How does a delta zig-zag transformer eliminate or minimize harmonic currents?

One of the questions we get a lot is: how a delta zigzag transformer eliminates or minimizes harmonic currents.

To start that conversation, let's talk about positive, negative and zero sequence harmonics. We've talked about these in the past when we talked about phase shifting and cancellation of harmonics.

Let's look at this PowerPoint slide. In this Sequence Harmonics slide we basically talk about 1, 2, 3, 4, 5, 6. Those are harmonic orders and then we go positive, negative, 0, positive, negative, 0. If you look at the positive sequence harmonics it goes A, followed by B, followed by C rotating. You can see the negative sequence harmonics too. The zero sequence harmonics are the ones we're going to focus on for this discussion. They all rotate together, and they rotate in this case counterclockwise. So, zero sequence harmonics in the chart, are going to be here, here and here.

Let's look at a standard Delta Y Transformer. Often we put a Delta Y Transformer into trap or recirculate the third harmonic currents in the Delta winding. You can see what goes on here, the current goes out on one leg, comes back on the neutral, out on the second leg, back on the neutral, out on the third leg back on the neutral. What happens in the primary is let's say there's 10 amps going here and 10 amps going here that means none of the current comes out of the transformer. So if you look at the third harmonics on the secondary they add up to three times. On the primary they're basically zero, but and this is very important, the fifth and seventh and all the all the negative and zero and negative and positive sequence harmonics will go right through the transformer harmonic mitigation.

Let's look at a harmonic mitigating or a Delta zigzag Transformer. The third harmonics don't couple and here's why. Basically if we look at each leg of the core of the transformer we have some current flowing on that first winding from Phase A. We also have some current flowing on the same leg but from Phase C. Here and here these arrows are opposite of each other, and they cancel each other out on this leg.

It basically doesn't circulate back to the load, but it doesn't get into the primary winding. The same happens on the middle leg and the same on the third leg. So essentially, what happens is, when you talk about the third harmonics or the zero sequence harmonics they circulate through and back to the load but they do not go back to the primary on that Delta zigzag Transformer. What happens is phase shifting again, fifth, seventh, and all the other harmonics go back through thirds. The multiples of the third don't. And what's important about that is if you use that strategically you can do things like phase shifting.

I'm going to show you a transformer though here. What's interesting, if you look at this transformer and just look at the primary of it or look at the input of it you would never know this is a Delta zigzag. Even if you kind of knew what you were looking for and you kind of paid attention to the back of it you may not notice it. But the way that I showed you the windings are done, that's done on the secondary, so we're taking on each leg of the core. We're taking a little bit from C and so forth. If you look at that transformer again it looks very similar. But how we deal with the third harmonics through that Delta zigzag winding is important to understand how they're made up, what we're doing with them and then if we use them with other Transformers to do the phase shifting.  This one gives us a Z-er degree phase shift, mix it with a 30, we can cancel out that fifth and seventh and so forth. There are lots of things we could do with Delta zigzag Transformers or other zigzags.

We'll have other videos about the different phase shifts you can get but for this one it's a Z-er degree phase shift and we're dealing with the third harmonic uh currents.