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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The over load inverse time relay is shown in fig 26. It consists of an upper electromagnet that has been provided with two windings one primary and the other secondary. Primary is connected to a current transformer in the line which is under protection and is provided with eight tappings. These tappings are connected to a plug setting bridge by which the number of turns to be used can be adjusted in order to have the desired current setting. The second winding called secondary is energized by the induction effect and is wound over the central limb of the upper magnet as well as it is spread over the two limbs of the lower magnet. By this method, the leakage flux from the upper magnet entering the disc have been displaced in phase from the flux entering the disc from the lower magnet. read more

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Distance relays are characterized by having two input quantities proportional to the voltage and current at a particular point in the power system, referred to as the relaying point. Ideal static distance relays have characteristics independent of actual magnitudes of voltage and current but dependent only on their ratio and phase angle between them. The versatile family of distance relays includes impedance relays, reactance relays and mho relays. The measurement of impedance, reactance or admittance is done by comparing input current and voltage. Hence distance relays have voltage and current as input quantities. In a static distance relay it is necessary that the two input quantities are similar i.e., voltage/voltage or current/current because they are not electrically separate as read more

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The differential relay measures the phasor difference between two similar electrical quantities(voltage-voltage or current-current). The block diagram for such a relay is shown in fig 24. Inputs I and II are supplied to the comparator. The output of the comparator (phase difference of inputs I and II) is amplified and used to operate the relay. The static differential relays are most commonly used for the protection of generators and transformers for any type of internal faults (two-and three-phase faults, earth faults with solidly grounded neutral or low resistance grounded neutral inter turn faults). These relays are advantageous over electromagnetic differential relays as they are very compact, highly sensitive for internal faults and have absolute stability for heavy through read more

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For obvious reasons of obtaining selectivity over-current relays are made directional. Directional relay senses direction of power flow by means of phase angle between V and I, When the phase angle between V and I exceeds certain predetermined value,the directional relay operates with a condition that the current is above the pickup value. Thus directional relay is a double actuating quantity relay with one input as current I from CT and the other input V from PT. Fig 23 represents the static directional relay with two inputs V and I. The inputs are supplied to the phase comparator. A phase shifter is included in the voltage input circuit, whose output is fed to the phase comparator so that the output from phase comparator under phase faults/earth fault condition is maximum. The output read more

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The over-current relays, even though simplest of all types of electro-mechanical relays, are the most difficult static relays. Static over current relays are of two types: (i) Instantaneous over-current relays and (ii) Time over-current relay. Static instantaneous Over-Current Relay The block diagram of an instantaneous over-current relay is shown in fig 21. The same construction may be used for under-voltage, over-voltage and earth fault relays too. The secondaries of the line CT’s are connected to a summation circuit (not shown in the fig). The output of this summation CT is fed to an auxiliary CT, whose output is rectified smoothened and supplied to the measuring unit (level detector). The measuring unit determines whether the quantity has attained the threshold value (set value) read more

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A static relay refers to a relay in which there is no armature or other moving element and response is developed by electronic, magnetic and other components without mechanical motion. The solid-state components used are transistors, diodes, resistors, capacitors and so on. Static circuits accomplish the function of comparison and measurement. A relay using combination of both static and electro-magnetic units is also called a static relay provided that static units accomplish the response. In static relays, the measurement is performed by electronic, magnetic, optical or other components without mechanical motion. Additional electro-mechanical relay units may be employed in output stage as auxiliary relays. A protective system is formed by static relays and electro-mechanical auxiliary read more

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