Part I Protection Philosophy of Electrical Equipments

8/9/2019 Part I Protection Philosophy of Electrical Equipments

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Need for protection of electrical equipment

All the electrical equipment must be protected against internal and external

faults, a well a abnormal conditions which may endanger the equipment or

the system. Appropriate relays and devices have to be provided, to detect the

fault and potentially dangerous conditions and to isolate the concern

equipment at the earliest in order to minimize the damage.

The basic requirements of sensitivity, selectivity and speed

have to be met by provision of carefully selected relays and relaying

schemes. Since failure of a protection to operate cannot be ruled out, and it

can lead to damage of costly equipment, it is necessary to provide back upsin the form of redundancy of relays, local or upstream back up relay etc.

However, continuity of power generation is also of utmost importance, and it

dictates that the risk of tripping (due to maloperation of the protection) when

a tripping is not really necessary, be minimized. This risk is related directly

to te number of relays and other elements in the protection scheme and its

complexity. Therefore, superfluous relays and devices must not to be

provided, and the protection schemes must be made as simple as possible.

Redundancy and back ups must also be minimized and tendency of over-

protection checked.The protection relays must be connected to automatically isolate the

endangered equipment if the damage is imminent or short circuit has already

occurred. However, in cases where the equipment or system is not

immediately endangered and can continue in service for some more time, the

protective devices should be connected to initiate only an alarm. This would

enable the plant operator to take the corrective steps and prevent a tripping,

or to prepare for outage of equipment (by changing over to standby

provisions or bringing down the unit load).

It is also important that the control room operator is not burdened

or confused with too many alarms. The alarm fascia on the UCB is thereforeto be used for annunciating only those abnormal conditions for which the

UCB operator has to take some distinct preventive or restorative action.

While tripping of all main equipment has to be annunciated in UCB, the

cause of tripping need not to be annunciated unless the UCB operator must

know it immediately to decide his future course of action. It is generally

adequate to register the cause of DAS printout, CRT display and / or with


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hand reset flags on protective and auxiliary relays in relay panels or

switchgear.

INTRODUCTION TO MOTORS:

In present there is wide range of motors and its

characteristics are taken into existence. The present day tendency is to

employ motors to limit their thermal margins and characteristics of motor

starting current to flow for time in excess of motor starting time.

Usually in most of applications we use induction motor due

to its inherent good characteristics, easy speed control and good protection

scheme. The three phase induction motor principle is that the three phase

voltage produces current in the stator windings which sets up a rotating

magnetic field. The field flux cuts the short circuited rotor conductor andinduces a current in it. The interaction of flux and current produces torque

which causes rotation. A torque increase until it reaches maximum value, the

value at this point is known as full load torque. A further increase in speed

causes the torque to decrease until it would become zero if 100% speed

could be reached. At zero speed the torque is in excess of that demanded by

the fan and hence the motor accelerates. The speed increases studily as the

excess torque is roughly the same value up to 30% speed. After 40% speed

there is a large excess of torque so that the machine accelerates quickly until

it delivers the amount of torque required by the fan. If the fan dampers wereclosed then the required torque is far less, the excess torque and therefore the

acceleration is greater and the machine runs up to speed quickly and delivers

the amount of the torque required by the fan.

The squirrel cage induction motor operation is simple to

understand as well as the speed and torque variation is simple.

The protection of motor will be so far simple. While

applying protection motor characteristics should be carefully considered.

The conditions for which motor protection is required can be divided into

two categories.

1.

External conditions2.

Internal faults

•

Unbalanced supply voltages

•

Under voltage

•

Single phasing


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These are some of the external conditions due to which fault

occurs.

Internal faults occurs due to

•

Bearing failures

• Internal fault

• Earth faults

The motor should develop the protection against all the faults. The

protection applied for a particular machine depends on its size and the nature

of load to which it is connected. However, all the motors should be provided

with over load and unbalanced voltage protection. This can often be

provided in a single relay.

