ABB RET670 Applications Manual page 473

1MRK504116-UUS C
Application manual
IRefMult: If a binary input ENMULT is activated the reference current value can be
multiplied by the factor IRefMult. The activation could be used in case of deviating
ambient temperature 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 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
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
Section 3
IED application
. Then the transformer is
o0
(Equation 368)
467