PROTECTIVE RELAYING FOR TRANSMISSION AND DISTRIBUTION LINES.doc

8/17/2019 PROTECTIVE RELAYING FOR TRANSMISSION AND DISTRIBUTION LINES.doc

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PROTECTIVE RELAYING FOR TRANSMISSION AND

DISTRIBUTION LINES

(Basic concept, te!ino"o#$ %&nctions o% CTs ' PTs, i!potant t$pes o% e"a$s etc, it)

e%eence to tans!ission ' *isti+&tion s$ste! potection-

- E- Y-S-.a*a&n,

C)ie% En#inee, U-P-S-E-B-/-0 INTRODUCTION

The power system equipment employed in generation, transmission and distributionof electrical power is prone to accidents, some of which can be very disastrous. Such

accidents are inevitable due to deterioration of insulation, lightning stroks, entry of birds and

rodents into the equipment and human error in system operation. The capital investment putin a power system is large. To save this equipment from abnormal system running conditions

and breakdowns generally known a “faults” and to ensure satisfactory operation of power

supply system, constant watch is required to be had on the equipment and running

conditions. n development of abnormal conditions ! faults a very quick assessment !analysis of condition and remedial action is required to be taken so much so that it generally

becomes humanly impossible to do so before taking place of damage to equipment ! property and life. "s such, special equipment referred as protective #ear equipment isrequired to be employed to sense the trouble and disconnect the faulty equipment section

from the rest of the system in time to minimi$e the trouble.

The power supply system consists of generating stations, transmission lines,substations, distribution lines, distribution substations and the distribution mains etc.

%ifferent relays ! schemes are employed for the various equipment used in the power system

based on principles most suited to the particular circumstances. The scope of the presentassignment is to discuss the protection relays schemes and related sub&ect as applied to the

transmission ' distribution lines and substations.

1-0 TYPES OF FAULTS

The types of faults can be broadly classified into (i) three phase, (ii) double phase to ground

(iii) phase to phase (iv) phase to ground and (v) three phase to ground ma&ority of faults are phase to ground in general.

2-0 ESSENTIAL OF PROTECTION

*.+ The protection is required to be reliable to ensure safety of equipment, property and

life and proper functioning of the unfaulted system. There are many components in a

protection in operative or mal operative. Though complete reliability isimpossible to achieve but it can be increased to great etent by taking following

measures

(i) /se of very good quality relays.

(ii) "ppropriate application(iii) 0roper application of 1Ts ' 0Ts.

(iv) 0roper maintenance and frequent ' regular checking and testing.

(v) 2mploying well trained staff to handle the equipment.(vi) /se of ferrules ' numbering to 3dentify wires ! cables and terminals to

ensure proper handling during maintenance ! testing.

(vii) 0roper '.1. supply arrangement and its frequent maintenance.


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(viii) "nalysis of trippings and takings appropriate measures such as changing the

settings as demanded by the situation4.(i) Supervision of 0T, 1T supplies and trip healthy circuits.

() 5aintaining proper environment e.g. temperature, moisture etc.

(i) 0roper coordination of protective gear schemes.

2-1 Se"ecti3it$

verlapping $ones of protection are arranged to ensure protection of each ' every

part of the system. 6hile proper selectivity on the part of these overlapping $ones ! protection schemes may ensure proper functioning of the power system, improper

selectivity may cause unnecessary plant outage. The schemes are classified into two

on this basis-(i) Unit S$ste!4 (Such as differential protection)

• These are absolutely selective.

(ii) 7on /nit Systems (Such as over current)

• These are relatively selective.

Some of the points which may increase selectivity if taken care are-

(a) 0roper application of 1.Ts., 0.Ts. and protection schemes.

(b) 2nsuring desired operating characteristics.

(c) 0roper co-ordination of overlapping $ones.

2-2 Spee* o% Opeation45

The protection should operate quickly to limit the etent of damage. 8ast clearanceof fault improves systems stability also. 9ut, this should not be at the cost of

selectivity as in that case mal -trippings will take place causing unnecessary andfrequent supply interruptions and at times, deteriorate the system stability as well.

