<p style="text-align:justify"><span style="font-size:14px">The electrical equipment will be damaged due to faults, they will be required to be repaired and till then there will be an outage of the portion of the power system. The duty of protection and maintenance engineers is to reduce down time (time elapsed from clearance of the fault to resumption of the portion of the power system back to consumers) to minimum.</span></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"><strong>Damage due to faults are of two kinds:</strong></span></p>

<p>&nbsp;</p>

<ol>
	<li style="text-align:justify">
	<p><span style="font-size:14px"><strong><u>Thermal Damage:</u> </strong>Fault current magnitude of 2 to 5 times (or even up to 8 times) the CMR (Continuous Maximum Rating) of the electrical equipment will heat the conductor (heat produced is proportional to I<sup>2</sup>Rt, I= Current passing through the conductor, R= Resistance of the conductor and t=time for which current is passed) and hence the insulation around it. When the current equal to CMR is passing, the conductor and insulation around it are also heated because of which the temperature rises. But at the same time heat is dissipated which is proportional to (T-T<sub>R</sub>)<sup>4</sup>. The heat is dissipated either naturally or using forced cooling means. As heat is dissipated at a higher rate than heat generated, temperature will stabilize at certain value. This equilibrium temperature reached for current equal to CMR will be withstood by the insulation of the equipment because the equipment is properly so designed. But when the fault current, as mentioned, passes through the equipment, the heat will generate at a faster rate and the equilibrium temperature reached in this case will be higher. This temperature will be certainly more than withstand limit of the insulation of the equipment, insulation will thermally break down, resulting into another fault if the remedial actions of the first fault are not taken. As thermal break down process is a function of time, the protective relaying has to act just before the time (leaving 10 to 20 % time margin) at which the insulation is going to be damaged. The time of operation of protective relay should, obviously, not more than this time at which insulation is damaged. But it should not be very earlier than this time. This is because the fault may be of transient nature and it may vanish before the withstand temperature limit of insulation is reached. In this case unnecessary operation of a relay and consequent tripping of a breaker will lead to unwanted outage and discomfort to consumers. Therefore, the function of protective gear is very critical. The relays used are time-delayed overcurrent relays in this case.</span></p>
	</li>
</ol>

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

<ol start="2">
	<li>
	<p style="text-align:justify"><span style="font-size:14px"><strong><u>Electrodynamic Damage:</u></strong></span>&nbsp;<span style="font-size:14px">When the fault current exceeds 8 to 10 times the full load rating of the equipment, this kind of damage can occur. This fault current flows through parallel conductors placed in transformers or in slots of generators, the repellent force is produced which would destruct the whole equipment structurally within few (8 to 10 or in certain cases little more) cycles (one cycle in 50 Hz a. c. system equals 20 milliseconds) and the equipment may burst. The structural or mechanical design of equipment is made such that these electrodynamic repellent forces produced by normal operating current of the equipment and those produced by smaller (2 to 8 times CMR) magnitudes of fault currents can be withstood by the equipment and it will not be electrodynamically damaged (the equipment can be damaged thermally as discussed, but this happens after delay in time, for which protective scheme is there). But for large short-circuit currents the devastating damage envisaged is to be avoided by using instantaneous overcurrent relay which operates within 1 to 3 cycles.</span></p>
	</li>
</ol>

<p style="margin-left:72px; text-align:justify">&nbsp;</p>

<p style="text-align:justify"><span style="font-size:14px">It would be worthwhile to have an idea of probability of incidence of faults on the different elements of a power system (Table 4.1).</span><span style="font-size:11pt">&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span></p>

<table align="center" border="1" cellspacing="0" dir="ltr" style="width:800px">
	<caption>
	<div><span style="font-size:28px">Table 4.1: Fault Statistics</span></div>
	</caption>
	<thead>
		<tr>
			<th scope="col" style="border-color:black; vertical-align:top; width:235px">
			<div><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Element</span></div>
			</th>
			<th scope="col" style="vertical-align:top; width:191px">
			<div><span style="font-size:28px">&nbsp;&nbsp;&nbsp; Percentage of Faults</span></div>
			</th>
		</tr>
	</thead>
	<tbody>
		<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:235px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp; Overhead Transmission Lines</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:191px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 45-55</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:235px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp; Underground Cables</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:191px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 10-12</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:235px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp; Switchgears</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:191px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 10-15</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:235px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp; Power Transformers</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:191px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 10-15</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:235px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp; 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:191px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;1-3</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:235px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp; Control Circuits</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:191px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 2-4</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:235px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp; Miscellaneous</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:191px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 7-10</span></div>
			</td>
		</tr>
	</tbody>
</table>

