<p style="text-align:justify"><span style="font-size:14px">The time delay in operation of a protective relay is required to achieve selectivity and back-up protection as we shall see later.</span></p>

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

<ol>
	<li style="text-align:justify">
	<p><span style="font-size:14px"><strong>Definite Time Overcurrent Relays:</strong></span></p>
	</li>
</ol>

<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><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16251265521625126552.jpg" style="height:480px; width:800px" /></strong></span></p>

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

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

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">The definite time overcurrent relay operates at the end of a definite set time, once the current passing through a relay exceeds a pre-set value (known as pick-up current). The characteristic of a definite time overcurrent relay is as shown in figure 6.4.</span></p>

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">Range of pick-up setting is usually 50-200 % of relay rating (1 amp or 5 amp), however, wider range is available in numerical equivalents. Generally, an instantaneous high-set unit is provided as an in-built arrangement in the same relay casing. The setting range of such a unit is usually 400-2000 % of relay rating (which is same as C. T. secondary rating). Operation of high-set unit can be blocked if required. Time setting of a definite-time unit ranges from 0.1 sec to 60 sec.</span></p>

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">Definite time overcurrent relays are used for overload alarm in generator protection and for stalling protection in induction motors.</span></p>

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

<ol start="2">
	<li style="text-align:justify">
	<p><span style="font-size:14px"><strong>Inverse Time Overcurrent Relays:</strong></span></p>
	</li>
</ol>

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">In these relays time of operation of a relay is inversely proportional to the current passing through coil of a protective relay. Once again, we will not discuss the electromagnetic construction of such relays as it is obsolete. We will see the static construction later.</span></p>

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">Current setting range available for phase relays [the relays operating in case of phase (L-L or L-L-L) faults] is 50-200 % of relay rating in seven equal steps (50-75-100-125-150-175-200 % of relay rating). For ground relays [the relays operating in case of ground (L-g or L-L-g) faults], however construction is the same, the setting ranges available are 5-20 % (5-7.5-10-12.5-15-17.5-20 %), 10-40 % and 20-80 % (all in seven equal steps) of relay rating. In-built high-set unit in conjunction with time-delayed unit is also, obviously, provided in these relays. The wider range (say, 10-400 % in case of phase and ground relays) and closer steps (say, steps of 1 %) are offered by numerical relay manufacturers.</span></p>

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

<p style="margin-left:72px; text-align:justify"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16251266781625126678.jpg" style="height:719px; width:800px" /></p>

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">Figure 6.5 shows three characteristics on the same graph. Normal inverse characteristic follows the equation It = k. If we use this characteristic, time setting of relays progressively towards source starting from load becomes difficult if desired selectivity is to be maintained. The definite time characteristic is a solution in this case, but the time of operation of relays located near source will be objectionably large. The hybrid of the two (which is used in practice) is the normal inverse IDMT (Inverse Definite Minimum Time) characteristic. </span></p>

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">The glossary of common terms used in conjunction with IDMT overcurrent relays is given as follows:</span></p>

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

<ol style="list-style-type:lower-roman">
	<li style="text-align:justify">
	<p><span style="font-size:14px">Plug-setting (PS): This is expressed in terms of either ampere or percentage of relay rating. Plug-setting is the threshold above which the relay will start operating.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Plug Setting Multiplier (PSM): The multiple of plug-setting current is known as PSM.&nbsp;&nbsp;<span class="math-tex">\(PSM=\frac{I_f}{P.S.}\)</span></span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Where I<sub>f</sub> = Fault current in primary of CT</span></p>
	</li>
</ol>

<p style="text-align:justify"><span style="font-size:14px">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;And P. S. = Plug setting in CT primary terms</span></p>

<p style="text-align:justify"><span style="font-size:14px">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Or, I<sub>f</sub> = Fault current in CT secondary</span></p>

<p style="text-align:justify"><span style="font-size:14px">&nbsp; &nbsp; &nbsp; &nbsp; And P. S. = Plug setting in CT secondary terms.</span></p>

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

<p style="text-align:justify"><span style="font-size:14px">The characteristics are generally plotted in terms of operating time v/s PSM.</span></p>

