Application - ABB RELION REX640 Technical Manual

1MRS759142 C
7.2.6
REX640
Technical Manual

Application

The insulation used in transformer windings is paper oil insulation that can degrade
over a period of time. The deterioration of insulation is a time function of temperature,
moisture and oxygen content. With modern oil preservation systems, the moisture and
oxygen contributions to insulation deterioration can be minimized, leaving insulation
temperature as the controlling parameter.
Since, in most of the transformers the temperature distribution is not uniform, the part
that is operating at the highest (hot-spot) temperature normally undergoes the greatest
deterioration. Hot-spot temperature of winding needs to be monitored continuously in
order to not exceed the flashover value of transformer insulation oil. The first step in
monitoring the insulation age is the calculation of hot-spot temperature of the
transformer.
A simplified representation of the variation of temperature inside the transformer is
shown in the Figure 697
GUID-55603C93-725A-4405-8D79-983C9F4354F5 V1 EN
Figure 697: 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
Condition monitoring functions
(IEC 60076-7).
Section 7
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