estimation is larger than the set
Maximum temperature
, the START output is
activated.
Current reference
and
Temperature rise
setting values are used in the final
temperature estimation together with the ambient temperature. It is suggested
to set these values to the maximum steady state current allowed for the line or
cable under emergency operation for a few hours per years. Current values with
the corresponding conductor temperatures are given in cable manuals. These values
are given for conditions such as ground temperatures, ambient air temperature, the
way of cable laying and ground thermal resistivity.
Thermal counter
The actual temperature at the actual execution cycle is calculated as:
Θ Θ Θ Θ
∆
n n final n
t
e= + −
( )
⋅ −
− −
−
1 1
1
τ
(Equation 7)
Θ
n
calculated present temperature
Θ
n-1
calculated temperature at previous time step
Θ
final
calculated final temperature with actual current
Δt time step between calculation of actual temperature
t thermal time constant for the protected device (line or cable), set
Time con-
stant
The actual temperature of the protected component (line or cable) is calculated by
adding the ambient temperature to the calculated temperature, as shown above.
The ambient temperature can be given a constant value or it can be measured. The
calculated component temperature can be monitored as it is exported from the
function as a real figure.
When the component temperature reaches the set alarm level
Alarm value
, the
output signal ALARM is set. When the component temperature reaches the set trip
level
Maximum temperature
, the OPERATE output is activated. The OPERATE signal
pulse length is fixed to 100 ms.
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:
t
operate
final operate
final n
= − ⋅
−
−
τ
ln
Θ Θ
Θ Θ
(Equation 8)
Caused by the thermal overload protection function, there can be a lockout to
reconnect the tripped circuit after operating. The lockout output BLK_CLOSE is
activated at the same time when the OPERATE output is activated and is not reset
until the device temperature has cooled down below the set value of the
Reclose
temperature
setting. The
Maximum temperature
value must be set at least two
degrees above the set value of
Reclose temperature
.
Protection functions
1MRS758755 C
268 REC615 & RER615
Technical Manual
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