CHAPTER
2 (week 5 -research )
LITERATURE
REVIEW
This paper that was published by Newagepublisher.com
titled Auto Re-Closing says that It is well realized
that the transient faults which are most frequent in occurrence do no permanent
damage to the system as they are transitory in nature. These faults disappear
if the line is disconnected from the system momentarily in order to allow the
arc to extinguish. After the arc path has become sufficiently deionized, the
line can be reclosed to restore normal service. The type of fault could be a
flashover across an insulator. Reclosing could also achieve the same thing with
semi-permanent faults but with a delayed action, e.g., a small tree branch
falling on the line, in which case the cause of the fault would not be removed
by the immediate tripping of the circuit breaker but could be burnt away during
a time delayed trip and thus the line reclosed to restore normal service. Now
should the fault be permanent, reclosing is of no use, as the fault still
remains on reclosing and the fault has to be attended personally. It simply
means that if the fault does not disappear after the first trip and closure,
double or triple-shot reclosing is used in some cases before pulling the line
out of service.
Experience
shows that nearly 80% of the faults are cleared after the first trip, 10% stay
in for the second reclosure which is made after a time delay, 3% require the
third reclosure and about 7% are permanent faults which are not cleared and
result in lockout of the reclosing relay. When a line is fed from both ends,
the breakers at the two ends trip simultaneously on occurrence of the fault,
the generators at the two ends of the line drift apart in phase, the breakers
must be reclosed before the generators drift too far apart for synchronism to
be maintained, such a reclosure increases the stability limit considerably.
In
present day power systems, automatic reclosing finds wide application. It
therefore follows that to effect fault clearance and subsequent reclosure, it
is often necessary to operate sequentially several items of switchgear.
Recently logical design principles have been applied for the control of
autoreclose switching sequences in large substations. The obvious advantages
are continuous supply except for short duration when tripping and reclosure
operations are being performed, this renders the substation unattended. The
success of rapid reclosure to a large extent depends on the speed of operation
of the protections. This is so because high speed protection decreases the
amount of damage incurred and thus increases the probability of successful
operation.
In
some cases application of automatic reclosing enables us to use very simple but
high speed protections of the lines. With instantaneous protection being
applied indiscriminate tripping of several circuit breakers is possible but the
provision of auto-reclose makes it a selective operation. is essential that the
system dead time be kept to a few cycles so that the generators do not drift
apart. High speed protection such as pilot wire carrier or distance must be
used to obtain operating times of one or two cycles. It is therefore desired
that the reclosure be of the single shot type. High speed reclosure in high
voltage circuits improves the stability to a considerable extent on
single-circuit ties.
Overcurrent
protection with in turn automatic reclosure is enables us to realize high-speed
protection of every section against transient as well as permanent faults. This
is achieved by installing high speed nonselective overcurrent protections at
all head parts of the sections. The pickup value for all such protections is
selected for faults inside the full length of the given section and not for
faults beyond the step down transformers if one such exists. Different zones of
protection. Any nonselective operation of faults outside the given section is
corrected by automatic reclosing. When the line is protected by composite
overcurrent protection, the first stage of the protection can be employed as
high speed nonselective overcurrent protection. Consider a permanent fault of
system). The high speed relays of breakers 1 and 2 will operate. Practically
immediately the automatic recloser circuit ties subjected to single circuit
faults the continuity through the healthy circuit prevents the generators from
drifting apart so fast and increase in the stability limit is thus moderate.
Nevertheless, it is sometimes important. However, when the faults occur
simultaneously on both the circuits the stability limit increases again
considerably.
line
are opened after fault incidence, independent of the fault type, and are reclosed
after a predetermined time period following the initial circuit breaker
opening. For a single circuit interconnectors between two power systems, the
opening of all the three phases of the circuit breaker makes the generators in
each group start to drift apart in relation to each other, since no interchange
of synchronizing power can take place.
On
the other hand single-phase auto-reclosure is one in which only the faulted
phase is opened in the presence of a single-phase fault and reclosed after a controlled
delay period. For multiphase faults, all three phases are opened and reclosure
is not attempted. In case of single-phase faults which are in majority,
synchronizing power can still be interchanged through the healthy phases. In
the case of single-phase auto-reclosing each phase of the circuit breaker has
to be segregated and provided with its own closing and tripping mechanism. Also
it is necessary to fit phase selecting relays that will detect and select the
faulty phase. Thus single-phase auto-reclosing is more complex and expensive as
compared to
three-phase
auto-reclosing. When single-phase auto-reclose is used the faulty phase must be
reenergized for a longer interval of time, than in the case of three-phase
auto-reclose, owing to the capacitive coupling between the faulty phase and the
healthy conductors which tends to increase the duration of the arc. The
advantage claimed for single-phase reclosing is that on a system with
transformer neutrals grounded solidly at each substation, the interruption of
one phase to clear a ground fault causes negligible interference with the load
because the interrupted phase current now flows in the ground through neutral
points until the fault current is cleared and the faulted phase reclosed. The
main drawback is its longer deionizing time which can cause interference with
communication circuits and, in certain cases may operation of earth relays in
double circuit lines owing to the flow of zero sequence currents.
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