ABB Relion REC670, Applications Manual

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transformer MVA rating is based on maximum allowable temperature of the insulation.
Design standards express temperature limits for transformers exceeds ambient
temperature. Use of ambient temperature as a base ensures that a transformer has
adequate thermal capacity and independent of daily environmental conditions.
Transformers in the power system are designed to withstand certain overload
conditions. The permissible transformer load level is highly depends on the
transformer cooling system. Both IEEE and IEC standards have established
transformer thermal model for all cooling system types and described formulae for
transformer temperature calculation.
Table 51: Different cooling systems used in transformers
Cooling system Description
OF (non-directed oil flow) Pumped oil from the radiators or heat exchangers
flows freely inside the tank and not forced to flow
through the windings.
ON (non-directed oil flow) Oil from the radiators or heat exchangers flows
freely inside the tank and not forced to flow through
the windings.
OD (non-directed oil flow) Part of pumped oil from the radiators or heat
exchangers is forced to flow through the windings.
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
Section 17
1MRK 511 401-UUS A
Monitoring
546 Bay control REC670 2.2 ANSI
Application manual