Figure 214: Spreadsheet Calculation For Dual Time Constant Thermal Characteristic; Figure 215: Dual Time Constant Thermal Characteristic - GE MiCOM P40 Technical Manual

Chapter 13 - Current Protection Functions
E00728

Figure 214: Spreadsheet calculation for dual time constant thermal characteristic

100000
10000
V00629

Figure 215: 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.
380
1000
100
10
1
1
Current as a Multiple of Thermal Setting
Time constant 1 = 5 mins
Time constant 2 = 120 mins
Pre-overload current = 0.9 pu
Thermal setting = 1 Amp
P446SV
Figures based
on equation
10
P446SV-TM-EN-1