GE P54E - Figure 110: Spreadsheet Calculation for Dual Time Constant Thermal Characteristic; Figure 111: Dual Time Constant Thermal Characteristic

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Figures based
on equation
Figure 110: Spreadsheet calculation for dual time constant thermal characteristic
1
1
0
100
1000
10000
100000
1 10
Current as a Multiple of Thermal Setting
O
p
e
r
a
t
i
n
g
T
i
m
e
(
s
e
c
o
n
d
s
)
Time constant 1 = 5 mins
T
ime constant 2 = 120 mins
Pre-overload current = 0.9 pu
Thermal setting = 1 Amp
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Figure 111: Dual time constant thermal characteristic
The curr
ent 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
1
= 5 minutes
●
t
2
= 120 minutes
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.
Chapter 9 - Current Protection Functions P54A/B/C/E
216 P54xMED-TM-EN-1