IS 2026 ( Part 6 ) : 1994

Indian Standard

POWERTRANSFORMERS PART 5 TRANSFORMER/REACTOR BUSHINGS - MINIMUM EXTERNAL

CLEARANCE IN AIR - SPECIFICATION

( Second Reprint OCTOBER 1997 )

UDC 621*315*626*042*4 : 621*314.212+621-318.43

Q BIS 1994

BUREAU OF INDIAN STANDARDS MANAK BHAVAN, 9 BAHADUR SljAH ZAFAR MARG

NEW DELHI 110002

May 1994 Price Group 4

Transformers Sectional Committee, ETD 16

FOREWORD

This Ir,dian Standard was adopted by the Bureau of Indian Standards, after the draft finalized by the Transformers Sectional Committee had been approved by the Electrotechnical Division Council.

In the preparation of this standard considerable assistance has been derived from IEC Pub 76-3-l ( 1987 ) ‘Power Transformer : Part 3 lnsulation levels and dielectric tests External clearances in air’ issued by the International Electrotechninal Commission ( IEC ).

For the purpose of deciding whether a particular requirement of this standard is complied wilh, the final value, observed or calculated, expressing the result of a test or analvsis shall be rounded off in accordance with IS 2 : 1960 ‘Rules for rounding off numerical values ( revised )‘. The number of significant places retained in the rounded off value shall be the same as that of the specified value in this standard.

Indian Standard

IS 2026 ( Part 5 ) : 1994

POWERTRANSFORMERS PART 5 TRANSFORMER/REACTOR BUSHINGS-MINIMUM EXTERNAL

CLEARANCE IN AIR - SPECIFICATION

P SCOPE

This Indian Standard specifies clearances in air between live paIts of bushings on oil-immer;sed power transformers and reactors and to object at earth potential.

NOTE - Wherever the word ‘transformer’ appears in the text, read as ‘transformer/reactor’.

2 REFERENCES

The Indian Standards listed in Annex A are necessary adjuncts to this standard.

3 GENERAL

3.1 Clearances m air are understood to mean, in this standard, distances where the electrostatic field is free from disturbance by insulator bodies. The standard does not deal with the requirements of effective flashover distance or creepage distance along the bushing insulators nor does it consider the risk From intrusion of birds or animals.

3.2 The insulation levels and dielectric tests which are specified in IS 2026 ( Part 3 ) : 1981 shall apply for oil immersed transformers to the internal insulation only.

3.3 It is reasonable that the rated withstand voltage values which are specified for the inter- nal insulation of the transformer should also be taken as a reference for its external insulation. This may, however, not be true in all cases. A failure of the non-self-restoring internal insula- tion is catastrophic and normally leads to the transformer being out of service for a long period, while an external flashover may involve only a short interruption of service without causing lasting damage. Therefore, it may be that, for increased safety, higher test voltages are specified by the user for the internal insula- tion of the transformer than for the external insulation of other components in the system. If such a distinction is made, the external clearances on the transformer should be referred instead to those specified for external insulation withstand voltages.

3.4 When establishing the requirements of the present standard in the higher voltage ranges, it has been recognised that the bushing ends normally have a rounded electrode shape. The clearance requirements are valid between such

rounded electrodes. It is assumed that conduc- tor clamps with their associated shield electrodes are suitably shaped so that they do not reduce the flashover voltage. It is also assumed that the arrangement of incoming conductors does not reduce the effective clearances provided by the transformer itself.

NOTE - If the user intends to make his connection in a particular way which is likely to reduce the effective clearances, this should be mentioned in the enquiry.

3.5 In general, the provision of adequate clearances in air becomes technically difficult mainly at high system voltages, particularly for relatively small units, or when the installation space is restricted. The principle follo\bed in the recommendations of this standard is to provide ample, non-critical clearances which are satis- factory without further discussion or proof under various system conditions and in different climates. Where extensive previous experience has indicated that smaller clearances are adequate, there is obviously no need to change.

3.6 The recommended clearances are referred to the rated withstand voltage values of the internal insulation of the transformer, unless otherwise specified in the enquiry and order. When the clearances of the transformer are equal to or larger than the values specified in this standard and the bushings have properly selected ratings according to IS 2099 : 1986 then the external insulation of the transformer shall be regarded satisfactory without further testing.

NOTES

1 The impulse withstand strength of the external insulation is polarity dependent, in contract to what is assumed for the internal insulation. The tests prescribed for the internal insulation of the trans- former do not automatically verify that the external insulation is satisfactory. The recommended clearances are dimensioned for the more onerous polarity ( positive ).

2 It is recognised that in some cases’good experience with well documented national practice allows the continuing use of established smaller clearances with- out further discussion or testing.