2-6 Sensiti3it$

Sensitivity, is a measure of impedance presented by protection to the current and

voltage transformers. 3f this current high, the ratio ' phase angle errors of instrumenttransformer will increases and the effective settings of protection will go up. The

protection should be sensitive enough to ensure its reliable operation under minimum

fault conditions and stable during maimum load ! through fault condition.

2-7 8ones o% Potection4

:one of protection is the part of system protected by a certain protection scheme.These $ones are arranged to overlap each other to avoid blind spots and ensure

complete protection. The $one boundaries generally correspond to the locations of

1T' 6here the 1Ts are located on both the side of 1.9s., the $ones naturally

overlap but otherwise blind spots occur. This problem is overcome by some of $one provision.


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Fi#4/ Location o% C-T

C89 Cic&it Potection :one, B89 B&s Ba Potection 8one, F9 Fa&"t-

3n case (c) fig.l fault 8 is not covered in circuit protection and therefore it is taken care of by

covering it in 9us protection scheme.

2-; Pi!a$ an* Bac


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(i) 0ickup

(ii) drop out(iii) both at pick = up and drop out

2-> Re"a$ Opeation In*icatos

These are -

(i) 5ecteiieaily operated.(ii) 2lectrically operated These can be -

(a) Series connected.(b) Shunt connected.

2-? Intena" Connections o% Re"a$ Contact S$ste!s

These are several methods. Some of the common ones are

(i) Series sealing.

(ii) Shunt reinforcing.(iii) Shunt repeat.

(iv) Shunt sealing,

2-/0 Tip Cic&it S&pe3ision

Several methods are used to indicate the healthy condition of trip circuit.

6-0 TYPES OF RELAYS (ELECTRO 5 MAGNETIC

6-/ Attacte* A!at&e T$pe

These are very simple (8ig.>) and therefore widely used.They work as ?yes@ or ?no? ?magnitude? and ?ratios? relays and are used generally as

auiliary, over current, under current over voltage, under voltage and impedance

measuring relays.

Fi#-1 Attaacte* A!at&e T$pe Re"a$s

The relays have either a hinged armature or plunger type construction and are governed by

equation -

8 A (B3 B +)

8 A 8orce B A 1onstant,

3 A C5S value of current and


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B A Cestraining force.

/nder threshold condition constant+ ==k

K I

The relay operation is influenced by -

(i) "mpere - turns developed by relay operating coil,

(ii) Si$e of air gap between core and armature,

(iii) Cestraining force on the armature-

These are inherently instantaneous, but small time delays of the order of +DD ms can be

easily produced by magnetic damping. Slugs (bends of conducting material such as copper)are provided at armature and or at the 4heel4 end of the relay so as to achieve delay in build

up of flu and its decay to enable delay at pick - up and at drop out respectively.

6-1 In*&ction Disc T$pe

These have either shaded pole or watt - metric type electro - magnets and discs. These work

on 8erraris principle (a torque is produced by two flued with phase displacement between

them which is proportional to the produce of their magnitudes and phase displacement).

θ φ φ Sin K T >+=

3f +φ and >φ are produce by 3+ ' 3> of the same main current 3 and fied by shading

arrangement, then it 8ollows that

Torque A B 3>

B + decreases with increase of current due to magnetic saturation. This phenomenon is made

use of to obtain 4%efinite 5inimum Time4 in Time ! current characteristic.

Fi#42 In*&ction Disc T$pe Re"a$

6-2 In*&ction C&p T$pe

These relays (8ig.E) are etensions of induction disc type relays. They are used for very high

speed operation along - with polari$ation and ! or differential winding as required.


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6-6 Reactance an* M@O Re"a$s

Ceactance and 5; type relays ate produced by manipulating current and voltage coilarrangement and relative phase displacement angles between the various flues. The induction cup

relays can be made to measure pure reactance or impedance.

6-7 Ba"ance* Bea! T$pe

These are similar to attracted armature type. 7ow a- days they are superseded by moresophisticated induction disc and induction cup type relays. The limitations of these relays were

poor reset ! operate ratio, susceptibility to phase displacement between the two energi$ingquantities.

