and two non-directional overcurrent relays. The scheme is shown in Fig. 14.32(d).

(a)

B

A

A

B

B

0.7 0.5 0.3 0.1

(c)

A

B

A

0.7 0.5 0.3 0.1

(b)

(d)

0.50.1G

0.7

0.7

F

C

0.1 0.5D

0.3

0.3 E

���������� (�)��������,�#+���#�����!*(�)����#����!�����!,����!*

(�)����#����!������!,����!*��!(�)��������,0��%��#�����5�,�%����

Consider a fault as shown in Fig. 14.32(d). The fault will be fed as shown by long arrows.The relays at locations CDE and FG will start moving. The relay at E will operate first as thishas minimum operating time out of these relays; thereby after a time of 0.3 sec. The relays at

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 55 / 57

RING MAIN FEEDER PROTECTION

��4 ������������� � ��

feeder 1 corresponds to load current and after some time the non-directional relay in feeder 2will operate, thereby isolating feeder 2 from the source.

Load

1

2

�������������#�����#�#"�������� "��!��,�

Protection of Ring Mains

As shown in Fig. 14.32, four substations are inter-connected and fed through one source. Therelays at A and B are non-directional relays. The coordination can be achieved by opening thering at A and considering the system as a radial feeder connected to one source (Fig. 14.32(b)).

The relays used are directional overcurrent relays with the relay near end A havingminimum time of operation. Next open the ring at B as shown in Fig. 14.32(c).

The total protection scheme consists of six directional overcurrent relays and two non-directional overcurrent relays. The scheme is shown in Fig. 14.32(d).

(a)

B

A

A

B

B

0.7 0.5 0.3 0.1

(c)

A

B

A

0.7 0.5 0.3 0.1

(b)

(d)

0.50.1G

0.7

0.7

F

C

0.1 0.5D

0.3

0.3 E

(�)��������,�#+���#�����!*(�)����#����!�����!,����!*

(�)����#����!������!,����!*��!(�)��������,0��%��#�����5�,�%����

Consider a fault as shown in Fig. 14.32(d). The fault will be fed as shown by long arrows.The relays at locations CDE and FG will start moving. The relay at E will operate first as thishas minimum operating time out of these relays; thereby after a time of 0.3 sec. The relays at

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 56 / 57

OVER VOLTAGE PROTECTION

D-8\N-SYSTEM1\SYS16-1.PM6.5

�:� %�%" #�"���'!*%#��+� %,�

units to flashover, thereby a path is provided to the surge to the ground through the coilelement and the non-linear resistor element. Because of the high frequency of the surge, thecoil develops sufficient voltage across its terminals to cause the by-pass gap to flashover. Withthis the coil is removed from the circuit and the voltage across the LA is the IR drop due to thenon-linear element. This condition continues till power frequency currents follow the preionizedpath. For power frequency the impedance of the coil is very low and, therefore, the arc willbecome unstable and the current will be transferred to the coil (Fig. 16.13 (b)). The magneticfield developed by the follow current in the coil reacts with this current in the arcs of the gapassemblies, causing them to be driven into arc quenching chambers which are an integral partof the gap unit. The arc is extinguished at the first current zero by cooling and lengthening thearc and also because the current and voltage are almost in phase. Thus the diverter comesback to normal state after discharging the surge to the ground successfully.

Line

Impulsecurent

Pre-ionizingtip

Gapunit

Magneticcoil

By-passgap

Gapunit

Pre-ionizingtip

Thyriteshuntingresistors

Thyritevalveelements

Line

Powerfollowcurrent

Pre-ionizingtip

By-passgap

Magneticcoil

Gapunit

Pre-ionizingtip

ThyriteshuntingresistorsF

lux

Thyritevalveelements

(b)(a)

Gap unit

� 0���� ���������.�������9�������(������ ��������0��.

�� �������� ������6�����>����/� ������7

Location of Lightning Arresters: The normal practice is to locate the lightning arresteras close as possible to the equipment to be protected. The following are the reasons for thepractice: (i) This reduces the chances of surges entering the circuit between the protectiveequipment and the equipment to be protected. (ii) If there is a distance between the two, asteep fronted wave after being incident on the lightning arrester, which sparks overcorresponding to its spark-over voltage, enters the transformer after travelling over the leadbetween the two. The wave suffers reflection at the terminal and, therefore, the total voltageat the terminal of the transformer is the sum of reflected and the incident voltage whichapproaches nearly twice the incident voltage i.e., the transformer may experience a surgetwice as high as that of the lightning arrester. If the lightning arrester is right at the terminals

K NITEESH KUMAR (RGMCET(EEE), NANDYAL)POWER SYSTEM PROTECTION 15/01/2017 57 / 57


Recommended