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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In this type of protection, transmission lines are used to carry protective currents at carrier frequency (30 to 200 kc/s) or at an ultra high frequency (above 900 mega cycles). When the transmission lines carry protective currents at carrier frequency it is called carrier current-pilot protection and when it carries protective current at ultra high frequency its called as microwave pilot wire protection. As no separate pilot wires are used, the transmission lines are used to carry both power current as well as protective carrier currents it causes a great saving. Fig 14 represents a phase blocking system schematic arrangement of the equipment required at both ends of the transmission line. Each end of the transmission line consists of a network which transforms CT output currents into read more

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The distance impedance relays are normally used for phase- to-phase faults as for such faults the loop impedance between the phase is obtained which remains constant, but in case of earth fault the loop impedance consists of impedance of one line and the impedance of the earth fault which is a variable factor. Practically it has been estimated that for earth fault loop impedance is approximately 1.5 times the impedance of earth phase (phase and neutral), however for a multiple earth system this factor may be taken 1.25. Hence for earth faults special connections can be made, so the impedance relay may be used for both phase and earth fault From this it appears that the system will require two – impedance relays with voltage restraint elements energized, one for phase faults and read more

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