ABB RELION REX640 Technical Manual page 368

Section 4
Protection functions
362
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 1 final n 1
− −
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Θ calculated present temperature
n
Θ calculated temperature at previous time step
n-1
Θ calculated final temperature with actual current
final
Δt time step between calculation of actual temperature
thermal time constant for the protected device (line or cable), set
t
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:
Θ − Θ

final operate
t τ ln
= − ⋅
operate 
Θ − Θ
 final
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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.
The time to lockout release is calculated, that is, the calculation of the cooling time to
a set value. The calculated temperature can be reset to its initial value (the Initial
temperature setting) via a control parameter that is located under the clear menu. This
t
∆
 
−
e
1
τ
⋅ −
 
 


n 
1MRS759142 C
(Equation 31)
Time constant
(Equation 32)
REX640
Technical Manual