ABB RET650 Technical Manual page 182

Section 8
Current protection
176
Q > Q
If
final
EQUATION1172 V1 EN
(
Q = Q + Q - Q
-
n n 1 final
EQUATION1173 V1 EN
Q < Q
If
final
EQUATION1174 V1 EN
(
Q = Q - Q
n final final
EQUATION1175 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 (steady state) temperature with the actual current
Q
final
is the time step between calculation of the actual and final temperature
Dt
is the set thermal time constant Tau1 or Tau2 for the protected transformer
t
The calculated transformer relative temperature can be monitored as it is exported
from the function as a real figure HEATCONT.
When the transformer temperature reaches any of the set alarm levels Alarm1 or
Alarm2 the corresponding output signals ALARM1 or ALARM2 are activated. When
the temperature of the object reaches the set trip level which corresponds to
continuous current equal to ITrip the output signal TRIP is activated.
There is also a calculation of the present time to operation with the present current.
This calculation is only performed if the final temperature is calculated to be above the
operation temperature:
æ
Q
t
= - × ç final
t ln
ç
operate
Q
è
final
EQUATION1176 V1 EN
The calculated time to trip can be monitored as it is exported from the function as a real
figure TTRIP.
After a trip, caused by the thermal overload protection, there can be a lockout to
reconnect the tripped circuit. The output lockout signal LOCKOUT is activated when
n
æ ö
D
t
)
-
× -
t
1 e
ç ÷
-
n 1
è ø
n
D
t
)
-
- Q ×
t
e
-
n 1
ö
- Q
operate
÷
÷
- Q
ø
n
1MRK 504 135-UEN A
(Equation 50)
(Equation 51)
(Equation 52)
(Equation 53)
(Equation 54)
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