6-; Mo3in# Coi" T$pe

0olari$ed %.1. moving coil relays are amongst most sensitive and are widely used for sensitive

and accurate measurement. They are used for distance and differential protections. 9eing

inherently suitable for %.1. only, they are used in 1T ! 0.T circuiteries with rectifier arrangement.

These relays are however more epensive then the induction cup type.

(a) Cotary 5oving 1oil (b) "lally 5oving 1oil

8ig F 5oving 1oil Celays

6-= Po"ai:e* Mo3in# Ion T$pe

This is also an etension of attracted armature type with a difference that it is polari$ed and more

sensitive. They are inverse of moving coil type as the coil here is stationary and iron armature is

movable. Stationary coil arrangement affords ample winding space and a robust relay there by permitting very high with stand to pickup ratio.

7-0 TYPES OF PROTECTIONS AND USE

The following types of protections are employed on transmission ! distribution systems

7-/ O3e C&ent Potection Usin#4

(i) 3nstantaneous over current

(ii) 3nverse time over current. @

(iii) 3%5T over current,

(iv) %irectional over current(v) 1urrent balance,

(vi) 0ower 9alance relays.


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7-1 @i#) Spee* Distance Potection 5 Usin# *istance Re"a$ it)4

(i) 3mpedance,(ii) 5odified impedance,

(iii) Ceactance,

(iv) 5;,

(v) ffset 5; and

(vi) Some special characteristics.

7-2 Pi"ot ie Potections

(i) 6ire pilot.

(ii) 1arrier pilot,

(iii) 5icrowave pilot protections.

;-0 SECONDARY SYSTEM PROTECTION

The secondary substations ( GGkH ' 9elow) generally vampires a row in coming and outgoing

lines with step - down transformers. The GG kH and ** kH ("lso *I.F kH tc.) sides are generallycontrolled by minimum oil and bulk oil circuit breakers if the capacity of transformers is F 5H"

or more otherwise ruses are J generally employed. The ++ kH side is controlled through ++ kHswitchgear.

Harious makes of + 3kH switchgears in use in /0S29 are

(a) 2nglish 2lectric, (b) #.2.1. (c) 9.3.1.. (d) 1romptons ' #reeves (e) 7.#.2.8. (8) Kyoti(g) Siemens, (h) Switchgear ' (&) 9.;.2.L. (k) Ceyrol (l) South wales corp. (m) Holtas

(n) 8erguson ' 0ailen .

The trip principles used are generally three

(i) Trip coils operable directly from 1.T. secondary current.

(ii) Trip coils operable from secondary current through relay - 1alled series trip .

(iii) Trip coils operable from %.1. ( >E, ++DH etc.) battery through relay contacts -called shunttrip.

3n case (i) the tripping takes place when the current flows in trip coils eceeds the setting valueforcing the plunger there by to move which strikes the tripping - rod -mechanism.

3n case (ii) current, when in ecess of set value , causes the relay disc to rotate , on closing of relaycontacts, the instantaneous unit is energi$ed opening there by the crip coil shorting contacts. This

allows the fault current to flow through the trip coil and operate the mechanism. The trip coil

ratings are F " and > or +" for over current ' 2arth fault cases respectively.

The trip circuit is operated from %.1 battery through current relay in cases (iii) the trip coils are of

F" ' > or K " for over current ' earth fault cases.

The over current and earth fault relays used may have ratings as below -


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;-/ Case (ii

(a OC

(i) 1urrent setting FDM to >DDM ad&ustable in I equal steps of >FM

(ii) Time settings to * sec at +D times current setting,

(+ EF

(i) 1urrent settings +DM to EDM ad&ustable in I equal steps of FM .

(ii) Time setting D to * sec at +D times current setting.

;-1 Case (iii

(a 0C

(i) 1urrent setting FDM to >DDM ad&ustable in I equal steps of >FM .

(ii) Time setting FDM to >DDM at&fustabie in I equal steps of >FM .

(+ EF

(i) 1urrent setting +DM to EDM ad&ustable in F equal steps .

(ii) Time setting D to * sec Nt +D times current setting.

The above relays have ad&ustable inverse time current characteristic with a definite

minimum time. The starting and operating currents are of the order of +D* to +DFM and+*M of current setting respectively.

