ABB RELION 670 Series Applications Manual page 182

Section 7
Current protection
Operation : Sets the mode of operation. Disabled switches off the complete function.
GlobalBaseSel : Selects the global base value group used by the function to define (IBase), (UBase)
and (SBase).
IRef : Reference level of the current given in % of IBase . 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 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.
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
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.
176
. Then the transformer is disconnected from the grid (no load). After
o0
t
DQ
ln
o 0 ot
. The thermal time constant can now be
ot
Tau2 and Tau1 .
1MRK 505 337-UUS A
IRefMult can be
(Equation 41)
Application manual