setting corresponding to the rated 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
o0
. Then the transformer is 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
ot
. The thermal time constant can
now be estimated as:
EQUATION1180 V1 EN-US (Equation 33)
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:
1MRK 504 169-UEN A Section 7
Current protection
Transformer protection RET650 2.2 IEC 135
Application manual
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