Whenever there is an unbalance in circuit, the unbalanced currents will have a negative phase sequence component. A negative phase sequence (or phase unbalance) relay is essentially provided for the protection of generators and motors against unbalanced loading that may arise due to phase-to-phase faults. Such relay has a filter circuit, which is responsive only to the negative sequence components. Since small magnitude over-current can cause dangerous conditions, it becomes necessary to have low setting of such relays. An earth relay can also provide the desired protection but only in case when there is a fault between any phase and earth. For phase-to-phase faults an earth relay cannot provide necessary protection and hence negative phase sequence relay is required. Fig. 19 a. read more

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Fig. 18 shows the arrangement of voltage balance protection. In this scheme of protection, two similar current transformers are connected at either end of the element to be protected (e.g. an alternator winding) by means of pilot of wires. The secondaries of current transformers are connected in series with a relay in such a way that under normal conditions, their induced e.m.f’s are in opposition Under healthy conditions, equal currents will flow in both primary windings. Therefore, the secondary voltages of the two transformers are balanced against each other and no current will flow through the relay-operating coil. When a fault occurs in they protected zone, the currents in the two primaries will differ from one another and their secondary voltages will no longer be in balance. read more

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The biased beam relay also called percentage differential relay is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected section. It’s called percentage differential relay because the ratio of differential operating current to average restraining current is a fixed percentage. It’s called bias relay because restraining known as biased coil produces the bias force. Fig 17 a, shows the schematic arrangements of biased beam relay. It is essentially an over current balanced beam type relay with an additional restraining coil. The restraining coil produces a bias force in the opposite direction to the operating force. Under normal and through load conditions, the bias force due to restraining coil is greater than read more

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Fig 16 a shows an arrangement of an over current relay connected to operate as a differential relay. A pair of identical current transfonners is fitted on either end of the section to be protected (alternator winding in this case). The secondaries of CT’s are connected in series in such a way that they carry the induced currents in the same direction. The operating coil of over current relay is connected across the CT secondary circuit. This differential relay compares the current at the two ends of the alternator winding. Under normal operating conditions, suppose the alternator winding carries a normal current of 1000 A. Then the current in the two secondaries of CT’s are equal as in figure. These currents will merely circulate between the two CT’s and no current will flow read more

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Relays, which depend on excess of current for their operation, are less sensitive because they cannot make correct distinction between load conditions and minor fault conditions. In order to overcome this difficulty differential relays are used. Differential relay is one that operates when the phasor difference of two or more similar electrical quantities exceed a predetermined value. Thus a current differential relay is one that compares the current entering a section of the system with current leaving the section. Under normal operating conditions, the two currents are equal but as soon as fault occurs, this condition no longer applies. The difference between the incoming and outgoing currents is arranged to flow through relay operating coil. If this difference is equal to or grater read more

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Fig. 14 shows the schematic arrangement of a definite-distance type impedance relay. It consists of a pivoted beam F and two electromagnets energized respectively by a current and voltage transformer in the protected circuit. The armatures of the two electromagnets are mechanically coupled to the beam on the opposite sides of the fulcrum. The beam is provided with a bridging piece for the trip contacts. The relay is so designed that, the torque produced by the two electromagnets are in the opposite direction. Operation. Under normal operating conditions, the pull due to the voltage element is greater than that of the current element. Therefore, the relay remains open. When a fault occurs in the protected zone, the applied voltage to the relay decreases whereas the current increases. read more

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Distance relays are those in which the operations are governed by the ratio of applied voltage to current in the protected circuit. It is also called Impedance relay. In this the torque produced by a voltage element opposes the torque produced by a current element. The relay will operate when the ratio V/I is less than a pre-determined value. Fig.13 illustrates the basic principal of operation of an Impedance relay. The voltage element of the relay is excited through a potential transformer (P.T.) from the line to be protected. The current element of the relay excited from a current transformer (C.T) in series with the line. The portion AB of the line is the protected zone. Under normal condition the impedance of the protected zone is ZL. The relay closes when the impedance of the read more

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An inverse time relay is one in which the operating time is approximately inversely proportional to the magnitude of the actuating quantity. Fig. 10.a show the time current characteristics of an inverse current relay. At values of current less than pickup, the relay never operates. At higher values, the time of operation of the relay decreases steadily with the increase of current. The inverse-time delay can be achieved by associating mechanical accessories with relays. In an induction relay, the inverse-time delay can be achieved by positioning a permanent magnet in such a way that relay disc cuts the flux between the poles of the magnet. When the disc moves, the current set up in it produce a drag on the disc, which slows its motion. In other types of relays, the inverse time delay read more

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