Principles of Symmetrical Components Part 2c

Short URL of this page: http://gpac.link/1YD0cSa

Resources Section

Web Application for Symmetrical Componets: http://personal.strath.ac.uk/steven.m.blair/seq/

Do you have a question? Click on the "Questions & Answers forum" and ask away!

In this video tutorial we're going to explore the visualization of symmetrical components through this web application app here. The way that this is set up is that you have this input which is your system phasors. And if you can just imagine the red phasors is phase A current, the yellow phasors are phase B current, and the blue phasors is phase C current, and all three phasors were rotating in the counterclockwise direction, we have an ABC phase sequence. And not only that, but we have a balanced system.

So, when we have a balanced system, our positive sequence component is going to be identical to our system phasors, but our negative sequence component or zero sequence component are both going to be zero. So, for a balanced system we should only have our positive sequence component and it should look exactly like our system phasors.

What would happen if we change our system phasors? Would we get the same result? Well, let's see. Let's make our system, which is a balanced system into an unbalanced system, which means we could either change the magnitude of one of the phases or we change the angle. So, let's just pick up phase A and B, change the magnitude. Then guess what? As soon as we change the magnitude and angle of phasors A current we see that the positive sequence component changes, but at the same time we also have a negative sequence component and a zero sequence component.

Now, let's look at some of the characteristics of the decomposition of the unbalanced system. Remember, the decomposition is just a fancy word of saying the symmetrical components of the unbalanced system, which is these guys right here.

Well, first of all we said that a system was an ABC phase rotation, or phase sequence, right? Well, let's look at our positive sequence component. Well, positive sequence component is also an ABC phase sequence, but our negative sequence component is the opposite phase sequence of our system sequence. So, our negative sequence component would be an ACB phase sequence, which we see here. And our zero sequence component, well, it has no phase sequence, and in here it doesn't actually show all three phasors. But, if you can just imagine that all three phasors are overlapping each other and it has the same angles. So, this right here would be just our zero sequence component. So, as we continue to move our phasors we see that the sequence component, they all change to match this to decompose the unbalance of our system. Now, let's put it back to where it was before, which was a balanced system.

Let's look at a different perspective. Now, I've superimposed the symmetrical component in our system phasors directly and look at the characteristics of symmetrical components that way. Because this is a balanced system, we would only get positive sequence component, and positive sequence component would look exactly like our balanced system. As soon as we change phase A current look what happens. Now, we get a decomposition of symmetrical components, which means that we get symmetrical components that equal our system unbalance. So, in the previous tutorial we said that this was phase A positive sequence component. This was phase A negative sequence component. And this right here was phase A zero sequence component.

Now, let's try to look at some of the characteristics of this example. We know that, for a positive sequence component, it should have the same phase sequence as our system phasors, right? And you can see here these three phasors are our positive sequence component. So, this is positive sequence component for phase A current. This is positive sequence component for phase B current, and this here is positive sequence component for phase C current. For negative sequence component, this is our negative sequence component for phase A current. This here is our negative sequence component for phase B current, and then this line here is our negative sequence component for phase C current.

You would have to imagine them being at the same origin to see how the phase C sequence of negative sequence component changes. Now, before we separate that out and actually see how it looks like, let's analyze our zero sequence components. In our previous tutorial we said that our zero sequence component should have no phase sequence, and it has the same magnitude for A, B, and C but the zero sequence components are all overlapping each other. So, in here we see that this right here is our zero sequence component for phase A current. Now, we should see some similar phasors for phase B and phase C zero sequence component. We see that over here. This right here is our zero sequence phase C component, and you can see that it has the same magnitude as our phase A zero sequence component. And they look like they will overlap each other if they were on the same axis. You see this one right here? That is our zero sequence component for phase C current, and it has the same magnitude and it looks like it will overlap phase A and phase B zero sequence component.

Now let's separate these out and see how the sequence components look like about its own axes. Here they are again. We've separated out and as you can see our zero sequence components do in fact overlap each other, and it has this magnitude and this angle. And our native sequence component you can see that it's a balanced system. What are the characteristics of the balanced system? Well, we said that the magnitude of the phasors is gonna be exactly the same, which they are, and then we also said that the angle displacement between each phase is 120 degrees. So, if you take this angle here and you subtract this angle there you're gonna get 120 degrees. So, we know that negative sequence component is a balanced set. Our positive sequence component is also a balanced set, because the magnitude of all phasors is the same and they are displaced by 120 degrees. So, if we keep on moving this around you see that our symmetrical components actually change based off of the unbalance of the system. Let's put it back to where it was. Let's put it back to a balanced system.

I hope that this example provided an intuitive perspective of symmetrical components. In the next part we'll actually calculate positive sequence component, negative sequence component, and zero sequence component based off of the magnitude and angle of our system phasors. You'll find the URL of this particular web application below this video. Take a look at that, play with it and try to get an intuitive feel for how symmetrical components actually work and grasp the idea that it's only decomposing the unbalance in our system.

I'm gonna repeat one more time what symmetrical components are doing. It's taking the unbalanced system that we have, this unbalanced system, and it's creating balanced set of three components. Technically, zero sequence component is not a balanced set but it's creating a balanced set of components. The positive sequence component is a balanced set. The negative sequence component is also a balanced set. The zero sequence component, not sure if we can call it a balanced set, but the idea is that this balanced set of phasors represent the unbalance of the system. And we could use this information to describe how our system is reacting or use this information to set relays and breakers and so forth.

Again, if you have any questions, there's gonna be a forum link on the bottom of this video. Please go ahead and ask your questions on that forum, and if you haven't subscribed already, please go ahead and click on the bottom right corner of this screen. There is a subscribe button. Thank you. This video was brought to you by generalpac.com.


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