The ways used for the protection of generators and transformers may be used, with slight modifications, for the busbars and contours. The modifications area unit is necessary to address the protection issues arising out of the bigger length of lines and an oversized range of circuits connected to a conductor. though differential protection is often used, it becomes too high-priced for extended lines because of the bigger length of pilot wires needed. as luck would have it, less costly ways area unit offered that area unit moderately effective in providing protection for the busbars and contours. during this chapter, we tend to shall focus our attention on the varied ways of protection of busbars and contours.
Busbars within the generating stations and sub-stations type a crucial link between the incoming and outgoing circuits. If a fault happens on a conductor, right smart injury and disruption of offer can occur unless some sort of quick-acting automatic protection is provided to isolate the faulty conductor. The conductor zone, for the aim of protection, includes not solely the busbars themselves however additionally the uninflected switches, circuit breakers, and also the associated connections.
Improved relaying ways are developed, reducing the chance of incorrect operation. the 2 most ordinarily used schemes for conductor protection area unit : (i) Differential protection (ii) Fault bus protection
The fundamental methodology for conductor protection is that the differential theme within which currents getting into and departure the bus area unit totalized. throughout traditional load condition, the addition of those currents is adequate to zero. once a fault happens, the fault current upsets the balance and produces a differential current to work a relay.
The conductor is fed by a generator and provides a load to 2 lines. The secondaries of current transformers within the generator lead, in line one and in line two are all connected in parallel. The protecting relay is connected across this parallel association. All CTs should be of identical quantitative relation within the theme despite the capacities of the assorted circuits. beneath traditional load conditions or external fault conditions, the total of the currents coming into the bus is adequate those exploit it and no current flows through the relay. If a fault happens among the protected zone, the currents coming into the bus can now not be adequate those exploit it.
Fault bus protection:
Fault Bus protection. it’s doable to style a station in order that the faults that develop ar largely earth-faults. this may be achieved by providing earthed metal barrier (known as fault bus) close every conductor throughout its entire length within the bus structure. With this arrangement, each fault which may occur should involve an association between a conductor Associate in Nursing an earthed metal half. By leading the flow of earth-fault current, it’s doable to find the faults and confirm their location. this kind of protection is thought as fault bus protection.
The metal construction or fault bus is earthed through a current electrical device. A relay is connected across the secondary of this CT. beneath traditional in operation conditions, there’s no current be due to fault bus to the bottom and also the relay remains dead. A fault involving an association between a conductor and earthed construction can end in current flow to ground through the fault bus, inflicting the relay to work. The operation of the relay can trip all breakers connecting instrumentation to the bus.
Protection of Lines:
The chance of faults occurring on the lines is way additional thanks to their bigger length and exposure to atmospherical conditions. This has demanded several protection schemes that haven’t any application to the relatively straightforward cases of alternators and transformers. the necessities of line protection are :
- Within the event of a short-circuit, the electrical fuse nighest to the fault ought to open, all different circuit breakers remaining during a closed position.
- Just in case the closest breaker to the fault fails to open, back-up protection ought to be provided by the adjacent circuit breakers.
- The relay operational time ought to be even as short as doable so as to preserve system stability, while not excess tripping of circuits.
The protection of lines presents a retardant quite totally different from the protection of the station equipment like generators, transformers, and busbars. whereas differential protection is a perfect technique for lines, it’s far more pricy to use. the 2 ends of a line could also be many kilometers apart and to check the 2 currents, an expensive pilot-wire circuit is needed. This expense could also be even however generally, less expensive ways are used. The common ways of line protection are :
- Time-graded overcurrent protection
- Differential protection
- Distance protection
Time-graded overcurrent protection:
In this theme of overcurrent protection, time discrimination is incorporated. In alternative words, the time setting of relays is therefore stratified that within the event of a fault, the tiniest potential a part of the system is isolated. we have a tendency to shall discuss a couple of vital cases.
1. Radial feeder. the most characteristic of a radial system is that power will flow solely in one direction, from the generator or provide finish to the load. it’s the disadvantage that continuity of providing can’t be maintained at the receiving finish within the event of a fault. Time-graded protection of a radial feeder is achieved by mistreatment
(i) definite time relays and (ii) inverse time relays.
