Benefits of SFRA - Case · PDF fileBenefits of SFRA - Case Studies 1 ... LV MAGNETIC BALANCE...

Benefits of SFRA - Case Studies

1

6th International Conference on Large Power

Transformers- Modern Trends

B B Ahir

Gujarat Energy Transmission Corporation Limited

Outline

Condition Monitoring in GETCO

Why SFRA ?

Case study (1):SFRA - How it helped to find a fault in winding

Case study (2): SFRA - How it helped to find a fault in core

Case study (3): SFRA - How it helped to find a fault in core

Conclusion

2

Condition Monitoring in GETCO

3

Technology / Tool Online Offline

Insulation Resistance and Polarisation Index √

Routine Low Voltage tests √

Capacitance & Tan δ measurements √(Bushing)

√

Infrared Thermograph √

Measurement of Moisture in Oil √ √

DGA √ √

SFRA √

Acoustic Partial Discharge √

Measurement moisture content in active parts of Transformer √

Transformer

Why SFRA ?

4

To assess mechanical integrity of a transformer

To detect core displacement and winding deformation

due to : Large electromagnetic forces from fault current

Transformer transportation and relocation

Winding shrinkage causing release of clamping structure

To detect broken or loosened clamping structure

Hoop Buckling

Shorted turns and open winding

Case study (1):

5

Location : 220 kV Kapadwanj S/s

Rating : 220/66kV, 100 MVA

Date : 22.01.2012

Initially SFRA along with Low Voltage Test and DGA

OSR relay operated due to LV side Isolator support insulator flash over

Case study (1):

LV MAGNETIC BALANCE ( Post fault – Abnormal )

APPLIED

VOLTAGE

Tap 1 Tap 17

r - n y - n b - n r-n y-n b-n

r – n 255 V 255 0 0 255 0 0

y – n 251 V 174 251 74 175 251 68

b – n 256 V 27 225 256 24 226 256

H2 O2 N2 CH4 C2H2 C2H4 C2H6 CO CO2

119 3657 14212 11 36 27 1 169 570

DGA Results ( Post Tripping )

Nature of incipient fault Major Key

Gas

Minor Key

Gas

Arcing C2H2 , H2 CH4, C2H4

Low voltage magnetic balance test found abnormal in R phase and DGA indicates arcing

Case study (1):

7

HV – N at Tap No.1 ( Post fault - Abnormal)

500Hz to 2MHz with +/- 3 dB: Tap and Main winding

HV R

Phase

HV R phase shifted -3dB with reference to other phase in main and tap winding portion

Case study (1):

8

LV – N at Tap No.17 ( Post fault - Abnormal)

500Hz to 2MHz with +/- 3 dB: Tap and Main winding

LV R

Phase

20Hz – 10KHz – Core

deformation/open ckt./

Shorted turns / residual

magnetism

LV R phase shifted widely with reference to other phase in main and tap winding portion.

It seems like open circuit.

Case study (1):

9

SFRA LV R phase found abnormal – defect in

tapped winding ( 500Hz to 2 MHz) and open

winding ( 5Hz to 100kHz ) with +/- 3 dB

variation.

Problem reflected in low voltage magnetic

balance and DGA ( Arcing ) also.

Based on above abnormal results, decided to

internal inspection of this transformer.

Analysis

Case study (1): Internal Inspection

10

LV R phase connecting leads of preselector switch of OLTC and its fixing

assembly burnt.

Burnt link and fixing assembly replaced

Re-insulation of leads

Drying process with oil filtration

Take SFRA and other supporting test

Burned connecting leads

Case study (1):

11

HV – N at Tap No.1 ( Normal – After rectification )

HV R

Phase

All phases are identical after rectification of problem

Case study (1):

12

LV – N at Tap No.17 ( Normal – After rectification)

LV R

Phase

All phases are identical after rectification of problem

Case study (1):

LV MAGNETIC BALANCE ( Normal – After rectification )

APPLIED

VOLTAGE

Tap 1 Tap 17

r - n y - n b - n r-n y-n b-n

r – n 255 V 255 224 30 255 225 25

y – n 252 V 176 252 76 175 252 69

b – n 255 V 28 226 255 26 226 255

Low voltage test found normal after rectification

Case study (1):

14

Cause of failure

The heavy fault current due to flash over of LV side

isolator support insulator caused R phase pre-selector

switch LV connection two nos. of leads between

position 3 & 12 to burn.

SFRA plays a role as a supporting test to confirm the

fault with other test.

Case study (2):

15

Location : 220kV Khanpur Substation

Rating : 220/66kV , 100 MVA

Date : 21.04.08

High key gases in routine DGA test

Low voltage test - √

Case study (2):

16

DGA Results ( on 22.12.07 , Normal )


7 3405 12150 21 Nil 48 15 34 292

Nature of incipient fault Major Key

Gas

Minor Key

Gas

Thermal Fault - Over heated oil CH4, C2H4 C2H6 , H2


355 2988 13928 472 <1 538 214 57 1085

DGA Results ( on 21.04.08 , Abnormal )

DGA indicates thermal fault as per high key gases method

Case study (2): Low voltage Test

MAGNETIZING CURRENT ( Post fault - Normal)

CURRENT

MEASURED

HV WINDING LV WINDING

R 3.2 mA 29.3 mA

Y 2.6 mA 26.3 mA

B 4.1 mA 35.2 mA

MAGNETIC BALANCE ( Post fault - Normal )

APPLIED

VOLTAGE R - N Y - N B - N r-n y-n b-n

R - N 255 V 255 193 56 87 65 18

Y - N 252 V 170 252 82 59 87 28

B - N 256 V 58 190 256 16 69 87

Low voltage test found normal after fault


SHORT CIRCUIT CURRENT ( Post fault - Normal )

