ABB REC670 Applications Manual page 210

Section 8
Current protection
204
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. 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-US
1MRK 511 401-UUS A
. Then the transformer is
o0
(Equation 61)
Bay control REC670 2.2 ANSI
Application manual