8 Chitapon TR Winding Buckling

Experience on detecting and locating a winding buckling in power transformer

Case studies

By Mr.Chitapon Jedwanna [email protected]

Transformer Maintenance and Workshop Department Transmission System Maintenance Division

Electricity Generating Authority of Thailand (EGAT)

1

Content

Introduction to EGAT Principle of winding buckling in power

transformer Case studies of the winding buckling in

power transformer in 2012 Conclusion

2

Introduction to EGAT

3

About EGAT

Electricity Generating Authority of Thailand (EGAT) is a state enterprise under the Ministry of Energy for generating, providing, transmissioning electrical power in Thailand

4

EGAT Generating capacity

Generating capacity in Thailand (July,2013)

EGAT 45% ,

15,010.13 MW

IPP & SPP 48% ,

15,906.29 MW

Overseas 7% ,

2,404.60 MW

Total generating capacity is 33,321.02 MW 5

EGAT Transmission system

Nominal voltage Number of HV Substation Transformer (MVA rating) Transmission line (Circuit-km)500 kV 11 20,849.99 4,094.86230 kV 71 50,960.04 14,116.24115 kV 130 14,617.49 13,785.43132 kV 133.40 8.7169 kV - 18.80

300 kV HVDC 388.02 23.07Total 212 86,948.94 32,047.10

(August,2013)

There are 622 units and 86,948.94 MVA rating of power transformers in the transmission system.

6

Principle of winding buckling in power transformer

7

Stress acting on the power transformer

Dielectric stresses Such as system overvoltage ,Transient impulse

conditions. Thermal stresses Such as overload ,malfunction of cooling equipment. Mechanical stresses Such as inrush current ,External short circuit

conditions.

8

External short circuit

During the external short circuit, the windings of the power transformer are subjected to both radial and axial forces due to the short circuit current.

9

Effect of radial forces on the windings

Fig. shows the tangential stress caused by radial forces in core-type power transformer.

Inward radial force => Compressive stress

Outward radial force => Tensile stress

10

Effect of radial forces on the windings Radial forces result in : Buckling for inner windings Increased radius for outer windings Spiraling of end turns in helical windings

Inner winding Outer winding 11

Effect of radial forces on the windings

Buckling Spiraling 12

Winding buckling

The presence of buckled segment , which changes the coupling of the leakage flux to the winding and a geometric of the winding , results in the change of ;

Leakage reactance. Winding capacitance. Frequency response of

winding.

13

Case studies of the winding buckling in power transformer

in 2012 Case 1. LV winding (X2-X0) buckling of power transformer, 50 MVA ,115-22 kV, Vector group Dyn1, Year 1990 Case 2. LV winding (X2-X0) buckling of power transformer, 50 MVA, 115-22 kV, Vector group Dyn1, Year 1994

14

50 MVA and 115 kV Power transformer

Fig. shows the power transformer ,50MVA , 115-22 kV ,Vector group Dyn1 which found LV winding buckling (x2-x0) of both cases.

15

50 MVA and 115 kV Power transformer

Fig. shows the sequence from core to outward of winding in the 50 MVA power transformer of both cases study.

16

On-site testing : Single phase leakage impedance

Case 1 : Energized WDG. H1-H2 /

Shorted WDG.X2-X0


Shorted WDG.X2-X0

Tap First test ()

Latest test () (21 May,2012)

% Deviation

7L 124.330 128.711 3.52% 2R 89.950 92.892 3.27% 10R 67.400 69.870 3.66%

Tap First test ()

Latest test () (17 September, 2012)

% Deviation

7L 123.410 126.590 2.58% 2R 89.780 92.205 2.70% 10R 66.620 68.815 3.29%

For both cases , the single phase leakage impedance (Energized WDG.H1-H2/Shorted WDG.X2-X0) was found different from the first test value.

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Fig. shows the testing circuit for single phase leakage impedance of Dyn1 power transformer

Energized WDG. H1-H2 / Shorted WDG.X2-X0

Testing current : 2 A

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Shorted WDG. X3-X0


Shorted WDG. X3-X0

Tap First test ()


% Deviation

7L 123.290 123.156 -0.11% 2R 89.350 89.290 -0.07% 10R 66.620 66.630 0.01%

Tap First test ()


% Deviation

7L 123.320 122.940 -0.31% 2R 89.135 89.225 0.10% 10R 66.470 66.300 -0.26%

For both cases ,the single phase leakage impedance (Energized WDG.H2-H3/Shorted WDG.X3-X0) was found normal.

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Shorted WDG. X1-X0


Shorted WDG. X1-X0

Tap First test ()


% Deviation

7L 123.580 123.228 -0.28% 2R 89.400 89.760 0.40% 10R 66.770 66.725 -0.06%

Tap First test ()


% Deviation

7L 123.160 122.330 -0.67% 2R 89.650 88.840 -0.90% 10R 66.315 66.060 -0.38%

For both cases ,the single phase leakage impedance (Energized WDG.H3-H1/Shorted WDG.X1-X0) was found normal.