Definite time relays:
The time of operation of every relay is mounted and is freelance of the operational current. so relay D has Associate in Nursing operational time of 0·5 second whereas, for alternative relays, the time delay is in turn enhanced by the 0·5 second. If a fault happens within the section American state, it’ll be cleared in 0·5 second by the relay AND gate breaker at D as a result of all alternative relays have higher operational time. during this means, solely section American state of the system are isolated. If the relay at D fails to trip, the relay at C can operate when a time delay of 0·5 second i.e. when one second from the prevalence of the fault.
The disadvantage of this method is that if there are a variety of feeders serial, the tripping time for faults close to the availability finish becomes high (2 seconds during this case). However, in most cases, it’s necessary to limit the utmost tripping time to a pair of seconds.
Inverse time relays:
With this arrangement, the farther the fuse from the generating station, the shorter is its relay in operation time. The 3 relays at A, B and C area unit assumed to possess inverse-time characteristics. A fault in section B.C. can provide relay times which can permit breaker at B to trip before the breaker at A.
Parallel feeders. wherever continuity of providing is especially necessary, 2 parallel feeders could also be put in. If a fault happens on one feeder, it is often disconnected from the system and continuity of provide are often maintained from the opposite feeder. The parallel feeders can’t be protected by non-directional overcurrent relays solely. it’s necessary to use directional relays additionally and to grade the time setting of relays for selective trippings.
- Every feeder includes a non-directional overcurrent relay at the generator finish. These relays ought to have the inverse-time characteristic.
- Every feeder includes a reverse power or directional relay at the sub-station finish. These relays ought to be instant kind and operate only if power flows within the reverse direction i.e. within the direction of the arrow at P and Q.
Ring main system:
During this system, varied power stations or sub-stations are interconnected by alternate routes, so forming a closed ring. just in case of injury to any section of the ring, that section could also be disconnected for repairs, and power is provided from each end of the ring, thereby maintaining continuity of providing.
The single line diagram of a typical ring main system consisting of 1 generator G provision four sub-stations S1, S2, S3, and S4. during this arrangement, power will flow in each direction beneath fault conditions. Therefore, it’s necessary to grade in each direction around the ring and conjointly to use directional relays. so as that solely the faulty section of the ring is isolated beneath fault conditions, the categories of relays and their time settings ought to be as follows :
- The 2 lines going away the generating station ought to be equipped with non-directional overcurrent relays (relays at A and J during this case).
- At every sub-station, reverse power or directional relays ought to be placed in each incoming and outgoing lines (relays at B, C, D, E, F, G, H and that I during this case).
- There ought to be correct relative time-setting of the relays. As associate degree example, going around the loop G S1 S2 S3 S4 G; the outgoing relays (viz at A, C, E, G and I) are set with decreasing limits e.g.
Suppose a brief circuit happens at the purpose. so as to confirm property, it’s desired that solely circuit breakers at E and Fought to receptive clear the fault whereas alternative sections of the ring ought to be intact to keep up continuity of providing. In fact, the on top of arrangement accomplishes this job.
The differential pilot-wire protection is predicated on the principle that beneath traditional conditions, this getting into one finish of a line is up to that effort the opposite finish. As presently as a fault happens between the 2 ends, this condition now not holds and also the distinction of incoming and outgoing currents is organized to flow through a relay that operates the electrical fuse to isolate the faulty line. There are many differential protection schemes in use for the lines.
- The Merz-price voltage balance system
- Translate scheme
The Merz-Price voltage balance system:
Merz-Price voltage balance system for the protection of a 3-phase line. Identical current transformers are placed in every section at each end of the road. The try of CTs in every line is connected serially with a relay in such some way that beneath traditional conditions, their secondary voltages are equal and con i.e., they balance one another.
Under healthy conditions, current getting into the road at one-end is up to that effort it at the opposite finish. Therefore, equal and opposite voltages are iatrogenic within the secondaries of the CTs at the 2 ends of the road. The result’s that no current flows through the relays. Suppose a fault happens at purpose F on the road. this can cause a bigger current to flow through CT1 than through CT2. Consequently, their secondary voltages become unequal and current flows through the pilot wires and relays.
- This system can be used for ring mains as well as parallel feeders.
- This system provides instantaneous protection for ground faults. This decreases the possibility of these faults involving other phases.
