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3 The third harmonic in transformers3.1 Star-delta transformerConsider a star-delta transformer, supplyingidentical non-linear loads connected betweenphase and neutral (see fig. 22). Each of theseloads generates a third order harmonic current.Remember that these currents (I3), containingthird order harmonics, are equal.Third order harmonic currents in the transformerprimary windings are also therefore identical toone another, and are noted I'3.In each node of the primary delta, the thirdharmonic currents compensate for one another,and the current in the line therefore contains nothird harmonics.Third order harmonic currents are not thereforetransmitted to the network. Instead, thesecurrents circulate in the transformer primarywindings and therefore cause an additionaltemperature rise.I'3PrimaryI'3I'3SecondaryI3I3I3Fig. 22: Third order harmonic currents in a star-deltatransformer3.2 Transformer with zigzag secondaryConsider a transformer with zigzag secondary,supplying identical non-linear loads connectedbetween phase and neutral (see fig. 23). Each ofthese loads generates a third order harmoniccurrent (marked I3 in the diagram). Rememberthat these third order harmonic currents are equal.It is easy to see from this diagram that theampere-turns on a single core at the secondarycancel one another out. As a result, there are nothird order harmonic currents circulating at theprimary.PrimarySecondaryI3I3I3I3I3I3Fig. 23: Third order harmonic currents in a transformer with zigzag secondaryCahier Technique Schneider Electric no. 202 / p.11