=-0 RELAY APPLICATION

=-/ Non 5 *iectiona" O3e C&ent an* Eat) Fa&"t

These relays are used either as main or back - up protections.

(i) 3%5T CelaysLimits of accuracy as set by 3S *>*+ is shown in shows application to a

sectional radial feeder. 3t seems mat with assumed relay settings and

tolerances as per, the minimum permissible time grading interval for

successive locations works out to D.F seconds. This implies that with largenumber of sections and increased fault levels , the clearance time towards

source will be considerable with good quality relays (such as 22 1%# ++)

the time intervals can be reduced to D.*F to D.E seconds.

(ii Co!+ine* IDMT ' @i#) Set Instantaneo&s

"dding of high set 3nstantaneous relay is advantageous on long lines withsmall source impedance and on transformer feeders as it make possible to

reduce the tripping time under the maimum short circuit conditions. This

would allow shortening of grading time between successive breakers. The

effect is to reduce operating time to D.D+ sec in shaded areas. 6hileselecting over current unit setting, care has to be taken of 4transient over

reach4 defined as

M+DD"

9-" reachovertransient ×

=

6here " A pick up current in steady state r.m.s amps,

9 A Steady state equivalent r.m.s current &ust sufficient to operate

the relay when the current is fully offset


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1" +I has transient over teacn from *M to FM, Transient over reach is not of much importance in case

of applications on transformers and transformer feeders,(iii) %efinite Time ver 1urrent

They are preferably where system fault current various widely (due to change in source

impedance). There being relatively small change in time with variation fault current, the grading of

several relays in series is easier ' surer.

These relays are also preferred on locations where high transient peaks of current are eperiencedin routine e.g. group of motors starting simultaneously and. direct on line.

The following points need attention

(a) The relay should have required setting range ' transient over reach.

J 3t should have high drop off to pick up ratio (say ODM to PDM) where it can seefaults of ad&oining section.

J 3t should have a continues thermal rating not less than rated current of the circuit

and a short time setting not less than maimum system fault current seen by the

relay for a duration equal to the maimum time delay setting possible on the relay.

=-1 Diectiona" O3e C&ent An* Eat) Fa&"t%irectional features improve selectivity. %irectional features can be introduced in ali the relaysdiscussed under I.+ above. This should be used where-

• 3nstantaneous protection is used and maimum back feed current is more than ODM of the

maimum for end current.

• 5aimum 9ack feed current is more than >FM of minimum for end current and time over

current units are used.

• 0ick up a desired below about twice the full load current, and the loan current is in the

directional unit non-trip direction.

These can be used on a bank of parallel operating transformer in the absence of unit protection.

This protection first for main or bank up on interconnected or ring main as (8ig.+F).

>-0 RELAY SETTING CALCULATIONS

>-/ Ti!e Settin# M&"tip"ies (TMS

Time is dependent on value of T.5S available and location from source. ;owever, it is not

kept $ero to avoid spurious tripping due to vibration and for allowing safer contact gap.

Sequential setting intervals between the relays can be kept as D.E sec.

T.5.S for inverse time relay M T

T =

6here, T A Cequired time of operation of relay.Tm A Time obtained from relay characteristics curve at T.5.S A 3 ' plug setting

5ultiplier (0.S.5) equivalent to maimum fault current. e,g. 3f A *DDD", Celay settingcorresponding to primary current A +FD"

Then< 0.S.5. A >D+FD

*DDD=

8rom standard curves of 3%T relay


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T5 A >.> see, -

3f required time setting A +.+ sec,

F.D>.>

+.+T.S.5Then< ==

>-1 C&ent An* Ti!e Ga*in#

>-1-/ C&ent Ga*in#

"s for eample "s there is sufficient margin in the 1.T ratio of 1.9s (*) ' (E) a +DDMsetting is possible and may be selected. The pick-up values of these will then be FDD '

>DD " respectively.

9reaker (>) has full load current of OI "mps. ' 1.T ratio IF!F amps. 6e can select a+>FM setting. 3t gives PE amps allowing a small margin of overload.

