Tettex MIDAS 2881, Operating Instructions Manual

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Applications Guide 119
The losses recorded should be less than 100mW. If the current or watts-loss is appreciably higher
than normal, then a second test is made after moving the collar down one petticoat. This procedure
can be followed as far down the bushing as necessary to determine how far down the fault has
progressed.
Variable Frequency Test (DFR)
The measurement of high tan delta (power frequency) at low frequencies could be an indication of
moisture in paper. In addition tan delta value increasing rapidly along frequency can be an indication
of other problems, for example contact issues.
Frequency response measurement for a new oil impregnated paper bushing
14.2 Transformers
14.2.1 Power and Distribution Transformers
The dissipation factor test for distribution transformers (rated ≤ 500kVA) and power transformers
(rated > 500kVA) is a very comprehensive test for detecting moisture, carbonization, and other forms
of contamination of windings, bushings, and liquid insulations.
Power and distribution transformers exist as single-phase or three-phase design. For insulation
purposes transformers can be further classified as dry type which have air or gas as insulation and
cooling medium, or as liquid-filled constructions which have mineral oil, Askarel
®
or other synthetic
materials.
The scope of the dissipation factor test for transformers is to determine the capacitance (insulation)
between the individual windings and between the windings and ground.
To eliminate any effect of winding inductance on the insulation measurements all terminals of each
winding, including neutrals, must be connected together. Check also for possible arrester elements in
the tap changer.
Before any measurement is performed the transformer must be deenergized and completely
isolated from the power system. The transformer housing must be properly grounded.
References and standards for the dissipation factor tests can be found in:
IEC60076-1 (2000) clause 10.1.3 “Measurement of the dissipation factor of the insulation”
IEEE Std C57.12.90-1999 clause 10.10 “Insulation power-factor tests”
Test Levels