Transformer protection 2012-1 (2).ppt

8/20/2019 Transformer protection 2012-1 (2).ppt

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1 > Transformer Protection

Protected

Zone

RE

F

Relay only operates for earth faults within protected zone.

Uses high impedance principle.

Stability level usually ma!imum through fault level of

transformer

Star Winding REF


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"# restricted E$F

protection trips

both %# and "# brea&er

Recommended setting '()

rated

Restricted E/F Protection

* + , -


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* + , -

"# restricted E$F protection trips both %# and "# brea&er

Recommended setting '() rated

Restricted E/F Protection


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Protected zone

REF

Source

elta winding cannot supply zero se/uence current tosystem

Stability ,onsider ma! "# fault level

Recommended setting less than 0() minimum earth fault

level

Delta Winding Restricted Earth Fault


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Protected

Circuit

R

Z

M

RCT

RL

Z

M

RCT

RLIF

IS

RS

T

VS

RL RL

High Impedance Principle

#oltage *cross Relay ,ircuit #S 1 IF 2R,3 4 5R"6

Stabilising resistor RS3 limits spill current to IS 2relay setting6

RS3 1 7 RR where RR 1 relay burden

,3 &nee point

#8P 1 5#S 1 5IF 2R,3 4 5R"6

#S

IS


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uring internal faults the high impedance relay circuit

constitutes an e!cessive burden to the ,39s.

* very high voltage develops across the relay circuit and

the ,39s.

,auses damage to insulation of ,3: secondarywinding and relay.

;agnitude of pea& voltage #P is given by an appro!imate

formula 2based on e!perimental results6

#P 1 5 √ 5#8 2#F 7 #86

0

The Use Of on!"inear Resistors #$etrosils%


7/437 > Transformer Protection

IOP

RST

VSVM

RR

;etrosil ,haracteristic

# 1 , ? β

Suitable values of , @β

chosen based on

;a! secondary current under fault conditions

Relay setting voltage

on!"inear Resistors #$etrosils%



1MVA(5%)

11000V415V

1600/1RCT =

4!Ω

"0MVA

RS1600/1

RCT =

4"Ω

MCA#14

IS = 01

A$& Core '/06'$$('41Ω/$)100$ Lo*

,alculate

'6 Setting voltage 2#S6

56 #alue of stabilising

resistor re/uired06 Effective setting

A6 Pea& voltage

developed by ,39s

for internal fault

REF Protection E&ample



Earth fault calculation 7

Using B(;#* base

Source impedance 1 ' p.u.

3ransformer impedance 1 (.(C ! B( 1 A p.u.

'

3otal impedance 1 'A

p.u.

?' 1 ' 1 (.(D'A p.u. 'A

+ase current 1 B( ! '(

√ 0 ! A'C

1 '''5 *mps

?F 1 0 ! (.(D'A ! '''5 1 50BA( *mps 2primary6

1 'A. *mps 2secondary6

' P.U.' A

I'

A

I5

A

I(

Se/uence iagram

Solution #'%



2'6 Setting voltage

#S 1 IF 2R,3 4 5R"6

*ssuming GearthH ,3 saturates:

R,3 1 A.B ohms

5R" 1 5 ! '(( ! D.A' ! '(70

1 '.AB5 ohms Setting voltage 1 'A. 2A.B 4 '.AB56

1 0. #olts

256 Stabilising Resistor 2RS6

RS 1 #S 7 '

IS IS5



2A6 Pea& voltage 1 5√5#8 2#F 7 #86

#F 1 'A. ! #S 1 'A. ! 0 1 '0A #olts

IS

For IEarth9 ,3: #8 1 '.A ! 50 1 00( #olts 2from graph6

#PE*8 1 5√5 ! 00( ! 2'0A 7 00(6

1

3hus: ;etrosil voltage limiter will be re/uired.

Solution



Differential Protection



Jverall differential protection may be Kustified for

larger transformers 2generally > C;#*6.

Provides fast operation on any winding

;easuring principle

+ased on the same circulating current principle as the

restricted earth fault protection %owever: it employs the biasing techni/ue: to maintain

stability for heavy through fault current

+iasing allows mismatch between ,3 outputs.

?t is essential for transformers with tap changing

facility.

*nother important re/uirement of transformer

differential protection is immunity to magnetising in

rush current.




+?*S+?*S I2I1

JPER*3E

I1 -

I2

ifferential,urrent

JPER*3E

RES3R*?-

;ean 3hrough,urrent

I1 +I2

2

I1 -

I2

)iased Differential Scheme



"#

R

%#

PRJ3E,3E

ZJ-E

,orrect application of differential protection re/uires ,3 ratio

and winding connections to match those of transformer.

,3 secondary circuit should be a GreplicaH of primary system.

,onsider

2'6 ifference in current magnitude

256 Phase shift

206 Zero se/uence currents




P' P5 *5 *' a' a5 P5 P'

* + ,

Differential *onnections +nd polarit, mar-ings



RR

R

P' P5 *5 *' a' a5 P5 P'

S5 S' P' P5

?nterposing ,3 provides #ector correction

Ratio correction

Zero se/uence compensation

S' S5 S5 S'

Use of Interposing *T



Interposing current transformers


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S5S'

P'P5*'*5 a5a'P5

'C;#*

$ '''C($C B(($C

P'

y'

S'S5


Liven above -eed to consider 7

2'6


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RR

R

P' P5 *5 *' a' a5 P5 P'

S' S5 P' P5

A.5*

S' S5 S5 S'

A.0D*

25.C6 2C6

'C($C B(($C

?t is usual to connect '' ,.3.9s in and utilise a $ interposing ,.3. 2this

method reduces lead #* burden on the line ,.3.9s6

,urrent from side 1 A.0D *mp3hus: current re/uired from winding of int. ,.3. 1 A.0D *mp

,urrent input to winding of int. ,.3. 1 A.5 *mp

Re/uired int ,.3. ratio 1 A.5 $ A.0D 1 A.5 $ 5.C5

√0

;ay also be e!pressed as C $ 5.C

Effect of Tap *hanger


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Effect of Tap *hanger

e.g. *ssume 4C): 7'C)

?nterposing ,.3. ratio should be based on mid tap position

;id 3ap 27C)6 1 5.D

Primary current 2'C ;#*6 1 '0B *mp

Secondary current 1 A. *mp ?nterposing ,.3. ratio re/uired 1 A.5 $ A.

√0

2 $ 6

1 A.5 $ 5.

;ay also be e!pressed as C $ 5.D

,ompared with C $ 5.C based on nominal voltage

*om.ined Differential and


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3o differential relay

S5

S'

P'

P5REF

S5

S'P'

P5

S5S'

P' P5a5a'*'*5

*om.ined Differential and Restricted Earth Fault Protection

Three Winding Transformer


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5C;#

*

''8#

0;#

*

'058# '(($C0(($C

C(;#

*

008#

'((($C

A.C

5.BB

'(.00

5.BB

C.C'

C

C

All interposing C.T. ratio’s referto o!!on "#A $ase %&'"#A(

Three Winding Transformer

Traditional Use of Interposing *T


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y'270(M6

RR

R

Nd''240(M6

?nterposing ,3 provides

#ector correction

Ratio correction

Zero se/uence compensation

Traditional Use of Interposing *T

Integral ectorial and Ratio


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Differentialelement

Numeric Relay

Virtual interposing CT

Vectorialcorrection

Ratio

correction

Virtual interposing CT

Integral ectorial and Ratio*ompensation

Power transformer

Date post:	07-Aug-2018
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Transformer protection 2012-1 (2).ppt

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