9reaker (l) covering meter would be of normal pattern with a pick up of +DFM and basedupon a 1.T ratio of +FD!F would represent a primary current settings of +FI amp.

"s most of the relays are on G.G kH side, we choose G.G kH as voltage base. n this basis+FI amp at E+F H are equal to P,O amps.

3t is noticed that there is ample current grading even considering pick up errors of +.DF to

+.* times relay setting,Celay + 0ickup at P.O ".

Celay > 0ick up between PE+.DF to PE+.* A +DD to +>F".

Celay * 0ick up between >>D+.DF to >DD+.* A >+D to >FD".Celay E 0ick up between FDD+.DF to FDD+.* A F>D to GFD".

(Celay E is with dual characteristics to cater for faults close to bus-bar second setting atEDM in which case pick-up will be F>D D.E to GFDD.E A >+D to >GD "mp.)

3f we add an instantaneous element to relay +, its current setting must be chosen for valueabove starting current of motor which is G times here and so setting of O times ;ill load will be o.k.

3f we add an instantaneous element to (>), the choice will depend upon maimum short circuit

current on L.H side of transformer so as not to reflected on ;.H side. 3f through fault current is say

+*DD", a setting of +GDD" is o.k.

>-1-1 Ti!e Ga*in#

(a) The time setting of relay (+) and fuse are not ad&ustable and these can be plottedfrom manufacturers curve.

(b) 0ick-up time ' current setting of relay (>) is greater than setting of relay (l). The

relay (>) can be set at lowest T.5.S ie.D.+. " curve can be plotted by noting time of operation at various fault currents.

(c) 0ick-up setting of relay (*) is>DD" (+DDM setting chosen)< 0.S.5 at +GDD "mp

(1ut-off value of relay (>) O>DD

+GDD=


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8rom standard curves, time setting on T.5.S of +.D at O times 0.S.5 is *.F sec. "llowing

discriminating interval of D.E sec. to operating times of relay (>) at +GDD "mp. (which is D.>E fromstandard curves)< T.5.S of relay(*)

+P.D*.*

D.ED.>E=

+

7ow relay pick up range is >+D to >GD amp. at T."+S D.+P and, at twice the current

A EDD amp, operating time A D.+P sec at +DQ>DD A >DD amp. Celay operating time 8rom curve onsec, and at >DR>DD A EDDD amps it is D.E> sec, so a curve can be plotted for relay (*)

5aimum short circuit currents A >PDD amps ' 0.5.S. A>DD

>PDDA +E.F, from standard curves,

time setting on a T.5.S of +.D at +E.F times plug setting A >.F sec.

T.5.S of relay (E) A sec*F.DF.>

E.DEO.D=

+

6here D.EO A operating time of relay (*) at >PDD amp. " curve can be drawn now.Similarly a curve can be drawn for lower setting of Celay (*)

Similarly a curve can be drawn for lower setting of Celay (E)

?-0 LINE PROTECTION BY DISTANCE RELAYS

%istance relay schemes offer considerable economic and technical advantages. This form of protection is simple to supply and is of high speed class. These can be used both primary and back

up protection,

?-/ Pincip"e o% Distance Re"a$in#

The relay compares local current and voltage in corresponding phase or suitable components of

them. 8or a fault the end of impedance :, the line drop will be +:. 3t follows that the voltage

current ratio will be . Z I

V = 8or an internal fault

I

V is less than :. Since : is proportional to

line length between the relay and the fault, it is also measure of distance to fault.

?-1 Re"a$ C)aacteistics

(i I!pe*ance Re"a$

This is simplest form of distance relay and respond only to the magnitudes of theimpedance given by applied voltage and current. The characteristic is a circle with radius

: on C-: diagram. This relay as well as others to be described here may take variousforms eg. " balanced beam relay, an induction disc relay , a transducer relay, a moving coil

relay and so on.

(ii Diectiona" Re"a$

These are not strictly depend on impedance for operation but belong to the same family

and they are dependent on the phase angle between H ' 3. n C-:

diagram the characteristic is a straight line through origin.