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

<p style="text-align:justify"><span style="font-size:14px">Faults occurring in overhead transmission lines are sharing the highest percentage. Therefore, the statistics of different kinds of faults occurring in an overhead transmission line will be, really, interesting (Table 4.2).</span><span style="font-size:11pt">&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span></p>

<table align="center" border="1" cellspacing="0" dir="ltr" style="width:800px">
	<caption>
	<div><span style="font-size:28px">Table 4.2: Statistics of Faults occurring in Overhead Lines</span></div>
	</caption>
	<thead>
		<tr>
			<th scope="col" style="border-color:black; vertical-align:top; width:169px">
			<div><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;Type of Fault</span></div>
			</th>
			<th scope="col" style="vertical-align:top; width:219px">
			<div><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Percentage Occurrence</span></div>
			</th>
		</tr>
	</thead>
	<tbody>
		<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:169px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; L-g Faults</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:219px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;80-90</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:169px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; L-L Faults</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:219px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 10-12</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:169px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; L-L-g Faults</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:219px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 3-5</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:169px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;L-L-L-g Faults</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:219px">
			<div style="text-align:center"><span style="font-size:28px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 0.1-0.2</span></div>
			</td>
		</tr>
	</tbody>
</table>

<p style="text-align:justify"><span style="font-size:11pt"><span style="color:white">It is very clear tha<span style="font-size:14px">t </span></span><span style="font-size:14px">Line to Ground faults are the most commonly occurring faults.</span></span></p>

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

<p style="text-align:justify"><span style="font-size:14px">It is very necessary to learn that 85% of faults listed in Table 4.2 are transient in nature. For example, the flashover occurs across the line insulator due to lightning or switching overvoltage. The overvoltage due to any of these two causes decays to normal voltage within milliseconds (maximum 5 milliseconds). However, the arc due to flashover maintains up to about 0.3 second. At the end of this time, the air around insulator regains its dielectric strength and fault is automatically rectified or self-healed. This is one kind of transient fault. In this case, either the relay should not operate, and breaker should not trip up to 0.3 second or if tripping occurs, circuit breaker is automatically reclosed using auto-recloser relays.</span></p>

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

<p style="text-align:justify"><span style="font-size:14px">For the remaining 15% sustained faults, maintenance staff have to rectify the fault. (Obviously first locate the fault in a long transmission line which was time consuming as one has to patrol on the route of line to locate the fault. With the advent of numerical relays, this task has been made very easy as the relay indicates the location very accurately). </span></p>

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

<p style="text-align:justify"><span style="font-size:14px">To summarize, it can be stated that the critical clearing time of the faults by protective relays is very important. Later operation damages the electrical equipment and too earlier operation unnecessarily trip the breaker which may result in power system instability. Hence, tripping a breaker unnecessarily is many a times as equally harmful as not tripping a breaker in case of fault. Therefore, the role of protection engineer is very crucial. </span></p>

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

<p style="text-align:justify"><span style="font-size:14px">Other important consequences of occurrence of faults are:</span></p>

<ul>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Because of damage or destruction of electrical equipment, they will be required to be repaired which results in long down time before portion of power system is resumed back to service.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Voltage drops.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Loss of revenue to electricity company and loss of production (and hence revenue) to industries.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Inconvenience caused to consumers.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Power system instability: The synchronism between machines working in power system could be lost because of fault occurrence and if this asynchronous operation is beyond transient stability limit of the generator, it will be isolated from bus because of operation of pole-slipping protection. If this is repeated for all generators of the system, there will be cascaded tripping (as it is called) and complete dark-out in the state grid.</span></p>
	</li>
</ul>

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

<p style="text-align:justify"><span style="font-size:14px">Therefore, proper protective system has to be designed to avoid these consequences or (if avoiding is not possible) minimize the effects of these consequences. For this purpose, minimum faulted portion of the power system has to be selectively isolated from the remnant of the healthy power system as quickly as it is possible. This is so often quite complicated, and we will enter into these complicacies little later and will also appreciate how modern Numerical protective relays make this task possible.</span></p>