Electrical apparatus of testing of breakdown voltage of insulating oils
IEC 156 automatic oil test set
The electrical apparatus consists of the following units:
(a) Voltage regulator
(b) Step-up transformer
(c) Switching system
(d) Energy limiting devices
Two or more of these units may be integrated in any equipment system.

3.1 Voltage regulator
Uniform increase of voltage with time by manual means is difficult and, for this reason, automatic control is essential.

Voltage control may be achieved by one of the following methods:
(a) Variable ratio auto-transformer
(b) Electronic regulator
(c) Generator-field regulation
(d) Induction regulator
(e) Resistive type voltage divider

3.2 Step-up transformer
The test voltage is obtained by using a step-up transformer supplied from an a.c. (48Hz to 62Hz) voltage source whose value is gradually increased. The controls of the variable low-voltage source shall be capable of varying the test voltage smoothly, uniformly and without overshoots or transients. Incremental increases (produced, for example, by a variable auto-transformer) shall not exceed 2% of the expected breakdown voltage.

The voltage applied to the electrodes of the liquid-filled cell shall have an approximately sinusoidal waveform, such that the peak factor is within the following limits: 1,41±0,07.

The centre-point of the secondary winding of the transformer should be connected to earth.

3.3 Current-limiting resistors
To protect the equipment and to avoid excessive decomposition of the liquid at the instant of breakdown, a resistance limiting the breakdown current may be inserted in series with the test cell.

The short-circuit current of the transformer and associated circuits shall be within the range of 10mA to 25mA for all voltages higher than 15kV. This may be achieved by a combination of resistors in either or both the primary and secondary circuits of the high-voltage transformer.

3.4 Switching system
3.4.1 Basic requirements
The circuit shall be opened automatically if an established arc occurs. The primary circuit of the step-up transformer shall be fitted with a circuit-breaker operated by the current resulting from the breakdown of the sample, and shall break the voltage within 10ms. The circuit may be
opened manually if a transient spark (audible or visible) occurs between the electrodes.

NOTE
The sensitivity of the current-sensing element depends on the energy-limiting device employed and only approximate guidance can be given. Normally, triggering of cut-off by a current of 4mA maintained for 5ms is acceptable, while fast energy-limiting (see 3.4.2) triggering by a transient current of 1 A maintained for 1uS has been found satisfactory.

3.4.2 Special requirements for silicone liquids
Silicone liquids can give rise to solid decomposition products through the action of electric discharges, which may cause gross errors in the observed results. In such cases, all feasible steps shall be taken to minimize the energy available for dissipation in the breakdown discharge.

Whilst current limiting as above, combined with isolation of the step-up transformer primary within 10ms, is adequate for hydrocarbons. More satisfactory performance for silicone liquids is obtained by short circuiting of the primary circuit of the transformer by a low-impedance or by use of a low-voltage device for detection of breakdown acting within a few microseconds. This device may be of either analogue (for example, modulating amplifier) or switching (for example, thyristor) type. By the use of this device, the output voltage of the step-up transformer shall be
reduced to zero within 1ms of detection of breakdown, and shall not thereafter increase again until the next step of the test sequence is commenced.

3.5 Measuring device
For the purpose of this standard, the magnitude of the test voltage is defined as its peak value divided by square root 2.

This voltage may be measured by means of a peak-voltmeter or by means of another type of voltmeter connected to the input or output side of the testing transformer, or to a special winding provided thereon; the instrument then used shall be calibrated against a standard up to the full voltage which it is desired to measure.

A method of calibration which has been found satisfactory is the use of a transfer standard. This is an auxiliary measuring device which is connected in place of the test cell between the high-voltage terminals to which it presents the same impedance as the filled test cell. The auxiliary device is separately calibrated against a primary standard, for example, a sphere gap
in accordance with IEC 52 (see also IEC 60).