Dielectric Breakdown Voltage Testing - Part 7Technical Briefs
Dielectric Breakdown Voltage Testing Part 7 – IEC 60156 for New and In-service Mineral Insulating Oil
In Parts 1-6 of our series, we discussed testing insulating oils for dielectric breakdown voltage. Aside from a brief description of the IEC method, we have so far provided information on just the two ASTM standard methods, D877 and D1816. In the final installment regarding this topic, we are going to discuss the IEC method from Standard 60156, including values used in IEC standards for unused and in-service mineral oil. Let’s start with a brief review of the differences in how this method is run.
Standard 60156 uses electrodes that are similar geometrically to the VDE electrodes used in ASTM D1816. The spherical electrodes are spaced 2.5 mm apart, and the rate of voltage increase is 2,000 volts per second. The method in Standard 60156 allows the optional use of an impeller, operating in similar fashion to the one described for D1816, except that it operates at 250 to 300 rpm. The IEC method also allows use of a magnetic stirrer operating at a similar rate, if there is no significant chance that magnetic particles will be removed from the oil. The presence of magnetic particles would affect dielectric breakdown in the transformer, so removal of those particles by the stirrer during the analysis would yield unrepresentative values.
Values for dielectric breakdown voltage obtained on purified and installed oil using the IEC 60156 method are typically considerably higher than those obtained using the two ASTM methods. Frequently, for properly maintained transformers, the breakdown voltage from the test may exceed the maximum reportable by the instrument. Many dielectric meters report a maximum voltage of 60 kV. This is generally sufficient for evaluating new oil from a supplier or in-service oil. For oil being processed for installation, or if a more precise value is required for in-service insulating oil, use of a dielectric breakdown voltage instrument that measures to higher values is advisable.
Conditions that affect the IEC method dielectric breakdown voltage determination are essentially the same as those that affect the D1816 dielectric measurement. The test is sensitive to moisture, oxidation of the oil, and contamination of the oil by non-dielectric materials and suspended particles. Dissolved gas content of the oil may also affect the results obtained by this method, as is the case with D1816, but the depression of the dielectric breakdown voltage values is not as significant for the IEC method.
IEC lists 60156 values in two Standards. The international industry specification for unused oil is IEC 60296, Fluids for electrotechnical applications – Unused mineral insulating oils for transformers and switchgear. In that Standard, the specification calls for unused oil, as received in bulk from the supplier, to have a minimum dielectric breakdown voltage value of 30 kV, as determined by 60156. This Standard also indicates that the oil received from the supplier must measure a minimum value of 70 kV after vacuum filtration in the laboratory. This 2003 Standard has been revised. The new Standard is IEC 60296:2012. Please reference the 2012 revision for current values.
Dielectric breakdown voltage is also addressed in IEC 60422, Mineral Insulating Oils In Electrical Equipment – Supervision and Maintenance Guide. For new oil in new equipment after filling but prior to energizing, the Standard lists acceptable values for dielectric breakdown voltage as indicated in this table:
IEC 60422 also lists Good, Fair, and Poor values for oil in transformers that are in service. These values for dielectric breakdown voltage are indicated in this table:
For in-service transformers, IEC 60422 includes recommendations for continuing normal monitoring if 60156 values are Good according to the table above. If values are Fair, more frequent sampling and cross checking with other tests is recommended. Those other tests include moisture content, particle count distribution, dissipation factor or resistivity, and acid number. If values are Poor, the Standard recommends that the oil be reconditioned to return it to good condition. This 2005 Standard has been revised. The new Standard is IEC 60422:2013. Please reference the 2013 revision for current values.
As is the case with other methods of determining dielectric breakdown voltage, the test is sensitive to the temperature of the oil being tested. In order to obtain reproducible and valid results, the test should not be run on hot oil out of an oil processor or in-service equipment.