Voltage Dependence of Dissipation Factor Measurements on High Voltage Instrument Transformers

Dissipation Factor is defined as a routine test in virtually every ANSI/IEEE standard applicable to oil filled High Voltage Instrument Transformers (C57.13, C57.13.5, etc.). These standards, recommend performing Dissipation Factor tests at rated voltage and 10 kV rms, both values at rated frequency. There is a clause which states the 10 kV is optional as the values are useful as a reference for commissioning. For years, the industry has performed the Dissipation Factor at the 10 kV value. This is not a problem when performing the tests at the factory as the test set is a permanent fixture in the test laboratory. Performing the test onsite is a different story. Performing Dissipation Factor tests onsite required bringing the test set, normally a 10 kV power supply and measurement set, to the site and running power to operate the system. Recently various manufactures of high voltage testing equipment have introduced new portable Dissipation Factor test sets which operate at 2.5 kV rms. Most of these sets are battery powered which make them ideal to use for onsite commissioning tests as well as routine yearly evaluation tests. The question which arises from performing Dissipation Factor at 2.5 kV verses 10 kV is, what are the differences in Dissipation Factor values at the different voltage levels? The standards require a maximum Dissipation Factor of 0.5%. To test this, a variety of High Voltage Instrument Transformers from different manufactures were tested at both 10 kV and 2.5 kV. The style of transformers tested included line to line and line to ground voltage transformers, current transformers and station service voltage transformers. Dissipation Factor tests were performed on each transformer using the same 10 kV and 2.5 kV Dissipation Factor test set. This paper presents the procedures and results to provide a comprehensive look at the differences and similarities of performing Dissipation Factor tests on a wide variety of high Voltage Instrument Transformers at both 10 kV and 2.5 kV.