ABB REC670 Applications Manual page 513

1MRK 511 401-UEN A
Bay control REC670 2.2 IEC
Application manual
Table 51: 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:
• 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 215 shows the complex transformer temperature distribution. The
assumptions made are:
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
507