This system is similar to voltage balance system except that here balance or opposition is between the voltages induced in the secondary windings wound on the relay magnet and not between the secondary voltages of the line current transformers. This permits to use current transformers of normal design and eliminates one of the most serious limitations of original voltage balance system, namely its limitation to the system operating at voltages not exceeding 33 kV. In a 3-phase system one relay is placed at each end of each phase of the 3- phase line. It can be simplified by combining currents derived from all the phases in a single relay at each end, using the principles of summation transformer in fig 8. A summation transformer is a device that reproduces the poly phase line currents read more

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Figure shows the single line diagram of Merz Price voltage balance system for the protection of 3-phse line. Identical current transformers are placed in each phase at both ends of the line. The pair of CTs in each line is connected in series with a relay in such a way that under normal conditions, their secondary voltages are equal and in opposition i.e. they balance each other Under healthy conditions, current entering the line at one-end is equal to that leaving it at the other end. Therefore equal and opposite voltages are induced in the secondaries of the CTs at the two ends of the line. The result is that no current flows through the relays. When a fault occurs at point F on the line as shown in Fig 6. It will cause a greater current to flow through CT1 than through CT2. read more

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The differential pilot-wire protection is based on the principle that under normal conditions, the current entering one end of a line is equal to that leaving the other end. As soon as a fault occurs between the two ends this condition no longer holds and the difference of incoming and outgoing currents is arranged to flow through a relay which operates the circuit breaker to isolate the faulty line. There are several differential protection schemes for the lines. They are: 1. Merz-Price voltage balance system 2. Translay scheme Share and Enjoy: read more

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Ring main system – In this system, various power stations or sub-stations are interconnected alternate routes, thus forming a closed ring. In case of damage to any section of the ring, that section may be disconnected for repairs and power will be supplied from both ends of the ring, thereby maintaining continuity of supply Fig 5 shows the single line diagram of a typical ring main system consisting of one generator G supplying four sub-stations S1, S2, S3 and S4. In this arrangement power can flow in both directions under fault conditions. So, it is necessary to grade in both directions round the ring and also to use directional relays. In order that only faulty section of the ring is isolated under fault conditions, the types of relays and their time settings should be as read more

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PARALLEL FEEDER – Where continuity of supply is particularly necessary, two parallel feeders may be installed. If a fault occurs on one feeder, it can be disconnected from the system and continuity of supply can be maintained from the other feeder. The parallel feeder cannot be protected by non directional relay over current relays only. It is necessary to use directional relay also and to grade the time setting of relays for selective tripping. Fig above shows the system where two feeders are connected in parallel between the generating station and the sub-station. The protection of this system requires that (i) Each feeder has a non directional over current relay at the generator end. These relays should have inverse -time characteristic. (ii) Each feeder has a reverse power or read more

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