ABB RELION REX640 Technical Manual page 1392

Condition monitoring functions
Figure 790: Transformer thermal diagram
The winding hot-spot temperature depends on the load losses, type of cooling
and ambient temperature. Usually the conductors near the top of the winding
experience the maximum leakage field and the highest surrounding oil temperature
(due to convection effect). It is natural to consider that the top conductors contain
the hottest spot. Measurements have shown that the hottest spot can also be
moved to lower conductors.
Direct measurement of hot-spot temperature with fiber optic probes became
available in mid 1980s. Experience has shown that there might be gradients of
more than 10 °C between different locations at the top of a transformer winding.
It is unlikely that the insertion of one to three sensors detect the real hot-spot.
Direct measurement of hot-spot temperature is difficult and it is calculated using
the empirical formulae.
The hot-spot temperature calculation is dependent upon the thermal properties
of the winding material, cooling oil and the cooling mode of the transformer. The
thermal model may also differ from IEC and IEEE markets, though the underlying
principle of temperature calculation remains the same. The transformer constants
are decided based on a user defined setting to select the preferred type of thermal
model (IEC, IEEE or Manual).
For most transformers in service, the top oil temperature inside a winding is
difficult to measure. On the other hand, the top oil temperature at the top of the
tank is well known, either by measurement or by calculation. The winding hot-spot
temperature rise above the top oil temperature is calculated. By adding this value to
the measured or calculated top oil temperature, the winding hot-spot temperature
is obtained. A pictorial representation of the location of top oil temperature sensor
is shown in
1392
Figure 791
.
1MRS759142 F
REX640
Technical Manual