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 infig 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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In this type of protection time settings of relays is so graded that in the event of fault, the smallest possible part of the system is isolated. Following are the important cases. RADIAL FEEDER The main characteristics of this system is that power can flow only one direction, from generator or supply end to the load .It has the disadvantage that continuity of the supply cannot be maintained at the receiving end in the event of fault .Time graded protection of a radial feeder can be achieved by using (i) Definite time relay (ii) Inverse time relay (i)Using Definite time relay Fig .2 shows the over current protection of radial feeder by definite time relays .The time of operation of each relay is fixed and is independent of the operating current Thus relay D has an operating time of read more

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The directional power relay is not suitable under short circuit conditions because as short circuit occurs the system voltage falls to a low value resulting in insufficient torque to cause relay operations. This difficulty is overcome in the directional over current relay, which is independent of system voltage and power factor. Constructional details: – Figure shows the constructional details of a typical induction type directional over current relay. It consists of two relay elements mounted on a common case viz. (i) directional element and (ii) non-directional element. (i) Directional element: It is essentially a directional power relay, which operates when power flows in a specific direction. The potential of this element is connected through a potential transformer (PT.) to read more

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The step of relay operates when the, power in the circuit flows, in a specific direction. A directional power relay is so designed that it obtains its operating torque by the interaction of magnetic field derived from both voltage and current source of the circuit it protects. The direction of torque depends upon the current relative to voltage. Constructional Details:- Figure shows the essential pails of a typical induction type directional power relay. It consists of an aluminum disc, which is free to rotate in between the poles of two electromagnet. The upper electromagnet carries a winding called potential coil on the central limb, which is connected through a potential transformer (PT.) to the circuit voltage source. The lower electromagnet has a separate winding called current read more

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