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

<ol start="3" style="list-style-type:lower-roman">
	<li style="text-align:justify">
	<p><span style="font-size:14px">Time Multiplier Setting (TMS): The operating time for IDMT relays is varying w. r. t. the current magnitude. If it is desired to change the time of operation of the relay at same current magnitude, TMS (time-setting) can be varied. If the time of operation of relay for the current I amp is t seconds with TMS = 1.0, time of operation of the relay for the same current with TMS = 0.5 will be 0.5 t seconds. The operating time is usually specified in seconds at PSM = 10 and TMS = 1.0.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Resetting time: The time within which the relay returns to its normal state from its fully operated state, once the current is discontinued, is known as resetting time. The resetting time is specified for TMS = 1.0.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Overshoot or Overtravel: When the current is discontinued by clearing the fault, the relay should start resetting. But due to mechanical inertia in the electromechanical relays and due to electrical inertia provided by inductance of relay circuitry (current does not immediately reduce to zero, once the current is discontinued, but reduces to zero exponentially), the relay keeps on counting time for a short while (before start of reset). This short time is known as </span><span style="font-size:14px">&ldquo;overshoot&rdquo;. This time is of the order of 5 cycles in electromagnetic relays, whereas it is of the order of 1 cycle in static and numerical relays.</span></p>
	</li>
</ol>

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

<p style="margin-left:72px; text-align:justify"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16251268111625126811.jpg" style="height:1153px; width:800px" /></p>

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">The characteristic curves of normal inverse IDMT overcurrent relays are shown in figure 6.6. The time of operation of a normal inverse IDMT overcurrent relay can be calculated using the approximate mathematical formula as follows:</span></p>

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px"><span class="math-tex">\(Time\hspace{0.1cm}of\hspace{0.1cm}operation\)</span></span></p>

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px"><span class="math-tex">\(=\frac{3}{log_{10}PSM}\times\hspace{0.1cm}TMS\)</span></span></p>

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

<ol start="3">
	<li style="text-align:justify">
	<p><span style="font-size:14px"><strong>&nbsp;Very Inverse IDMT Overcurrent Relays:</strong></span></p>
	</li>
</ol>

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

<p style="text-align:justify"><span style="font-size:14px"><strong><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16251268441625126844.jpg" style="height:964px; width:800px" /></strong></span></p>

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">The characteristic of these relays is steeper than that of normal inverse IDMT relays (refer figure 6.7). We will discuss the application of such relays in transmission line protection.</span></p>

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

<ol start="4">
	<li style="text-align:justify">
	<p><span style="font-size:14px"><strong>&nbsp;Extremely Inverse IDMT Overcurrent Relays:</strong></span></p>
	</li>
</ol>

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">The characteristics of these relays is yet more steep than very inverse IDMT relays (refer figure 6.7). The significance of such a characteristic will be discussed later.</span></p>

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

<ol start="5">
	<li style="text-align:justify">
	<p><span style="font-size:14px"><strong>&nbsp;Normal Inverse Self Powered IDMT Relays:</strong></span></p>
	</li>
</ol>

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

<p style="margin-left:72px; text-align:justify"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16251269031625126903.jpg" style="height:584px; width:800px" /></p>

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">These relays are used in unattended substations where auxiliary d. c. supply is not available. The basic concept of such a relay is shown by figure 6.8. The tripping power in case of such relays is taken from C. T. secondary. The unit (IAS) is provided with a normally closed heavy duty contact. Under normal (no-fault) condition, a C. T. secondary current will not pass through the trip coil of the breaker. When the IDMT unit operates (or high-set unit operates), IAS is energized, IAS-1 opens, C. T. secondary current flows through the trip coil, breaker trips and fault is cleared.</span></p>

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

<ol start="6">
	<li style="text-align:justify">
	<p><span style="font-size:14px"><strong>&nbsp;Voltage Controlled Normal Inverse IDMT Overcurrent Relays:</strong></span></p>
	</li>
</ol>

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

<p style="margin-left:72px; text-align:justify"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16251270141625127014.jpg" style="height:780px; width:800px" /></p>

