<p style="text-align:justify"><span style="font-size:14px">L-L, L-g and L-L-g faults are unsymmetrical faults. When such faults occur in power system network, negative and zero sequence voltages and currents are produced. When negative sequence currents flow in the network, rotating magnetic field (RMF) is produced in all rotating machines (synchronous generators, synchronous motors and induction motors) which rotate at synchronous speed in reverse direction w. r. t. RMF produced by positive sequence currents. The physical rotor is always rotating in the same direction as RMF due to positive sequence currents. Slip due to positive RMF is zero in synchronous machines and about 4 % (0.04 pu) in induction motors. Hence, slip frequency eddy currents are nil in synchronous machines and negligible in induction machines. Therefore, there is no eddy current loss (or negligible loss) in rotor iron in all rotating machines under normal operating condition as there is only positive sequence current present. But slip frequency due to negative RMF (due to presence of negative sequence currents in case of unbalanced faults) is 200 % (196 % in case of induction motors) and hence large slip frequency eddy currents flow in rotor windings as well as rotor iron. Rotor of synchronous generators, as discussed, is not laminated and hence large eddy currents and large eddy current loss (eddy current loss is proportional to f<sup>2 </sup>and sf being 200 % or 100 Hz which gives rise to very large eddy current loss). This loss manifests in terms of heat and development of large temperature due to heat. This heat can make the rotor red hot if protective actions are not taken early. Also, if the temperature developed is more than withstand limit of the insulation used, the insulation of the rotor winding will be thermally damaged resulting in rotor earth fault.</span></p>

<p style="text-align:justify"><span style="font-size:14px">As a thumb rule, if heat produced by positive sequence current I<sub>1</sub> is H [proportional to (I<sub>1</sub>)<sup>2</sup>Rt], heat produced by equal magnitude of negative sequence current I<sub>2</sub> will be 6H. Thus, even if the fault current due to unsymmetrical fault is less than CMR of the equipment, it will be harmful to all rotating machines.</span></p>

<p style="text-align:justify"><span style="font-size:14px">Zero sequence current is third harmonic current. Refer figure 4.33. It can be appreciated that the third harmonic or zero sequence currents (or voltages) are in phase. The hysteresis loss is proportional to frequency, f; but the eddy current loss is proportional to square of the frequency, f<sup>2</sup>. Hence large iron losses are produced because of zero sequence currents. The magnitude of zero sequence current may be less than that of fundamental (positive sequence) current, but as eddy current loss is proportional to f<sup>2</sup>, the heat produced will be quite high. Hence zero sequence currents are harmful to rotating machines and transformers as well.</span></p>

<p style="text-align:justify"><span style="font-size:11pt"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16161527141616152714.jpg" style="height:451px; width:800px" /></span></p>

<p style="text-align:justify"><span style="font-size:14px">Thus, when we will enter in power system protection, we will see that fault currents are usually larger than CMR of the equipment, but in certain cases, the fault currents are smaller than CMR of the equipment and yet they are harmful.</span></p>