Tettex MIDAS 2881 Operating Instructions Manual Download Page 134

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124

Applications Guide

Measuring Data Interpretation

If available the dissipation factor and the capacitances should be compared with factory data, with
previous test results and with test results on similar units.

Capacitance is a function of winding geometry, and is expected to be stable with temperature and
age. A change of capacitance is an indication of winding movement or distortion such as might occur
as a result of a through fault. Such a fault affects mainly the C

and C

insulations.

Increased dissipation factor values normally indicate some general condition such as contaminated
oil. An increase in both dissipation factor and capacitance indicates that contamination is likely to be
water.

Modem oil-filled power transformers should have insulation power factors of 0.5% or less at 20°C.
There should be a justification by the manufacturer for higher values, and assurance that they are not
the result of incomplete drying. Older power and distribution transformers may have power factors
higher than 0.5%.

Abnormal power factors are occasionally recorded for inter-winding insulations of two-winding
transformers. These may be the result of improper (high-resistance) grounding of the transformer
tank, or the use of grounded electrostatic shielding between transformer windings. In this case, as a
result of the ground shield, the inter-winding capacitance is practically non-existent except for stray
capacitances between bushing leads.

Although the bushings are included in C

, C

, the effect of a single bushing on the measuring value

may be small, depending upon the relative capacitance of the bushing and the overall C

, C

component. It is possible that a defective bushing may go undetected in an overall test because of the
masking effect of the winding capacitance. It is imperative that separate tests should be performed on
all transformer bushings.

The Transformer windings must remain short-circuited for all bushing tests and all bushings
connected to deenergized windings shall be connect to the V-point (if not done by the test
mode).

Bushings with potential or dissipation factor taps may be tested separately. See also section “Test
Procedure Bushings”.

Note:

For regular comparison measurements normal UST- mode measurements should be

sufficiently accurate and are reproducible when measured directly afterwards or after a longer period
of time.

To obtain the exact dissipation factor of the capacitance C1 obtained by a UST- measurement on a
transformer, an additional calculation should be made to correct for the dissipation factor error.

This dissipation factor error is caused by the physical phenomenon, that
there is always the influence of one or more parallel capacitances C2 to
R

shunt

(e.g. C

or C

on a two winding transformer), which can not be

excluded during an UST- mode measurement.

The dissipation factor error caused by the influence of the always parallel
capacitance C2 to R

shunt

(e.g. C

or C

), is in most cases negligible.

Only in the rare cases that the R

shunt

will switch to the highest value of

200

Ω

and the parallel capacitance (C2) to the R

shunt

is relatively high,

the error becomes significant. The R

shunt will

switch to the highest value

of 200

Ω

, when the current measured through the R

shunt

is smaller than

3mA. This will only occur when the capacitance C1 to be measured has a
very low value (e.g. 50~100pF) or when the test voltage applied to the
capacitance to be measured C1 is very low.

Therefore the following formula may be used to calculate the dissipation factor error :

Dissipation factor error = R

shunt

* C2.

The exact dissipation factor will be the dissipation factor as displayed by MIDAS subtracted by the
dissipation factor error :

Tan



(exact) = Tan



(Midas288X) - R

shunt

* C2.