<p style="text-align:justify"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16253849601625384960.jpg" style="height:386px; width:800px" /></p>

<p style="text-align:justify">&nbsp;</p>

<p style="text-align:justify"><span style="font-size:14px">The radial feeder that we have been discussing so far is suffering from a major disadvantage. Refer figure 7.2. It is obvious that the loads are fed from all buses A, B, C, and D. Now, in case of fault in line section CD, relay R<sub>3</sub> will operate to trip breaker CB<sub>3</sub>. It can be very easily appreciated that all loads fed from bus D will be devoid of power. In case of fault in line section BC, relay R<sub>2</sub> will operate, CB<sub>2</sub> will open and fault will be cleared but, at the same time, all loads fed from bus C and D will not receive electric power till the fault is rectified. In case of fault in line segment AB, probably 80 % of power system will be in dark. Also, there will be a serious problem of power system instability.</span></p>

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<p style="text-align:justify"><span style="font-size:14px"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16253850231625385023.jpg" style="height:251px; width:800px" /></span></p>

<p style="text-align:justify">&nbsp;</p>

<p style="text-align:justify"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16253850661625385066.jpg" style="height:554px; width:900px" /></p>

<p style="text-align:justify">&nbsp;</p>

<p style="text-align:justify"><span style="font-size:14px">This problem can be solved if the same radial feeder as in figure is fed from both the ends as shown in figure 7.17. But in this case, there will be a problem of setting of overcurrent relays if simple inverse-time overcurrent relays are used. The radial system shown in figure 7.17 can be simplified to be two radial feeders fed from one end as shown in figure 7.18. In first radial network, R<sub>5</sub> is a last relay towards load. R<sub>3</sub> backs up R<sub>5</sub> and R<sub>1</sub> backs up R<sub>3</sub>. In second radial feeder, the ascending order of time setting will be of relays R<sub>2</sub>-R<sub>4</sub>-R<sub>6</sub>. The time settings are shown in terms of seconds near relays for discriminative operation of relays.</span></p>

<p style="text-align:justify">&nbsp;</p>

<p style="text-align:justify"><span style="font-size:14px">Now, if simple overcurrent relays are used, relays R<sub>1</sub>, R<sub>2</sub> and R<sub>5</sub> will operate tripping respective breakers for the fault at F<sub>1</sub>. For the fault at F<sub>2</sub>, R<sub>2</sub> and R<sub>5</sub> will operate. Similarly, for the fault at F<sub>3</sub>, R<sub>2</sub>, R<sub>5</sub> and R<sub>6</sub> would have operated. This is contrary to requirement of selectivity of protective system, as healthy feeders are isolated and loads of Buses B and C are devoid of power. Whereas, for the fault at F<sub>1</sub>, the requirement calls for tripping breakers CB<sub>1</sub> and CB<sub>2</sub> (relays R<sub>1</sub> and R<sub>2</sub> only should operate). Similarly, for fault at F<sub>2</sub>, relays R<sub>3</sub> and R<sub>4</sub> should operate as primary protection and none other. Similar is the case for fault at F<sub>3</sub>. If the breakers are made to trip for fault locations as above, only faulty feeder will be isolated and yet all loads are continued to be fed, may be with minor load shedding. This is possible only if we add one more discriminative feature, i.e., direction of fault current. If the current flows away from the bus AND if it is more than plug-setting of the relay, then only the relay should operate. If the direction of flow of the fault current is non-operative (i.e., towards the bus), the relay should block even if the current is more than plug-setting. These operative directions are shown by arrowheads in figure 7.17. It can be appreciated that, for the fault at F<sub>2</sub>, the direction of flow of fault current (fed from G<sub>1</sub>) is contrary to the operative direction for the relay R<sub>2</sub> and correct (operative) for the relay R<sub>3</sub>. Hence, R<sub>3</sub> will operate (R<sub>2</sub> will not operate) and R<sub>1</sub> will offer back-up. Similarly, for the same fault location and for fault current fed from G<sub>2</sub>, R<sub>4</sub> will operate and R<sub>5</sub> will not operate. Thus, if R<sub>2</sub>, R<sub>3</sub>, R<sub>4</sub> and R<sub>5</sub> are directional overcurrent relays, the requirement of selectivity and discrimination between faults in different sections is achieved. The directional property is not required for R<sub>1</sub> and R<sub>6</sub> as current does not reverse in these two relays for fault locations F<sub>1</sub>, F<sub>2</sub> or F<sub>3</sub>.</span></p>

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<p style="text-align:justify"><span style="font-size:14px">The requirement of this directional discrimination is applicable in case of different power system networks depicted below:</span></p>

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<p style="text-align:justify"><span style="font-size:14px"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16253851141625385114.jpg" style="height:580px; width:800px" /></span></p>

