ABB Relion 670 Series Applications Manual page 174

Section 7
Current protection
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.
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
ILowTau1 . ILowTau1 is set in % of IBase1 .
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
ILowTau2 . ILowTau2 is set in % of IBase2 .
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.
LockoutReset : 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.
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.
set in % of the trip heat content level.
168
t
DQ
ln
o 0 ot
IHighTau1 . IHighTau1 is set in % of IBase1 .
IHighTau2 . IHighTau2 is set in % of IBase2 .
Tau2 and Tau1 .
LockoutReset is set in % of the trip heat
1MRK 511 358-UUS A
(Equation 61)
ThetaInit: is
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