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Section 7
Current protection
IRef : Reference level of the current given in %. When the current is equal to IRef the final
(steady state) heat content is equal to 1. It is suggested to give a setting corresponding to the
rated current of the transformer winding.
IRefMult : If a binary input ENMULT is activated the reference current value can be multiplied by
the factor
from the reference value. In the standard for loading of a transformer an ambient temperature
of 20°C is used. For lower ambient temperatures the load ability is increased and vice versa.
IRefMult can be set within a range: 0.01 - 10.00.
IBase1 : Base current for setting given as percentage of IBase . This setting shall be related to
the status with no COOLING input. It is suggested to give a setting corresponding to the rated
current of the transformer with natural cooling (OA).
IBase2 : Base current for setting given as percentage of IBase . This setting shall be related to
the status with activated COOLING input. It is suggested to give a setting corresponding to
the rated current of the transformer with forced cooling (FOA). If the transformer has no
forced cooling
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
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
time constant can now be estimated as:
t =
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 value
Tau2High : Multiplication factor to adjust the time constant Tau2 if the current is higher than
the set value
172
IRefMult . The activation could be used in case of deviating ambient temperature
IBase2 can be set equal to IBase1 .
t
DQ -
DQ
ln
o
0
ot
IHighTau1 . IHighTau1 is set in % of IBase1 .
ILowTau1 . ILowTau1 is set in % of IBase1 .
IHighTau2 . IHighTau2 is set in % of IBase2 .
. Then the transformer is disconnected
o0
Tau2 and Tau1 .
1MRK 505 370-UEN D
. The thermal
ot
(Equation 38)
Busbar protection REB670
Application manual
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