NUMBER OF SHOTS
There are no clear cut rules for defining the number of shots for a particular application. Generally medium voltage
systems use only two or three shot Autoreclose schemes. However, in certain countries, for specific applications, a
four-shot scheme is used. A four-shot scheme has the advantage that the final dead time can be set sufficiently
long to allow any thunderstorms to pass before reclosing for the final time. This arrangement prevents
unnecessary lockout for consecutive transient faults.
Typically, the first trip, and sometimes the second, will result from instantaneous protection. Since most faults are
transient, the subsequent trips will be time delayed, all with increasing dead times to clear semi-permanent faults.
An important consideration is the ability of the circuit breaker to perform several trip-close operations in quick
succession and the affect of these operations on the circuit maintenance period.
On EHV transmission circuits with high fault levels, only one re-closure is normally applied, because of the damage
that could be caused by multiple re-closures.
DEAD TIMER SETTING
The choice of dead time is dependent on the system. The main factors that can influence the choice of dead time
Stability and synchronism requirements
The type of circuit breaker
Fault deionising time
The protection reset time
STABILITY AND SYNCHRONISM REQUIREMENTS
It may be that the power transfer level on a specific feeder is such that the systems at either end of the feeder
could quickly fall out of synchronism if the feeder is opened. If this is the case, it is usually necessary to reclose the
feeder as quickly as possible to prevent loss of synchronism. This is called high speed autoreclosing (HSAR). In this
situation, the dead time setting should be adjusted to the minimum time necessary. This time setting should
comply with the minimum dead time limitations imposed by the circuit breaker and associated protection, which
should be enough to allow complete deionisation of the fault path and restoration of the full voltage withstand
level. Typical HSAR dead time values are between 0.3 and 0.5 seconds.
On a closely interconnected transmission system, where alternative power transfer paths usually hold the overall
system in synchronism even when a specific feeder opens, or on a radial supply system where there are no
stability implications, it is often preferred to leave a feeder open for a few seconds after fault clearance. This allows
the system to stabilise, and reduces the shock to the system on re-closure. This is called slow or delayed auto-
reclosing (DAR). The dead time setting for DAR is usually selected for operational convenience.
When HSAR is not required, the dead time chosen for the first re-closure following a fault trip is not critical. It
should be long enough to allow any resulting transients resulting to decay, but not so long as to cause major
inconvenience to consumers who are affected by the loss of the feeder. The setting chosen often depends on
service experience with the specific feeder.
Typical first shot dead time settings on 11 kV distribution systems are 5 to 10 seconds. In situations where two
parallel circuits from one substation are carried on the same towers, it is often arranged for the dead times on the
Chapter 13 - Autoreclose