How can I eliminate or cancel harmonics with phase shifting transformers?

So one of the questions we get a lot is around phase shifting. How can I use phase shifting transformers to cancel out and negate harmonics on my power system?

We have a couple different things set up here. We have six pulse drives, we have 12 pulse phase shifting, we have 24 pulse phase shifting, we have an 18 pulse drive. So we have a lot of different combinations of things that we can show you to show you how phase shifting works.

But let's start with this simple diagram to show you how phase shifting works. Taking two six pulse drives and converting it to a 12 pulse system. If you notice, the drives down at the bottom are producing the same exact current. But what happens is, when you go through a delta-delta transformer on the left, or it could be a delta zigzag, or it could be just a reactor, that current looks the same. But the drive on the right, the harmonic current looks different. Or the phase-- the actual current looks different.

So what's happening is the harmonics are being shifted, and the 5th and 7th from the one on the left are canceling with the 5th and 7th on the right. And we have a much more expanded version of this that we can show you, but for this purpose, let's talk about just simple phase cancellation and harmonic cancellation here.

So what we end up with is two six pulse drives, delta-delta delta-wye transformer, we end up with a 12 pulse system. Now, the next thing we want to show you in the slides is the waveform for going from a six pulse, where we have four drives running together, to a 24 pulse. And you can see how much cleaner the current looks. The current is the one a little bit to the right here.

And now let's look at an 18 pulse drive. So on an 18 pulse drive, again, the harmonic currents look a lot cleaner than the six pulse drive.

Now what if you had a commercial building? If you had a commercial building and you picked a couple strategic locations for transformers, in a normal standard case, most people are going to pick delta-wye transformers. All of your harmonics from each floor are going to add up. So let's say you have three rooms or four rooms or five rooms on each floor that have a whole bunch of computers. All those harmonics are going to add up together, and you're going to end up with 12 units of 5th harmonic, 7th harmonic current.

If you just switch one to a delta zigzag, which is a zer0-degree phase shift, you'd have five plus four or nine minus three, or six units of harmonic current. And so essentially, you've cut your harmonic current in half just by selecting one transformer.

Now if we do the same thing but pick the bigger transformer that has more load, we go four plus three, seven minus five, or two. We've cut our harmonic current from, essentially, 12 units, or let's say it's 120 amps, down to two units or maybe 20 amps. So it's a significant amount of reduction. So any phase shifting can help, and we can cancel out the harmonics.

And let me just go the opposite way that says, a lot of people say that, well, if I have a whole bunch of harmonic current on my system from multiple drives and computers, do the harmonics naturally cancel each other out? And the answer is no, they don't naturally cancel because it's just one plus one equals two, two plus two equals four, and so forth.

The only benefit of having a whole lot of harmonic current on the system is the impedance of the system starts to look more like a line reactor and cancel those harmonics.

So what we're going to do now is show you a demonstration, showing you waveforms with harmonic cancellation. So what I'll do is I'll set up our drives with six pulse drives at first. And with those six pulse drives, we'll put all six pulse-- all the six pulse drives on top of each other so that with that harmonic current, it's going to add up and it's going to be essentially multiple additional harmonic currents on top of each other.

So let's look at the waveform first, and I'll turn on all the drives.

So here's our voltage and current, for example, for phase A. And you can see the harmonic distortion in the waveform. And as we've seen many times, this is a six pulse current. So if we look at the harmonic spectrum, we'll see the total harmonic distortion on the current's pretty high.

And then we also have-- how much 5th harmonic current do we have here? Let's take a look. So we have about 41 and a half amps of 5th harmonic current. 7th harmonic current, let's see what we have there. So we have 14.2 amps of 7th harmonic current.

So now, let's look back at the waveform once. And I'm going to switch the drives off. And now what I'm going to do is I'm going to change over to make it a 12 pulse system.

So now when I run two drives together, you can see already, the waveform looks a lot more linear, a lot more like 60 hertz. So let's look at the harmonic spectrum now. And what we'll do is we'll see that the 5th and 7th are pretty much going away. Now this works especially good when the harmonics from each driver are about equal. So right now we have about 1.9 amps of 5th harmonic current.

What I'll do now is I'll turn one of the drives off. OK, and if you take a look in the top right hand corner here, we see the 5th harmonic current or the 300 hertz current is-- H5 is at 33 degrees. We have about 9 and a half amps. Now I'm going to turn the other drive on, which is going to negate that. And so we've negated it.

Now I'm going to turn the first drive off and leave the second one on. And what you can see here is we have about the same magnitude of current. But remember how our phase angle was about 30-something degrees? We're at minus 164. So the difference in those phase angles is just about 180 degrees. And if we do the math, that 180-degree difference is what allows us to cancel the harmonics.

So now what I'm going to do is I'm going to turn on all four drives, and I'm going to do a zero, a 15, a minus 15, and a 30-degree phase shift, and I'm going to show you what that current looks like all together now. So now we can look at the waveform and voltage and current for phase A, and look how nice and clean the current looks. That's what a 24 pulse system would look like. That's what a medium voltage drive current would look like.

So again, how are we doing that? Let's switch over to this other camera here real quick.

And what we'll do is we just see, we have one transformer that has a zero-degree shift. This is isolation transformer delta-delta or delta zigzag. We have one that's a plus 15 degrees. We have one that's a minus 15 degrees. And we have one that's a 30-degree phase shift. So delta-wye or standard transformer like that.

So when you think about phase shifting, you can do a lot of cancellation on a system level. You might shift one motor control center versus the other motor control center, and then get the benefit of canceling 5th and 7th harmonic currents. You might take a commercial building and cancel 5th and 7th for multiple floors. 11th and 13th can also be done if you're doing 18 pulse. Or 5th, 7th, 11th, 13th, 17th, 19th can be done if you do 24 pulse.

So lots of really good benefits. One of my favorite ways to get rid of harmonic current on a power system.