It is important for an overcurrent starter, which has picked up, to reliably reset at the
maximum load current
IBmax, if for example the fault is tripped by a downstream protection.
Taking due account of the reset ratio of 0.95, the lowest permissible setting is given by:
max
min
(
)
1.25
0.95
B
A
N
I
I
Istart
I
The maximum setting
(IStart)max is derived from the minimum fault current IK for a fault at
the end of the next section of line:
max
min
(
)
K
N
Istart
I
I
Should the above relationships result in
(IStart)max being lower than (IStart)min, the
underimpedance starters must be used instead.
Underimpedance starters
GUID-AB1404EC-2679-4CA9-AA8B-F84A320C432E v1
The underimpedance starters are enabled by selecting
UZ as the StartMode parameter. The
following parameters then have to be set:
•
XA
•
XB
•
RA
•
RB
•
RLoad
•
AngleLoad
The parameters
RLoad and AngleLoad define the permissible load area.
Section 5
1MRK 505 406-UEN B
Bay protection functions
38
Bay protection functions REB500
Technical manual
© Copyright 2019 ABB. All rights reserved