Gassing of insulating liquids under electrical stress and ionization

Describes two procedures each using different apparatus to measure the tendency of insulating liquids to evolve or absorb gas when subjected to electrical stress.

Das Gasen von Isolierflüssigkeiten unter elektrischer Beanspruchung und Ionisation

Gassing des isolants liquides sous contrainte électrique et ionisation

Décrit deux méthodes, utilisant chacune des appareillages différents pour évaluer la tendance des isolants liquides à émettre ou absorber des gaz lorsqu'ils sont soumis à une contrainte électrique.

Gassing of insulating liquids under electrical stress and ionization (IEC 60628:1985)

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SIST HD 488 S1:1999
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SIST HD 488 S1:1999
Gassing of insulating liquids under electrical stress and ionization (IEC
Gassing of insulating liquids under electrical stress and ionization

Das Gasen von Isolierflüssigkeiten unter elektrischer Beanspruchung und Ionisation

Gassing des isolants liquides sous contrainte électrique et ionisation
Ta slovenski standard je istoveten z: HD 488 S1:1987
29.040.10 Izolacijska olja Insulating oils
SIST HD 488 S1:1999 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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Deuxième édition
Second edition
Gassing des isolants liquides
sous contrainte électrique
et ionisation
Gassing of insulating liquids
under electrical stress and ionization
© IEC 1985 Droits de reproduction réservés — Copyright - all rights reserved

Aucune partie de cette publication ne peut être reproduite ni No part of this publication may be reproduced or utilized in

utilisée sous quelque forme que ce soit et par aucun any form or by any means, electronic or mechanical,

procédé, électronique ou mécanique, y compris la photo- including photocopying and microfilm, without permission in

copie et les microfilms, sans l'accord écrit de l'éditeur. writing from the publisher.

International Electrotechnical Commission 3, rue de Varembé Geneva, Switzerland

Telefax: +41 22 919 0300 e-mail: inmail(cr^ IEC web site http: //

Commission Electrotechnique Internationale
International Electrotechnical Commission
IEC MensAyH apomeR 3neKTpoTexHH4ecKaR I4OMHCCHfl
Pour prix, voir catalogue en vigueur
For price, see current catalogue
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Publication 628 de la CEI IEC Publication 628
(Deuxième édition - 1985) (Second edition - 1985)
Gassing des isolants liquides sous Gassing of insulating liquids under
contrainte électrique et ionisation electrical stress and ionization
Correction du texte anglais seulement.
Correction of the English text only.
Page 17
Page 17
au lieu de: instead of:
13.1.1 Glass cell precision bore (see Figure 4) made of borosilicate glass
tubing with permittivity of 5 + 0.2 (at 50 Hz and 80 °C) and dimensions
as follows:
13.1.1 Glass cell precision bore (see Figure 4) made of borosilicate glass
tubing with a relative permittivity of 5 + 0.2 (at 50 Hz and 80 °C) and
dimensions as follows:
Octobre 1986 October 1986
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628 ©I E C 1985 — 3 —
1. Scope 7
2. General notes on the methods 7
3. Outline of method 9
4. Apparatus 9
5. Reagents 11
6. Preparation of the apparatus 11
7. Procedure 13
8. Calculation of the results 15
9. Number of tests 15
10. Report 15
11. Precision 15
12. Outline of method 17
13. Apparatus 17
14. Reagents 21
15. Preparation of apparatus 21
16. Procedure 23
17. Calculation of the results 25
18. Number of tests 25
19. Report 25
20. Precision 27
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628 © I E C 1985 — 5 —

I) The formal decisions or agreements of the I E C on technical matters, prepared by Technical Committees on which all

the National Committees having a special interest therein are represented, express, as nearly as possible, an international

consensus of opinion on the subjects dealt with.

2) They have the form of recommendations for international use and they are accepted by the National Committees in that


3) In order to promote international unification, the I E C expresses the wish that all National Committees should adopt

the text of the I E C recommendation for their national rules in so far as national conditions will permit. Any divergence

between the I E C recommendation and the corresponding national rules should, as far as possible, be clearly indicated

in the latter.

This standard has been prepared by Sub-Committee 10A: Hydrocarbon Insulating Oils, of I E C

Technical Committee No. 10: Fluids for Electrotechnical Applications.
This publication is the second edition of I E C Publication 628.
The text of this standard is based on the following documents:
Six Months' Rule Report on Voting
10A(CO)53 10A(CO)60

Further information can be found in the Report on Voting indicated in the table above.

Other publications quoted:
ISO Standard 653 (1980): Long Solid-stem Thermometers for Precision Use.

Heat-treated Steels, Alloy Steels and Free-cutting Steels — Part XIII: Wrought Stainless

ISO Standard 683/13 (1974):
ISO Standard 4803 (1978): Laboratory Glassware — Borosilicate Glass Tubing.
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628 © I E C 1985 — 7 —
1. Scope

This standard describes two procedures each using different apparatus to measure the

tendency of insulating liquids to evolve or absorb gas when subjected, in cells having specific

geometries, to electrical stress of sufficient intensity to cause an electric discharge through a

gas phase in which a gas-oil interface is located.

