Figure 69: Spreadsheet Calculation For Dual Time Constant Thermal Characteristic; Figure 70: Dual Time Constant Thermal Characteristic - GE MiCOM P40 Agile Technical Manual

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P14x
E00728

Figure 69: Spreadsheet calculation for dual time constant thermal characteristic

100000
10000
V00629

Figure 70: Dual time constant thermal characteristic

The current setting is calculated as:
Thermal Trip = Permissible continuous loading of the transformer item/CT ratio.
For an oil-filled transformer with rating 400 to 1600 kVA, the approximate time constants are:
t
= 5 minutes
1
t
= 120 minutes
2
An alarm can be raised on reaching a thermal state corresponding to a percentage of the trip threshold. A typical
setting might be "Thermal Alarm" = 70% of thermal capacity.
P14xEd1-TM-EN-1
1000
100
10
1
1
Current as a Multiple of Thermal Setting
Chapter 6 - Current Protection Functions
Time constant 1 = 5 mins
Time constant 2 = 120 mins
Pre-overload current = 0.9 pu
Thermal setting = 1 Amp
Figures based
on equation
10
141

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