Insulating oil of transformers and current transformers fulfills the purpose of insulating as well as cooling. On a regular schedule, it must be subject to an oil test to determine its breakdown voltage.
The oil test is essential and in most countries even mandatory, since the dielectric oil deteriorates in its isolating and cooling qualities due to ageing and pollution by dust particles or humidity, potentially endangering operational facilities and staff.
- Breakdown during an oil test
International standards define the test sequence and procedure of such an oil test.
Periodic execution of oil tests also makes sense from an economic point of view, as damage to the insulation can be avoided by prompt substitution of the dielectric oil.
Lifetime of plant is increased and the requirement for new investment can be delayed.
Principle of the oil test
To determine the insulating property of the dielectric oil, an oil sample is taken from the device under test, and its breakdown voltage is measured on-site.
In the vessel, two standard-compliant test electrodes with a typical clearance of 2.5 mm are surrounded by the insulating oil.
During the oil test, a test voltage is applied to the electrodes. The test voltage is continuously increased up to the breakdown voltage with a constant slew rate of e.g. 2 kV/s.
Breakdown occures in an electric arc, leading to a collapse of the test voltage.
Immediatly after ignition of the arc, the test voltage is switched off automatically.
Ultra fast switch off is crucial, as the energy that is brought into the oil and is burning it during the breakdown, must be limited to keep the additional pollution by carbonisation as low as possible.
The root mean square value of the test voltage is measured at the very instant of the breakdown and is reported as the breakdown voltage.
After the test is completed, the insulating oil is stirred automatically and the test sequence is performed repeatedly.
The resulting breakdown voltage is calculated as mean value of the individual measurements.
The lower the resulting breakdown voltage, the poorer the quality of the oil!
New oil should always be tested before it is used. Following that, the oil condition should be checked in the course of regular maintenance on at least a yearly basis;
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