(a Diectiona" (+ P"ain I!pe*ance (c An#"e I!pe*ance

Fi#4 ; C)aacteistic o% I!pe*ance Re"a$s


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(iii M@O Re"a$

These combine the feature (i) and (ii) above. The operation depends upon both themagnitude and phase angle of impressed impedance. The characteristic is a circle passing

through origin on C-B. diagram.

(i3 O%%set M@O Re"a$

3t is modified form of (iii) above and have the characteristic does not pass through origin.

(3 O)! Re"a$

These have straight line characteristic on polar diagram and operate when the ratio I

V

gives value of comple impedance which lie on side of this characteristic.

(3i Reactance Re"a$

3t is special case of (v) and has a polar characteristic as straight line to the resistance ais.

?-2 App"ication O% Distance Re"a$

(i I!pe*ance it) Diectiona"

3mpedance relay is non-directional so a directional unit is required along with to make thescheme directionally selective. The relay can operate for terminal faults ecept for a three phase ne in which case the operation becomes uncertain. The difficulty also eists in

relation to susceptibility of relays to system power swings. 8urther, under eternal fault

condition. n an interconnected system it is possible for a sudden reversal of current to

take place when one 1.9 opens on a double circuit line and unless the timings of thedirectional and impedance relays are properly coordinated, a spurious tripping may take

place. 3n this case both the directional and impedance relays require slow operating and

fast resetting times.

3f only one impedance and one directional relay is used and switching is employed to

obtain $one 33 and $one 333 by means of timer than starting must be by means of directionalrelays. " directional relay is liable to operate under normal load conditions and therefore

such an arrangement is not very satisfactory.

(ii I!pe*ance it) M@O

This scheme is less prove to normal loads and power swings. ;owever, some problems

are encountered as regards directional control as discussed earlier.

(iii Reactance it) M@O

The difficulties here are also similar to the ones discussed above. The use or reactance

relay is particularly suited to short lines having high source to line impedance ratio since it

can operate down to a lower voltage on account of the fact that reactance relay obtains its polari$ation input from system current and not the voltage. The reach of relay does not

vary with fault resistance theoretically. ;owever when fault resistance are comparable, the

reach of the relay is modified by the value of the load and its p.f and it may either over-reach or under-reach. "lso, when the fault current is being fed from both ends of a line

with two current not in phase, the fault resistance will appear as comple impedance. Tthis

will positive on higher source angle side relay, causing the relay to under reach and

negative on the lower source angle side causing this relay to over reach.


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(i3 M@O it) O%%set M@O

9eing inherently directional, the use of 5; relay avoids use of separate directional relayand therefore the difficulties associated with it. The reach point varies with the fault angle

as the impedance measurement is not constant at all angles.

;ence, those are better suited to power swings and heavy load conditions limitation for

terminal fault is there unless special measure is provided such as 4memory4 circuit. "rcresistance causes under reach.

(a Reactance Re"a$ (+ Con*&ctance Re"a$ (c M@O Re"a$

Fi#4 = Vaio&s Re"a$ C)aacteistic

The ellipticalisation of 5; characteristic improves of the scheme on

power swings and heavily loaded lines, but worsens the response to fault resistance.

This scheme has found wide application due its better over ++ performance.

(3 Statin# Re"a$

The primary function is to control the timing relay for etending reach to second and other $ones (8igO). They also provide directional discrimination when used with non-

directional relay. This means that a variety of combinations can be had eg.

(a) %irectional arc with impedance measuring relays.

(b) !1 with 5; measuring relays.(c) 5; with any distance relay including reactance type etc.

Fi#-> T)ee Step M@O Distance Re"a$ B"oc


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(3i Specia" C)aacteistics

1omple characteristics relay such as hyperbolic, rectangular, elliptical,

combination of various other characteristics etc are available and may be used under

various situations.

