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SLIDE 1

Transformer

Protection

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SLIDE 2

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SLIDE 3

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SLIDE 4

maximum fuse rating or 

circuit breaker setting

125% of full load current

480

240/120V

set ≤ 2·5Ifl

set ≤ 1·25Ifl

Maximum fuse rating

or circuit breaker 

setting 125% of fullload current

Set≤

2.5 I fl

Set ≤ 1.25 I fl

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SLIDE 5

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SLIDE 6

fuse rating

< 2·5Ifl

Relay set < 3IflSI

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SLIDE 7

r 

y

b

R Y B

r 

y

b

Differential Protection

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SLIDE 8

When applying relays for the protection of

transformers, the following points must be

considered:

•

Line currents present on either side

of star delta transformers will differ in

magnitude and/or phase.

• Any sudden change in the

energisation of the transformer will

produce an inrush magnetising current into

or out of one winding. This can occur when

energising a previously disconnectedtransformer or when a sudden appreciable

drop in transformer load occurs.

• Delta connected windings do not

provide a path for zero sequence currents

during earth fault conditions.

• Tap changers used for voltage

regulation purposes cause the turns ratioand hence the voltage and current

magnitudes to vary.

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SLIDE 9

Magnetising Inrush for a Small Transformer 

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SLIDE 10

Relaypoint

1

2

1

1

1

0

Relaypoint

Fault current flow in transformers

Generator Relay

0

point

1

1

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SLIDE 11

External fault

Stability

Internal fault

Operation

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SLIDE 12

Current Transformer Connections and Secondary Current Flow

External fault: maloperation of relay

Consider and modify connections for correct operation

Internal Fault: Operation of relay

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SLIDE 13

10

9

8

7

6

5

4

3

2

1

 2 4 6 8 10 12 14 16 18 20

Bias Setting

40%

30%

20%

Operating

current I2 (pu)

Bias current I1 (pu)

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SLIDE 14

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SLIDE 17

Winding arrangement & current distribution

3I

I

I

2I

I I

I

3I

I

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SLIDE 18

C.T. Connections

O.C. Relays

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SLIDE 19

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SLIDE 20

87

ZEBZEA

 A B

Vf 

R ACT RLA RLB RBCT

RR

If 

IR

High impedance differential

(Principle of operation)

Saturation has the effect of lowering the excitation

impedance and is deemed to have taken place in CT Buntil, at the limit, the shunt impedance becomes zero and

the CT has no output.

Vf  = If  (RLB + RB CT)

Current through relay IR = Vf  / (RR + RLB + RB CT)

If RR is small IR ≈ If which is not acceptable

If RR is large IR is reduced and without incurring

signif icant error 

IR can be reduced below any specified relay setting

RR

)CTBRLBR(.f I

RRf V

 RI 

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SLIDE 21

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SLIDE 22

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SLIDE 23

Percentage of Transformer Winding

Protected Against Earth Faults

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SLIDE 24

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SLIDE 25

Buchholz Relay

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SLIDE 26

(3)

( 3 )

86

87

52 - 2

**

63

49

*

52-1

HV bus

151 G

51 G

connection

51

( 3 )

trips 52 - 2

trips 52 - 1

( 2 )

( 1 )

(1)

50G

connection

(3)

50/51/51N

(1)50N

Protection diagram

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SLIDE 27

R

Y

B

HV CTs

LV CTs

 Auto transformer 

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SLIDE 28

Operating

Bias coils

coil

Three-winding transformer 

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SLIDE 29

B

POWER TRANSFORMER

B

R

r 

R

y

Differential Protection

B B

E / F Relay

Core Balance CT

R

b

B B

E / F Relay

EarthingTransformer 

 Aux. CT

Biased dif ferential & earth fault protection

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SLIDE 30

600/1

13.8 kV 132 kV

EF

displacement

Circuit breaker

Winding temp.

WT

Trip coilTC

CB

Restricted earth

SEF

Neutral

Standby

NDearthfault

leakage

Earth FaultREL

NDR

TR 

20/1

Trip Relay

BTBucholz trip

OvercurrentOC

TR

CB6000/20

1000/5

B.T. W.T.

  144 MVA

RRR protectiondifferential

SEF

R

R

REL

OC / EF

R

R R TC

CB

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SLIDE 31

Differential Protection with Supplementary

Earth Fault Protection

Interposing CTs

Ratio 1 / 0.577

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SLIDE 32

Typical Arrangement of Differential Protectionwith Supplementary Earth Fault Protection

Interposing CTs

Ratio 1 / 0.577

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SLIDE 33

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SLIDE 34

Differential Characterist ics

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SLIDE 35

(a)

(b)

Transformer Feeder Circuits

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SLIDE 36

Restricted earth fault

Tripping relay

 _

N O

+

+

 _

B

Y

R

B T

 _FTS

W TB G

H.V. Balanced

E/F often used

 _

Directional o/c & E/F

 _

R Y B

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SLIDE 37

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SLIDE 40

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SLIDE 41

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SLIDE 42

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SLIDE 43

11kV

1.8A

200 / 1875A

1750A

875A

R

8.8A

4.4A

0.9A

66kV

4.4A

1500 / 0.577

9090A

9090A

Y B

7.0A

3.5A

0.9A

3.5A

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r 

y

b

R Y B

r 

y

b