Tettex MIDAS 2881, Operating Instructions Manual

Page: 130 / 164

120 Applications Guide
The decision about the applied test voltage is in most cases easy since the tested equipment is
generally rated above 15 kV. In case of equipment rated 15 kV or lower, consideration should be
given to include testing at slightly above (10..25%) the operating line-to-ground voltage.
IEEE C57.12.90 recommends that, for insulation dissipation factor tests, the voltage should not
exceed one-half the low-frequency test voltage given in IEEE C57.12.00. The lowest low-frequency
test voltage given in C57.12.00 is 10 kV which correspond to a nominal system voltage of 1.2 kV.
Therefore, in accordance with IEEE, an insulation dissipation factor test voltage of 5 kV could be
applied to 1.2 kV transformer.
The following sections try to illustrate three typical applications of testing the insulation properties of
transformers. First an ordinary two winding transformer is presented, then an autotransformer is
visualized and finally a three winding transformer is explained.