GE Multilin L60, Инструкция по эксплуатации

Страница: 412 / 752

5-282 L60 Line Phase Comparison System GE Multilin
5.6 CONTROL ELEMENTS 5 SETTINGS
5
The thermal overload protection element corresponds to the IEC 255-8 standard and is used to detect thermal overload
conditions in protected power system elements. Choosing an appropriate time constant element can be used to protect dif-
ferent elements of the power system. The cold curve characteristic is applied when the previous averaged load current over
the last 5 cycles is less than 10% of the base current. If this current is greater or equal than 10% than the base current, then
the hot curve characteristic is applied.
The IEC255-8 cold curve is defined as follows:
(EQ 5.27)
The IEC255-8 hot curve is defined as follows:
(EQ 5.28)
In the above equations,
• t
op
= time to operate
• τ
op
= thermal protection trip time constant
• I = measured overload RMS current
• I
p
= measured load RMS current before overload occurs
• k = IEC 255-8 k-factor applied to I
B
, defining maximum permissible current above nominal current
• I
B
= protected element base (nominal) current
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 5.29)
In the above equation,
• τ
rst
= thermal protection trip time constant
• T
min
is a minimum reset time setting
t
op

op
I
2
I
2
kI
B
 
2
–
--------------------------
 
 
 
ln

=
t
op

op
I
2
I
p
2
–
I
2
kI
B
 
2
–
--------------------------
 
 
 
ln

=
t
rst

rst
kI
B
 
2
I
2
kI
B
 
2
–
-----------------------------
 
 
 
ln

T
min
+ =