Video: Electricity Transmission Networks

In this lesson, we will learn how to describe how national electricity grids work and how they respond to changes in demand.


Video Transcript

In this video, we’re talking about electricity transmission networks. Whenever we do something in our home that requires electricity, say turning on a lamp or flipping on the television, we’re relying on this network in order to provide us with instant electrical power. In this lesson, we’ll see different ways that that power is generated and also how it makes its way from the place where it’s created to our homes and businesses.

We can start off by defining a term, the electrical grid, sometimes also called the national grid. This is the system that’s responsible for transporting electricity from where it’s generated to where it’s consumed or used. There are three main parts or components to the electrical grid. The first part has to do with the generation of electrical energy. This creation often takes place at what’s called a power plant. These are facilities that are made to take one form of energy and convert it into electrical energy. The power plant we see here converts nuclear energy to electrical energy, while other types of power plants use energy that comes from burning materials such as coal or gas. And more recently, methods using wind energy or solar energy, energy from the sun, had been shown to be able to create electrical energy.

Regardless of the means used, once electricity is generated, it moves into the next section of the electrical grid. This is the transmission portion of the electrical grid, which is responsible for transmitting electrical energy over large distances. We need this component of the electrical grid because generating electricity is something that we often do at a large scale. A single power plant, for example, can generate enough electricity to supply hundreds of thousands of homes. But of course, we need to get that electricity from the plant to the homes. And we like to do it as efficiently as possible. That’s where the transmission network comes in.

Once electrical energy has been generated and transmitted, it enters the final component of the national grid. This is known as the distribution phase. It’s at this point that the electrical energy is conditioned and then delivered specifically to individual homes and businesses. In a little while, we’ll talk in more detail about each one of these three phases, generation, transmission, and distribution. For now, though, let’s notice a few interesting facts about the overall system, the overall electrical grid. Interesting fact number one, we could call it, is that energy is not stored in the grid. It’s not like a battery where the energy is saved up or stored until we need it. The national grid doesn’t work like this. Instead, the only electrical energy that’s available at the distribution phase for homes and businesses is electricity that’s currently being generated on the generation side.

We’re not yet able to save up extra energy that’s generated for use later. Building a battery that big is not energy-efficient. All this means that when we plug in an electrical appliance in our home, the energy that powers that appliance is being generated at that moment somewhere. And that brings us to a second interesting fact about this network. Within the national grid, energy is supplied to meet demand. Let’s consider a single homeowner in one of these homes over here that’s having electricity distributed to it. That homeowner, when the home is connected to the national grid, can reasonably expect that any number of electrical appliance they want to use will be powered. In other words, power will be supplied to those things.

We don’t get notes from the electricity company telling us what’s the maximum amount of electrical power we can use. Instead, the company adjusts the amount of electricity generated at the generation phase to meet our demand. Now, if we take these two facts together, that energy in the grid is not stored and that it’s applied to meet demand, then that shows us that there needs to be significant flexibility in the generation of electrical energy. In other words, if homes and businesses start to use more electricity, then the power company needs to generate more. And it needs to do so very rapidly. This fact has a large impact on how the national grid works.

In addition to these two interesting facts, let’s add a third one. And this is that electrical energy is often transmitted over distances of hundreds of kilometers. Over distances this large, the efficiency or the energy losses involved in that transmission become very important. And as we’ll see, a strategy has been adopted to minimize energy lost during the transmission phase. Each one of these facts relates to a specific section of the electrical grid. So let’s look at each one in a bit more detail.

First, electricity generation, we’ve seen how there are a number of different methods that we can use to generate electrical energy. We’ve also seen that generating enough electricity to meet or exceed demand is important and that that electrical demand can change over time, sometimes very quickly. This means that when we’re generating electricity, we need to set up a system that can respond to these changes. Say, for example, that we have one power plant operating in our electrical grid and at the moment the electricity generated is enough to meet demand. But then imagine that the demand increases. It goes up so much so that the one power plant is no longer able to meet the need. In that case, we would need to bring a second power plant online to start it, generating electricity to meet the increased demand. And of course, we would want this to be able to happen quickly, without much delay.