There are various types and sizes of motors used in a power station.These are used for various purposes as prime mover. Apart from the simple

motors used in different areas, there are HT motors used in conjunction with

various heavy duty equipments. These are FD, PA & other fans, boiler feed

pumps, CW pumps etc..These motors have certain special features like

cooling, auto starting, inter locking & controlling.

The basic principles underlying the operation of these fans are by

and large as explained in the previous sections. But the more important

factors that need attention is the start up these motors, lubrication etc.

Fans are used to produce the draught in the furnace or to handlethe pulverized fuel or to recirculate the flue gases or to provide cooling air

for various equipments.

The forced draught fan (FD) provides atmospheric air for

combustion and the induced draught fan (ID) handles the products of

combustion. Primary air fan (PA) handles cold atmospheric or hot air to

carry the pulverized fuel. Mill fans are used to produce draught in the

milling system.


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Mill-7ETO ash slurrySWBD#OCB

MILL

Coal conveyor

FD fan

VFD TRFR (ID

FAN) CH2VFD TRFR (IDFAN) CH1PA FAN

BCW pump

ECW pump (SG)

ECW pump (TG)

ESP ser. Tran 1

ESP ser. Tran 2

CEP

CEPSpare Tranfeeder

BUS PT

Spare motorfeeder(1200KW)CT service tran

CW pump

UST

FEEDER PT

29 MF4

28 TIE2A

27 MF4

26 MF4

25 MF4

24 MF4

23 MF5

22 MF2

21 TRF1

20 TFR1

19 MF1A

18 MF6

17 MF9

15 TRF3

14 TRF3

13 TRF3

11 MF3

10 MFE9 TIE1A8 TRF2

7 BPT1

6 MF2

5 TRF3

4 MF1D

3 TRF2

2 FPT1

1 1FI

I N C O M E R

F R O M

U A T

T i e t o s t n .

S w b d O C A

6 . 6

K V s w i t c h

g e a r


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DISCRIPTION OF SWITCH BOARD:

The given single line diagram illustrates the power distribution from

6.6 KV feeder to various sub feeders. The power generated at 6.6 KV and

step down to the 6.6 KV and from 6.6 KV feeder power distributed to 29 panels. In that many companies of ESP service transformer, ECW pump,

VFD transformer (ID fan), FD fan, coal conveyer, ash slurry switch board

and mill.

To the 6.6 KV bus there will be two supplies connected to the

vaccum circuit breakers. The switchgear is fed from a UAT(unit auxiliary

transformer ) rated 21KV/6.9KV,25MVA.The board is having an alternate

supply from station transformer source so that failure of any transformer

does not hamper the starting up/ shutting down, or normal operation of the

plant.

A proper protection is provided by using CT’s shown in single linediagram for the incomer. From the 6.6 KV bus all the equipments are

supplied through vaccum circuit breakers. Each having the protective relays

of over current, over load and the earth fault protections. CT’s are connected

so that the current in main supply is stepped down according to protective

relay magnitude so that we can give suitable protection. Economically in the

single line diagram as shown motors in pumps and mills are effectively

grounded through impedance to limit the fault current.

In ash slurry again this 6.6 KV stepped down to 433V and this

433V supply 2 degrees given to various motors. There will be five motorswhich will feed the ash and water to the ash pot which located 2 to 3 Km out

of plant. In the switch gear we also have a spare motor feeder and a spare

transformer feeder so that if there is failure of supply from UAT then it will

be fed from spare motor and transformer.

UAT RATINGS:

Rated voltage : 25MVA

No load voltage :

HV side : 21KVLV side : 6.9KV

Line current

HV side : 688.1A

LV side : 2094.3A


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TECHNICAL DATA OF MOTORS USED:

parameters FD fan PA Fan CEP MILL

Rating(KW) 1280 2940 962 583

Derated rating for

50C

1200 2750 900 525

Rated voltage(V) 6000 6600 6600 6600

Frequency(Hz) 50 50 50 50

Permissible

variation of

Voltage %

Freq %

+/-10.0

+/-3to5

+/-10.0

+/-3to5

+/-10.0

+/-3to5

+/-10.0

+/-3to5

Min perms starting

voltage %

80 80 85 90

Rated speed(RPM) 995 1493 1486 589

At Rated volt & freq

Full load current

(A)

