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-phase
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
369. When simultaneous faults occur, the phase selectors at the remote
protection IED - relative to the faults, see the A side in figure 369 - 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.
L1
L2
L3
Earth
IED IED
L1
L2
L3
Station A Station B
IEC06000309_2_en.vsd
IEC06000309 V2 EN-US
Figure 369: Simultaneous faults on two parallel lines
By using phase-segregated channels for the communication scheme, the correct
phase information in the protection IED near the faults can be transferred to the
other side protection IED. A correct single-pole trip can be achieved on both lines
and at both line IEDs.
ZC1PPSCH requires three individual channels between the protection IEDs on
each line in both directions. In case of single-phase faults, only one channel is
activated at a time. But in case of multi-phase faults, two or three channels are
activated simultaneously.
The following descriptions of the schemes generally presents one of the three
identical phases.
Section 16 1MRK 506 369-UEN B
Scheme communication
710 Line distance protection REL670 2.2 IEC
Application manual
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