Setting Guidelines For Single Time Constant Characteristic - GE MiCOM P40 Agile Technical Manual

Chapter 6 - Current Protection Functions
Note:
The thermal time constants given in the above tables are typical only. Reference should always be made to the plant
manufacturer for accurate information.
12.5.2

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
overload was applied from hot or cold.
The thermal time constant characteristic may be rewritten as:
θ θ −

( )
− / τ
=
t
e 
θ 1 −

e
where:
●
θ = thermal state = I
●
θ = pre-fault thermal state = I
p
I is the pre-fault thermal state
●
p
I is the full load current
●
FLC
Note:
A current of 105%Is (KI ) has to be applied for several time constants to cause a thermal state measurement of 100%.
FLC
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.
2
Area mm
25 – 50
70 – 120
150
185
240
300
Dry-type transformer <400 kVA
Dry-type transformers 400 – 800 kVA
Air-core Reactors
Capacitor Banks
Overhead Lines with cross section > 100 mm
Overhead Lines
Busbars
142

p


2 2 2
/K I
FLC
2 2 2
/K I
p FLC
6 - 11 kV
10 minutes
15 minutes
25 minutes
25 minutes
40 minutes
40 minutes
Plant type
2
22 kV
15 minutes 40 minutes
25 minutes 40 minutes
40 minutes 40 minutes
40 minutes 60 minutes
40 minutes 60 minutes
60 minutes 60 minutes
Time Constant (Minutes)
40
60 - 90
40
10
10
10
60
33 kV 66 kV
–
60 minutes
60 minutes
60 minutes
60 minutes
90 minutes
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