GE P54C User Manual

Note:
The thermal time constants giv
en in the above tables are typical only. Reference should always be made to the plant
manufacturer for accurate information.
7.5.2 SETTING GUIDELINES FOR SINGLE TIME CONSTANT CHARACTERISTIC
The time to trip varies depending on the load current carried before application of the overload, i.e. whether the
ov
erload was applied from hot or cold.
The thermal time constant characteristic may be rewritten as:
e
t
e
p−
( )
=
−
−






/ τ
θ θ
θ
1
where:
●
θ = thermal state = I
2
/K
2
I
FLC
2
●
θ
p
= pre-fault thermal state = I
p
2
/K
2
I
FLC
2
● I
p
is the pre-fault thermal state
● I
FLC
is the full load current
Note:
A current of 105%Is (KI
FLC
) has to be applied for several time constants to cause a thermal state measurement of 100%.
The current setting is calculated as:
Thermal Trip = Permissible continuous loading of the plant item/CT ratio.
The following tables show the approximate time constant in minutes, for different cable rated voltages with
various conductor cross-sectional areas, and other plant equipment.
Area mm
2
6 - 11 kV 22 kV 33 kV 66 kV
25 – 50 10 minutes 15 minutes 40 minutes –
70 – 120 15 minutes 25 minutes 40 minutes 60 minutes
150 25 minutes 40 minutes 40 minutes 60 minutes
185 25 minutes 40 minutes 60 minutes 60 minutes
240 40 minutes 40 minutes 60 minutes 60 minutes
300 40 minutes 60 minutes 60 minutes 90 minutes
Plant type Time Constant (Minutes)
Dry-type transformer <400 kVA 40
Dry-type transformers 400 – 800 kVA 60 - 90
Air-core Reactors 40
Capacitor Banks 10
Overhead Lines with cross section > 100 mm
2
10
Overhead Lines 10
Busbars 60
P54A/B/C/E Chapter 9 - Current Protection Functions
P54xMED-TM-EN-1 217

GE P54C Specifications

General

Brand	GE
Model	P54C
Category	Relays
Language	English