3.7 If a clearance smaller than that according to the paragraph above has been used for a contract, a type test may be required on an arrangement simulating the actual clearance, or

1

IS 2026 ( Part 5 ) : i994

on the transformer itself. Recommended test procedures for such cases are given.

the standard voltages which appear in Table 2 of IS 2026 ( Part 3 ) : 1981.

3.8 If the transformer is specified for operation at an altitude higher than 1000 m, the clearance requirements have to be increased by 1 percent for every 100 m by which the altitude exceeds 1 000 m. Requirements are given for the follow- ing clearances:

- clearance phase-to-earth and phase-to- neutral;

- clearance *phase-to-phase between phases of the same winding; and

- clearance between a line terminal of the high voltage winding and a line terminal of a lower voltage winding.

3.9 It follows from the above that the ‘recommended’ values are in effect <minimum’ values. The design clearances shall be stated on the outline drawing. These are nominal values subject to normal manufacturing tolerances and they have to be selected so that the actual clearances will be at least equal to the specified values. These statements shall be taken as proof that the transformer complies with the recommendations of the standard, or with the modified values which may have been agreed for the particular contract.

4 BUSHING CLEARANCE REQUIREMENTS AS DETERMINED BY TRANSFORMER INSULATION WITHSTAND VOLTAGES

4.0 The requirements are formulated as descri- bed below, depending on the U, voltage value of the winding, and on the method of specifying the transformer insulation.

Indian Standards IS 2165 ( Part 1 ) : 1977 deals with phase-to-earth insulation co-ordination and IS 2165 ( Part 2) : 1983 deals with phase-to- phase insulation co-ordination, principles and rules. The above two Indian Standards make use of different reference withstand voltage for different ranges of Urn voltages. In the ranges ,4 and B ( U, ( 300 kV ) the reference voltages are power frequency voltage and lightining impulse voltage. In range C ( U, > 300 kV ) the reference voltage for external CiearanCe iS

a switching impulse voltage. For further details, see 3. The conversion from rated with- stand voltages to clearances in this standard is generally along the lines of 1’S 2165 ( Part 1 ) : 197; and IS 2165 ( Part 2 ) : 1983.

. . . 4.1 Wmdlags with U, < 300 kV

The same distance shall apply for clearances phase-to-earth, phase-to-neutral, phase-to- phase, and towards terminals of a lower Voltage winding. The recommended minimum clea- rances are given in Table 1 with reference to

If a type test on a reduced clearance is to be conducted this shall be lightning impulse test, dry with positive impulse, three shots, with the test voltage according to Table 1.

4.2 Windings with CT, 3 300 kV - Transformer Specified According to Method 1

For equipment with U, in this range, IS 2165 ( Part 2 ) : 1977 specifies different requirements for phase-to-earth and phase-to-phase insulation. Air insulation clearances are referred to switching impulse conditions. Transformers which are specified according to Method 1 in IS 2026 (Part 3 ) : 1981 have, however, no rated switching impulse withstand level.

The recommended minimum distances in Table 2 have been chosen, directly on the basis of U, values, from the selection of standard clearances appearing in Table 3. This is because it is assumed that the requirements for external insulation should be the same, irrespective of the use of Method 1 or Method 2 as described in IS 2026 ( Part 3 ) : 1981 for the tests on the internal insulation.

The phase-to-earth clearances in Table 2 also apply for clearances towards the terminals of a lower voltage winding. ( An unusual voltage ratio or connection may, however, require special consideration of the clearances. )

4.2.1 Type Test Procedure

If a type test on a reduced clearance is to be conducted this shall be carried out as described in 4.3.3. The test voltage is selected as follows:

- determine the standard clearance value;

- by reference to the appropriate curve of Fig. 1 or Fig. 2, establish the switchmg impulse voltage correspondjng to this standard clearance. This is the test voltage to be used.

4.3 Windings with [lnr > 300 kV - Transformer Specified According to Method 2

Transformers specified according to Method 2 have a rated switching impulse withstand voltage assigned to the highest voltage winding. This value, which applies bttween phase and earth, is also used as a reference for the external insulation. The internal insulation is verified b\ a test with negative test voltage on the tested phase, and with I.5 times test voltage between the phases on three-phase transformers.

