Woodward
Manual MRI3-ITER GB
16
DOK-TD-MRI3 ITERE Rev.A
4.4 Algorithm
Based on the defined thermal model, one can de-duce that an energy Q is stored in the electrical
equipment.
After expiry of a long time and with a constant cur-rent load, a stationary condition will be achieved
in which the temperature of the electrical equipment does not increase anymore. The heat supplied
for a unit of time is equal to that released by cooling down (steady energy balance).
Q
released
= Q
supplied
The supplied thermal energy as well as the temperature
of the electrical equipment in stationary
condition are in proportion to the square of the phase current (e.g. ohmic losses and iron losses in
the transformer).
Q
I² or
I²
Since the pickup value in the MRI3-ITE is deter-mined by I
B
, the following relation is effective:
n
k²
(I
B
k)²
For this purpose, the temperature T that really prevails in the electrical equipment needs to be
known. This temperature
(in%) is described in the thermal replica through the temperature equiv-
alent k
I
B
. When being loaded with the maximum permissible operating current B in stationary
condition, the electrical equipment reaches the maximum allowed operating temperature
B
. For
this load, the temperature equivalent is defined as k
2
100%.
%
I
K
l
B
· 100%
I.e.: At a load with I = 0.9 x (k
I
B
) and k
I
B
= 1.2, and according to the before indicated definition,
the temperature reaches 81% of the maximum permissible operating temperature. For an
electrical equipment that - after initial load - will be loaded beyond the admissible operating current
(I > k•I
B
), the following temperature curve will result:
Figure 4.3: Warming up of an electrical item
