OTS PB and OTS AF series of insulating oil test sets

Check the gap between the electrodes and make sure the vessel is cleaned according to the standards.

Megger offers a voltage check metre that can be fitted to the instrument in place of the measuring vessel. Doing so allows you to compare the voltage shown on the check metre with that shown on the instrument display. Check meters are not sufficiently accurate to use as a calibration standard. Still, they provide an excellent way of detecting changes in instrument calibration. You should record the check meter readings each time you carry out a voltage check to identify changes quickly. If any significant change is detected, you should not use the instrument until you have returned it to Megger or an accredited service centre for servicing and recalibration.

Indicators that you need to send your OTS into Megger or to an accredited service centre for repair include your OTS not booting up or not building voltage.

This extract of a chart comparing standards shows that each standard specifies different conditions that must be met if the test results are to be accepted as valid. You can find the full chart in our ‘Guide to insulating oil dielectric breakdown testing’.

Mean is the average of the breakdown values recorded in the test sequence. For example, if the breakdown values are 33 kV, 37 kV, 32 kV, 35 kV, 38 kV, and 34 kV, the mean value would be the total of these results – 209 – divided by the number of results – 6 – which gives a mean value of 209/6 = 34.83 kV. (Note that in this example, there are six results as required by the IEC standard. The ASTM standards require either five or ten results.)

Range of breakdown voltage is referred to in the ASTM standards. For example, D877 specifies that the test sequence must be repeated if the range of breakdown voltages recorded is more than 92 % of their mean value. Two examples will make this easier to understand.

In the first example, the breakdown voltages recorded are 43, 45, 52, 40, and 38 kV. The lowest value is 40 kV and the highest is 52 kV, so the range is 12 kV. The mean of the recorded values is 43.6 kV, so the range is only 12/43.6 x 100 % = 27.5 % of the mean value. These test results are, therefore, valid.
 
In the second example, the breakdown voltages recorded are 33, 45, 52, 18, and 20 kV. The lowest value is 18 kV and the highest is 52 kV, so the range is 34 kV. The mean of the recorded values is 33.6 kV, so the range is 34/33.6 x 100 % = 101 %. This is above the 92 % limit, which means that the test must be repeated.

Standard deviation: IEC 60156, there is a graphical representation of standard deviation – otherwise known as the coefficient of variation – versus the mean breakdown voltage. Calculation of the mean has already been covered, but what about the standard deviation? IEC 60156 does not explain how to calculate this. The procedure, however, is to calculate the difference between each of the six test results and the mean value of those test results, then square each of the differences and add them together. Divide the figure obtained by 2, and then take the square root. The final answer is the standard deviation for the set of test results.

IEC 60156 states that, for the test results to be considered valid, the following procedure must be followed:

• Perform six tests
• Calculate the mean of the results
• Calculate the standard deviation (see above)
• Divide the standard deviation by the average value, noting that scatter is expected and acceptable (see the chart at the end of IEC 60156)
• If the value is acceptable, conclude testing
• If not, perform six more tests
• Repeat the calculations using all 12 results

An insulating fluid manufacturer normally quotes typical new and in-service fluid breakdown values in its data sheets. In addition, the test standards refer to oil condition standards that provide guidance about the acceptability of results.

D877 is usually only recommended to accept new oil from a supplier. However, some oil testing laboratories still recommend its use for specific in-service applications. In these cases, a breakdown voltage of 30 kV or more is usually considered acceptable, with values below 25 kV unacceptable. Values between 25 and 30 kV are considered questionable. For new oil, a minimum value of 30 kV is normally specified.

D1816 is more widely used and is accepted by the IEEE as the test method to be used for dielectric breakdown testing for the acceptance and maintenance of insulating oil. The IEEE C57.106 standard incorporates the D1816 limits – which are shown below – for new and in-service oil. Note that the values provided in this table are for mineral oil.

IEEE C57.106-2006

IEEE Guide for acceptance and maintenance of insulating oil in equipment

IEC 60156 uses acceptance values that are contained in two further standards: IEC 60296 and IEC 60422. 

IEC 60296, fluids for electrotechnical applications: Unused mineral insulating oils for transformers and switchgear. As its title indicates, this standard applies only to new, unused oil as received from the manufacturer, which must have a dielectric breakdown voltage of 30 kV or more, determined using the IEC 60156 test method. Oil that has been vacuum filtered in a laboratory must have a minimum dielectric breakdown voltage of 70 kV.

IEC 60422, mineral insulating oils in electrical equipment: Supervision and maintenance guide. This standard prescribes acceptable dielectric breakdown values for new oil (after filling but before energising) and for in-service oil. The values are:

The IEC recommends that if values are in the ‘fair’ range, testing should be performed more frequently, and that the test results should be cross checked with other testing methods. If the test results are in the ‘poor’ range, the oil must be brought back into a good state by reconditioning. This might, for example, involve filtering and drying the oil.