<p style="text-align:justify"><span style="font-size:14px">If delta-wye transformer (figure 4.36) is present between source and a fault (L-L-L-g, L-L-L, L-L, L-L-g or L-g), the relation between positive sequence currents on primary and secondary sides of a power transformer can be seen in figure 4.37.</span></p>

<p style="text-align:justify"><span style="font-size:14px"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16161542591616154259.jpg" style="height:545px; width:800px" /><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16161542781616154278.jpg" style="height:545px; width:800px" /></span></p>

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

<p style="text-align:justify"><span style="font-size:14px">By kirchoff&rsquo;s current law, it can be written as,</span></p>

<p style="text-align:justify"><span style="font-size:14px"><span class="math-tex">\(I_{RL}=I_R-I_B\)</span>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;<span class="math-tex">\(I_{YL}=I_Y-I_R\)</span>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;<span class="math-tex">\(I_{BL}=I_B-I_Y\)</span></span></p>

<p style="text-align:justify"><span style="font-size:14px">Comparing primary and secondary line currents, secondary line current lead respective (say, R-phase) primary line current by 30<sup>0</sup>. This delta-wye connection is also known as DY-11 or 11 o&rsquo;clock connection. This 30<sup>0</sup> phase-shift is there in any kind of fault (or for that matter normal load currents also) i.e.</span></p>

<p style="text-align:justify"><span style="font-size:14px">&nbsp;L-L-L-g, L-L-g, L-L or L-g. While carrying out fault analysis, this phase-shift has to be considered. It was not mentioned in analysis of three phase fault because the three line currents after phase shift were yet balanced. The calculation shown was of magnitude of fault currents only.</span></p>

<p style="text-align:justify"><span style="font-size:14px">There is another kind of delta-wye connection also which is known as DY-1 connection. In this connection I<sub>r1</sub> will lag I<sub>RL1</sub>, hence there is -30<sup>0</sup> phase shift. Unless specifically mentioned, we will assume delta-wye connection to be DY-11.</span></p>

<p style="text-align:justify"><span style="font-size:14px">The relation between negative sequence currents on both sides of a power transformer is different. This is shown in figure 4.38. Secondary line current, in this case, lag respective primary line current by 30<sup>0</sup>. </span></p>

<p style="text-align:justify"><span style="font-size:14px"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16164835781616483578.jpg" style="height:545px; width:800px" /></span></p>

<p style="text-align:justify"><span style="font-size:14px">For zero sequence values,</span></p>

<p style="text-align:justify"><span style="font-size:14px"><span class="math-tex">\(I_{RL0}=I_{R0}-I_{B0}=0\)</span></span></p>

<p style="text-align:justify"><span style="font-size:14px">Similarly, I<sub>YL0</sub> and I<sub>&shy;BL0</sub> are zero. This means that zero sequence currents which are present in star side are not reflected in delta side. Repeating the same sentence in different words, third harmonic currents present in wye side are not transformed to delta (source) side. Thus, third (and hence all triplen) harmonics are suppressed by delta-wye connection and damages to electrical equipment due to third harmonic currents (or voltages) is avoided. This is one important reason why delta-wye connected transformers are used in power systems.</span></p>

<p style="text-align:justify"><span style="font-size:14px">&nbsp;</span></p>