<p style="text-align:justify">&nbsp;</p>

<p style="text-align:justify"><span style="font-size:14px">As shown in figure 7.19, most of the transmission lines are parallel feeders running on single series of transmission towers. For fault at F<sub>1</sub>, the fault current flows in paths AF<sub>1</sub> and CDBF<sub>1</sub>. Hence R<sub>1</sub> will operate tripping CB<sub>1</sub>. Also, R<sub>3</sub> and R<sub>4</sub> will operate if the relays are simple overcurrent relays. This is contrary to requirement of selectivity of protective relaying system as healthy feeder CD will also be isolated. Top of R<sub>4</sub> cannot be set more than that of R<sub>3</sub>, as for fault at F<sub>2</sub> (when fault current flow path is ABDF<sub>2</sub>), the requirement will call for Top of R<sub>3</sub> to be more than that of R<sub>4</sub>. Therefore, TMS of both relays R<sub>3</sub> and R<sub>4</sub> are set to be same and are provided with directional discrimination. This means that relays R<sub>3</sub> and R<sub>4</sub> are directional IDMT overcurrent relays with operative direction as shown by arrowheads in figure 7.19 (fault current direction away from bus is operative for these relays) and R<sub>1</sub> and R<sub>2</sub> are non-directional simple overcurrent relays (as the direction of current flow does not reverse in these relays for faults at F<sub>1</sub> or F<sub>2</sub>). It can be appreciated that with addition of directional sense in the relays, only faulty feeder is isolated and healthy feeder keeps on feeding power to load. No doubt, the healthy feeder will be overloaded for a while until necessary load-shedding is done at load end. The transmission line can be easily overloaded (up to even 200 %) for few minutes as the insulation is air and porcelain. Porcelain can withstand high temperatures and hot air due to increase in temperature (due to overload) is replaced by cool air. For a fault at F<sub>2</sub>, if any component of switchgear at relaying point R<sub>4</sub> fail, R<sub>1</sub> will offer back-up. Similarly, R<sub>2</sub> backs up R<sub>3</sub>. This will require proper time grading while setting the relays.</span></p>

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<p style="text-align:justify"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16253851611625385161.jpg" style="height:1056px; width:800px" /></p>

<p style="text-align:justify"><span style="font-size:14px">The third kind of power system network where directional discrimination is necessary is Ring Main Network as shown in figure 7.20. In this network, if there is a fault at F ( in line BC) , the fault current will pass from generator to fault via two paths:</span></p>

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<ol>
	<li style="list-style-type:none">
	<ol style="list-style-type:lower-alpha">
		<li style="text-align:justify">
		<p><span style="font-size:14px">Relays R<sub>1</sub> &ndash; R<sub>8</sub> &ndash; R<sub>7</sub> &ndash; F.</span></p>
		</li>
		<li style="text-align:justify">
		<p><span style="font-size:14px">Relays R<sub>2</sub> &ndash; R<sub>3</sub> &ndash; R<sub>4</sub> &ndash; R<sub>5</sub> &ndash; R<sub>6</sub> &ndash; F.</span></p>
		</li>
	</ol>
	</li>
</ol>

<p style="text-align:justify">&nbsp;</p>

<p style="text-align:justify"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16253853141625385314.jpg" style="height:395px; width:800px" /></p>

<p style="text-align:justify">&nbsp;</p>

<p style="text-align:justify">&nbsp;</p>

<p style="text-align:justify"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16253853371625385337.jpg" style="height:384px; width:800px" /></p>

<p style="text-align:justify">&nbsp;</p>

<p style="text-align:justify"><span style="font-size:14px">With reference to the requirement of selectivity, relays R<sub>6</sub> and R<sub>7</sub> only should operate. For fault in line AB, relays R<sub>1</sub> and R<sub>8</sub> shall operate and no other relay should operate. Hence, once again, the relays R<sub>3</sub> to R<sub>8</sub> are directional overcurrent relays (with operative direction of fault current away from the bus as shown by arrowheads in figure 7.20) and R<sub>1</sub>, R<sub>2</sub> are non-directional overcurrent relays. If directional feature is used in this way, selective tripping of breakers will occur for fault anywhere in transmission lines shown in figure 7.20. Back-up protection is also offered if proper grading of relays is carried out while relay setting exercise. For a fault at F ( in line BC), if R<sub>6</sub> fails, R<sub>4</sub> backs up and if R<sub>7</sub> fails, R<sub>1</sub> will offer back-up.&nbsp;Figure 7.21 shows cascaded parallel feeder fed from one end, which is one more application of directional discrimination in relays. The operative direction for relays is shown by arrowheads in figure 7.21. The directional requirement in cascaded parallel feeder fed from both ends is shown in figure 7.22.</span></p>