P54x/EN OP/La4 Operation
(OP) 5-82
MiCOM P543, P544, P545 & P546
OP
1.27.1 Single time constant characteristic
This characteristic is used to protect cables, dry type transformers (e.g. type AN), and
capacitor banks.
The thermal time characteristic is given by:
t = - log
e
2
- (K.
FLC
)
2
(
2
- p
2
)
Where:
t = Time to trip, following application of the overload current, ;
= Heating and cooling time constant of the protected plant;
= Largest phase current;
FLC = Full load current rating (relay setting ‘Thermal Trip’);
k = 1.05 constant, allows continuous operation up to <1.05 FLC;
P = Steady state pre-loading before application of the overload.
The time to trip varies depending on the load current carried before application of the
overload, i.e. whether the overload was applied from 'hot” or “cold”.
The thermal time constant characteristic may be rewritten as:
e
(-t/)
=
-
p
- 1
Where:
= 2/k2 FLC2
and
p = p2/k2 FLC2
Where is the thermal state and is p the pre-fault thermal state.
Note: A current of 105%s (kFLC) has to be applied for several time
constants to cause a thermal state measurement of 100%.
1.27.2 Dual time constant characteristic (typical
ly not applied for MiCOM P54x)
This characteristic is used to protect oil-filled transformers with natural air cooling (e.g. type
ONAN). The thermal model is similar to that with the single time constant, except that two
timer constants must be set.
For marginal overloading, heat will flow from the windings into the bulk of the insulating oil.
Therefore, at low current, the replica curve is dominated by the long time constant for the oil.
This provides protection against a general rise in oil temperature.
For severe overloading, heat accumulates in the transformer windings, with little opportunity
for dissipation into the surrounding insulating oil. Therefore, at high current, the replica curve
is dominated by the short time constant for the windings. This provides protection against
hot spots developing within the transformer windings.
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