ABB Relion 670 Series Applications Manual page 186

Section 7
Current protection
GlobalBaseSel: Selects the global base value group used by the function to define IBase, VBase and
SBase. Note that this function will only use IBase value.
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
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
least 30 minutes the temperature of the oil is measured again. Now the oil temperature above the
ambient temperature is DQ
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 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.
180
. Then the transformer is disconnected from the grid (no load). After a time t of at
o0
. The thermal time constant can now be estimated as:
ot
t
DQ
ln
o 0 ot
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1MRK 505 370-UUS Rev. K
(Equation 41)
Busbar protection REB670
Application manual