Auto Reclosing Schemes:-
Types
of Faults:-
i) Transient
Faults:-
These are cleared by
the immediate tripping of circuit breakers and do not recur when the line is
re-energised.
ii) Semi-permanent
Faults:-
These
require a time interval to disappear before a line is charged again.
iii) Permanent
Faults:-
These are to be located and repaired before the line is re-energised.
About
80-90% of the faults occurring are transient in nature. Hence the automatic reclosure of breaker
(after tripping on fault) will result in the line being successfully
re-energised, thereby
a) Decreasing outage time
b) Improving reliability
c) Improving system stability
d) Reduces fault damage and maintenance time
Dead
Time:-
The time between the Auto-reclosing scheme being
energised and the 1st
reclosure of the circuit breaker . This is normally set at 1 Sec.
Reclaim
Time:-
The time following a successful closing operation
measured from the instant the auto-reclosing relay closing contacts making
which must elapse before the auto-reclosing relay initiated another reclosing
attempt. In other words, it may be said
to be the time between 1st and 2nd re-closure.
Types
of Auto-reclosing schemes (based on phase):
a) Three phase Auto-reclosing:
This type of
auto-reclosing causes an immediate drift apart of the two systems and hence no
interchange of synchronizing power can take place during the dead time.
b) Single Phase Auto-reclosing:
In this only the
faulty phase (which already has tripped on SLG fault) is reclosed without
causing interruption in interchange of synchronising power between two systems through other two
healthy phases.
Types
of Auto-reclosing schemes (case on attempts of reclosure):
a) Single Shot Auto-reclosing:-
In this scheme,
breaker is reclosed only once on a given fault before lockout of circuit
breaker occurs. High speed
auto-reclosing for EHV system is invariably single shot.
b) Multi-shot Auto-reclosing:-
In this scheme, more
than one reclosing attempt is made for a given fault before lockout of the
circuit breaker occurs. Repeated closure
attempts with high fault level would seriously affect the circuit breaker, equipment
and system stability. The factors that
must be taken into account:-
i)
Circuit
Breaker Limitations:-
Ability of circuit
breaker to perform several trip close operations in quick succession.
ii) System Conditions:-
In the percentage of
the semi-permanent faults (which could be burnt out) is moderate, for example
on the lines through the forest, multishot auto-reclosing is followed.
Types
of Auto-reclosing (depending on speed):
I) High speed Auto-reclosing:
This aids in fast restoration of supply but should be done by taking
into account the following factors:-
i)
System
disturbance time can be tolerated without loss of system stability
ii)
Characteristics
of protection schemes and circuit breaker.
II) Low Speed or Delayed Auto-reclosing:-
This is suitable for
highly interconnected systems where the loss of a single line is unlikely to
cause two sections of the system to drift apart and loose synchronism.
For EHV
Systems:-
a) Choice of Dead Time:
Lower
limit is decided by deionising time of
circuit breaker.
Upper
limit is decided by transient stability and synchronism.
Long
transmission lines require longer dead time for single phase faults.
The dead time for
high speed auto-reclosing scheme with EHV system is 0.3-0.8 Sec.
b) Choice for reclaim time:-
This should not be
set to such a low value that the operating cycle of breaker is exceeded when
two fault incident occurs close together.
The reclaim time will be in the range of 10-30 Sec., depending on the
breaker opening and closing mechanisms.
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