(a, O%%set M@O (+, E""iptica" (c, Rectan#&"a

Fi#4 ? So!e Ot)e C)aacteistic

Special characteristics are produced by manipulation of electrical quantities

Fi#4 /0 Sc)e!atic Vo"ta#e S/an* S1

HL A Line Holtage

3L A Line 1urrent

5a A "mplitude 1omparator 5 p A 0hase "ngle 1omparator

S+ ' S> A %erived Signals

1 A 1omparator.


15/18

Ta+"e4 Dei3ati3e Vo"ta#es S/ an* S1

S-No Distance

Sc)e!e

A!p"it&*e Co!paato P)ase An#"e Co!paato

Opeatin#

&antit$

Restainin# &antit$ Opeatin# &antit$ Restainin#

&antit$

+ %irectional

R

L

L

Z

V I +

R

L

L

Z

V I −

3L:C HL

> 0lain

3mpedance L

I

R

L

Z

V ( ) L R L V Z I − ( ) L R L V Z I +

* "ngle

3mpedance R

L

L

Z

V I −

R

L

Z

V ( ) L R L V Z I − ( ) R L Z I

E Ceactance

R

L

L

Z

V I −

R

L

X

V ( )φ SinV Z I L R L − ( ) R L Z I

F 1onductance

R

V I

L

L −

R

V L ( )φ CosV Z I L R L − ( ) R L Z I

G 5; L I

R

L

L

Z

V I −

( ) L R L V Z I − HL

I ffset 5; L I L

R

LnL

Z

V −

( )θ φ −− (CosV Z I L R L )( R L L Z nI V +

O 2lliptical

(* input) L

I

a

I Z V

a

I Z V L R L L R L

>>

???−

+−

/0-0 DISTANCE SC@EMES USED IN U-P-S-E-B

(i) "S2" 5ake C:29 and C:2

(ii) 9rown 9everi 5ake L* 6"S LG f and L*6S LG f

(iii) Siemens 5ake C *:>E, C >:>Ea, C +:>F.(iv) 2nglish. 2lectric 5ake SSCC*H, 55*H, 55*t, CC*H.

(v) Static Celays of various makes.

//-0 CURRENT TRANSFORMERS

These are used to provide proportional currents for protection schemes at the same time

isolating the primary -and secondary circuits electricity. These are required to give fairly


16/18

proportional reproduction of secondary currents over a large value of primary currents. 3n a

1.T input tc .To 1.T approimates to a current source and therefore primary windingimpedance can be neglected,

1.Ts can be two types vi$.< ;igh reactance or low reactance type. The first one is wousid

primary type and the second one toroidal type.

The eciting current depends upon core material used and amount of flu required for a

certain burden. This may be obtained directly from magneti$ing characteristic since thesecondary e.m.f and therefore the flu developed is proportional to the product of secondary

current and burden impedance. The core material used in 1Ts are

(i) ;ot rolled silicon steel and cold rolled silicon steel.

(ii) 7ickel iron alloy. The knee point occurs at high flu density in case of (i) and (ii) and

are used for protection 1.T and at low flu density in case (iii) and this is used for

1.T in metering ! instrumentation.

" 1.T is required to develop a voltage of magnitude( ) R LCT f

R R R I ++=

where 3f A fault current.k

C 1T A 1.T secondary resistance.C L A Load resistance

C C A 3mpedance+ of the relaying scheme at current 3f through it.

This method is .B. 3f operating time of the relay is more than D. + sec. 9ut with

fast protections operating in transient and subtransient impedance regions, R

X ratio, is to be

taken in to consideration to ta?ke in to account the effect of saturation of 1.T.

The 1.T are classified according to the allowable percentage errors in secondary current

reproduction. The classification as per the 3ndian standards (3S >IDF are given in thefollowing tables + and >.

/1-0 VOLTAGE TRANSFORMERS

The H.Ts are used to provide proportional voltage in secondary circuit at the same time

isolating secondary circuit from the primary electrically.

The H.Ts are required to maintain protection accuracy between FM of rated J voltage and the

highest possible system voltage depending upon the system being effectively earthed andnon- effectively earthed.

" secondary winding is connected in broken or open delta for use in earth fault protection

whenever such a protection is required to be used. 3n that case two windings may be required

to be had in the H.T., the second winding connected in broken delta.

