6.6.7 Operation principle
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 LFPTTR/LCPTTR (26) function. The temperature is displayed either in
Celsius or Fahrenheit, depending on whether LFPTTR/LCPTTR (26) function is selected.
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 16)
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
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:
( )
1 1
1
t
n n final n
e
t
D
-
- -
æ ö
Q = Q + Q - Q × -
ç ÷
è ø
EQUATION1168 V1 EN
(Equation 17)
where:
Q
n
is the calculated present temperature,
Q
n-1
is the calculated temperature at the previous time step,
Q
final
is the calculated final temperature with the actual current,
Dt
is the time step between calculation of the actual temperature and
t
is the set thermal time constant for the protected device (line or cable)
The calculated component temperature is available as a real figure signal, TEMP.
1MRK 511 287-UUS A Section 6
Current protection
133
Technical manual
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