ABB RELION 670 SERIES Applications Manual page 661

1MRK 506 369-UUS -
Line distance protection REL670 2.2 ANSI
Application manual
Single-pole tripping and single-phase automatic reclosing is a way of limiting the effect
of a single-phase line fault on power system operation. Especially at higher voltage levels,
the majority of faults are of single-phase type (around 90%). To maintain system stability
in power systems with limited meshing or parallel routing single-phase auto reclosing is
of particular value. During the single-phase dead time the system is still capable of
transmitting load on the two healthy phases and the system is still synchronized. It requires
that each circuit breaker pole can be operated individually, which is usually the case for
higher transmission voltages.
A somewhat longer dead time may be required for single-phase reclosing compared to
high-speed three-phase reclosing. This is due to the influence on the fault arc from the
voltage and the current in the non-faulted phases.
To maximize the availability of the power system it is possible to choose single-phase
tripping and automatic reclosing during single-phase faults and three-phase tripping and
automatic reclosing during multi-phase faults. Three-phase automatic reclosing can be
performed with or without the use of synchrocheck.
During the single-phase dead time there is an equivalent "series"-fault in the system
resulting in a flow of zero sequence current. It is therefore necessary to coordinate the
residual current protections (ground fault protection) with the single-phase tripping and
the auto reclosing function. Attention shall also be paid to "pole discrepancy" that arises
when circuit breakers are provided with single-phase operating devices. These breakers
need pole discrepancy protection. They must also be coordinated with the single-phase
auto recloser and blocked during the dead time when a normal discrepancy occurs.
Alternatively, they should use a trip time longer than the set single-phase dead time.
For the individual line breakers and auto reclosing equipment, the auto reclosing dead
time expression is used. This is the dead time setting for the auto recloser. During
simultaneous tripping and reclosing at the two line ends, auto reclosing dead time is
approximately equal to the line dead time. Otherwise these two times may differ as one
line end might have a slower trip than the other end which means that the line will not be
dead until both ends have opened.
If the fault is permanent, the line protection will trip again when reclosing is attempted in
order to clear the fault.
It is common to use one automatic reclosing function per line circuit breaker (CB). When
one CB per line end is used, then there is one auto- recloser per line end. If auto reclosers
are included in duplicated line protection, which means two auto reclosers per CB, one
should take measures to avoid uncoordinated reclosing commands. In breaker-and-a-
half, double-breaker and ring bus arrangements, two CBs per line end are operated. One
auto recloser per CB is recommended. Arranged in such a way, that sequential reclosing
of the two CBs can be arranged with a priority circuit available in the auto recloser. In case
of a permanent fault and unsuccessful reclosing of the first CB, reclosing of the second CB
is cancelled and thus the stress on the power system is limited.
Section 15
Control
655