ABB REG670 Applications Manual page 659

1MRK 502 071-UEN -
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
• Laminated steel core with copper or aluminium windings
• Solid refined paper insulation
• Highly refined mineral oil as insulating and cooling medium for the entire
transformer
The oil is cooled by a separate cooling system using air or water. The core, windings
and insulation have specific thermal capabilities.
Losses in the winding and core can cause temperature rises in the transformer, which
is transferred to the insulating oil. Failure to limit these temperature rises to the
thermal capability of the insulation and core materials can cause premature failure of
the transformer.
A transformer is rated at the power output. It can continuously deliver at rated voltage
and frequency without exceeding the specified temperature limit. This temperature
rise is based on thermal limitations of the core, winding and insulation. Therefore,
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 70: Different cooling systems used in transformers
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:
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
653