<p style="text-align:justify"><span style="font-size:14px">In preceding sections, the settings of overcurrent and earth-fault relays are calculated in case of radial feeder or parallel feeder networks fed from source at one end only. However, actual power system networks are interconnected having sources at multiple ends. The procedure of calculating settings of overcurrent and earth-fault relays is quite complicated and will involve the use of certain algorithms and a digital computer. We will not enter into these computer algorithms but will certainly solve the setting problems of comparatively simple interconnected networks so that the complicacy can be appreciated. The following steps are required to be followed for solving any problem of setting of overcurrent relays in an interconnected power system:</span></p>

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<ol>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Power-flow Analysis: The active and reactive power (P + j Q) flowing in every transmission line is to be calculated. Gauss-Seidel method, Newton-Raphson method and Fast Decoupled method are well-known methods for load flow analysis. But the requirement is yet complex. For a directional relay under consideration, it will be required to find out the configuration of network that makes the maximum current to flow through the relay in operative direction. For this purpose, line contingencies are to be considered. Usual practice is to consider single line contingency (tripping of one line at a time). This means 50 load-flow calculations for each relay if there are 50 transmission lines in a network. That is the reason why computer is required to suggest the worst</span></p>
	</li>
</ol>

<p style="margin-left:24px; text-align:justify">&nbsp;</p>

<p style="margin-left:24px; text-align:justify"><span style="font-size:14px">(i.e., one offering the maximum load current) configuration.</span></p>

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<ol start="2">
	<li style="text-align:justify">
	<p><span style="font-size:14px">Fault Current Calculations: The fault currents for L-L-L-g and L-g faults at each bus are required to be calculated. Once again, this requires multiple calculation exercises for single line contingency as discussed. The highest of the fault currents so calculated is to be used for a given relay.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Finding out Primary-Backup Pairs in a power system network: If there are 50 transmission lines in a power system network, there will be 100 overcurrent and 100 earth-fault relays. A relay under consideration may be backed up by (say) 11 (or more or less) relays. The back-up relay that is going to operate fastest shall operate 0.25 second later than a primary relay under consideration. Even if a single relay is missed, it may lead to series of unwanted operations of protective relaying system. Hence, a computer algorithm is resorted to for finding out primary-backup relay pairs. A well-known tool for the purpose is a Link-net structure using three one-dimensional vectors, namely, LIST (bus), NEXT (relay) and FAR (relay).</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Calculation of Plug-Setting.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Calculation of Time Setting.</span></p>
	</li>
</ol>

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

<p style="text-align:justify"><span style="font-size:14px">Now, we will calculate the relay settings for the simplest interconnected network containing only two buses, two transmission lines and four overcurrent relays. The reader will appreciate the complicacy involved and hence understand how complicated would be, the relay setting exercise in a state grid having 50 lines and 100 relays.</span></p>