Considering some of the options we have for generating electrical power, we can see that some choices are better than others for rapidly responding to increased electrical need. Here’s what we mean by that. Say that on the day the electrical need increased — so that we needed to bring a second power plant online — on that day, the weather was very cloudy. So not much sunlight was coming through to the ground. And also we could imagine that it was a very calm day with very little wind. Well, this scenario shows us that generating electricity through these means is not the most reliable way of doing it. If the wind isn’t blowing or the sun isn’t shining, then we won’t be able to get electricity from these methods. But then, even if we do have a reliable source of electrical energy, we may not be able to access that source until sometime in the future.

For example, a nuclear power plant is a reliable source of electrical energy. But in starting up a nuclear power plant, once it’s been stopped, it will take over 24 hours for the plant to be able to supply electricity. So it’s not able to offer a quick response to rising electrical demand. But it turns out there is a type of power plant that meets the conditions of both being reliable as well as starting up quickly. Gas-powered plants are able to work regardless of the weather or other conditions so long as the system is working properly and there’s enough fuel. And they’re able to start up in less than 10 minutes time. So for an electrical system experiencing rapidly rising demand, bringing gas power plants online is an effective way of meeting this need.

As we saw, once electricity is generated at a power plant, it moves into the transmission phase of this grid. One of the most important goals of this transmission phase is to minimize the electrical energy lost. The way that this happens is through a device known as a transformer. The way a transformer works is it connects one wire, which carries a particular current and a particular voltage. And it connects that wire with a second one which it then induces to carry a different amount of voltage and a different amount of current. So if the current in the first wire is 𝐼 one and the potential difference across that wire is 𝑉 one, we can call the current coming out of the second wire 𝐼 two, and the potential difference across that wire 𝑉 two.

Now, here’s why transformers are so useful when it comes to transmitting electrical power. Think for a moment about the electrical current that travels through these wires. We know that the more current flows in the wire, the more the wire will heat up. That is, it will lose energy due to heating. And, of course, that’s not how we want this energy to be used. We want it to go towards powering homes and businesses. So here’s what we’d like to do. We’d like the current in our transmission wires to be as small as possible, which will mean as little energy as possible is lost due to heating. The smaller the current, the more efficiently we can transmit electrical energy. And this brings us back to our transformer.

When electricity is generated at a power plant, it’s generated at a certain potential difference and with a certain current. Now, those values are suitable based on the power plant being used. But they may not be the best values for transmitting that electricity across long distances. So before sending the electricity into the transmission part of our network, we pass it through a transformer, which is designed to lower the electrical current coming out, which has the effect of raising the potential difference. And here’s the reason why that’s so.

Recall the equation for electrical power, which says that power is equal to potential difference multiplied by current. When it comes to our transformer, the transformer is able to modify current and voltage. But it doesn’t have an effect on the overall electrical power. The power that goes into the transformer is the power that comes out minus the very small losses due to inefficiencies. So based on our equation for electrical power, we can write that the potential difference going into our transformer multiplied by the current going in is equal to the potential difference coming out, 𝑉 two, multiplied by the current coming out, 𝐼 two.

Now, based on the electricity generated by our power plant, this left side is a constant. And that means if we want to drop the current coming out of our transformer and we do because that will let us transmit this electricity more efficiently, then we’re going to need to increase the potential difference coming out. All this to say when electricity makes it into the long distance transmission lines of our grid, in order to make it travel more effectively, we’ve raised its potential difference very high by dropping its current very low. And when we say the potential difference is very high, we’re talking in the range of hundreds of thousands of volts, one to 800 kilovolts, approximately.

