The three-phase current measuring protection has an I
2
t characteristic with settable
time constant and a thermal memory..
An alarm pickup gives early warning to allow operators to take action well before the
line is tripped.
7.7.2 Principle of operation
The sampled analog phase currents are pre-processed and for each phase current the
RMS value is derived. These phase current values are fed to the thermal overload
protection, one time constant function LPTTR. (26).
From the largest of the three-phase currents a final temperature is calculated according
to the expression:
2
final ref
ref
I
T
I
æ ö
Q = ×
ç ÷
ç ÷
è ø
EQUATION1167 V1 EN (Equation 92)
where:
I is the largest phase current,
I
ref
is a given reference current and
T
ref
is steady state temperature rise corresponding to I
ref
The ambient temperature is added to the calculated final temperature. If this
temperature is larger than the set operate temperature level, TripTemp, a PICKUP
output signal is activated.
The actual temperature at the actual execution cycle is calculated as:
Section 7 1MRK505222-UUS C
Current protection
450
Technical reference manual
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