20

On-site testing : Winding capacitance (pF)

Case 1 :

Position First test Latest test (21 May,2012)

%Deviation

CHL+CH 10305.0 10234.6 -0.68% CH 3280.2 3269.4 -0.33% CHL+CL 18089.5 18993.1 5.00% CL 11049.7 12031.6 8.89%

Case 2 :

Position First test Last test (17september,2012.)

%Deviation

CHL+CH 10311.0 10208.0 -1% CH 3287.4 3282.7 -0.14% CHL+CL 18082.0 18978.4 4.96% CL 11058.0 12052.6 8.99%

From capacitance value, it indicates that the LV winding of the transformer possibly buckle because capacitance value of CL, CHL+CL deviated from first test, but CH was still normal.

21

On-site testing : Winding capacitance (pF)

Fig. shows a connection to measure the winding capacitance. (Testing voltage = 10 kV)

22

Case 1 : the buckling of LV winding (X2-X0) of case study 1

23


24


25

On-site testing : Frequency Response Analysis (FRA) for Case study 1

26

=

=

in

out

in

out

VVH

VVdBH

1

10

tan)(

log20)(

Fig. shows a transfer function (H) of frequency response analysis (FRA).


The FRA result in phase degree of LV winding X2-X0 (Red curve) was different from other LV windings in frequency range of 100 kHz-1000 kHz 27


The FRA result in dB of LV winding X2-X0 (Red curve) was different from other LV windings in frequency range of 100 kHz-1000 kHz 28


-180

-120

-60

0

60

120

180

10 100 1000 10000 100000 1000000

Frequency (Hz)

Phas

e (d

egre

e)

EN X2 - X0 Open H1-H2-H3 EN X2 - X0 Open H1-H2-H3

The FRA result in phase degree of LV winding X2-X0 (Red curve) was different from the FRA result in same winding of the sister unit (Black curve) in frequency range of 100 kHz-1000 kHz .

Failed unit Sister unit (Normal)

29


Frequency response analysis

-120

-100

-80

-60

-40

-20

0

10 100 1000 10000 100000 1000000

Frequency (Hz)

Am

plitu

de (d

B)

The FRA result in dB of LV winding X2-X0 (Red curve) was different from the FRA result in same winding of the sister unit (Black curve) in frequency range of 100 kHz-1000 kHz .

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Conclusion

31

Conclusion

According to two case studies mentioned earlier, it can be concluded that the single phase leakage impedance, winding capacitance measurement and frequency response analysis (FRA) can be used to detect and locate the winding buckling in the power transformer.

32

Conclusion

In addition, EGAT has established the recommended criteria value and maintenance policy for all transformers with the same rating manufactured by particular manufacturer in order to prevent the transformer breakdown. Such criteria value and maintenance policy can be explained as follows,

If the transformer is subjected to the external fault in the

system, the single phase leakage impedance and winding capacitance have to be measured to check the winding condition.

33

Conclusion

If the single phase leakage impedance is deviated more than 2% and the value of winding capacitance increase comparing with those of the commissioning test or first test , the transformer have to be de-energized and the winding buckling may be confirmed by using frequency response analysis (FRA).

A neutral grounding reactor (NGR) shall be installed to prevent the winding deformation which is basically caused by external ground fault, particularly the transformers manufactured before 1996.This is because it is found that the winding design was not good enough to withstand the external fault condition.

34

Literature

[1] Giorgio Bertagnolli , Short-circuit duty of power transformer, ABB handbook.

[2] EGAT testing standard , Testing standard for Power transformer , 2012.

[3] Thomas Fogelberg , Surviving a short-circuit : Short-circuit withstand capability of power transformers ,2008

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Thank you for your attention.

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Experience on detecting and locating a winding buckling in power transformer Case studiesContentIntroduction to EGATAbout EGATEGAT Generating capacity EGAT Transmission systemPrinciple of winding buckling in power transformerStress acting on the power transformerExternal short circuit Effect of radial forces on the windingsEffect of radial forces on the windingsEffect of radial forces on the windingsWinding bucklingCase studies of the winding buckling in power transformer in 201250 MVA and 115 kV Power transformer50 MVA and 115 kV Power transformerOn-site testing : Single phase leakage impedanceOn-site testing : Single phase leakage impedanceOn-site testing : Single phase leakage impedanceOn-site testing : Single phase leakage impedanceOn-site testing : Winding capacitance (pF)On-site testing : Winding capacitance (pF)Case 1 : the buckling of LV winding (X2-X0) of case study 1 Case 2 : the buckling of LV winding (X2-X0) of case study 2 Case 2 : the buckling of LV winding (X2-X0) of case study 2 On-site testing : Frequency Response Analysis (FRA) for Case study 1On-site testing : Frequency Response Analysis (FRA) for Case study 1On-site testing : Frequency Response Analysis (FRA) for Case study 1On-site testing : Frequency Response Analysis (FRA) for Case study 1On-site testing : Frequency Response Analysis (FRA) for Case study 1ConclusionConclusionConclusionConclusionLiteratureThank you for your attention.

8 Chitapon TR Winding Buckling

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