The methods described in this standard are suitable for purchase specifications, general

selection of insulating liquids, product development and quality assurance.

WARNING! Attention is called to national regulations associated with the use of high

voltage, hydrogen and solvents.
2. General notes on the methods

2.1 These methods indicate whether insulating liquids are gas absorbing or gas evolving under the

test conditions. The gassing behaviour of any one insulating liquid is primarily a function of

its chemistry but changes in certain test parameters can modify the results significantly.

2.2 These methods can operate under a variety of gas phase, temperature and voltage stress

conditions. In order to establish uniform criteria of measurement, specific test conditions are

specified which experience has shown to be most informative of the general performance

expected from the liquid dielectric in electrical equipment should ionization occur.

At present, however, though it is generally agreed that gas absorbency of the impregnant

has a positive effect in minimizing ionization problems in impregnated insulation systems used

at high electrical stress, correlation of gassing-cell test results with equipment performance is

limited. Engineering judgement is necessary in interpreting the test results in relation to any

intended application.

2.3 Both methods, have been originally designed for the range of gassing rates characteristic of

mineral insulating liquids. The use of these methods with other liquids may require some

adaptations in the dimensions of the test cell.
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628 © I E C 1985 — 9 —
Outline of method

This method determines the gassing tendency of an insulating liquid under a hydrogen

atmosphere and expresses the results in terms of gassing rate over a relatively short test period.

After being dried and saturated with hydrogen gas, the insulating liquid and the hydrogen

pocket above the liquid are subjected in the specified cell to a radial electrical stress under the

following experimental conditions:
— voltage: 10 kV;
— frequency: 50 Hz or 60 Hz;
— temperature: 80 °C;
— test duration: 120 min at 50 Hz or 100 min at 60 Hz.

The rate of evolution or absorption of gas resulting from reactions at the gas-oil interface,

is calculated as volume per unit of time from changes in pressure with time.
4.1 Gassing-cell and gas-burette assembly

The gassing-cell illustrated in Figure 1, page 28, with dimensions given in Figure 2, page 29,

consists of the following components:

— Cell made of borosilicate glass with a relative permittivity of 5 ± 0.2 at 80 °C measured

at a stated frequency (50 Hz or 60 Hz). The part under stress is constructed of 16 ± 0.2

mm inside diameter and 18 ± 0.2 mm outside diameter precision selected lightwall tubing

according to ISO Standard 4803. This cell has an outer electrode (earth) 60 mm high made

of solvent-resistant silver paint with a vertical slit for observing the oil level and a copper

band for earth connection.

— Hollow high-voltage electrode made of 10 ± 0.1 mm outside diameter centreless-ground

and polished stainless steel seamless tubing No. 11 according to ISO Standard 683/XI11

and containing a 1.0 mm stainless steel capillary tubing as a gas passage.

The electrode shall be supported and centred by a precision machined 24/29 recessed

polytetrafluoroethylene plug.
A 3.0 mm needle valve (E) with gas inlet is on top of the electrode.

Note. — After repeated tests at 80 °C, the shape of the polytetrafluoroethylene plug should be checked because it may

deform and no longer be leak-tight.

Gas burette (Figure 1) made of 7 mm outside diameter borosilicate glass tubing with an

etched scale (mm), tapered glass joint 10/19 (G) for connecting to the gassing-cell, a

by-pass stopcock (D) and three glass bulbs (A, B and C). The correlation between the

reading (mm) and the volume (mm ) must be known.

Note. — Increased capacity of gas-burette is required for highly gas absorbing liquids.

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628 © I E C 1985 — 11 —
4.2 Heating device

A transparent oil bath, preferably filled with silicone liquid, with thermostatic control and

liquid circulating system to maintain the bath medium at
80±0.5 °C. The bath may be
equipped with suitable supports for holding the gassing-cell and gas burette.

Note. — If the level of the oil filling drops below a defined minimum, the high voltage should be disconnected

automatically by safety switches. The bath may be provided with an effective circulating cooling system to

allow rapid cooling after the test.
4.3 Transparent safety shield

Fitted with safety electrical interlock switches to protect the operator from parts under high

4.4 High-voltage transformer

The transformer and its controlling equipment shall be of such size and design that, with

a filled gassing-cell in the circuit, the peak factor (ratio of peak value to r.m.s. value) of the

test voltage shall not differ by more than ±5% from that of a sinusoidal wave while

maintaining 10 kV ± 2%.
4.5 Thermometer
Any convenient thermometer for measuring a temperature of 80 ±
0.1 °C (e.g. ISO Stan-
dard 653 - STL/0.1/60/85).
4.6 Syringe
A convenient glass syringe, volume 10 cm3.
5. Reagents
Hydrogen with oxygen content less than 10 mm 3/dm 3
and water content less than 2 mm3/dm3
from a cylinder with two-stage pressure reducer and a fine flow regulator.
5.2 Dibutyl phthalate, technical grade.
5.3 1,1,1-trichloroethane, technical grade.
5.4 n-heptane, analytical grade.
Silicone vacuum grease.
6. Preparation of the apparatus
General remark:

