IEC 60156:2018 pdf download

IEC 60156:2018 pdf download.Insulating liquids – Determination of the breakdown voltage at power frequency – Test method
1 Scope
This document specifies the method for determining the dielectric breakdown voltage of insulating liquids at power frequency. The test procedure is performed in a specified apparatus, where the oil sample is subjected to an increasing AC electrical field until breakdown occurs. The method applies to all types of insulating liquids of nominal viscosity up to 350 mm 2 /s at 40 °C. It is appropriate both for acceptance testing on unused liquids at the time of their delivery and for establishing the condition of samples taken in monitoring and maintenance of equipment.
4 Electrical apparatus
4.1 General The electrical apparatus consists of the following units: 1 ) voltage regulator, 2) step-up transformer, 3) switching system, 4) current-limiting resistors, 5) measuring device. Two or more of these units may be integrated in any equipment system. 4.2 Voltage regulator The test voltage shall be increased with an automatic control of the required uniform voltage rate of rise. The device should not introduce harmonics disturbances (< 3%) and the AC source should be free from harmonics.4.3 Step-up transformer The test voltage is obtained by using a step-up or resonant transformer supplied from an AC source using 48 Hz to 62 Hz (sinusoidal waveform). The voltage source value is constantly 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 or an amplifier) shall not exceed 2 % of the expected breakdown voltage. The centre-point of the secondary winding of the transformer should be connected to earth. 4.4 Switching system The circuit shall be opened automatically if a sustained arc between the electrodes occurs and the voltage between the electrodes collapses to a voltage less than 500 V. The primary circuit of the step-up transformer shall be fitted with a circuit-breaker operated by the current sensing device, resulting from the breakdown of the sample and shall break the voltage within 1 0 ms. The sensitivity of the current or voltage sensing element depends on the energy-limiting device employed and only approximate guidance can be given. A cut-off time of < 1 00 µs, as given in the previous edition of this document, is needed to perform multiple breakdowns on silicone liquids. 4.5 Current-limiting resistors To protect the equipment and to avoid excessive decomposition at the instant of breakdown of liquids such as silicone or ester liquids, a resistance limiting the breakdown current shall be inserted in series with the test cell. The short-circuit current of the transformer and associated circuits shall be within the range of 1 0 mA to 25 mA for all voltages higher than 1 5 kV. This may be achieved by a combination of resistors in either or both the primary and secondary circuits of the high-voltage transformer.The output voltage of the step-up transformer may be measured by means of a measuring system consisting of a voltage divider or a measuring winding of the step-up transformer coupled with a peak-voltmeter. The measuring system shall be calibrated up to the upper scale voltage to be measured. 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 an impedance similar to the one of the sample liquid. The auxiliary device is separately calibrated against a primary standard [2],[3].