ABB Relion 650 Series Applications Manual page 93

1MRK 505 287-UUS B
current of the transformer with forced cooling (FOA). If the transformer has no forced cooling
IBase2 can be set equal to IBase1 .
Tau1 : The thermal time constant of the protected transformer, related to IBase1 (no cooling) given
in minutes.
Tau2 : The thermal time constant of the protected transformer, related to IBase2 (with cooling)
given in minutes.
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
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
estimated as:
t =
DQ -
ln
EQUATION1180 V1 EN-US
If the transformer has forced cooling (FOA) the measurement should be made both with and
without the forced cooling in operation, giving
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
Tau1Low : Multiplication factor to adjust the time constant Tau1 if the current is lower than the set
ILowTau1 . ILowTau1 is set in % of IBase1 .
value
Tau2High : Multiplication factor to adjust the time constant Tau2 if the current is higher than the
set value
Tau2Low : Multiplication factor to adjust the time constant Tau2 if the current is lower than the set
ILowTau2 . ILowTau2 is set in % of IBase2 .
value
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.
Busbar protection REB650
Application manual
. Then the transformer is disconnected from the grid (no load). After
o0
t
DQ
ln
o 0 ot
IHighTau1 . IHighTau1 is set in % of IBase1 .
IHighTau2 . IHighTau2 is set in % of IBase2 .
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. The thermal time constant can now be
ot
Tau2 and Tau1 .
Section 7
Current protection
(Equation 16)
87