For thk external insulation the phase-to-phahe withstand voltage is defined differently according

2

IS 2026 ( Part 5 ) : 1994

Table 1 Recommeded Minimum Clearances from Bushing Live Parts on Power Transformers Having Windings witb Highest Voltage for Equipment U, <’ 300 kV

( Altitude Up to 1 000 m ) ( Clause 4.1 )

Rated Lighthing Impulse Withstand Voltage ( peak )

r---- ----_h_--_--_.-_

Highest Voltage Rated Short for Equipment Duration

Power Fre- C rms ) quency With-

stand Voltage

(1) kV

< 1’1

3’6

7’2

12

17.5

24

36

52

72’5

( rms ) (2) kV

123

245

3

10

20

28

38

50

70

95

140

165

230

275

325

360

395

Distribution Other Trans- Transformers formers

(3) (4) kV kV

-

20

40

60

1.5

95

145

-

40

60

75

95

125

170

0

325 630

450 830

550 1 050

650 1 250

750 1 450

850 1 600

950 1 800

Minimum Clearance r_--_---_- h_-- _-__ __~ With Reference to Distribution Transformers

(5) mm

-

-

60

90

125

170

275 I

L 15t

Wit:oRgr$ernce

Transformers

(6) mm

-

60

90

125

170

225

315

3

Table 2 Recommended Minimum Clearances from Bushing Live Parts on Power Transformers Having Windings with Highest Voltage for Equipment U, > 300 kV, Specified

According to Method 1 ( Altitude Up to 1 000 m ) ( Clauses 4.1 and 4.2 )

Highest Voltage for EF;pment

( rms )

(1) kV

Rated Short Dura- tion Power Fre-

quency Withstand Voltage ( rms )

(2) kV .

Rated Lightn- Minimum Clearance ing Impulse r------- h-_-----~ Withstand Phase-to- Phase-to-

Voltage Earth Earth

(p3) (4) (5) kV mm mm

460

9.50

1 050

1 050

1 175

1 900

2 300

2 300

2 700

2 250

2 650

2 650

3 100

1 300

1 425

2 700

3 100

3 100

3 500

3

IS 2025 ( Part 5 ) : 1994

Table 3 Recommended Minimum Clearances from Bushing Live Parts on Power Transformers Having Windings with Highest Voltage for Equipment iJ, > 300 kV, Specified

According to Method 2 ( Altitude Up to 1 000 m ) ( Clauses 4.3, 4.3.1.3.~ and-4.3.2 )

Highest Voltage Rated Switching for Eq;i;ment Impulse Withstand

Rated Lightning

Voltage Impulse With-

(rms) ( peak ) stand Voltage

( peak 1

(1) (2) kV kV

750

300

650

362

950

420

1050

525

1175

765

(3) kV

650

950

(4) (5) mm mm

1 900 2 250

Voltage (6)

mm

1600

1 750

1050

1175

2 300 2 650 1 950

2 700 3 100 2200

1300 2400

1425

1550

1800

1950

3 100 3 5co 2 650

3 700 4 200 2 850

5 000 5 800 3 300

5 800 6700 3 600

LMinimum Clearance ~---_-----_----h ---------7 Phase-to-Earth, Phase-to-Phase, To Other Based on Swit- Based on Swit- Winding, thing Impulse thing Impulse Based on

Withstand Withstand Vojtage

Lightning Voltage Impulse

Withstand

Dm 1

5

4

3

2

1

I i i i i i i i 500 1000 1500 2ooq

6

kV

Fan. I CLEARANCE PHASE-TO-EARTH BASED ON RATED SWITCHING IMPULSE WITHSTAND VOLTAGE

4

IS 2026 ( Part 5 ) : 1994

Om

a

1000 1500 2000 2500

FIG. 2 CLEARANCE PHASE-TO-PHASE BASED ON SWITCHING IMPULSE VOLTAGE APPEARING BETWEEN PHASE

to IS 2165 ( Part 2 ) : 1983 ( see Table 4 ). An appropriate test procedure involves positive polarity impulses for a configuration phase-to- earth, and opposite polarity impulses for phase- to-phase clearances ( see 4.3.3 ). This has been considered for the clearance values given in Table 3.

4.3.1 Clearance Phase-to-Earth, Phase-to-Neutral and Phase-to-Phase Between Phases of the Same Winding

The clearance from the high voltage bushing top to earth ( tank, conservator, cooling equip- ment, switchyard structures ) or to the neutral terminal is determined from column 4 of Table 3. The clearance between bushing caps of difference phases is determined from column 5 of Table 3.

4.3.2 Clearance Between Terminals of Different Windings

The clearance between terminals of different windings of the transformer shall be checked with regard to both switching, impulse and lightning impulse conditions. The switching impulse withstand requirement is based on the precalculated difference voltage which appears

between the two terminals under consideration during the switching impulse test [ see 14 of IS 2026 ( Part 3 ) : 1981 ] according to the turns ratio between the windings. This voltage.value gives the clearance which is required with regard to the switching impulse condition. Fig. 2 is used to find the recommended clearance if the terminals receive opposite polarity voltages and the ratio between their voltage amplitudes is 2 or less. In other cases, Fig. 1 applies.

