ABB REG670 Applications Manual page 375

1MRK 502 071-UEN -
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
The thermal time constant should be obtained from the transformer manufacturers
manuals. The thermal time constant is dependent on the cooling and the amount of oil.
Normal time constants for medium and large transformers (according to IEC 60076-7)
are about 2.5 hours for naturally cooled transformers and 1.5 hours for forced cooled
transformers.
The time constant can be estimated from measurements of the oil temperature during
a cooling sequence (described in IEC 60076-7). It is assumed that the transformer is
operated at a certain load level with a constant oil temperature (steady state operation).
The oil temperature above the ambient temperature is DQ
disconnected from the grid (no load). After a time t of at least 30 minutes the
temperature of the oil is measured again. Now the oil temperature above the ambient
temperature is DQ
. The thermal time constant can now be estimated as:
ot
t
t =
DQ - DQ
ln ln
o 0 ot
EQUATION1180 V1 EN
If the transformer has forced cooling (FOA) the measurement should be made both
with and without the forced cooling in operation, giving Tau2 and Tau1.
The time constants can be changed if the current is higher than a set value or lower than
a set value. If the current is high it is assumed that the forced cooling is activated while
it is deactivated at low current. The setting of the parameters below enables automatic
adjustment of the time constant.
Tau1High: Multiplication factor to adjust the time constant Tau1 if the current is
higher than the set value IHighTau1. IHighTau1 is set in % of IBase1.
Tau1Low: Multiplication factor to adjust the time constant Tau1 if the current is lower
than the set value ILowTau1. ILowTau1 is set in % of IBase1.
Tau2High: Multiplication factor to adjust the time constant Tau2 if the current is
higher than the set value IHighTau2. IHighTau2 is set in % of IBase2.
Tau2Low: Multiplication factor to adjust the time constant Tau2 if the current is lower
than the set value ILowTau2. ILowTau2 is set in % of IBase2.
The possibility to change time constant with the current value as the base can be useful
in different applications. Below some examples are given:
• In case a total interruption (low current) of the protected transformer all cooling
possibilities will be inactive. This can result in a changed value of the time
constant.
• If other components (motors) are included in the thermal protection, there is a risk
of overheating of that equipment in case of very high current. The thermal time
constant is often smaller for a motor than for the transformer.
Section 9
Current protection
. Then the transformer is
o0
(Equation 210)
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