No load current

126.5

42.0

273.0

59.0

96.5

31.0

68.0

33.0

p. f. at load100%

75%

50%

no load

starting

0.86

0.83

0.76

0.04

0.13

0.97

0.9

0.87

0.05

0.11

0.85

0.82

0.74

0.017

0.208

0.72

0.66

0.54

0.0179

0.356

Starting current (%)

at

100% voltage

min stating voltage

600

460at80%@

RV

600

460at80%

@RV

600

498at85%

@RV

450

405at90%

@RVEfficiency at rated

volta

100%

75%

50%

96.6

96.6

96.0

96.8

96.8

96.4

95.8

95.8

95.2

94.1

94.1

93.2

CT Ratio 150/1A 315/1A 125/1A 75/1A


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LISTING OF PROTECTION RELAYS OF MOTORS:

FD FAN :

• Definite time over current alarm relay – CTU 32

• Inverse time over load relay – CDGM 12

• Instantaneous over current relay – CAG 37

• Sensitive earth fault relay – CTUM 13

PA FAN:

•

Definite time over current alarm relay – CTU 32



CEP:




•

Sensitive earth fault relay – CTUM 13

MILL:






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ABBRIVATION:

FIRST LETTER – OPERATING QUANTITY:

A-

Phase angle comparison.B- Balanced currents

C-

Currents (amps)

D - Differential

E -Direction

F -Frequency

I - Directional current

K – Rate of rise of current

M - Manual

O – Oil pressure

P – Poly phase VA

R – Reactive VA

S – Slip frequency

T – Temperature

V – Potential (volts)

W – Watts (power)

X – Reactance

Y – Admittance

Z – Impedance

SECOND LETTER – MOVEMENT:

A – Attracted armature

B – Buchholz

C – Induction cup

D – Induction disc

G – Galvanometer (moving coil)

I – Transactor

J – Mixed typesM – Magnate (polarized)

P – Plug

R – Rectifier

S – Synchronous motor

T – Transistor

W – Weight


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THIRD LETTER – APPLICATION:

A – Auxiliary

B – Testing

C – Carrier or counting

D – Directional

E – Earth (ground)

F – Flag and alarm indicator

G – General or generator

H – Harmonic restraint

I – Inter lockedJ – Tripping

JE – Tripping (elect – reset)

JH – Tripping (hand – reset)

JS – Tripping (self – reset)

JC – Control

K – Check alarm

L – Load limiting

M – Semaphore or motor

N – Negative sequence

O – Out of stepP – Potential

Q – Alarm

R – Reclosing

S – Synchronizing

T – Timer or transformer

U – Definite time

V – Voltage restraint

W – Pilot wire

WA – InterposingWJ – Inter Tripping

X – Supervisory

Y – Flash back (back fire)

Z – Special application

ZS – Zero sequence


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FOURTH LETTER:

M – Special variations

First Figure: Indicates the number of units in the relay essential to its

operation – not including seal – in auxiliary units.

Second Figure: Indicates particular characteristics of one of a group of

similar relay e.g. CDG 11, CDG 12, CDG 13 and CDG 14 are all inverse

time over current relays but with different characteristic curves.

For example•

CDG

C-currents

D- Induction disc

G- General or Generator

• CTUM

C-current

T-Transistor

U-Definite timeM-special variations

RELAYS:A relay is a device which detects the fault and supplies information to

the breaker for circuit interruption. A typical relay circuit can be divided into

in to three parts.

1.

The primary winding of a current transformer which is connected in

series with circuit to be protected. The primary winding often

consists of main conductor itself.2. Second circuit is secondary winding of CT connected to relay

operating coil.

3.

The third circuit is tripping circuit, which consists of source of

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

Under normal load conditions, the emf of secondary winding of CT

is closed to relay contacts. This keeps the trip coil of circuit breaker


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unenergized. Consequently, the contacts of breaker remain closed

and it carries the normal load current.

