ABB REG316 4 Series Operating Instructions Manual page 260

ABB Switzerland Ltd
E. Setting instructions
Settings:
Initial temperature rise
Temperature rise for alarm
Temperature rise for tripping
No. of phase currents measured NrOfPhases
Reference current
Thermal time constant
The overtemperature function guards against inadmissible tem-
perature rise caused by overcurrents. The temperature rise is
modelled on the basis of the influence of the current flowing
through the protected unit on a thermal image of the protected
unit. In contrast to the overload protection, this function can pro-
tect units of any power rating and thermal capacity. It monitors
the temperature rise and not the absolute temperature. It takes
account therefore neither of the ambient temperature nor the
effectiveness of a cooling system.
The protection operates with a thermal image of the temperature
rise. A current change causes the temperature of the protected
unit to rise from an initial value to a final value according to one
or several exponential functions. The various influences on the
temperature rise are the thermal response of, for example in the
case of a power transformer, the cooling water, the oil, the
windings etc. One exponential function such as that of the
transformer oil is always more dominant than the others. The
thermal image used in the protection for modelling the transient
temperature rise operates according to an exponential function.
The excursion of the temperature rise modelled by the protection
is determined by the following:
· the final steady-state temperature corresponding to the cur-
rent
· the increased temperature rise due to the transfer functions.
The protection assumes that at the rated current I
tected unit, the temperature rise represents 100 %. Neglecting
any compensation of the A/D channel or the base current I
protection measures a current I
of the c.t's:
I
N2
=
I I
R
GN
I
N 1
3-222
REG 316*4 1MRB520049-Uen / Rev. F
Theta-Begin
Theta-Warn
Theta-Trip
IB-Setting
TimeConstant
determined by the rated current
R
of the pro-
GN
, the
B