ABB REC670 Applications Manual page 331

1MRK 511 401-UUS A
Bay control REC670 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
Section 14
Control
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