E00728
Figures based
on equation
Figure 69: Spreadsheet calculation for dual time constant thermal characteristic
1
10
1
00
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
Ti
me constant 2 = 120 mins
Pre-overload current = 0.9 pu
Thermal setting = 1 Amp
V00629
Figure 70: Dual time constant thermal characteristic
The current setting is calculated as:
Thermal Trip = P
ermissible 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.
P14x Chapter 6 - Current Protection Functions
P14xEd1-TM-EN-1 141
To Next Page
Loading...
Need help?
General
