REG 316*4 1MRB520049-Uen / Rev. F ABB Switzerland Ltd
3-75
Overcurrent setting “Current”
The current setting is chosen such that neither false tripping nor
false signals can occur during normal operation and yet the
minimum fault current is detected. The setting must therefore be
between the maximum short-time load current and the minimum
fault current and allow for the tolerance on the protection setting
and also its reset ratio. The maximum short-time load current is
a parameter of the power system concerned and must take ac-
count of switching operations, load surges and fast response
excitation (Fig. 3.5.5.2).
I
N
I
Maximum short-time
load current
“C "urrent
"Delay"
Minimum fault current
HEST 905 013 C
Fig. 3.5.5.2 Operating characteristic of a definite time overcur-
rent function
I
N
= rated current of the protection
Delay
The delay is used to achieve discrimination of the overcurrent
function. It is set according to the grading table for all the over-
current units on the power system. The zone of protection of the
voltage-controlled overcurrent function extends from the c.t’s
supplying it in the generator star-point to the location of the next
downstream overcurrent relay.
To Next Page
Loading...