NOTE;If Fig. 1 and Fig. 2 are compared, it appenrs that a phase-to-phase clearance withstands a higher voltage difference than the same.distance would do in a phase-to-earth configuration. The reason is that in the phase-to-phase configuration the two terminals are supposed to have opposite polarity, and the maximum dielectric gradient at either of them ( which is largely determined by the voltage to earth ) is relatively lower.

It is assumed also that the electrodes have a rounded shape. The clearance shall, however, also fulfil the lightning impulse withstand requirement, which presupposes that the lower voltage winding terminal is at earth potential when rated lightning impulse withstand voltage is applied to the high voltage terminal. The dis- tance requirement in column 6 of Table 3 and Fig. 3 corresponding to this rated lightning

5

----

IS 2026 ( Part 5 ) : 1994

FIG. 3 CLEARANCE BASED ON LIGHTNING IMPULSE VOLTAGE

impulse voltage, has therefore, to be fulfilled between the two terminals. The higher of the two clearance estimates shall apply. The s\\,itching impulse test on the transformer will induce voltages between phases of other star- connected windings as well. It shall be checked wliether this condition requires a larger phase- to-phase clearance in such a winding than as prescribed for this \vinding alone such as in 4.1.

4.3.3 Type Test Procedure

If a tape test on a reduce clearance is to be conducted, the test procedure shall be as foilows:

A test on a configuration phase-to-earth or phase-to-netural, or towards a terminal of a lower voltage winding shall consist of a switching impulse test, dry, with positive polarity on the line terminal of the winding ( the higher voltage winding ). The counter electrode shall be earthed. If the tested terminal belongs to a three-phase winding, the other line terminals shall also be earthed.

NOTE - This test ih not generally feasible on complete three-phase transformers and may, thcre- for-e, have to be conducted on ;I model simulating the actual configuration of the transformer.

Tests on the phase-to-phase clearances of a three-phase transformer shall consist of switch- ing impulse tests, dry, with half of the specified

test voltage, positive, on one line terminal, the other half, negative on another line terminal, and the third terminal earth [ see 10, 12 and 14 of IS 2165 ( Part 2 ) : 1983 ] The combinations of phase-to-earth and phase-to-phase test volt- ages, according to IS 2165 ( Part 2 ) : 1983 are reproduced in Table 4. The values for U, = 525 kV and 800 kV are subject to review.

When the outer phases are placed symmetrically with respect to the middle phase, it is sufficient to make two separate tests, one with positive polarity on the middle phase, and the other with positive polarity on an outer phase, the middle phase having negative polarity. If the line terminal arrangement is asymmetrical, it may be necessary to perform more than two tests.

Each test shall consist of 15 applications of impulse voltage with wave shape 250/2 500 in accordance with IS 2071 ( Part 1 ) : 1993.

NOTE - The above test procedure for phase-to- phase external clearances, which follows IS 2165 ( Part 2 ) : 1983 differs in several’ respects from the switching impulse test procedure specified for the internal insulation of the transformer in 14 of IS 2165 ( Part 2 ) . 1983. The two test procedures do not replace each other.

6

IS 2026 ( Part 5 ) : 1994

Table 4 Combinations of Switching Impulse Test Voltages Phase-to-Earth and Phase-to-Phase for Different Values of U,. The Phase-to-Phase

Voltage Value for a Type Test According to 4.3.3 is Selected According to the Rated Switching Impulse Withstand Voltage,

Phase-to-Earth Assigned to the Transformer

( Clauses 4.3 and 4.3.3 )

Highest Voltage for Eqc:yment

zI:11 (ims 1

kV

(1)

Rated Switching Impulse Withstaud Voltage Phase-to-

Earth (peak 1

kV

(2)

Rated Switching Impulse With- stand Voltage Phase-to-Phase

( peak ) kV

(3)

1 175

1 300

1 300

1 425

I 425

1800

2 400

2 550

ANNEX A

( Clause 2 )

LIST OF REFERRED INDIAN STANDARDS

IS No. Tiile IS No.

2026 Power transformers: Part 3 ( Part 3 ) : 1981 Insulation levels and dielectric 2165

tests ( second revision ) ( Part 2 ) : 1983

2099 : 1986 Bushing for alternating voltages above 1 000 V ($rst revision )

2165 Insulation cordination:Part 1 2071 ( Part 1 ) : 1977 Phase-to-earth insulation

coordination ( second ( Part 1 ) : 1993

revision )

Title

Insulation co-ordination: Part 2 Phase-to-Dhase insula- tion co-ordination, principles and rules

High voltage test techniques; Part 1 General definition and test requirements revision )

( second

7

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This Indian Standard has been developed from Dot: No. ETD 16 ( 3486 J

Amendments Issued Since Publication

Amend No. Date of Issue Text Affected

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