Rev. 32
37/114
5.6.
49 Function. Thermal Image Protection
Thermal image is a measure of heating and cooling of an electric machine. Unlike overcurrent
protection, it does not start counting the time when it detects a fault, but it is continuously determining
the thermal state of the machine that monitors. The trip time depends on the thermal constants
adjusted, the current flowing and the prior thermal state of the machine.
The thermal image is calculated based on the following equation:
θ = 100 x (I/I
t
)
2
x (1
– e
-
t/ζ
) + θ’
0
x e
-
t/ζ
Where:
I, maximum R.M.S. current of three phases
I
t,
adjusted tap current
ζ, thermal constant
θ’
0
, initial thermal state
The trip time is given by the equation:
t =
ζ x ln x {[(I/I
t
)
2
–
(θ’
0
/ 100)] / [(I/I
t
)
2
- 1]}
The accuracy of the tripping time is 5% of the theoretical time.
The algorithm uses the maximum of the three phase currents. If the maximum is greater than 15% of
the adjusted Current Tap, heating thermal constant is applied. If the maximum is less than 15% of the
adjusted Current Tap cooling thermal constant is taken into account.
The overload function trips when the thermal image reaches the value of 100%. This value is reached
in time when the current is equal to the function adjusted in thermal function.
It provides an adjustable level of thermal imaging to generate an alarm. If the trip occurs, the function
is reset when the thermal image falls below the set alarm level.
As the current measurement algorithm used is R.M.S., in the thermal model is taken into account the
heat produced by the harmonics.
This protection function is adjusted by setting five different parameters
:
Function
Description
Minimum
Maximum
Pitch
Unit
Default
49
Thermal image protection function
Function
Enable
-
-
Yes/No
-
No
Current Tap
0.10
2.40
0.01
xIn
1.20
ζ heating
3
600
1
min
3
ζ cooling
1
6
1
ζ heating
1
Alarm
20
99
1
%
80
Содержание SIA-B
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