CURRENT

MEASURED


R 3.54 A 10.38 A

Y 3.55 A 10.18 A

B 3.48 A 10.41 A

Low voltage test found normal after fault

19

HV - N ( Post fault - Abnormal)

Case study (2):




magnetism

HV B Phase

B phase curve differ in core area in compare with other two phases which shows core

related issue

Case study (2):

20

HV - N ( LV Shorted ) ( Post fault - Abnormal )

HV B Phase

LV short curve nullify effect of core, winding part seems identical and normal

Case study (2):

21

LV – N ( Post fault - Abnormal)

LV B Phase




magnetism

B phase curve differ in core dominated area, which shows problem in core area

Case study (2):

22

SFRA showed abnormality in the core.

IR between core , frame and earth was measured.

Core to Frame : 2000MΩ

Frame to Earth : 8000MΩ

Core to Earth : 0.03MΩ

As the Core to Earth insulation found quite low, internal

inspection carried out but nothing seen abnormal.

Further investigation was done by inserting a GI wire

between core and bottom of the tank and along the core

nut found under the B phase core limb which was

touching core and bottom.

Analysis

Case study (2): Schematic Diagram

23

Case study (2): Analysis

24

After removing the nut, again IR between core , frame

and earth was measured.

Core to Frame : 2000MΩ

Frame to Earth : 8000MΩ

Core to Earth : 2000MΩ

Normally core earthing provided on top of the tank. The

nut was providing another earthing and it caused thermal

fault due to circulating current between core and earth.

25

HV - N ( Normal – After rectification)

Case study (2):

After attending the problem all phase curves found identical

Case study (2):

26

HV - N ( LV Shorted ) ( Normal – After rectification )


Case study (2):

27

LV – N ( Normal – After rectification)


Case study (2):

28

Conclusion

A metallic nut below the core at bottom of the tank was

found out with the help of SFRA which created parallel

path to earth and led to thermal fault.

Such kind of unbelievable fault can be identified by

SFRA.

In this case SFRA played a vital role as a supporting tool

with DGA to detect the faulty area and to reach up to the

root cause of the fault.

Case study (3):

29

Location : 66kV Chhala Substation

Rating : 66/11.55kV , 10 MVA

Date : 01.09.08

Tripped on differential relay due to failure of 66kV class

Y phase bushing with reflected of 11kV feeder


MAGNETIZING CURRENT ( Post fault - Abnormal )

CURRENT

MEASURED


R 1.8 mA 117.5 mA

Y 13.5 mA 133 mA

B 1.8 mA 135 mA

30

MAGNETIC BALANCE ( Post fault - Abnormal )

APPLIED

VOLTAGE R - Y Y - B B - R r-n y-n b-n

R – Y 433 V 433 388 176 41 37 16

Y – B 431 V 230 431 296 21 40 38

B - R 437 V 309 296 437 19 38 41

Low voltage test found abnormal after fault

31

HV PH – PH ( Post fault - Abnormal )

Case study (3):




magnetism

Y-BR-Y B-R

All three phases response are differ in core related area as mentioned frequency

32

HV PH – PH ( LV Shorted ) ( Post fault – Normal )

Case study (3):

In LV short condition core area nullify and winding area curves are identical in all

phases

33

LV PH – N ( Post fault - Abnormal )

Case study (3):




magnetism

R Y B

All three phases response are differ in core related area as mentioned frequency

Case study (2): Analysis

34

SFRA and low voltage revealed problem inside core

area.

Internal inspection carried out by removing top plate of

transformer.

2 Nos. of bolts clamping the top channel with tank were

loose and top yoke channel got lifted upwards by about

25mm which caused a little core deformation.

Case study (2): Internal Inspection

35 After attending the problem, SFRA was taken.

Yoke

Bolt

Yoke

Channel

36

HV PH – PH ( Normal – After rectification )

Case study (3):

All three phases response are identical in core dominated area after rectification of

fault

37

HV PH – PH ( LV Shorted ) ( Normal – After rectification )

Case study (3):

This response was identical from initially as problem was in core area after rectification

of fault

38

LV PH – N ( Normal – After rectification )

Case study (3):

All three phases response are identical now in core area after rectification of fault


MAGNETIC BALANCE ( Normal – After rectification)

APPLIED

VOLTAGE R - Y Y - B B - R r-n y-n b-n

R – Y 442 V 442 365 79 41 35 6

Y – B 450 V 256 450 189 23 42 17

B - R 445 V 44 396 445 4 37 41

39

MAGNETIZING CURRENT ( Normal – After rectification)

CURRENT

MEASURED


R 2.4 mA 56.2 mA

Y 1.9 mA 28.5 mA

B 2.2 mA 55.6 mA

Low voltage test found normal after rectification of fault

Case study (3):

40

Cause of Failure

Heavy reflected fault caused axial and radial forces

exerted on transformer active part.

Core structure got disturbed and it was reflected in SFRA

plot.

Conclusion

41

SFRA responses clearly indicate faulty conditions of transformer and it

helped for root cause analysis.

SFRA helps to detect fault by just compare with different phase with

different task without having a initial results of same transformer or

same design transformer.

SFRA plays vital role as supporting test for final conclusion as its

deliver diversity of information in only one test.

SFRA is very useful tool to detect heavy through fault current leads to

mechanical deformation of core or winding, internal connections and

contacts inside transformer.

GETCO has adopted such diagnosis tool while fault as well as in

routine practice while shifting of transformer.

Thanks………

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