Transformer Protection Calculations 6v163i

Transformer Protection

SLIDE 1

SLIDE 2

SLIDE 3

Maximum fuse fuse rating maximum rating or or circuit breaker circuit breaker setting setting 125% of full 125% of full load current load current

480 240/120V

Set 2.5 I fl set ≤≤2·5I fl

set Set≤≤1·25I 1.25fl I fl

SLIDE 4

SLIDE 5

fuse rating < 2·5Ifl

SI

Relay set < 3Ifl

SLIDE 6

Differential Protection

r

r

y

y

b

b

R

Y

SLIDE 7

B

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 disconnected transformer 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 ratio and hence the voltage and current magnitudes to vary. SLIDE 8

Magnetising Inrush for a Small Transformer

SLIDE 9

Generator

Relay point 1 1 0

Relay point 2

Relay point 1

1

0

1

1

Fault current flow in transformers

SLIDE 10

External fault Stability

Internal fault Operation SLIDE 11

Internal Fault:

Operation of relay

External fault: maloperation of relay Consider and modify connections for correct operation

Current Transformer Connections and Secondary Current Flow SLIDE 12

Operating current I2 (pu) 10 Bias Setting

9 8

40% 7 6 30%

5 4 20% 3 2 1

2

4

6

8

10

Bias current I1 (pu)

SLIDE 13

12

14

16

18

20

SLIDE 14

Schematic of 1-pole of differential relay system Transformer

Bias coil

Harmonic bias coil

Operating coil

SLIDE 15

Star-Delta-Star Station Transformer

250 / 1

132 kV 30MVA 11 kV

SLIDE 16

2500 / 1

2I

3I

I

I

I

3I I

I

I

Winding arrangement & current distribution SLIDE 17

C.T. Connections

O.C. Relays

SLIDE 18

SLIDE 19

High impedance differential (Principle of operation)

A

B RACT

RLA

RLB If

RR

ZEA

RBCT

ZEB

Vf 87

IR

Saturation has the effect of lowering the excitation impedance and is deemed to have taken place in CT B until, 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 significant error IR =

(R +R ) Vf = If . LB B CT RR RR

IR can be reduced below any specified relay setting SLIDE 20

SLIDE 21

SLIDE 22

Percentage of Transformer Winding Protected Against Earth Faults

SLIDE 23

SLIDE 24

Buchholz Relay

SLIDE 25

HV bus (3)

*

connection

(3)

52-1

(1) 50N /51N

50/51 (1) 50G

49

87 (3) 51

63

(2) 51 G trips 52 - 1 86

(1) 151 G trips 52 - 2

52 - 2

**

(3) connection

Protection diagram SLIDE 26

HV CTs

R Y

LV CTs

B

Auto transformer

SLIDE 27

Bias coils

Operating coil

Three-winding transformer

SLIDE 28

POWER TRANSFORMER

Earthing Transformer

Core Balance CT

Aux. CT

E / F Relay

Differential Protection B

B

B

B B

B R

R r

R y

b

E / F Relay

Biased differential & earth fault protection

SLIDE 29

CB

144 MVA 13.8 kV 132 kV

6000/20

TR Trip Relay BT Bucholz trip OC Overcurrent EF Earth Fault REL Restricted earth leakage SEF Standby earthfault ND Neutral displacement CB Circuit breaker TC Trip coil WT Winding temp.

B.T. W.T.

CB

REL

600/1

R

R R

20/1

R 1000/5 OC / EF

R SEF R

ND

TR

R

SLIDE 30

R

R

differential protection

TC

Interposing CTs Ratio 1 / 0.577

Differential Protection with Supplementary Earth Fault Protection

SLIDE 31

Interposing CTs Ratio 1 / 0.577

Typical Arrangement of Differential Protection with Supplementary Earth Fault Protection SLIDE 32

SLIDE 33

Differential Characteristics

SLIDE 34

(a)

(b)

Transformer Feeder Circuits SLIDE 35

H.V. Balanced E/F often used RYB

R Y B BT

_

BG WT

_ Restricted earth fault

_ FTS

Directional o/c & E/F NO

+ _

+ _ Tripping relay

SLIDE 36

SLIDE 37

SLIDE 38

SLIDE 39

SLIDE 40

SLIDE 41

SLIDE 42

11kV

66kV 875A

200 / 1

1500 / 0.577 9090A

1750A 875A

9090A 4.4A

3.5A 7.0A

8.8A 4.4A

3.5A

Y

R

0.9A

1.8A

SLIDE 43

B

0.9A

r

r

y

y

b

b

R

Y

SLIDE 44

B