Some transformer oil tests can be carried out in the field, using portable test apparatus. Other tests, such as dissolved gas, normally require a sample to be sent to a laboratory. Electronic on-line dissolved gas detectors can be connected to important or distressed transformers to continually monitor gas generation trends.
To determine the insulating property of the dielectric oil, an oil sample is taken from the device under test, and its is measured on-site according the following test sequence:
In the vessel, two standard-compliant test electrodes with a typical clearance of 2.5mm are surrounded by the insulating oil.
During the 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. 2kV/s.
Breakdown occurs in an electric arc, leading to a collapse of the test voltage.
Immediately 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.