Operation Principle - ABB REL650 series Technical Manual

1MRK 506 335-UUS A
7.9.7
Technical manual

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
æ ö
I
Q = ×
ç ÷
T
ç ÷
final ref
I
è ø
ref
EQUATION1167 V1 EN
where:
I is the largest phase current,
I is a given reference current and
ref
T 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:
(
Q = Q + Q - Q
-
n n 1 final
EQUATION1168 V1 EN
where:
is the calculated present temperature,
Q
n
is the calculated temperature at the previous time step,
Q
n-1
is the calculated final temperature with the actual current,
Q
final
is the time step between calculation of the actual temperature and
Dt
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.
ref
æ ö
D
t
)
-
× -
t
1 e
ç ÷
-
n 1
è ø
Section 7
Current protection
(Equation 56)
(Equation 57)
253