When fault occurs, a large current flows through motors

•

Overload protection• Over current protection

• Earth fault protection

• Differential protection(for HT Motors)

• Locked rotor protection(for special motors)

The job of relay is to discriminate between a fault within its zone of

protection and all other system conditions. It must act energize the trip coil

of its associated circuit breaker and provide security against fault tripping for

fault outside zones. A relay is made secure and dependable by designing into

it a logical decision making capability such that it produce correct output.

When a fault occurs in a system results sudden rise in current towards

the fault associated with reduction of voltage and system. These power

signals are very high and are converted into lower level by instrument

transformer and fed to relays.

These relays decide depending on the logic built in energize trip coil

of circuit breaker whose contacts are series with faulty line, which move

apart very rapidly. As current through breaker passes through zero, the space

between contacts become a dielectric and disconnects the faulty section from

rest of the system. The entire process from time of initiation of fault through

its final clearance takes between 30 to100ms depending on protectionsystem used.

The common protective relays are Inverse time Overcurrent relay

usually fed from protective type CT. The most frequent type of fault is an

Earth fault through which phase connected. Over current relay with detect

earth fault current above relay setting. The earth fault current may be limited

in magnitude by neutral Earthing impedance or by earth connected

resistance.


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ABOUT THE RELAYS USED:

1. INSTANTANEOUS OVER CURRENT RELAY (CAG 37):

Features:

• Continuously variable current setting

• High drop off/ pickup ratio

• Low transient over-reach

Application:

The type CAG 37 is a highest instantaneous

Overcurrent unit with low transient over reach and a high drop off /

pick up ratio.

Because of its infinitely variable settings and immunity

to offset transients, this relay has special advantages for protection

of feeders connected to high MV sources. Where lines are fed from

high MV sources, the impedance of the line causes a sharp

reduction in the fault current as the distance between the fault and

source increases.

Conventional instantaneous over current protection gives

good discrimination an economy on these lines, but a relay set todetect symmetrical faults at the far end will over reach and cause

tripping for off set faults which are out side the protective zone.

The over current setting must, therefore, be raised in proportion to

the reach of the relay, with consequent loss of coverage for

symmetrical faults at the far end of the line.

General description:

The relay comprises of a standard DC hinged armature

unit fed via a single phase transformer and full wave bridge

rectifier. A residual screw is fitted with armature of the relay to

achieve a high drop off / pick up ratio. A potentiometer is

connected in parallel to the relay coil and adjustment of this varies


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the effective operating current of the relay over a range of 1 to 2.

The transformer primary winding is center tapped to give a further

1 to 2 adjustment of the relay operating current and together with

potentiometer give an over all adjustment of current setting of 1:4

ratio. Selection of transformer primary tapping is by means of link provided at the rare of the relay cradle. A surge diverter is

connected across the secondary winding of the transformer to limit

the secondary voltage.


14/25

ALARM TRIP

RYB

OVER CURRENT RELAY


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TECHNICAL DATA:

Current ratings: 1A or 5A

Settings : 200 – 800 %, } continuously500 – 2000 %} adjustable

Operating time: See Fig 1

Drop off / pick up ratio: Not less than 80% of setting current

Thermal rating

Continuous: Min setting current subject to max of

o 20A

Short time: Max setting time for 15 sec

Burden: 200 – 800 % 500 – 2000 %

Version Version

200% 800% 500% 2000%

At Rated current 0.22VA 0.054VA 0.03VA 0.011

At setting current 1.0 VA 3.5 VA 1VA 5VA

2. INVERSE TIME OVER LOAD RELAY (CDG 12):

Features:

• Two tabs with identical time / current characteristics

•

High torque, ensuring consistent timing even under adverse condition

• Very low over shoot

•

Simple construction, easily accessible

•

Comprehensive range of auxiliary unit ratings

•

Dust tight draw out case and tropicalised finish.


16/25

Application:

Stand by earth fault protection of neutral Earthing resistance and other

applications requiring long line delay.

