ABB REC670 Applications Manual page 211

1MRK 511 401-UUS A
Bay control REC670 2.2 ANSI
Application manual
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.
Tau1Low: Multiplication factor to adjust the time constant Tau1 if the current is lower
than the set value ILowTau1. ILowTau1 is set in % of IBase1.
Tau2High: Multiplication factor to adjust the time constant Tau2 if the current is higher
than the set value IHighTau2. IHighTau2 is set in % of IBase2.
Tau2Low: Multiplication factor to adjust the time constant Tau2 if the current is lower
than the set value ILowTau2. ILowTau2 is set in % of IBase2.
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. LockoutReset is set in % of the trip heat content level.
Section 8
Current protection
205