- This system provides instantaneous relaying which reduces the amount of damage to overhead conductors resulting from arcing faults.
- Accurate matching of current transformers is very essential.
- If there is a break in the pilot-wire circuit, the system will not operate.
- This system is very expensive owing to the greater length of pilot wires required.
- In case of long lines, charging current due to pilot-wire capacitance* effects may be sufficient to cause relay operation even under normal conditions.
- This system cannot be used for line voltages beyond 33 kV because of constructional difficulties in matching the current transformers.
This system is analogous to voltage balance system except that here balance or opposition is between the voltages elicited within the secondary windings wound on the relay magnets and not between the secondary voltages of the road current transformers. this allows to use current transformers of traditional style and eliminates one amongst the foremost serious limitations of the first voltage balance system, namely; its limitation to the system operative at voltages, not extraordinary thirty-three kV. the appliance of Translay theme for one section line has already been mentioned in Art. 21.20. this may be extended to the 3-phase system by applying one relay at every finish of every section of the 3-phase line.
The Transbay theme for the protection of a 3-phase line. The relays employed in the theme square measure primarily overcurrent induction sort relays. every relay has 2 magnetic force parts. The higher component carries a winding (11 or eleven a) that is energized as a summation electrical device from the secondaries of the road CTs connected within the phases of the road to be protected. The higher component additionally carries a secondary coil (12 or twelve a) that is connected asynchronous with the operative winding (13 or thirteen a) on the lower magnet.
- Once the feeder is sound, the currents at its 2 end square measure equal to the secondary currents in each set of CTs square measure equal.
- Consequently, the currents flowing within the relay primary eleven and eleven-a are going to be equal and that they can induce equal voltages within the secondary windings twelve and 12a.
- Since these windings square measure connected con, no current flows within them or in the operative windings thirteen and 13a. within the event of a fault on the protected line, the road current at one finish should carry a bigger current than that at the opposite finish.
- Suppose a fault F happens between phases R and Y and is fed from either side. this can energize solely section one of primary windings eleven and 11a and induce voltages within the secondary windings twelve and 12a.
- As these voltages square measure currently additive, therefore, current can flow into through operative coils thirteen, 13a and therefore the pilot circuit.
Now imagine that associate earth fault happens on section R. this can energize sections one, a pair of and three of the first windings eleven and 11a. once more if the fault is fed from each ends, the voltages elicited within the secondary windings twelve and 12a square measure additive and cause a current to flow through the operative coils thirteen, 13a. The relays, therefore, operate to open the circuit breakers at each end of the road.
- The system is economical as only two pilot wires are required for the protection of a 3-phase line.
- Current transformers of normal design can be used.
- The pilot wire capacitance currents do not affect the operation of relays.
Both time-graded and pilot-wire system isn’t appropriate for the protection of terribly long high voltage transmission lines. the previous offers Associate in Nursing unduly while the delay in fault clearance at the generating station finish once there square measure quite four or 5 sections and therefore the pilot-wire system becomes too costly due to the larger length of pilot wires needed. This has junction rectifier to the event of distance protection within which the action of relay depends upon the gap (or impedance) between purpose|the purpose wherever the relay is put in and therefore the point of fault.
The principle and operation of distance relays have already been mentioned in chapter twenty one. we tend to shall currently contemplate its application for the protection of transmission lines. an easy system consisting of lines asynchronous such power will flow solely from left to right. The relays at A, B and C square measure set to work for ohmic resistance but Z1, Z2, and Z3 severally.
It is clear that for the portion shown, the sole relay at B can operate. Similarly, if a fault happens inside section AB, then the sole relay at A can operate. during this manner, instant protection may be obtained for all conditions of operation. In actual observe, it’s uphill to get instant protection for the entire length of the road thanks to inaccuracies within the relay parts and instrument transformers.
In this theme of protection, 3 distance parts square measure used at every terminal. The zone one component covers the primary ninetieth of the road and is organized to trip outright for faults during this portion. The zone two component visits for faults within the remaining 100 percent of the road and for faults within the next line section, however, a time delay is introduced to stop the road from being tripped if the fault is within the next section. The zone three component provides back-up protection within the event a fault within the next section isn’t cleared by its breaker.