12 THERMAL OVERLOAD PROTECTION
The heat generated within an item of plant is the resistive loss. The thermal time characteristic is therefore based
on the equation I
2
Rt. O
ver-temperature conditions occur when currents in excess of their maximum rating are
allowed to flow for a period of time.
Temperature changes during heating follow exponential time constants. The device provides two characteristics
for thermal overload protection; a single time constant characteristic and a dual time constant characteristic. You
select these according to the application.
12.1 SINGLE TIME CONSTANT CHARACTERISTIC
This characteristic is used to protect cables, dry type transformers and capacitor banks.
The single constant thermal characteristic is given by the equation:
t
I KI
I I
e
FLC
p
= −
−
( )
−
τ
log
2
2
2 2
where:
● t = time to trip, following application of the ov
erload current I
●
t = heating and cooling time constant of the protected plant
● I = largest phase current
● I
FLC
full load current rating (the Thermal Trip setting)
● K = a constant with the value of 1.05
● I
p
= steady state pre-loading before application of the overload
12.2 DUAL TIME CONSTANT CHARACTERISTIC
This characteristic is used to protect equipment such as oil-filled transformers with natural air cooling. The thermal
model is similar to that with the single time constant, except that tw
o 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 levels, the replica curve is dominated by the short time
constant for the windings. This provides protection against hot spots developing within the transformer windings.
Overall, the dual time constant characteristic serves to protect the winding insulation from ageing and to minimise
gas production by overheated oil. Note however that the thermal model does not compensate for the effects of
ambient temperature change.
The dual time constant thermal characteristic is given by the equation:
0 4 0 6
1 2
2
2
2 2
. .
/ /
e e
I KI
I I
t t
FLC
p
−
( )
−
( )
+ =
−
( )
−
τ τ
where:
●
t
1
= heating and cooling time constant of the transformer windings
●
t
2
= heating and cooling time constant of the insulating oil
P14x Chapter 6 - Current Protection Functions
P14xEd1-TM-EN-1 139
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