<ul>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Z<sub>s</sub>/Z<sub>l</sub> Ratio: As already discussed,</span></p>
	</li>
</ul>

<p style="text-align:justify"><span style="font-size:14px"><span class="math-tex">\(I_f=\frac{E}{Z_s+Z_l}\)</span>&nbsp;.</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/16253776981625377698.jpg" style="height:366px; width:900px" /></p>

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<p style="text-align:justify"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16266936771626693677.jpg" style="height:960px; width:800px" /></p>

<p style="text-align:justify"><strong><span style="font-size:20px">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;Fig.7.10: Better Discriminetion Using Very Inverse and Extremly Characteristics</span></strong></p>

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

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

<p style="text-align:justify"><span style="font-size:14px">If Z<sub>s&nbsp;</sub>&gt;&gt; Z<sub>l&nbsp;</sub>(which occurs in end-zone feeders as shown in figure 7.9), the difference between fault current magnitude for a fault at F<sub>1</sub> (immediately after relaying point R<sub>3</sub>) and that for a fault at F<sub>2</sub> (at the end of feeder section as shown in figure) is insignificant. Hence there will be no significant difference in time of operation of relay R<sub>3</sub> for both these fault locations. Hence, the normal inverse overcurrent characteristic is not justified where&nbsp;Z<sub>s</sub>/Z<sub>l&nbsp;</sub><span class="math-tex">\(\geq\)</span>&nbsp;2. The remedy is to use a very inverse characteristic. Referring to figure 7.10, for the same difference in fault current magnitudes (PSM = 3&nbsp;and PSM = 5), the difference in time of operation of normal inverse overcurrent relay is 2 sec, that of very inverse overcurrent relay is 3.4 sec&nbsp; and that of extremely inverse overcurrent relay is 6.67 sec. This clearly shows that very inverse overcurrent relay provides better discrimination than normal inverse overcurrent relay and extremely inverse overcurrent relay is the best in this respect. Another useful advantage is closer matching of extremely inverse overcurrent relay with MCCB characteristic. Therefore, lower time of operation of relay R<sub>3</sub> can be achieved. Hence, consequently lower TMS of R<sub>2</sub> and R<sub>1</sub> too can be set. The selectivity is yet maintained.</span></p>

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

<ul>
	<li style="text-align:justify">
	<p><span style="font-size:14px">The second problem is the varying generating capacities to match with the load variations. No doubt, when the load is less at night, one cannot switch off the thermal power plant as starting time of this plant is about 7 hours from fully cool condition. But hydro power plant, gas turbine plant or diesel power plant can be switched off fast and can be quickly switched ON also when desired. At night, when the load is less, small number of generators are connected in the power system and hence the magnitude of fault current will be less. In the limiting case, the fault current with minimum generation (least number of generators connected in the system) may be less than the load current with maximum generation (maximum number of generators connected). However, the fault current is less than the CMR of equipment, it is harmful because:</span></p>

	<ol>
		<li style="list-style-type:none">
		<ol style="list-style-type:lower-roman">
			<li style="text-align:justify">
			<p><span style="font-size:14px">The voltage has dropped (practically zero) at fault point.</span></p>
			</li>
			<li style="text-align:justify">
			<p><span style="font-size:14px">The flow of NPS (negative phase sequence) currents, in case of unsymmetrical faults give rise to double frequency eddy current flow in iron of the rotor of all machines overheating the rotor [heat produced due to X amp of NPS current is approximately 6 times that produced by same amount of positive sequence current (I<sup>2</sup>Rt)]. Hence negative sequence current of smaller magnitude than positive sequence current can prove to be more harmful.</span></p>
			</li>
			<li style="text-align:justify">
			<p><span style="font-size:14px">The flow of zero sequence current (which is third harmonic current) gives rise to eddy current losses (in machines and transformers) which are 9 times (proportional to f<sup>2</sup>) that due to positive sequence currents. Thus, again, lower magnitude of fault current containing negative and zero sequence components is harmful.</span></p>
			</li>
		</ol>
		</li>
	</ol>
	</li>
</ul>

<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/16253777671625377767.jpg" style="height:567px; width:900px" /></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/16253777971625377797.jpg" style="height:367px; width:900px" /></p>

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

<p style="text-align:justify"><span style="font-size:14px">As I<sub>Lmax&nbsp;</sub>&gt;&nbsp;I<sub>fmin</sub>, it is difficult to set a simple IDMT overcurrent relay. Instead, voltage-controlled overcurrent relay can be used and relay is set for I<sub>Lmax</sub>. When fault occurs, voltage collapses below 70 % of V<sub>R</sub> (rated voltage) and plug-setting reduces to 40 % of set value. If voltage-controlled relay is not available, an undervoltage relay can be used in conjunction with simple overcurrent relay. Overcurrent relay is set for I<sub>fmin</sub> and undervoltage relay for 70 % V<sub>R</sub>.&nbsp;Referring to figure 7.11(a), and 7.11(b), when the normal load current is flowing during the time of day, as I<sub>L</sub> &gt;&nbsp;I<sub>f</sub>, relay 51 will operate closing 51-1 [figure 7.11(b)]. But as voltage is normal, undervoltage relay 27 is energized opening 27-1. Hence, 86 is not triggered and no tripping of breaker as required. If fault occurs at night, 51 operates and 27 deenergizes (as voltage is less than 70 % of V<sub>R</sub>) closing both 51-1 and 27-1. 86 gets energized, 86-1 closes, 52-a is already closed, hence 52-TC is energized tripping breaker as required.</span></p>