Phân tích đáp ứng tần số trong thí nghiệm Máy Biến Áp

6th International Conference on Large Power
Transformers- Modern Trends

Benefits of SFRA - Case Studies

B B Ahir

Gujarat Energy Transmission Corporation Limited

1


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
Transformer
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
3


Why SFRA ?
 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

4

Case study (1):

Location : 220 kV Kapadwanj S/s
Rating : 220/66kV, 100 MVA
Date : 22.01.2012

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

Initially SFRA along with Low Voltage Test and DGA
5


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

DGA Results ( Post Tripping )
H2

O2

N2

CH4

C2H2

C2H4

C2H6

CO

CO2

119

3657

14212

11

36

27

1

169

570

Nature of incipient fault

Arcing

Major Key
Gas

Minor Key
Gas

C2H2 , H2

CH4, C2H4

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


Case study (1):
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
7


Case study (1):
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.
8


Case study (1):
Analysis


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.

9


Case study (1): Internal Inspection
 LV R phase connecting leads of preselector switch of OLTC and its fixing
assembly burnt.

Burned connecting leads






Burnt link and fixing assembly replaced
Re-insulation of leads
Drying process with oil filtration
Take SFRA and other supporting test

10


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

HV R

Phase

All phases are identical after rectification of problem

11


Case study (1):
LV – N at Tap No.17 ( Normal – After rectification)

LV R
Phase

All phases are identical after rectification of problem

12


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):
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.

14


Case study (2):
Location
Rating
Date

: 220kV Khanpur Substation
: 220/66kV , 100 MVA
: 21.04.08

High key gases in routine DGA test
Low voltage test -

√

15

Case study (2):
DGA Results ( on 22.12.07 , Normal )
H2

O2

N2

CH4

C2H2

C2H4

C2H6

CO

CO2

7

3405

12150

21

Nil

48

15

34

292

DGA Results ( on 21.04.08 , Abnormal )
H2

O2

N2

CH4

C2H2

C2H4

C2H6

CO

CO2

355

2988

13928

472

<1

538

214

57

1085

Nature of incipient fault

Major Key
Gas

Thermal Fault - Over heated oil

CH4, C2H4

Minor Key
Gas
C 2H 6 , H 2

DGA indicates thermal fault as per high key gases method

16


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


Case study (2): Low voltage Test

SHORT CIRCUIT CURRENT ( Post fault - Normal )

CURRENT
MEASURED

HV WINDING

LV WINDING

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


Case study (2):
HV - N ( Post fault - Abnormal)

20Hz – 10KHz – Core
deformation/open ckt./
Shorted turns / residual
magnetism

HV B Phase

B phase curve differ in core area in compare with other two phases which shows core
related issue
19


Case study (2):
HV - N ( LV Shorted ) ( Post fault - Abnormal )

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

20


Case study (2):
LV – N ( Post fault - Abnormal)

20Hz – 10KHz – Core
deformation/open ckt./
Shorted turns / residual
magnetism

LV B Phase
B phase curve differ in core dominated area, which shows problem in core area
21


Case study (2):
Analysis
 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.

22


Case study (2): Schematic Diagram

23


Case study (2): Analysis
 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.

24


Case study (2):
HV - N ( Normal – After rectification)

After attending the problem all phase curves found identical
25