v)Auto Reclosing Schemes:-Types of Faults:-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, therebya) 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):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.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.

vi)Carrier Communication Schemes:-The main disadvantage of conventional time-stepped distance protection is that the instantaneous Zone-1 of the protective scheme at each end of the protected line is set to cover 80% of the line and hence faults in the balance 20% of the line (at each end) are cleared in Zone-2 time, which is undesirable.The desirable scheme is the one wherein the relays clear the faults on the 100% of the protected line instantaneously and also provide backup for uncleared faults on adjacent lines. This can be achieved by interconnecting the distance relays are each end of the line by a signaling channel (which can be either pilots, a power line carrier communication channel, a radio link or a microwave channel).

Transfer trip is of two types:-i)Under-reaching scheme:-

The scheme in which the Zone-1 relay (set to cover about 80% of ZL) is used to send a signal to the remote end of the feeder for inter-tripping is termed as transfer trip under-reaching scheme.

To avoid mal-operation due to receipt of false signal, the receiving end relay operation is inter-locked with its Zone-3/starter operation i.e. the scheme operates either by its own Zone-1 relay operation or by receipt of carried and its Zone-3/starter operation.

Over-reaching scheme:-

This scheme is suitable for short lines where an underreaching Zone-1 would be too short to be of any practical use. In this scheme the relay set to reach beyond 100% of the line, is used to send an inter-tripping signal to the remote end of the line. It is essential that the receive relay contact be monitored by a directional relay to ensure that tripping does not take place unless the fault is within the protected section.

The disadvantage of this scheme is that there is no independent Zone-1 tripping. The fast tripping therefore relies entirely on signaling channel.

The disadvantages of these schemes is that the signal is transmitted over the fault line section. Distortion of the signal may occur due to attenuation introduced into the line by the fault.

Blocking schemes:-

In this scheme, a blocking signal is sent by the reverse looking directional unit ZR to prevent instantaneous tripping for Zone-2 & Zone-3 faults, external to the protected line. Here ZR must operate faster then forward looking Zone-3 units and the signaling channel must also be extremely fast is operation.

Though all the distance schemes with carrier inter-tripping/carrier blocking facility are procured, the same are yet to be commissioned.