GIC Update: Wye Delta

I still haven’t looked at the ground loop through transmission line problem in depth, but the more I think about it, the more something it doesn’t make sense. Either the Swedish grid where this happened on is designed differently than the North American grids, or the author of the paper missed something, or I’m missing something. I’m not seeing how it’s possible to get a ground loop between distant substation transformers without a direct connection, in this case, a neutral. Most transmission lines in North America do not have a neutral.

sinewaveTo understand why, we have to understand something about how electric power is delivered and how transformers work. The North American grid is based on Alternating Current (AC), which reverses polarity, with electrons flowing one way, then the other. This means AC voltage varies depending on where it is in this cycle. We can model AC voltage as a sine wave.

 

transformerAC makes transformers simpler. Transformers are based on inductance. A coil produces a magnetic field, inducing a current on a second coil. The catch is that inductance only happens if the magnetic field is changing. But with AC, the magnetic field in the primary coil is constantly changing, and this induces current in the secondary coil. If both coils have the same number of windings, the ratio is 1:1 and in a perfect transformer you’d have the same current induced in the secondary as you would the primary. If the secondary coil has less windings, current voltage is reduced. If the secondary coil has more windings, secondary voltage is increased.

Transformers have steel cores to make them more efficient. But they’re never 100% efficient, and tend to produce heat. Larger transformers usually contain electrical insulating oil that also serves as a coolant, and some transformers have cooling fins.

3phaseJust about all grids are three-phase, with each phase rotated 120° away from another. (360°/3 = 120°). US AC is 60 Hertz, which means it reverses polarity 60 times a second (makes it handy to keep plug-in clocks accurate – as long as electricity doesn’t blink and the frequency doesn’t vary). Three phase has some advantages, especially with electric motors.

 

phasorAn easy way to understand three-phase power is to use phasors. Well, phasor diagrams. Remember Vector from Despicable Me? A vector is a representation of force having direction and magnitude. A phasor diagram is a vector is a polar vector, with each phase represented by it’s direction in a 360° circle, with voltage represented as length.

 

In vector form we can use simple trigonometry calculate things like voltage between phases or to a grounding point. We can also use it to visualize transformer connections. Here is three wire Delta:

delta

And here is Wye:

wye

Important: “Ground” is not a vector here. It’s to indicate that all three transformers have a common ground.

Notice how a Delta looks like the Greek letter Delta, and Wye like an upside down letter “Y.” Also notice that three wire Delta doesn’t have a neutral. Wye does; three wire Delta doesn’t. And most North American transmission lines are three wire Delta.

Trivia Time. In Isaac Asimov’s Prelude to Foundation, Hari Sheldon and company end up at the Trantor South Pole,  in the district that generates power for the entire planet. The district’s name? Wye.

What usually happens is generation is in Wye, and the connection to step up current from generation to transmission voltages is known as Wye – Delta. Stepping it down from transmission voltages to Wye distribution is known as Delta – Wye. Often a short-hand of showing only the secondary side is used in diagrams, but you can see both primary and secondary diagrams used.

Look back at our transformer diagram a moment. Notice that you have to have a complete circuit on both sides. Changes in current on the secondary side will be stepped up to the primary side. So, if there is GIC on the secondary side, it will be stepped up to the primary side. The thing is, for this to happen the GIC has to be in a complete circuit on the secondary side. A difference in ground potential between two substation transformers connected by a transmission line isn’t a complete circuit because there’s no connection between transformers: the neutral only exists on the secondary side, not the primary.

This brings it back to inducing current on the conductor itself. But a ground loop over three wire Delta? I don’t see how.

There are other things I need to look at. GIC does exist and has damaged transformers though harmonics (worth another post, and now that we see how three-phase works, easier to understand). I have some suspicions that I need to look into, and if it can be shown that ground loops can exist between substation transformers over three-wire Delta, I’m willing to eat crow. I have to understand the how first.

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