ABB REB670 - Page 219

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
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:
0
ln ln
o ot
t
t =
DQ - DQ
EQUATION1180 V1 EN-US (Equation 38)
If the transformer has forced cooling (FOA) the measurement should be made both
with and without the forced cooling in operation, giving T
au2
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.
1MRK 505 370-UUS A Section 7
Current protection
Busbar protection REB670 2.2 ANSI 213
Application manual