<p style="text-align:justify"><span style="font-size:14px"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16251391421625139142.jpg" style="height:489px; width:800px" /></span></p>

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

<p style="text-align:justify"><span style="font-size:14px">Instead of using magnitude of fault current as a discriminating quantity for discriminating location of faults in different zones, time of operation of relays is used as a discriminating quantity for the same purpose. This solves all the problems discussed in foregoing section. For this purpose, relays R<sub>1</sub>, R<sub>2</sub> and R<sub>3</sub> of figure 7.2 are definite time overcurrent relays. The distance-time characteristic of such relays is shown in figure 7.4. The maximum pick-up current of relay R<sub>1</sub> is such that it reaches up to bus C for double line fault at bus C with minimum generation. This means that if the relay R<sub>1</sub> reaches beyond bus C, it is not objectionable but if it does not reach up to bus C, relay R<sub>1</sub> will not provide 100 % back-up to relay R<sub>2</sub>. Similarly, R<sub>2</sub> should reach up to bus D and R<sub>3</sub> should cover longest 415 V distributor emanating from bus D. The time setting for relay R<sub>3</sub> is t<sub>3</sub> sec (say 0.1 second). R<sub>3</sub> has to be coordinated with fuse so that for a fault in 415 V distributor, fuse melts and trips the distributor. Should the fuse fail to trip the distributor, R<sub>3</sub> will offer back-up protection. If there is a fault in line section CD, relay R<sub>3</sub> will operate and trip the breaker CB<sub>3</sub> at the end of time delay t<sub>3</sub> sec. Should any of the component of switchgear (CT, CB<sub>3</sub> or R<sub>3</sub>) fail, relay R<sub>2</sub> will give back-up protection. This is known as remote back-up. Obviously, time of operation of relay R<sub>2</sub> is set at t<sub>2</sub> sec (say 0.35 sec) i. e., more than time of operation of relay R<sub>3</sub>. The difference (t<sub>2</sub>-t<sub>3</sub>) is t<sub>d</sub>, the discrimination time margin. The reason for this discrimination time as 0.25 sec will be discussed later. Similarly, R<sub>1</sub> is set to operate after time delay t<sub>1</sub> sec (say 0.6 sec). There can be such 4 or 5 (or more) sections in series. In this case, the time of operation of a relay near power source is objectionably high. This is dangerous as the fault currents for faults in the vicinity of source are very large. Thus, the contradiction of the scheme of protection is that the heaviest fault is cleared the slowest. This difficulty can be overcome by introducing an instantaneous high-set unit in-built with definite time unit such that the faults (L-L-L fault) up to 80 % of line section from relaying point are cleared instantaneously. By the inclusion of such high-set instantaneous units, the saving in time of clearing the faults as shown by shaded portion in figure 7.4 is achieved. The faults beyond 80 % of line section are taken care of by time delayed unit.</span></p>

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

<p style="text-align:justify"><span style="font-size:14px">As instantaneous overcurrent relays are highly prone to Z<sub>s</sub>/Z<sub>l</sub> ratio, if Z<sub>l</sub> is too small (line is too short), Z<sub>s</sub>/Z<sub>l</sub> will be high. </span></p>

<p style="text-align:justify"><span style="font-size:14px">Therefore,</span></p>

<p style="text-align:justify"><span style="font-size:14px"><span class="math-tex">\(I_f=\frac{E}{Z_s+Z_l}\)</span>&nbsp;</span></p>

<p style="text-align:justify"><span style="font-size:14px">&nbsp; &nbsp;<span class="math-tex">\(\cong\frac{E}{Z_s}\)</span></span></p>

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

<p style="text-align:justify"><span style="font-size:14px">The instantaneous overcurrent relay cannot be that accurate as it can differentiate between fault immediately after relaying point in first line section and fault 10 (or 20) km away in the next line section. This can happen for very short transmission line just emanating from source. For example, a 220 kV transmission line from Ukai hydro power plant to Ukai thermal power plant is just 6 km long. In such a case, high-set instantaneous unit cannot be provided or if provided, it has to be blocked which results in very dangerously long time of operation of relay R<sub>1</sub> (in figure 7.4). Hence definite time overcurrent relays also do not find its application for radial feeder protection.</span></p>