<p style="text-align:justify"><span style="font-size:14px">If the country or a state is very small with reference to its geographical area, the transmission lines will be too short, resulting in very large fault currents. These fault currents are required to be limited or reduced. This can be done by using fault current limiting reactors. The advantage of using reactors is that there will be no power loss, but the disadvantage is that power factor worsens. Yet a very useful application of fault limiting reactors is discussed as follows:</span></p>

<p style="text-align:justify"><span style="font-size:14px">There is a power system network as shown in figure 4.19. All 66 kV breakers used are of symmetrical breaking capacity 750 MVA. After 5 years (or may be 10 years), 50 MVA capacity is required to be added as load demand has increased. This necessitated to install one more 66 kV incomer and one more 66/11 kV, 50 MVA power transformer. But as third parallel incomer has been added feeding more power to 66 kV bus, the fault current for a fault at 66 kV bus will increase. This will necessitate all the breakers (the new ones to be installed and the old ones which are already existing) to be of higher breaking capacity. The new ones can be purchased with higher breaking capacity, but the old breakers will have to be replaced with breakers having higher breaking capacity. This is not economically viable. In this case, a bus reactor can be installed as shown in figure 4.20. If need be, a similar fault limiting reactor can be installed at 11 kV bus too. This reactor will limit the fault current at 11 kV bus as required.</span></p>

<p style="text-align:justify"><span style="font-size:14px"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16160622531616062253.jpg" style="height:631px; width:800px" /><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16160622761616062276.jpg" style="height:569px; width:800px" /></span></p>

<p style="text-align:justify"><span style="font-size:14px"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16160623181616062318.jpg" style="height:491px; width:800px" /></span></p>

<p style="text-align:justify"><span style="font-size:14px">The same motive can be achieved in yet different way. Refer figure 4.21. The old installations are shown with full lines and the new installations are shown with dotted lines. If old breakers are not to be replaced with breakers having higher breaking capacity, the only requirement is an interlock such that:</span></p>

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

<ol>
	<li style="text-align:justify">
	<p><span style="font-size:14px">When both the incomer circuit breakers CB<sub>1</sub> and CB<sub>2</sub> are closed (ON), bus sectionalizing breaker cannot be made ON.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Bus sectionalizing breaker can be made ON only if either of the incomer circuit breaker is OFF.</span></p>
	</li>
</ol>

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

<p style="text-align:justify"><span style="font-size:14px">This interlock serves the purpose of limiting fault current as two incomers are never in parallel. Other advantages of using sectionalizing breaker are:</span></p>

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

<ul>
	<li style="text-align:justify">
	<p><span style="font-size:14px">The disadvantage of low power factor with use of bus reactor is not there when we use bus sectionalizing breaker.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">In case of bus fault, whole of the bus has to be evacuated when bus reactor is used. Whereas half of the bus can be used in case of bus fault when sectionalizing breaker is used, as the healthy part of the bus can be used as the concerned incomer breaker and sectionalizing breaker will be tripped by bus protection system.</span></p>
	</li>
</ul>