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">In case of comparatively smaller sized generators for which automatic voltage regulators are not provided, if a fault occurs, the steady-state fault current is less than CMR of the generator as the synchronous reactance is more than 100 %. This is particularly true in case of fault in transmission line and line relay and back-up overcurrent relay placed in secondary of GT fails. The back-up overcurrent relay in this case is in third line of defence and hence considerable delay in time of operation. By this time, the generator impedance has entered in steady-state region reducing the value of fault current below CMR of the generator. Under this condition, a simple inverse time overcurrent relay cannot be set. Voltage controlled overcurrent relay, in this case, will solve the problem of setting. The relay offers two time-current characteristic curves. First curve offers overload protection of the generator under normal voltage condition. When a fault occurs, voltage collapses below 70 % of rated voltage, which automatically shifts the plug-setting of a relay to 40 % of its original value. Thus, pick-up current is reduced which allows the operation of the relay at reduced fault current below CMR. Also, the time of operation of the relay at higher PSM gets reduced. The second curve, hence, offers the back-up protection for line fault as discussed. These two characteristic curves are shown in figure 6.9.</span></p>

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

<ol start="7">
	<li style="text-align:justify">
	<p><span style="font-size:14px"><strong>&nbsp;Long Time Inverse IDMT Overcurrent Relays: </strong></span></p>
	</li>
</ol>

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">These relays can be used for the dual (overcurrent cum overload) purpose. The time of operation of such relays is 30 seconds at PSM = 5, TMS = 1.0.</span></p>

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

<ol start="8">
	<li style="text-align:justify">
	<p><span style="font-size:14px"><strong>&nbsp;1.3 second Normal Inverse IDMT Overcurrent Relays:</strong></span></p>
	</li>
</ol>

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">&nbsp;These relays are faster in operation w. r. t. 3 second normal inverse relays. The time of operation of such relays is 1.3 seconds at PSM equal to 10 and TMS equal to 1.0.</span></p>

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">The general equation that covers the time of operation of major IDMT relays is given as follows:</span></p>

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px"><span class="math-tex">\(The\hspace{0.2cm}time\hspace{0.2cm}of\hspace{0.2cm}operation\hspace{0.2cm}of\hspace{0.2cm}a\hspace{0.2cm}relay\)</span></span></p>

<p style="margin-left:72px; text-align:justify"><span class="math-tex">\(=\frac{\beta}{PSM^\alpha-1}\times\hspace{0.1cm}TMS\)</span></p>

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

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

<p style="margin-left:72px; text-align:justify"><span style="font-size:14px">The value of &nbsp;<span class="math-tex">\(\alpha\)</span>&nbsp;and&nbsp;<span class="math-tex">\(\beta\)</span>&nbsp;are different for different characteristics. These values are given in table 6.2.</span></p>

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

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

<table cellspacing="0" class="MsoTableGrid" style="border-collapse:collapse; border:none; margin-left:88px; width:900px">
	<caption>
	<p>&nbsp;</p>

	<p><strong><span style="font-size:18px">Table 6.2: Major IDMT Overcurrent Relays</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:170px">
			<div style="text-align:center"><span style="font-size:28px">Type of IDMT Relay</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:94px">
			<div style="text-align:center"><span style="font-size:28px"><em><span class="math-tex">\(\alpha\)</span></em></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:95px">
			<div style="text-align:center"><span style="font-size:28px"><em><span class="math-tex">\(\beta\)</span></em></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; height:21px; vertical-align:top; width:170px">
			<div style="text-align:center"><span style="font-size:28px">Normal Inverse</span></div>
			</td>
			<td style="border-bottom:1px solid black; border-left:none; border-right:1px solid black; border-top:none; height:21px; vertical-align:top; width:94px">
			<div style="text-align:center"><span style="font-size:28px">0.02</span></div>
			</td>
			<td style="border-bottom:1px solid black; border-left:none; border-right:1px solid black; border-top:none; height:21px; vertical-align:top; width:95px">
			<div style="text-align:center"><span style="font-size:28px">0.14</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:170px">
			<div style="text-align:center"><span style="font-size:28px">Very Inverse</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:94px">
			<div style="text-align:center"><span style="font-size:28px">1.0</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:95px">
			<div style="text-align:center"><span style="font-size:28px">13.5</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:170px">
			<div style="text-align:center"><span style="font-size:28px">Extremely Inverse</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:94px">
			<div style="text-align:center"><span style="font-size:28px">2.0</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:95px">
			<div style="text-align:center"><span style="font-size:28px">80.0</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:170px">
			<div style="text-align:center"><span style="font-size:28px">Long Time Inverse</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:94px">
			<div style="text-align:center"><span style="font-size:28px">1.0</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:95px">
			<div style="text-align:center"><span style="font-size:28px">120.0</span></div>
			</td>
		</tr>
	</tbody>
</table>

<p>&nbsp;</p>

<p>&nbsp;</p>