Estudios descriptivos

Distance relays which respond to balanced 3 phase changes in impedance will be affected by system power swings. These swings or oscillations occur following a system disturbance such as load change or fault clearance. As the generators attempt to find a stable operating angle relative to each other, they overshoot the final position and continue oscillating until stability is achieved. The extend of the oscillations depends upon the severity of the disturbance, and the natural stability of the system. The oscillation rate is determined by the inertia of the system and impedance between different generators. When the generation at each end of a line protected by distance relays oscillates, the impedance seen by the relays varies along some locus. It will be seen that this locus can enter the distance relay characteristic, and

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cause relay operation if steps are not taken to prevent this. The general practise is to block the power swing in all zones of distance protection. The practise will be modified according to system studies.

Power line carrier aided transfer tripping (as enclosed in separate sheet) auto Reclosing and syrichronization.

Power line carrier aided Inter Tripping:

The zone I coverage of the distance protection will cover only the 80% of the protected line and will not cover the “end zones”, about 20% of the protected feeder length. If the fault occurs at the end zone of the protected line, one side the protection will clear the fault constantaneously, but other side will face it as zone & fault will clear after 0.4 see.

Faults remaining on the feeder for zone 2, time may cause the system to be come unstable. The fault will cause permanent lock out of the circuit breaker at each end of the line section, on usage of auto re closing in the feeder. The general practice is to avoid the auto reclosing if the carriers aided scheme is not adopted.

The unit scheme of protection is to compone the condition at both ends of the feeder.

Whenever the fault is internal or external to the protacted section. The simplerr way of speeding up fauet clearence at the terminal which clears an end zone fault in zone 2 time is to adopt a direct, accelerated transfortrip. The Direct transfer trip relay, scheme in which 2000 I relay is used to send a signal to the remove and of the feeder, the receive relay contact is corrected directly to the trip relay. The disadvantage of this scheme is the possibility of considered tripping by accidented operation or mal operation of signalling equipment.

The scheme is more secure by supervising the receive signal with the impedance zone 2 measurement operation before allowing tripping and the scheme is known as permissive under reach transfer trip scheme.

The accelerated transfer trip scheme is similar to Permissine under reach transfer trip scheme. In its principle of operation but it is applicable only to zone suitched distance relays which shares the same measuring which for both zone 1 & 2 and.

In the above relays the under reach zone 1 unit is arranged to send a signal to the remote end of the feeder in addition to tripping the local breaker. The receive relay contact is arranged to operate the range change relay which extends the reach of the measuring unit from zone 1 to zone 2. Immediately instead of at the end of the zone 2 time delay. This accelerate the fault clearance as the remote end.

Permissive under reach scheme general arrangement in distance relay.

Z1 - Zone Trip Contact

Z2M - Zone 2 measuring element contact Z2T - Zone 2 timer / Contact

Z3M - Zone 3 measuring element contact Z3T - Zone 3 timer / contact

TR - tripping relay.

Z₃M Z₂M Z₃T Z₂T Z₁

Z2T

Z3T TR

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RR

RR = Receive relay contact in carrier set, RRX = Receive and relay at Distance relay Reclosing and synchronizing:

The large majority of the over head line faults are transient and case can be cleared by momentarily de energising the line. It is therefore, feasible to improve service continuity by automatically reclosing the breaker after relay operation. The automatic reclosing will improve continuity of service and increase the availatility of transmission line, but certain pre cautions are to be taken.

1. The generator should never be connected to a system on automatic reclosing, time the angle of voltages across the breaker in the vicinity of the generator is an inadequate measure of the possible hazard associated with closing the breaker. The sudden change in power in the generator following closure is however a key indicator.

RR Z₃M Z₂M

Z₂M RRX

Z₃T Z₂T

Z₁

TR

Z₂T

RRX Z₃T

The desired attributes of a reclosing system vary with user requirements. In an area with high level of transient fault, (lightning incidences) more transmission time breakers will be successfully re closed on the first try. Single short re closing relays those which produce only one reclosure until reset are entirely satisfied.

Multiple shot reclosing relays are warrented on disribution circuits with significant free exposure, where an unsucessful reclosure would generally mean a customer outage.

The speed of tripping is a significant factor in the success of a reclosure on a transmission circuit. The faster the closing, the loss fault damage and/or degree of arc ionization, the less the shock to the system on reclosure and greater the likelihood of re energization with out subsequent tripping. The probability of sucessful reclosing is improved it reclosing occurs only after a high speed pilot trip. The high speed of pilot tripping is achieved with the help of communication channels like power line carrier communication protection couplers. Such a system will eliminate the high of probability of unsuccessful re-closure on non pilot trips, particularly for end-zone faults in which clearing OCCURS sequentially and the de energized time is short. The general practice of utilization of single shot re-closing is with the assistance of PLCC network, and on failure of PLCC network the function of re-closing is blocked in the relay.

A sample calculation to be adopted for re-closure for dead time and re-claim time is tabulated below. The longer death time is required because of the fact that the two phases remain energized tend to keep the arc conducting longer.

ASPER GENERAL PRACTICE ONE SHOT RECLOSE IS ADOPTED