important pay attention to the communication channel dependability to ensure that
proper signals are communicated during power system faults, the time during which
the protection schemes must perform their tasks flawlessly.
The logic supports the following communications schemes:
•
blocking scheme
• permissive schemes (overreach and underreach)
• direct intertrip
A permissive scheme is inherently faster and has better security against false tripping
than a blocking scheme. On the other hand, permissive scheme depends on a received
CR signal for a fast trip, so its dependability is lower than that of a blocking scheme.
When single-pole tripping is required on parallel lines, an unwanted three-pole trip can
occur for simultaneous faults near the line end (typical last 20%). Simultaneous faults
are one fault on each of the two lines but in different phases, see figure
386. When
simultaneous faults occur, the phase selectors at the remote protection IED - relative to
the faults, see the A side in figure 386 - cannot discriminate between the fault on the
protected line and on the parallel line. The phase selector must be set to cover the
whole line with a margin and will also detect a fault on the parallel line. Instantaneous
phase-selective tripping for simultaneous faults close to line end is not possible with
the information that is available locally in the remote protection IEDs relative to the
faults. The protection IED near the faults detects the faults on the protected line as a
forward fault, and on the parallel line in reverse direction. The directional phase
selector in the two IEDs near the faults can discriminate between the faults and issue
correct single-pole tripping commands.
A
B
C
Ground
IED IED
A
B
C
Station A Station B
ANSI06000309_2_en.vsd
ANSI06000309 V2 EN-US
Figure 386: Simultaneous faults on two parallel lines
1MRK 504 163-UUS A Section 16
Scheme communication
Transformer protection RET670 2.2 ANSI 821
Application manual
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