Maintaining Short Circuit Strength in Transformers

8/9/2019 Maintaining Short Circuit Strength in Transformers

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A Member of the Group

Maintaining Short Circuit

Strength in TransformersSecond Annual

Weidmann-ACTI Conference

November 12, 2003

Thomas A. Prevost

EHV Weidmann Industries Inc.


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Short Circuit Withstand Capability

A key factor throughout transformer life is the ability to

withstand short circuits.

Severe radial and axial forces can damage the insulation

integrity and deform windings.

Windings are pre-loaded to a pressure at least as high as the

maximum calculated axial short circuit force.

Tight windings mitigate short circuit damage due to

movement of the conductors.


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F

F

rigid clamping distance

transformer

windingcoil

pressboard

presspapercopper

F Clamping Pressure = f(moisture,temperature,age)


4/26A Member of the Group


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copper 12 x 3, R 1 mm

Kraft paper insulation 1 mm

pressboard spacer 2 mm

25

25

40

Spacers (2 mm thick) interspersed with paper insulated

copper conductor disk segments. Apparent pressed area: 625 mm2.


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Test-devices, with model-1 (left) and model-2 (right). The

models are processed under a constant pressure of 5 N/mm2

prior to assembly in the device.


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Variation of Spacer Thickness During Processing

94

96

98

100

102

104

106

108

110

112

As-Is ( 6% m/c, 1 N/mm^2) Compressed ( 6 % m/c, 10N/mm^2)

Vacuum Dried (10 N/mm^2) Oil Impregnated (10 N/mm^2)

Sp

acerStackHeight

(mm)

Effect of coil processing on the thickness of a 100 mm stack of

Transformerboard spacer material. Material was vacuum dried for 7

days at 0.5mbar under a load of 10 N/mm2.


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Effect of Temperature At the moment the test temperature increases to 60C (450 hours), a

sudden increase in pre-load pressure was observed resulting from

dimensional expansion of the components of the test-devices, due to th

different material expansion coefficients.

Pressboard: pb 4510-6/K

Steel: st = 1010-6/K

Copper: cu = 1710-6/K

When cellulose replaced by a 30 mm high stack of silicate glass, theclamping force decreased from 1.58 to 1.04 kN at a temperature

increase from 23 to 60C.


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5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

0 2 4 6 8 10

Time (minutes)

Pressure

(N/

mm)andtemp

erat

Air temperature

Board temperature

Pressure


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6.0

6.5

7.0

7.5

8.0

8.5

.

0 2 4 6 8 10

Time (minutes)

Pressure(N/mm)an

dtemperature(C/1

Air temperature

Board temperature

Oil temperature

Pressure


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Model Deg. C Oil Deg. C Pressure

25 25 No Change

80 25 Decrease

25 80 Increase

80 80 No Change


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t im e ( h )

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0

5

4

3

2

1

0

f o r c e ( k N )m o i s t u r e (% )

8

7

6

5

4

3

2

1

0

p r e s s u r e( N /m m )

m odel-1 (spacers)

m odel-2 (spacers and condu ctor segm ents)

moisture

Clamping force and moisture content versus time for the non-oiled

models.


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!"#$%&&'($

')*+,-..

Schematic of 550 kV BIL core and coil layout.


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Clamping Pressure vs Temperature

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

-20 0 20 40 60 80 100Temperature (Deg. C)

Pressure(N/mm^2)

Variation in static clamping pressure


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Effects of Aging Over the life of a transformer the cellulose insulation will be

subjected to thermal cycles which cause aging of the cellulose

material.

We have found in recent tests that the aging of cellulose material

has an even more pronounced effect on the thickness of material

under pressure.

Pronounced dual effect of material decomposition from

degradation of the cellulose polymer (de-polymerization) and

from the settling of the material due to pressure.


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Aging of Pressboard Under Compression

88

90

92

94

96

98

100

102

0 50 100 150 200 250 300

Aging Time (Days)

SpacerSt

ackHeight(mm)

135 Deg. C

150 Deg. C

Effect of aging on the thickness of a stack of Transformerboard.


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0

1

2

3

4

5

6

7

8

9

10

11

0 1 2 3 4 5 6 7 8 9 10

Time (days)

Pressure(N/m

m)andtempera

ture(C/10)

Air temperature

Board temperature

Pressure

100C

20C

60C

design pressure 2.5 N/mm2

Model was pre-dried under constant pressure of 5 N/mm^2The daily load cycling shoes an oscillation of the clamping pressure from 2 to 4 N/mm^2


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0

1

2

3

4

5

6

7

8

9

10

11

0 1 2 3 4 5 6 7 8 9 10

Time (days)

Pressure(N/m

m)andtempera

ture(C/10)

Air temperature

Board temperature

Pressure

100C

20C

60C

design pressure 2.5 N/mm2

Model was Pre-dried with no pressure.The loss of pressure during the first load cycle is significant.


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Conclusion

Units in Manufacture Power transformers must be clamped to a pressure at least as

great as the maximum anticipated short circuit force.

Spacer material should be of pure high density cellulose withthe surfaces milled to avoid excessive compression set.

Winding spacer material should be pre-stabilized by drying

under pressure. It is critical that the insulation structure is thoroughly dried

and oil impregnated prior to applying final pre-load pressure.

The effect of oil impregnation on the decease in pre-loadpressure has been found to be caused by thermal contraction.

It is best to pre-load the transformer at 30C to 50C to

assure the windings have a proper pre-load force over theoperating range of the transformer.


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What Does this Mean to My Transformers in Service?

Effects of Moisture

Moisture is a by-product of aging of cellulose insulation.An increase in moisture will result in an increase in clamping

pressure.

An increase in moisture degrades the insulation strength of theinsulation system.

Dryout of the insulation will lower the clamping pressure.


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What Does this Mean to My Transformers in Service?

Effects of Temperature

Cellulose has a TEK three times greater than copper or steel.An increase in temperature will result in an increase in

clamping pressure.

Daily load cycles will result in oscillations in the clampingpressure.

A transformer will have a better chance to withstand a short

circuit when heavily loaded.


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Field Processing of Transformers On units with high moisture, the moisture tends to hide the

loose winding problem, until such time as the unit is

reprocessed.

Due to todays higher transformer loading practice and in

order to prolong transformer life, more frequent monitoring

and oil processing in the field is now required.

Removal of moisture improves the overall dielectric system

and extends insulation life. Unfortunately, it can have a

detrimental effect on the remaining pre-load pressure of the

transformer windings.


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Maintaining Short Circuit Strength in Transformers