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.
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.
ResLo
is set in % of the trip
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.
7.5.3.1 Setting example
GUID-37AB7D3E-9380-49A4-A47D-067E3E5DC914 v1
Calculation of the operate time with the available current is performed only if the calculated
final temperature is greater than the operate level temperature.
ln
final operate
operate
final n
t
t
æ ö
Q - Q
= - ×
ç ÷
ç ÷
Q - Q
è ø
EQUATION1176 V1 EN-US (Equation 39)
where:
t
operate
is the time to operate
t
is the time constant
is the steady state heat content
I is the largest phase load current
Table continues on next page
1MRK 505 370-UEN D Section 7
Current protection
Busbar protection REB670 173
Application manual
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