IEC 60628 Measurement of gassing of insulating liquids under electrical stress and ionization
Section 2. Method A
3 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 Apparatus
4.1 Gassing-cell and gas-burette assembly
The gassing-cell illustrated in Figure 1, page 9, with dimensions given in Figure 2, page 10, 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/XIII 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 (mm3) must be known.
NOTE Increased capacity of gas-burette is required for highly gas absorbing liquids.
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 voltage.
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 Standard 653-STL/0.1/60/85).
4.6 Syringe
A convenient glass syringe, volume 10 cm3.
5 Reagents
5.1 Hydrogen with oxygen content less than 10 mm3/dm3 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.
5.5 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.
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 paper.
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 cm3 of the oil sample through a previously dried filter paper and rapidly introduce 5 +/- 0.1 cm3 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 9, 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 dm3/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.
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:
G = (B130 (or 110) - B10)K/t
where :
G = gassing tendency, in cubic millimetres per minute
B130 (or 110) = burette reading, in millimetres, at 130 (or 110) min of test
B10 = 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.
10 Report
The report shall include the following:
- IEC Publication 628-Method A;
- gassing tendency (mm3/min), mean value of duplicate tests;
- test voltage;
- test voltage frequency (50 Hz or 60 Hz);
- test temperature;
- test duration;
- gas phase.
11 Precision
Repeatability:
The results should be considered suspect if they differ by more than 0.3 + 0.26|G| (where |G| is the absolute value of the average of the duplicate results in cubic millimetres per minute).
NOTE Results on oils which are about neutral, will not achieve this repeatability.
