<p style="text-align:justify"><span style="font-size:14px"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16251152001625115200.jpg" style="height:585px; width:800px" /></span></p>

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

<p style="text-align:justify"><span style="font-size:14px">Figure 6.1 shows a single line diagram of typical power system network. Electrical power is generated at 15.75 kV (210 MW) and stepped up to 220 kV using generator transformer (GT). This power is transmitted at 220 kV (GT secondary voltage and transmission voltage decided by economic choice as discussed earlier). As the power to be transmitted further is less, economic voltage is 66 kV (say) now. Therefore, power is stepped down at receiving substation shown in figure 6.1. There are many equipments (not shown in figure) in this substation as discussed. Only power transformers (220/66 kV, 150 MVA) are shown. Capacities shown in figure 6.1 are typical. 66 kV transmission line terminates at 66/11 kV distribution substation. Economic choice might have been stepping down to 132 kV at receiving substation. In that case, transformation would have been 220 kV to 132 kV and 132 kV transmission further. Then there would have been 132/66 kV intermediate substation and 66 kV transmission as in figure 6.1.</span></p>

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

<p style="text-align:justify"><span style="font-size:14px">In distribution substation, voltage is stepped down to 11 kV (the distribution voltage). 11 kV feeders feed the pole mounted transformer or distribution transformer (11 kV/415 V). These distribution transformers are protected by drop-out (DO) fuse on primary side of transformer and by fuse or (in modern times) by MCCB (Molded Case Circuit Breaker) on secondary side of transformer. This protective gear is decided by economic reasons to be discussed later. 415 V distributors finally feed power to consumers.</span></p>

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

<p style="text-align:justify"><span style="font-size:14px">Until this power system operates normally, there are no problems. But faults and abnormalities are inevitable, and these will damage the equipment of power system network. Hence faults are to be cleared as quickly as possible. This function is carried out by components of protective gear [protective relays, circuit breakers, transducers (CT and PT) and related control circuit].</span></p>

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

<p style="text-align:justify"><span style="font-size:14px">There is a limit to the amount that can be spent on protective gear. Thumb rule says that protective relaying equipment shall not cost more than 5 % or at the most 10 % (if the equipment to be protected is very costly and important) of the total cost of equipment to be protected. Table 6.1 shows the break-up of costs of components of switchgear for protection of typical equipment. As circuit breaker is a device which is needed to manually make and break the circuit, it is a usual practice not to consider its cost in protective gear.</span></p>

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

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

<table align="center" cellspacing="0" class="MsoTableGrid" style="border-collapse:collapse; border:none">
	<caption>
	<p><strong><span style="font-size:20px">Table 6.1: Costs of Switchgear Components for Protecting An Equipment</span></strong></p>
	</caption>
	<tbody>
		<tr>
			<td style="border-bottom:1px solid black; border-left:1px solid black; border-right:1px solid black; border-top:1px solid black; vertical-align:top; width:274px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;Total Cost of Equipment to be Protected</span></div>
			</td>
			<td style="border-bottom:1px solid black; border-left:none; border-right:1px solid black; border-top:1px solid black; vertical-align:top; width:189px">
			<div style="text-align:center"><span style="font-size:28px">100 %</span></div>
			</td>
		</tr>
		<tr>
			<td style="border-bottom:1px solid black; border-left:1px solid black; border-right:1px solid black; border-top:none; vertical-align:top; width:274px">
			<div style="text-align:center"><span style="font-size:28px">Protective Relays</span></div>
			</td>
			<td style="border-bottom:1px solid black; border-left:none; border-right:1px solid black; border-top:none; vertical-align:top; width:189px">
			<div style="text-align:center"><span style="font-size:28px">0.55 %</span></div>
			</td>
		</tr>
		<tr>
			<td style="border-bottom:1px solid black; border-left:1px solid black; border-right:1px solid black; border-top:none; vertical-align:top; width:274px">
			<div style="text-align:center"><span style="font-size:28px">Control Panels and Associated Wiring</span></div>
			</td>
			<td style="border-bottom:1px solid black; border-left:none; border-right:1px solid black; border-top:none; vertical-align:top; width:189px">
			<div style="text-align:center"><span style="font-size:28px">0.40 %</span></div>
			</td>
		</tr>
		<tr>
			<td style="border-bottom:1px solid black; border-left:1px solid black; border-right:1px solid black; border-top:none; vertical-align:top; width:274px">
			<div style="text-align:center"><span style="font-size:28px">Control Room</span></div>
			</td>
			<td style="border-bottom:1px solid black; border-left:none; border-right:1px solid black; border-top:none; vertical-align:top; width:189px">
			<div style="text-align:center"><span style="font-size:28px">0.12 %</span></div>
			</td>
		</tr>
		<tr>
			<td style="border-bottom:1px solid black; border-left:1px solid black; border-right:1px solid black; border-top:none; vertical-align:top; width:274px">
			<div style="text-align:center"><span style="font-size:28px">Transducers (C. T. and P. T.)</span></div>
			</td>
			<td style="border-bottom:1px solid black; border-left:none; border-right:1px solid black; border-top:none; vertical-align:top; width:189px">
			<div style="text-align:center"><span style="font-size:28px">4.20 %</span></div>
			</td>
		</tr>
		<tr>
			<td style="border-bottom:1px solid black; border-left:1px solid black; border-right:1px solid black; border-top:none; vertical-align:top; width:274px">
			<div style="text-align:center"><span style="font-size:28px">Total Cost of Switchgear</span></div>
			</td>
			<td style="border-bottom:1px solid black; border-left:none; border-right:1px solid black; border-top:none; vertical-align:top; width:189px">
			<div style="text-align:center"><span style="font-size:28px">5.27 %</span></div>
			</td>
		</tr>
	</tbody>
</table>