GE T35 UR Series, Instruction Manual

Page: 284 / 568

5-162 T35 Transformer Protection System GE Multilin
5.7 CONTROL ELEMENTS 5 SETTINGS
5
To ensure element accuracy for high overcurrent conditions, the maximum value of I/(k x I
B
) is limited to 8, even when real-
istically it is exceeding this value.
The reset time of the thermal overload protection element is also time delayed using following formula:
(EQ 5.38)
In the above equation,
• τ
rst
= thermal protection trip time constant.
• T
min
is a minimum reset time setting
Figure 5–62: IEC 255-8 SAMPLE OPERATE AND RESET CURVES
The thermal overload protection element estimates accumulated thermal energy E using the following equations calculated
each power cycle. When current is greater than the pickup level, I
n
> k × I
B
, element starts increasing the thermal energy:
(EQ 5.39)
t
rst
τ
rst
kI
B
( )
2
I
2
kI
B
( )
2
–
-----------------------------
 
 
 
ln
×
T
min
+ =
,,SNS
W
PLQ
ƌUVW
ƌRS
7PLQ
$&'5
E
n
E
n 1
–
t Δ
t
op In
( )
---------------
+ =