ABB Relion 670 Series Applications Manual page 479

1MRK 511 401-UUS F
Table 52:
Cooling system
OF (non-directed oil flow)
ON (non-directed oil flow)
OD (non-directed oil flow)
In addition to types of cooling system used, size of the transformer also determines transformer
loading beyond the nameplate rating. Leakage flux density, short circuit force and high electric
stress on the insulation increases once size of the transformer increases. Hence, determination of
hot spot temperature becomes more complex. Therefore, large transformers are more vulnerable
than the smaller ones. Consequences of transformer failures are more severe for larger sizes than
for smaller ones.
As per IEC 60076 guidelines, reasonable risk degree for the expected duties are categorized into
three types:
• Distribution transformers: Only hot spot temperature in the windings and thermal
deterioration are considered.
• Medium power transformers: Hot spot temperature in the windings, thermal deterioration
and variations in the cooling modes are considered.
• Large power transformer: Hot spot temperature in the windings, thermal deterioration,
variations in the cooling modes and effects of stray leakage flux are considered.
Conductors on top of the winding experience the maximum leakage field and the highest
transformer oil temperature. It would be natural to consider that conductors at the top have the
hottest spot. However, measurements have shown that the hottest spot might be moved to
conductors in the lower part of the winding. Therefore, direct hot spot temperature measurement
is difficult. Hence, it should be calculated using the empirical formulae given by relevant
standards. The hot spot temperature shall be monitored continuously so that it will not exceed the
transformer oil flashover value.
Figure 220
• Oil temperature increases linearly from bottom to top irrespective of the cooling system.
• Winding temperature rise is parallel to the oil temperature rise with constant difference '
(average winding to average oil temperature gradient).
• The hot spot temperature rise is higher than the top winding temperature rise with the factor
called Hot Spot Factor (
Bay control REC670
Application manual
Different cooling systems used in transformers
shows the complex transformer temperature distribution. The assumptions made are:
H ).
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Description
Pumped oil from the radiators or heat exchangers flows
freely inside the tank and not forced to flow through
the windings.
Oil from the radiators or heat exchangers flows freely
inside the tank and not forced to flow through the
windings.
Part of pumped oil from the radiators or heat
exchangers is forced to flow through the windings.
Section 17
Monitoring
g '
473