ABB Relion REC670 Applications Manual page 152

Section 7
Current protection
t =
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.
Tau1High : Multiplication factor to adjust the time constant Tau1 if the current is higher than
the set value
Tau1Low : Multiplication factor to adjust the time constant Tau1 if the current is lower than the
set value
Tau2High : Multiplication factor to adjust the time constant Tau2 if the current is higher than
the set value
Tau2Low : Multiplication factor to adjust the time constant Tau2 if the current is lower than the
set value
The possibility to change time constant with the current value as the base can be useful in
different applications. Below some examples are given:
• In case a total interruption (low current) of the protected transformer all cooling
possibilities will be inactive. This can result in a changed value of the time constant.
• If other components (motors) are included in the thermal protection, there is a risk of
overheating of that equipment in case of very high current. The thermal time constant is
often smaller for a motor than for the transformer.
ITrip: The steady state current that the transformer can withstand. The setting is given in % of
IBase1 or IBase2 .
Alarm1 : Heat content level for activation of the signal ALARM1. ALARM1 is set in % of the trip
heat content level.
Alarm2 : Heat content level for activation of the output signal ALARM2. ALARM2 is set in % of
the trip heat content level.
ResLo : Lockout release level of heat content to release the lockout signal. When the thermal
overload protection trips a lock-out signal is activated. This signal is intended to block
switching on of the protected circuit transformer as long as the transformer temperature is
high. The signal is released when the estimated heat content is below the set value. This
temperature value should be chosen below the alarm temperature.
heat content level.
ThetaInit : Heat content before activation of the function. This setting can be set a little below
the alarm level. If the transformer is loaded before the activation of the protection function, its
temperature can be higher than the ambient temperature. The start point given in the setting
will prevent risk of no trip at overtemperature during the first moments after activation.
ThetaInit: is set in % of the trip heat content level.
Warning : If the calculated time to trip factor is below the setting Warning a warning signal is
activated. The setting is given in minutes.
146
t
DQ - DQ
ln
o 0 ot
IHighTau1 . IHighTau1 is set in % of IBase1 .
ILowTau1 . ILowTau1 is set in % of IBase1 .
IHighTau2 . IHighTau2 is set in % of IBase2 .
ILowTau2 . ILowTau2 is set in % of IBase2 .
Tau2 and Tau1 .
ResLo is set in % of the trip
1MRK 511 358-UEN A
(Equation 61)
Application manual