Principles of Symmetrical Components Part 2b

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Principles of Symmetrical Components Part 2b.

Now in part two, we started the conversation of the decomposition of symmetrical components, and we started to define the positive sequence component, the negative sequence component, and zero sequence component, as well as to describe some of the characteristics of these components. Now in Part 2b, we're going to look at some of the benefits of symmetrical components and how to visually decompose them.

This is where we left off last time. Now we've said this many times before, but I'm going to reiterate, because it's very important that symmetrical components are used to analyze unbalance in the power system. When we say unbalance, we primarily mean unbalanced faults or short circuits, so we're talking about line-to-ground fault or two-line-to-ground faults and a line-to-line faults, and we'll talk about this in greater detail later.

What do we mean when we say decompose? What do we mean by this term here? What we're saying is that if we take positive, negative, zero sequence components, and we actually add them up. So if we add positive sequence component with negative sequence component with zero sequence component, what we're going to get is our system unbalanced phasors. This is the same thing as saying our unbalanced system is broken down into positive sequence component, negative sequence component, and zero sequence component. Now, a lot of the papers and a lot of the books out there kind of missed this particular point, but I think it's very important to visually show what we mean by that.

Let's take our positive sequence phasor A. Okay? Now if I take our positive sequence phasor A and put it right there. Okay? Now, I add our negative sequence phasor A, put it right there, and I add that to our zero sequence phasor A. Look what happens. What we get is our original phasor. So all we're doing is we're taking IA positive sequence, we're adding that to IA negative sequence, and we're adding IA zero sequence, and we get our original phasor. Do you see that?

Now, let's do it with IB. We take our IB positive sequence component and place it there. Okay? Now we take our IB negative sequence component and add it to the positive sequence component, and we get that. Then lastly, we take our zero sequence component and add it to the previous component, and there you have it. What we get is our original phasor. So, IB positive sequence is added to IB negative sequence, which is added to IB zero sequence, and we get our original phasor.

Now, let's do the last one: IC. So IC, we put it there, and then positive sequence, so IC positive sequence was placed there. Now we're going to add IC negative sequence to the positive sequence, and then lastly we are going to add the zero sequence. And guess what? We get our original phasor. IC positive sequence is added to IC negative sequence, which is added to IC zero sequence, and we get our original phasor. What we're saying here is that we can take any unbalanced set, and we can derive the positive sequence component, the negative sequence component, and the zero sequence component.

Well, you might be asking, well, whoop-de-do, how can that help us? So how can that help ... How can symmetrical components help? Now I'm going to place all of the symmetrical components back to where they were. Let's say that we have our system here, and we have a breaker that's protecting this distribution line. Okay? This breaker is monitoring all of the currents that are flowing through this line, which is provided by this system, and then let's say that we have a fault at the end of the line. Okay? We don't know what the fault is, but we know that the system is providing the fault current in that direction. So the relays in this breaker senses the fault current. Okay?

Let's just suppose that this is the phasors that it sets, right, where the phase A current look like that, phase B current look like that, and phase C current look like that. Now, can this relay in this breaker confidently declare that this right here is a line-to-ground fault and more specifically it's a A-phase line-to-ground fault? It's going to have a hell of a hard time, because it only has these phasors to go off of. Now, the question is what if this breaker can decompose this unbalanced set of phasors into its positive sequence component, negative sequence component, and zero sequence component? Now, we're talking about some business here.

When we break down or decompose the unbalanced set into its sequence components, then we derive enough information from the sequence components to answer detailed questions. The whole idea is symmetrical components provides us with very, very useful information that we can use to our benefit. With this information, our power system can be way more reliable and way more secure, because we can confidently declare the types of faults, understand what's happening in the system from the information of sequence components. That concludes Part 2b.

In the next part, we'll go into more detail about how to actually derive the positive, negative, zero sequence components from this set. It's going to involve some mathematics, but it's very doable from a visual perspective as well.

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