Q
n-1
is the calculated temperature at the previous time step
Q
final
is the calculated final (steady state) temperature with the actual current
Dt is the time step between calculation of the actual and final temperature
t
is the set thermal time constant Tau1 or Tau2 for the protected transformer
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:
ln
final operate
operate
final n
t
t
æ ö
Q - Q
= - ×
ç ÷
ç ÷
Q - Q
è ø
EQUATION1176 V1 EN (Equation 22)
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
the temperature of the object is above the set lockout release temperature setting ResLo.
The time to lockout release is calculated, That is, a calculation of the cooling time to a
set value.
_
_
ln
final lockout release
lockout release
final n
t
t
æ ö
Q - Q
= - ×
ç ÷
ç ÷
Q - Q
è ø
EQUATION1177 V1 EN (Equation 23)
In the above equation, the final temperature is calculated according to equation 17.
Since the transformer normally is disconnected, the current I is zero and thereby the
Θ
final
is also zero. The calculated component temperature can be monitored as it is
exported from the function as a real figure, TRESLO.
1MRK 505 277-UUS C Section 7
Current protection
105
Technical Manual
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