<p style="text-align:justify"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16251283541625128354.jpg" style="height:616px; width:800px" /></p>

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<p style="text-align:justify"><span style="font-size:14px">Figure 6.17 shows a circuit diagram of static instantaneous overcurrent relay. A current passing in transmission line is stepped down by high voltage C. T. of ratio 100/1 (C. T. primary current depending upon maximum full load current of the transmission line). The secondary juice of the C. T. is fed to the input terminals 1 and 2 of a relay. As static relays cannot handle current of even 1 amp (this current can be even 20 amp when fault occurs), the current is further stepped down to a value usually 25 milliamperes by an input C. T. inbuilt in the static relay. This current passes through a resistor of value 40 ohms provided across secondary of input C. T. Thus, A voltage signal of 1 volt is made available which is equivalent to 1 amp (input to a relay) or 100 amp passing through the line. This signal is rectified so that 1 volt d. c. is fed further to a level detector. Op-amp 741 can be used as a level detector. Output from rectifier is fed to the non-inverting input of an op-amp. Reference voltage to an op-amp is a d. c. reference derived from a variable pot the output of which can be adjusted from 1-20 volts d. c. We have adjusted the pot at (say) 10 V d. c. It is very obvious that if the input from rectifier is more than 10 V, there will be an output available at the output terminal of op-amp. Output is a square wave of +V<sub>CC</sub> and - V<sub>CC</sub> volts. This means that 10 V (and hence 10 amp) is the setting of the relay. Output of op-amp [a pulse sensing circuit or differentiator can be added (not shown in the figure 6.17) between op-amp and an output circuit as a signal of a large band width is not desirable to be fed to the gate of the thyristor] is given to output circuit consisting of a thyristor (or S. C. R., i. e. silicon-controlled rectifier). When a positive rising pulse is fed to a gate of thyristor, it starts conducting and keeps on conducting until its anode circuit is disconnected. The output tripping relay R placed in the anode circuit operates when the thyristor starts conducting. The contacts of this relay can be used to trigger the a. c. tripping relay 86 and contacts of 86 will complete the trip coil circuit of the breaker finally tripping the breaker. Once the breaker trips, 52-a shown in figure 6.17 opens and thyristor stops conducting, resetting the relay.</span></p>