Understanding the Grid : Part I

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In the last article we saw what the war of currents was all about, who fought it, and why. In this and the next article, we will try to understand what the grid, as we know it, looks like, what makes it work, and what makes it fail.

understanding the grid part 1-3In the simplest of terms, the grid is nothing but a network of big fat wires that carry electricity from point A to point B. (In the scheme that Westinghouse imagined, the grid was meant to carry electricity from the power plants to the consumers. However, with the advent of distributed renewable energy technologies, the picture has changed quite a bit; consumers have now become “prosumers”. But more about that later in the series.) This network connects cities, states, and even countries, thus forming the state, national, and continental grid.

The wires themselves don’t care much about the voltage at which they carry electricity; they can carry it at 230V or 400,000V. But one thing is true: given a particular thickness of a wire, the higher the voltage, the lower the transmission losses, and vice versa. And the reason for it is very simple.

Power = V x I, or the product of voltage and current. So for a given amount of power, the higher the voltage, the lower the current. It’s like this… instead of sending a million packets of energy, you send a thousand packets of energy where each packet is 1,000 times more “powerful” than in the previous case.

Now on their way from point A to point B, these packets of energy are going to encounter some “resistance” right? So what will they do? They will expend some of their energy in overcoming this resistance. This loss is equal to I2 x R, or the product of the square of the current multiplied by the resistance of the wire. By increasing the voltage, we have already reduced the current, and therefore the term I2 as well. We can reduce R by using big fat wires, since the bigger the wire, the lower is its resistance.

To give you an analogy that is non-technical and therefore much easier to understand, assume that there is a company which has to get some work done. It can send out “normal” humans to do it, but then it will have to send out a million such humans. Alternatively, and magically enough, they can also send out “powerful aliens” who are the same size as the humans but a 1,000 times more powerful. So, the company will have to send out only a thousand of these. When dispatched, the people/aliens will come out of the company premises and start their “journey” on the road. If there are a million people, there will be a lot of crowding, quite naturally, and the people will experience a lot of “resistance”. On the contrary, the aliens will experience a lot less resistance since there will be only a 1,000 of them. So going with the aliens is obviously a better option, isn’t it? Now imagine that the company has a lot of resources at its disposal, which it decides to use to widen the roads. Now the resistance experienced by the aliens will be even lesser and their journey a lot smoother, won’t it? And consequently, the aliens will expend a lot less energy on the way and will have a lot more of it to do productive work when they reach their destinations, won’t they? That is exactly what transmitting at high voltages using big fat wires does!

Since these wires carry very high voltages, they can be fatal to anybody who comes in contact with them, and also simultaneously with the ground of course. That is what creates a “path” from the grid to the ground. The current that flows through the person (or animal) is almost always fatal. Unfortunately, low-hanging live wires do cause deaths in India. On the contrary, birds sitting on a live wire, without touching any other live wire or the ground, do not get a shock.

The wires are carried from one point to another with the help of towers as can be seen in the figures below.

understanding-the-grid-part-1

Figure 4.1

The big towers that you see in Figure 1 are the transmission towers, whereas the one in Figure 2 is a distribution tower. The difference between transmission and distribution is that the former is done at very high voltages over long distances, whereas the latter is done at relatively lower voltages over short distances. Distribution is the last leg of transmission so to speak, and it terminates in connections to the end consumers.

Picture3

Figure 4.2

 

understanding the grid part 1-2If you have noticed, the number of wires in transmission towers is 3 or 6, whereas in distribution towers it is 3 wires. It is because they carry “3-phase supply”. We have already looked at an AC waveform in the previous article. In 3-phase supply, each wire carries an AC waveform as well, of the same amplitude and frequency. (At least that is what it should be in the ideal condition. But sometimes the conditions on the ground deviate from the ideal.) However, the three waveforms are “phase shifted” with respect to each other. If one entire cycle can be thought of as having 360°, phase 2 lags phase 1 by 120°, and phase 3 lags phase 2 by 120°, which is what figure 3 shows. Talking in terms of time instead of phase, since the frequency is 50 Hz, each cycle takes 20 milli seconds to complete. Phase 2 begins its cycle 6.66 ms after phase 1 begins its cycle, and phase 3 begins its cycle 6.66 ms after phase 2 or 13.33 ms after phase 1. The 3 phases keep “going after one another” endlessly. It’s as simple after that.

In the case of transmission towers with 6 wires, there are two sets of 3-phase wires.

understanding the grid part 1-1In the movie 3 Idiots, Virus asks Raju in one scene, “Mr. Rastogi, can you explain to us how an induction motor starts?” The induction motor referred to in that dialogue (invented by Nikola Tesla and mentioned in this article) is the one that needs a 3-phase supply to function properly, and these motors are a big reason why 3-phase supply became popular in the first place. Almost all other appliances need a single-phase supply. So industrial consumers always get a 3-phase supply, and they use the three phases to power their motors. The supply to residential and commercial consumers is also 3-phase. However, the three phases are typically used to power different parts of the house. So for example, a couple of rooms are powered by one phase, while a couple of other rooms are powered by another phase. The third phase is used to power appliances like refrigerators and water heaters. Same for commercial consumers. So residential and commercial appliances don’t need/use a 3-phase supply like an induction motor does.

So that was a brief introduction about the grid. In the next article we will take a look at a few more aspects related and gain a deeper understanding of the grid.

Sustainably yours,
Prashant Karhade.
Writer, Publisher, Entrepreneur

4 Responses

  1. vijendra patani 3 years ago
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