<p style="text-align:justify"><span style="font-size:14px">The demand of electrical power is increasing at a very fast rate. This necessitates installation of transmission lines and distribution network reaching out to all urban and rural areas of the country. The voltage of transmission has reached 765 kV and still higher voltages are planned.</span></p>

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<p style="text-align:justify"><span style="font-size:14px">These transmission lines are required to be protected by comprehensive and quite involved protection schemes so that the interruption of power is reduced to minimum in terms of outage and the area affected. The protective scheme must be very fast and yet selective.</span></p>

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<p style="text-align:justify"><span style="font-size:14px">It is usual practice to protect feeders of 11 kV voltage rating and transmission lines of 66 kV by overcurrent and earth-fault relays as primary (and only) protection as distance protection scheme will not be economically viable for the purpose. 132 kV and 220 kV lines are part of interconnected power system network and forced outage of such lines would cost a lot to generation and transmission companies. Therefore, these lines are protected by more comprehensive distance protection schemes. Very important of these 132 kV and 220 kV lines, 400 kV and 765 kV transmission lines are protected by line differential (carrier current) protection. However, overcurrent and earth-fault relays are used as back-up or secondary relays for protection of such ehv and uhv lines.</span></p>

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<p style="text-align:justify"><span style="font-size:14px">We will discuss about details of overcurrent and earth-fault protection of transmission lines.</span></p>