Strength of insulation between windings

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SOFTWARE FOR CALCULATIONOF SAFETY FACTORS OF STRENGTH

OF INSULATION BETWEEN THE WINDINGS

AT THE EDGE OF THE WINDINGS

ENDINS

Version 01

BRIEF DESCRIPTION OF THEORETICALPRINCIPLES OF CALCULATION METHOD

ENDINS_CM.00

2005


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Copyright 2002 OSC VITAlteration, duplication and distribution are prohibited.All rights reserved.

The data comprised in the document can be altered by the authors withoutpreliminary notice.

OPEN STOCK COMPANY

UKRAINIAN RESEARCH, DESIGN AND TECHNOLOGICAL TRANSFORMER INSTITUTE

OSC VIT

11 Dnepropetrovskoye shosse69069, Zaporozhye, Ukraine

Telephone: (0612) 52-64-91, 52-75-80Fax: (0612) 52-74-21, 52-81-26E-mail: [email protected]

WWW: http://www.vit.zp.ua/

DXF is trademark of Autodesk, Inc.Microsoft and Windows are registered trademarks of Microsoft Corporation

All the other names mentioned in the manual, are trademarks or registered trademarks of appropri-

ate manufacturers.


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ENDINS_CM.04

mum breakdown intensities depending on the dimension of oil duct and on kind of testvoltage are listed in Table 1.

Table 1

Oil duct l, mm E1h,

kV/mm

1m,

kV/mm

si,

kV/mm

fli,

kV/mm5 7.85 8.70 16.67 24.16

6 7.42 8.22 15.75 22.82

8 6.76 7.49 14.35 20.80

10 6.27 6.95 13.32 19.31

12 5.90 6.53 12.52 18.15

14 5.59 6.20 11.88 17.21

16 5.34 5.92 11.34 16.43

18 5.13 5.68 10.88 15.77

20 4.94 5.47 10.49 15.20

22 4.78 5.29 10.14 14.70

24 4.63 5.13 9.83 14.25

26 4.50 4.99 9.56 13.85

28 4.38 4.86 9.31 13.49

30 4.28 4.74 9.08 13.16

2.4 Investigations have shown, that the most important parameter responsible forbreakdown of the oil gap in the oil-barrier insulation is not an intensity in the point, but theaverage intensity of electric field in oil along the field line from the surface of insulation ofone part to the surface of insulation of the neighbouring insulating part:

l

dlE

E

l

av

= 0 (1),

where Eav - is average intensity of electric field in oil along the field line from the sur-face of insulation of one part to the surface of insulation of the neighbouring insulatingpart;

E - is the present intensity in a small section dl of the field line;

l - is the length along the field line from the surface of insulation of one part tothe surface of insulation of the neighbouring insulating part.

Table 1 comprises the intensities, corresponding to a probability of breakdown being2-5%.

The indicated values are true for insulation in a zone of the shielded(with capacitivering) edge between the windings. If the capacitive ring is missing (unshielded edge of thewinding) there shall be used reduced values, obtained with a factor of 0.9.


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ENDINS_CM.05

3 DESCRIPTION OF CALCULATION METHOD OF ELECTRIC STRENGTH OF

INSULATION BETWEEN THE WINDINGS AT THE EDGE OF THE WINDINGS

3.1 Analysis of the effects on insulating gap in the operating mode and at test volt-ages for all test schemes and schemes of OLTC or NLTC (maximum, minimum, nomi-nal). Determination of the maximum values of effects.

3.2 Construction of a design model of the insulating gap.As a rule, design model of the insulating gap is chosen in several stages from preliminaryto the final one. In the preliminary design model there are adopted preliminary insulationdistance and preliminary insulation structure (dimensions of oil ducts near the windings,between capacitive ring (coil) and angle washer, between angle washers, between theother barriers, with an account for the background experience with similar transformers (ifany).

In the final design model there are adopted the insulation distance between the wind-ings and the gap insulation structure, ensuring sufficient electric strength of insulation atthe maximum values of the effects, determined in item 3.1.

3.3 Calculation of electric field.

Calculation of electric field of the insulating gap shall be made with ELAX -2D programat all stages of the design model change, till obtaining the final structure of insulation.

3.4 Construction of field lines in oil ducts of the design zone.In construction of the design model in ELAX -2D program, one should point out de-

tailed construction of field lines in oil ducts of the design zone.3.5 Calculation of safety factors of strength in oil ducts for the specified field lines at

all kinds of test voltages.Calculated safety factors of strength shall be found in the specified places under the

data of the present software on the basis of the results of calculations of electric field withELAX -2D program.

Calculations shall be made by formulae:

)(

)(

1

1

1lE

lEK

hav

hf

h= (2),)(

)(

1

1

1lE

lEK

mav

mf

m= (3),)(

)(

lE

lEK

avsi

fsi

si= (4),)(

)(

lE

lEK

avfli

ffli

fli= (5),

where K1h, K1m, Ksi, Kfli - are calculated safety factors of strength of the oil duct atone-hour voltage, one-minute voltage, switching impulse voltage and full-wave lightningimpulse voltage, respectively;

Ef1h (l), Ef1m(l), Efsi(l), Effli(l) areminimum breakdown intensities of oil duct on thelength of field line I at one-hour voltage, one-minute voltage, switching impulse voltage

and full-wave lightning impulse voltage, respectively;Eav1h, Eav1m, Eavsi, Eavfli - are calculated effecting intensities in oil duct on the length of

field line I at one-hour voltage, one-minute voltage, switching impulse voltage and full-wave lightning impulse voltage, respectively.

Electric strength of insulation of the construction being calculated is considered suffi-cient if the calculated safety factors are equal to or exceed 1.15.

In addition to calculation of safety factors of oil ducts there shall be checked the valueof the highest intensity in the point, which under the action of one-minute test voltageshall not exceed 10 kVeff./mm, as well as the highest intensity along the surfaces of insu-lating parts (in a design zone), which under the action of full-wave lightning impulse shallnot exceed 15 kVmax/mm on the length of field line 2-4 mm.

3.6 The results of calculations are true for the transformers, in which parameters ofinsulation measured direct prior to the tests meet the requirements given below.


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ENDINS_CM.06

3.6.1 Characteristics of transformer oil, taken from transformer tank before dielectrictests, should have the values not worse than the ones below:

- U bd 70 kV;

- t at 90 0.5% max;- Moisture content, % of mass 0.001;

- Gas content, % of volume 1.0;- Mechanical impurities quantity of particles with size over 5m should not ex-

ceed 2000 pieces in a volume of 100 ml.3.6.2 Insulating parts of the windings, as well as parts of the main insulation shall be

manufactured of materials free from internal partial discharges after technological treat-ment of insulation.

Strength of insulation between windings

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