General Description:

A non directional heavily damped induction relay which has an

adjustable long inverse definite minimum time / current characteristics. The

relay has a high torque moment combined with low over shoot. The relay

disc is so shaped that, as it rotates, the driving torque increases an off sets

the changing restraining torque of the control spring. This feature combine

with the high torque of the relay induces good contact pressure given at

currents near pick up. Damping of the disc moment is by a removable high

retentivity permanent magnate.

One unique method of winding the operating coil ensures that the time /

current characteristics are identical on each of the two current taps. Selection

of the required current setting is by means of a plug setting bridge which has

a single insulated plug. The higher current tap is automatically connected

GND

R

Y

B


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when the plug is with drawn from the plug, allowing the setting to be

changed while in service without risk of open circuiting the current

transformers.

The relay operating time can be adjusted by the moment of the disc

back stop which is controlled by rotating a knurled Moulded disc at the base

of graduated time multiplier scale.

Type CDG 12 is a single pole relay and is available in this version.

TECHNICAL DATA:

Current ratings: the operating coil can be supplied suitable for

Operation from 1A & 5A.

Settings: CT secondaries and the current setting available

Are 15 % and 20 % of rated current.

Starting current: 85 to 105 % of current settings.

Closing currents: 85 to 105 % of current settings.

Resetting current: The disc will completely reset at 80% or more of

Time / current characteristics given in fig.

Resetting time with time multiplier set at 1.0, the resetting

Time is 40 sec

Burden: 6.0 VA at current setting

Thermal rating: the relay will with stand twice the current setting

Continuously for 60 degree centigrade rise in coil

Temperature & 50 time the current setting for the

Operating time of the relay.


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19/25

3.DEFINITE TIME OVER CURRENT RELAY (CTU 32):

Features:

•

Consistent accuracy

•

Reliability•

Low burden

•

Reduced maintenance

•

Long life

•

Immunity to transience and surges

•

Fast reset

•

Exceptionally low over shoot

Application:

Type CTU relay can be used for definite time over current

protection against phase and earth fault on medium and low voltage

distributions.

These relays are particularly suitable on systems where there is a

wide variation in source impedance.

Another important protection of CTU relay is the field of stalling

protection of motors. When thermal over load relay does not provide

protection against stalling, separate definite time over current relay like CTU

can be used to provide the same.

General Description:

The operation is illustrated by block diagram shown in figure

When the positive peak of the input signal exceeds the reference

level the time delay circuit starts and after a preset time drives the outputrelay.


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Instantaneous high set unit, when fitted, uses alternate half cycle for

measurement and through a separate level detector drives a separate output

relay. The static circuitry is fully protected against high transient voltages.

TECHNICAL DATA:

A. C. Burden: 0.15 VA per phase at lowest setting.

2.0 VA per phase at highest setting.

INPUT

TRANSFORMER

HIGHSET LEVELDETECTOR

HIGHSET OUTPUT

CURRENTLEVEL

DETECTOR

TIME CONSTOUTPUT

T R A N S I E N T S U P P R E S S O

DEFINITE TIME OVER CURRENT RELAY


21/25

GN

A

50

R

Y

B

TO DDCMIS SPARE

51I1

51i2

50L

50

50N

CT

RY

B

SURGE


22/25

GN

A

V

C

B

CT

50

R

Y

B

TO DDCMIS SPARE

51i1

51i2

50

50N

RYB

87M


23/25

GN

A

RY

B

50

R

Y

B

TO DDCMIS SPARE

51I1

51i2

50

50N

CT

SURGE


24/25

52a

52Tc

51i1 51i1

86

50

50N2

86

51i1 51i2CsC N T

43SS

Tripping circuit for FD FAN, CEP & MILL


25/25

52a

52Tc

51i1 51i1

86

87 50N2

86

51I1 51i2CsC N T

43SS

Tripping circuit for PA FAN

TRIPPING CIRCUIT

Documents

Part I Protection Philosophy of Electrical Equipments