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Directional Relay

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54 views4 pages

Directional Relay

Uploaded by

Itiba
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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234 Power System Protection and Switchgear 5.6 REVERSE POWER OR DIRECTIONAL RELAY Figure 5.10(a) shows an electromechanical directional relay. A directional relay is energised by two quantities, namely voltage and current, Fluxes @, and ¢, are sel up by voltage and current, respectively. Eddy currents induced in the dise by ¢, interact, with @, and produce a torque. Similarly, ¢, also induces eddy currents in the disc, which interact with 4, and produce a torque. The resultant torque rotates the disc. ‘The torque is proportional to VI cos ¢, where @ is the phase angle between V and J. ‘The torque is maximum when voltage and current are in phase. To produce maximum orque during the fault condition, when the power factor is very poor, a compensating ‘winding and shading are provided, as shown in Fig. 5.10(3). Earlier it has been mentioned that the torque produced by an induction relay is given by T= 6; 6, sin 8 J, I, sin 8, where 6, and 9, are fluxes produced by I, and zh, respectively. The angle between 6, and 4, or J, and J, is @. Ione of the actuating quantities is voltage, the current flowing in the voltage coil lags behind voltage by approximately 90°. Assume this current to be 1, The load current I (say /,) lags Vby 4. Then the angle @ between /, and /, is equal to (90 — g), as shown in Fig. 5.10(b) - compunsaing SEF v Votage creut KN aA Shading al “roc Th ‘ 5 curt (2) Construction (0) Phasorfaoram Fig. 540 Induction dis type directional relay 1, sin (0~ 9) = 1 I, cos 8 VFcos 6 Votage ‘An induction cup construction can also be used {o produce a torque proportional to VI cos 6. The arrangement is shown in Fig 5.11, Two opposite r poles are energised by voltage and the other wo . poles by current. Here voltage is a polarising i quantity. The polarising quantity is one which é produces one of the two fluxes. The polaising quantity is taken a areference with respect tothe ther quantity which scent nts ese [ TTorgue produced is postive when cos ¢ is T positive, ic. ¢ is less than 90°. When ¢ is mor than 90° (between 90° and 180°), the torque is Fig. 5.11 induction cup type directional relay ‘Overcurrent Protection 235 negative. At a particular relay location, when power flows in the normal direction, the relay is connected to produce negative torque. The angle between the actuating quantities supplied to the relays is kept (180° — 9) to produce negative torque. If due to any reason, the power flows inthe reverse direction, the relay produces a positive torque and it operates. In this condition, the angle between the actuating quantities @ is kept less than 90° to produce a positive torque. This is shown in Fig. 5.12(). For normal flow of power, the relay is supplied with V and ~ J. For reverse flow, the actuating quantities become V and /. Torque becomes VI cos ¢, ic. positive, This can bbe achieved easily by reversing the current coil, as shown in Fig. 5.1206) Relaying units supplied with single actuating quantity discussed earlier are znon-directional overcurrent selays. Non-

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