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2.18.1 Thermal state determination
THM simulates the change of thermal state using an algorithm; THM can detect an overload
condition. Thermal state (θ) is determined with the following equation:
where
I
is the load current applied;
I
AOL
is the allowable overload current; is the thermal time
constant. The value of is set using setting [TTHM].
Thermal state 0% is the cold state, whereas 100% reflects the thermal limit—which is
the point at which no further temperature rise can be tolerated safely and the system should
be disconnected. The thermal limit for any given system is subject to setting [THM], which
determines the value of overload current. The THM signals a trip when the value of θ is at
100%.
THM determines
I
using the positive-sequence current (I
1
) and negative-sequence
current (I
2
) using Equation (2.18-2); and operates according to the characteristics defined in
IEC 60255-149† when the user has set scheme switch [THM-Itype] Equivalent.
where,
q: Unbalance factor; the unbalance factor is used for motor/generator protection; the
user should set q=0 when setting [THM-q] except when it is applied to
motor/generator protection.
†Note: If the user prefers to use the thermal characteristic for the THM function specified
in IEC 60255-8, set Max phase for the scheme switch [THM-Itype]. In this case,
THM is determined using the largest of the three-phase currents.
The period of time for a trip to occur depends not only on the degree of overload, but also
on the degree of load current as far as reaching the overload state. The period of the trip, that
is, depends on whether the overload is applied from ‘cold’ or ‘hot’. THM has thresholds for
alarm and trip; hence, the two thresholds exist independently.
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