As the gassing tendency of liquids may be strongly influenced by solvents, it is important

that no traces of solvent remain after the cleaning procedure.
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628 © I E C 1985 — 13 —

6.1 Clean the glass cell by first rinsing it inside and outside with 1,1,1-trichloroethane then with

n-heptane. Then, refill the cell with n-heptane and scrub with a stiff brush of polyamide fibres

to remove deposits from previous test.

Insert a smaller brush into the tapered joint (G) and scrub out silicone grease, taking care

that none of the grease enters the cell. Again rinse with n-heptane and blow dry with clean

compressed air.
Check the painted-on silver electrode, and touch up if necessary.

6.2 Clean the hollow electrode by blowing out the capillary tube with clean compressed air, rinsing

the oil off the entire electrode with 1,1,1-trichloroethane and wiping off any deposit with tissue


Polish the surface of the stainless steel shaft of the electrode with a suitable device, such as

a buffing wheel; wipe off the buffing compound carefully with tissue paper moistened with

1,1,1-trichloroethane. Rinse again first with 1,1,1-trichloroethane, then with n-heptane. Blow

dry with clean compressed air and complete drying in an oven at 80 °C.

6.3 Apply a light coat of silicone vacuum grease to the stopcock (D) and the standard tapered joint

(G) and assemble the glass cell and burette, but do not insert the electrode into the glass cell.

6.4 Fill the burette to the half-full mark with dibutyl phthalate.
6.5 Clean the syringe with n- heptane then blow dry with compressed air.
7. Procedure

7.1 Filter about 10 cm 3 of the oil sample through a previously dried filter paper and rapidly

introduce 5 ± 0.1 cm 3 of the filtered oil into the glass cell by means of the hypodermic syringe.

7.2 Lightly coat the polytetrafluoroethylene plug of the electrode with the test liquid (to act as a

gas-seal) and insert the electrode into the glass cell.

7.3 Check the bath temperature, which shall be maintained at 80±0.5 °C during the test.

7.4 Suspend the gassing-cell and gas burette assembly in the oil bath at the level indicated in

Figure 1, page 28, and connect the lead from the outside electrode to earth.

7.5 Attach the gas inlet and outlet connections. The gas outlet should lead outside the building,

either directly or through a fume hood.

7.6 Close the stopcock (D) and open the valve (E) to allow the saturating gas to bubble through

the test oil and the burette liquid at a steady rate of 3 dm 3/h for 60 min.

7.7 Open the stopcock (D) and continue bubbling the saturating gas through the test oil for an

additional 5 min.

7.8 After a total of 65 min of gas bubbling, first close the valve (E) and then the stopcock (D),

making certain the liquid levels in the two legs of the burette are equal.
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628 © I E C 1985 — 15 —
7.9 Connect the high-voltage lead to the centre electrode.

7.10 Place the transparent safety shield in position and take the burette reading after checking the

bath temperature.
7.11 Turn on the high-voltage and adjust to 10 kV.

7.12 Record the time and the burette level and check the observation slit on the outer electrode

for onset of the gassing reaction.
7.13 After 10 min, record the burette level.

7.14 After an additional 120 min (if 50 Hz) or 100 min (if 60 Hz) again record the burette level

and then turn off the high-voltage.
8. Calculation of the results
Calculate the gassing tendency in the presence of hydrogen as follows:
B10) KI t
G = (B 130 (or 110)—
G = gassing tendency, in cubic millimetres per minute
= burette reading, in millimetres, at 130 (or 110) min of test
B 10^ 130iao i io>
B 10 = burette reading, in millimetres, at 10 min of test
K = burette constant = cubic millimetres per millimetre burette reading
t = test time of computed gassing rate
t (min) = 130-10 = 120 min if 50 Hz
t (min) = 110-10 = 100 min if 60 Hz
Value of G will be positive if gas is evolved and negative if gas is absorbed.
9. Number of tests
Tests should be run in duplicate.
The report shall include the following:
— I E C Publication 628 — Method A;
gassing tendency (mm 3/min), mean value of duplicate tests;
— test voltage;
— test voltage frequency (50 Hz or 60 Hz);
— test temperature;
— test duration;
— gas phase.
11. Precision

The results should be considered suspect if they differ by more than 0.3 + 0.26 iGj (where IGI

is the absolute value of the average of the duplicate results in cubic millimetres per minute).

Results on oils which are about neutral, will not achieve this repeatability.
Note. —

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