Introduction to Current Transformers Part 2: CT Polarity

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Introduction to current transformers Part 2.

In part 1, we introduced the current transformer and we gave an example where we would want to use a CT. In part 2, we're going to talk about polarity marks. In part 3, we're going to talk about current transformer ratio.

Let's begin by drawing a Delta connected transformer like such. This is line A, this is line B, and this is line C.Then on the secondary side, suppose we had a wye connected transformer that was grounded.

On this wye connected side – on the secondary side of the power transformer, we have CTs connected on each phase. Let's suppose there is a relay that we want this CT to connect to. And on this relay, we have inputs for I_a, I_b, and I_c. And we have two contacts for each phase.

Okay so we've created our CTs on the secondary side of the power transformer, the wye connected side. We have our relay. Let's now talk about current flow.

In a steady state system, let's assume the current flows from the source which is on the left hand side. So here is our source. To the load. Suppose the load was on the right hand side. And the current flows from the source to the load in our steady state system.

The current will flow on each line, then on the secondary side, it will flow like this. Let's zoom back in to the CT that's connected to phase A. Okay so suppose the current in our steady state flows in this particular direction. That is key.

No in terms of polarity marks, we typically see them in two places. One is on the line side and the other on the CT side. To interpret this, we have to follow what's called the dot convention. In our dot convention, we have two rules.

Current into the dot and current out of the dot. The way to understand this is – if current flows into the primary side of this transformer – the primary side. So this is our primary current. If my primary current flows into the dot on the primary side, then the secondary current must flow out of the dot on the secondary side.

So let's connect our CTs in this particular way. Again, primary current flows into the dot on the primary side – then secondary current must flow out of the dot on the secondary side *(illustrated in the image above)*. Secondary current flows like this – it goes through this polarity mark here. Then out of the polarity mark and then completes this circulating current. Because there is a dot on this of the CT – then we would expect this contact here on the relay is the polarity side of the contact. So these two sides should match up.

Again, primary current flows into the dot on the primary side. Then secondary current must flow out of the dot on the secondary side *(illustrated in the image above)*. That's the rule of the dot convention.

And suppose that we have a dot on this side of the transformer – how we would be interpret that? Okay so it's fairly simple. The way that we would do it – the current that flows into the dot on the primary side must – then secondary current must flow out of the dot on the secondary side *(illustrated in the image below)*. So the dot comes out like that. So let's say the current flows this way.

Because we have the dot on this side now, we should expect the polarity side of relay to be like this – this is the polarity side contact of our relay. And that's how we should think about it. Okay so current that's flowing into the dot on the primary side, must flow out of the dot on the secondary side *(illustrated in the image above)*.

So now the question is – what if we had current flowing in a different direction on our primary side. Very simple – we just need a very good explanation. Current that's flowing like this. When current is flowing in this direction, we know that the CT comes first, then we have the dot *(illustrated in the image below)*.

So in this particular configuration, the current is flowing out of the dot on the primary side, which means current must flow into the dot on the secondary side *(illustrated in the image above)*. So it's going to be make a circulating path like this. So current that flows out of the dot on the primary side means that current must flow into the dot on the secondary side.

Now what if our dot on our secondary side was over here. How would we interpret that with the same current flow? Well first of all on our relay, we know that on our relay, this contact will be on polarity side because it matches with our CT.

So now current flows out of the dot on the primary side, means that current flows into the dot on the secondary side *(illustrated in the image above)*. So it'll make the connection like this. So in terms of the dot connection, there is really two rules that we need to memorize. What's goes into the dot must flow out of the dot.

Now as a teaser to other parts of this series, I'll show you show three current transformers are connected to the relay – the most typically example.

So this is a very typical arrangement of the current transformer connected in Wye. We'll talk about this particular of arrangement in later modules. In part 2, we talked about current transformer and the polarity markings of the CTs. We talked about different arrangements of these polarity marks and how to interpret them. And then as a teaser, we drew the wye connected three phase CTs.

In part 3, we'll talk about current transformer ratio and how to relate primary current with secondary current.

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