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 0.25 sec. when there is fault near the power station. Fig 11 illustrates the characteristic curve of the relays installed at the beginning of each section. Let there be a fault in between substation B and C at point F. The time- distance relay at sub-stations B will trip in 0.5 sec. while the relay at sub-section A and at power station will operate in 1.05 and 1.53 sec. respectively. In order that the fault may be cleared by the nearest circuit breaker first the circuit breaker operation time should also he included and if the current breaker opening time is assumed to be 0.25 sec. after its trip contacts energize, the breaker operation time is modified to be represented by dotted lines for sub-stations A and B. Further it will be obvious that the distance relays at substation A and at power station will act as back up relays for fault at point F, i.e. if the relay at station B fails to operate the relay of sub-station A will trip the circuit after 1.3 see. (including time of C.B. operation) and even relay at substation A fails to operate, then the relay at power station will trip the circuit in 1.53 sec

Now assume that the distance between the first sub-station and the power station is larger enough than the distance between the subsequent substations. Let the impedance relays be adjusted so that they operate, in 0.75 sec. (excluding time for CB operation)

For a fault at the far end of its own section, i.e. the relay at power station will trip is 0.75 sec for a fault at sub-station A. Similarly the relay at sub-station A will trip in 0.75 sec. for a fault at sub-station B. Let, there be a fault at point shown by dotted line in Fig 12. It will be clear that the relay at power station A will operate first in 0.975 sec. then the relay at sub-station A will trip in 1.025 sec. which is incorrect.

So, in order to provide clear discrimination it is necessary that the relay at generating station must have such adjustment that at must provide a discrimination of about 0.5 sec, at the far end of subsequent section as is shown in Fig 12. Thus, for a back up protection for section between sub-stations A and B the back up relay at power station will operate in 1.5 sec and if the time of operation of the circuit breaker is also added to it, the fault will be cleared an 1.75 sec which is rather high.

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