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Implementation of Thermal Model to IEC60255-149
Heating by overload current and cooling by dissipation of an electrical system follow exponential time
constants. The thermal characteristics of the electrical system can be shown by equation (1).
2
I
-
θ =
1
2
I
AOL
where:
θ = thermal state of the system as a percentage of allowable thermal capacity,
I = applied load current,
I
= allowable overload current of the stator,
AOL
τ = thermal time constant of the system.
The thermal stateθ is expressed as a percentage of the thermal capacity of the protected stator of
motor, where 0% represents the cold state and 100% represents the thermal limit, that is the point at
which no further temperature rise can be safely tolerated and the system should be disconnected. The
thermal limit for any given electrical plant is fixed by the thermal setting I
output when θ = 100%.
If current I is applied to a cold system, then θ will rise exponentially from 0% to (I
constant τ, as in Figure A-2. If θ = 100%, then the allowable thermal capacity of the system has been reached.
2
I
2
I
AOL
A thermal overload protection relay can be designed to model this function, giving tripping times
according to the IEC60255-8 'Hot' and 'Cold' curves.
I
Ln
t =τ·
I
2
I
-
I
2
-
Ln
t =τ·
2
I
I
-
where:
I
= prior load current.
P
-
t
τ
×
e
100
%
1
θ (%)
100%
×
100
%
2
2
AOL
2
I
P
2
AOL
182
(1)
ｔ
-
τ
θ
2
-
=
I
1
e
2
I
AOL
t (s)
Figure A-2
∙∙∙∙∙ Cold curve
(1)
∙∙∙∙∙ Hot curve
(2)
6 F 2 T 0 1 7 2
. The relay gives a trip
AOL
2
2
× 100%), with time
/I
AOL
×
100
%
- Quick Links:
- Inverse Time Overcurrent Protection
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