ABB REB670 Applications Manual page 38

Section 2
Application
32
The fast tripping time (shortest trip time is 5ms) of the low-impedance differential
protection function is especially advantageous for power system networks with high fault
levels or where fast fault clearance is required for power system stability.
All CT inputs are provided with a restraint feature. The operation is based on the well-
proven RADSS percentage restraint stabilization principle, with an extra stabilization
feature to stabilize for very heavy CT saturation. Stability for external faults is guaranteed
if a CT is not saturated for at least two milliseconds during each power system cycle.
The advanced open CT detection algorithm detects instantly the open CT secondary
circuits and prevents differential protection operation without any need for additional
check zone.
Differential protection zones in the IED include a sensitive operational pickup. This
sensitive operational pickup is designed to be able to detect internal busbar ground faults
in low impedance grounded power systems (that is, power systems where the ground-fault
current is limited to a certain level, typically between 300A and 2000A primary by a
neutral point reactor or resistor). Alternatively this sensitive pickup can be used when high
sensitivity is required from busbar differential protection (that is, energizing of the bus via
long line).
Overall operating characteristic of the differential function in the IED is shown in figure
2.
Diff Oper Level
Sensitive Oper Level
IEC06000142 V1 EN
Figure 2: IED operating characteristic
Sensitive
differential
protection
Operate
region
Sens Iin Block
s=0.53
1MRK 505 337-UUS -
Differential protection
operation characteristic
I [Primary Amps]
in
en06000142.vsd
Application manual