T"9L2-+ L353TS 8 2CCCS 8C 0CT21T37 1.T


17/18

Acc&ac$

C"ass

C&ent Eos at

Re"ate* Pi!a$

C&ent

P)ase Disp"ace!ent

ate* Pi!a$ C&ent

Co!posite Eos at

Rate* Acc&ac$ Li!it

Pi!a$ C&ent

0ercent 5inutes 0ercent

U + GD F

U* U +D± F +F

TABLE5II4 ACCURACY CLASS FOR METERING CTs

App"ication C"ass o% Acc&ac$

(a) 0recision testing or as a standard for testing laboratory current

transformers.

D.+

(b) 8or laboratory and test work in con&unction with high accuracyindicating instruments integrating meters and also for standard

for testing of industrial current transformers.

D.>

(D 8or precision industrial metering D.F

(d) 8or commercial and industrial metering D.F or +

(e) 8or use with indicating and graphic watt meters and ammeters + or *

(f) 8or purposes where the ratio of mass importance, foe eg

ammeters there approimate values are required.

* or F

/2-0 RECENT TRENDS

9efore +PODVs electric magnetic relays have been in use universally, through static relays hadcome in eistence much earlier. The power system having electromagnetic and static relays still

continue to have them. 3t is because mainly of two reasons, vi$

(i) The protection systems using electromagnetic and static relays are functioning in a verygood manner,

(ii) Ceplacement of these systems, which are very large in number, involves huge cost apart from

etensive work required in carrying out of the same. ;owever where ever replacement is

called for, the same is being done in line with the current technology. +POD?s saw a trendtowards use of individual solid state relays in place of electromagnetic ones for each

protective function.

6ith the introduction of microprocessor based multifunction digital protective relays in +POD?s,

the trend has changed and this technology is being preferred now a days in place of use of 4solid

state relays.


18/18

/2-/ A*3anta#es o% Micopocesso Base* Potecti3e Re"a$ (MPR

(i) Se"% Monitoin#4 These arc 4watch dog timers4 to continuously monitor their

own operating status. "ny potential malfunctioning is immediately identified and

reported to the distributed 1omputer 1ontrol System (%1S)! 0rogrammable Logic

1ontroller (0L1). " protective relaying system is required to operate only in case of

abnormal condition of running of power system ! equipment and therefore, anymalfunctioning of earlier types could be detected only on routine testing or on its failing

to operate when it was required to operate. The self monitoring feature of 50C make is

much advantageous,

(ii) Co!!&nication4 The digital relays are provided with serial data parts based on established

protocols compatible with the %1S ! 0L1 communication protocol used at the plant. This

allows these relays to use digital communication scheme to communicate to plant control system.

(iii) M&"tip"e Potecti3e F&nctions4 The older relays required an individual relay for each protective function. The digital relays provide multiple protective functions in one relay

there by reducing panel space and wiring costs,

(iv) Se"%5 Ca"i+ation4 %igital relays are provided with self- calibration routine which

can be initiated by selecting relay calibration mode in soft ware programming of therelay.

(v) Po#a!!a+"e Set points4 %efining $ones of protection for each primary and backuprelay, calculating settings, performing co-ordination analysis etc needed eperienced

engineers for application of previous type relay systems.

3n case of 50C this is a simpler &ob since digital relay uses widows based soft

ware programmed which provides tutorials and recommended set points for each

protective relay function based on system characteristics. 5ost of the soft ware

programmers also provide graphic display worksheets showing time currentcharacteristics of the relays in the form of graph to simplify co-ordination amongst multi

protective functions contained in one relay.

(vi) 2vent Storage O to +D selected wave forms can be stored by digital relays on anoscillograph record. This will show the condition of each of the selected wave

forms before and after the protective relay has operated. This information is

valuable as it helps determining the reason of operation of relay and cause of tripping. .

/2-1 App"ication Possi+i"ities an* Pe%eences

/se of 50C is fast gaining acceptance especially in new plant and systems on account of

the various advantages associated with it. /se of these is also being made in cases of renovation,, moderni$ation and remote control of the old!worn out power stations!power

systems, ;owever, their use-in. eisting systems is not finding .many acceptances due-, to

cost and other implications, particularly in 4 developing countries. 3t appears that it will takequite some tirne in charge over and till such time old systems will continue to run concurrently

with the 50Cs.

Documents

PROTECTIVE RELAYING FOR TRANSMISSION AND DISTRIBUTION LINES.doc