When a generating station supplies to three sub-stations radially, it is desired to isolate all those sub-stations beyond the fault point. This can be achieved by the use of time delayed over current relay but the main disadvantage is that if the fault is near the power station it will take much longer time to isolate the system and this may cause serious defects. However with the use of time-distance relay the fault can be cleared much early. Let the impedance of each section of the transmission line be the same. Let the time distance relay installed at the beginning of each section of the line be so adjusted that it provides discrimination with the circuit breaker on the following section which should be about 0.75 sec. At the power station end the relay is adjusted so as to trip in read more

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This relay operates at a constant time for all faults independent of the distance of occurrence of fault. For its application consider Fig 10 which represents a power station feeding transforming sub-station. The power transformer is protected independently by inverse-time over current relay. Let, it be required to provide a relay at the input side so that it wilt trip the circuit instantaneously for any fault on the transmission line and at the same time permit the use of time delayed inverse-time over current relay for the transformer This cannot be achieved with the use of instantaneous over current relay set to trip the circuit for a certain minimum short circuit current because the fault current depends on the number of sections in service and the number of sources feeding the read more

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Although the protection for cable feeders (only non-pilot wire type) can be used for transmission lines also, but generally such protections for transmission lines are not used as the transmission lines are longer and work at much higher voltage and there are frequent faults. For protection of transmission lines usually distance protections are used, the principle of operation of these relays as explained earlier depends on the fact that under fault conditions the ratio of voltage to current remains constant and is independent of the fault current and its power factor. As, a matter of fact such relays are designed to measure the impedance of the line (in some cases only reactance) or the ratio of voltage to current up to the fault point and it indicates the distance over which the fault read more

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The probability of faults occurring on the lines is much more due to their greater length and exposure to atmospheric conditions. This has called for many protective schemes which have no application to the comparatively simple cases of alternators and transformers. The requirements of line protection are: (i)In the event of a short circuit the circuit breaker closest to the fault should open all other circuit breakers remaining in a closed position (ii)In case the nearest breaker to the fault fails to open back up protection should be provided by the adjacent circuit breakers. (iii) The relay operating time should be just as short as possible in order to preserve system stability with out unnecessary tripping of circuits. Differential method is ideal for protection of lines but it is read more

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