So electricity is created at our generation site, the power plant, and then it’s passed through a transformer and then sent on through the transmission network of our grid. While it travels over these long distances, it has relatively low current and high potential difference. And after the electricity has been transmitted, it’s ready to be distributed to its end or final users. Now, before the electricity can be used in homes and businesses, it needs to be transformed using a transformer one more time. The reason for this is that while the electricity is in the transmission network, its potential difference is too high to be useful on a local level.

If electricity with potential differences this high were sent directly to individual homes, the result would be both dangerous and expensive. What’s needed is for the electricity to go through yet another transformer, that’s this element right here, whereas before our transformer increased the potential difference of our electricity. And by the way, that’s called a step up transformer when the potential difference is increased across it. Now, at this second transformer, we want to step down or decrease the voltage.

Now, the electricity we use in our homes, the electricity that comes out of the wall when we plug something in, has a potential difference on the order of 100 volts. So we can see that, indeed, a transformer will be needed to convert our electricity’s voltage from this high value to this lower, more manageable value. This is an overview of the electrical grid, also called the national grid. Now that we understand a bit about the three main sections of this system, let’s work through an example exercise.

The national grid transmits electrical power at a very high potential difference and a very low current. Why is this?

Okay, so when we talk about the national grid, we’re speaking of a very large-scale electrical network. This network has three main sections or parts to it. These parts are generation, where electricity is generated, transmission, where the electrical energy travels over large distances, and then distribution, where the electrical energy is delivered to its end users. Now, our particular question is asking about the transmission phase of this process.

During this phase, electrical energy is transmitted over large distances up to hundreds of kilometers. During this process, one of the most important considerations is the efficiency with which electricity can get from one point to another. That is, of all the electrical energy that goes into the transmission wires in our network, we want as much of that energy as possible to make it out the other end at the distribution phase. In other words, during transmission, we want to minimize the energy lost.

Now, if we picture a wire in our transmission network, we know that electrical current travels through that wire. And we also know that the more current flows in the wire, the more the wire heats up. Now, this heat is not a way that we want this electrical energy to be used. We prefer the energy make it to the distribution phase where it can be used in homes and businesses. So during the transmission phase, we would like our wire heating to be a small as possible. And the way to do that is to make the current running through these wires as small as possible. So the smaller our current 𝐼 is that runs through these transmission wires, the less the wires heat up, and therefore the less energy is lost due to heating.

But interestingly, because of the properties of electricity, if we want to change the current 𝐼, there’s another property we’ll need to change, and that’s the potential difference 𝑉. The reason for this comes down to a mathematical expression for electrical power. This equation tells us that power is equal to potential difference or voltage multiplied by current 𝐼. The electrical power moving through the transmission lines of our national grid depends on the power plant that was used to generate that electricity. For a given power plant, we can consider the electrical power 𝑃 in the transmission lines to be a constant. In other words, 𝑃 in this equation is fixed. That’s something that we’re given rather than something we can change.

This means that if we want to decrease the current running through our transmission lines and we do because that will minimize energy losses, then in order to do that, we’ll have to increase the potential difference of our electricity. It’s only by doing both of these things that we can decrease the current as well as keep the power constant. And so we now see why the national grid transmits electrical power both at very high potential difference and at very low current. We can write out that reason this way. We can say that while high current heats cables and loses energy, using less current and, therefore, more potential difference wastes less energy. So the reason for transmitting electrical power this way is ultimately to minimize energy loss.

Let’s take a moment now to summarize what we’ve learned about electricity transmission networks. Starting off, we saw that the national grid is a system that provides electricity to businesses and homes. The three main sections of this grid involve electricity generation, transmission, and then distribution. Concerning the generation phase, we saw that different power sources have greater reliability and shorter startup times than others. And lastly, we saw that devices called transformers change electrical current and voltage when electricity travels across the grid, in particular when it goes from the generation to the transmission phase and then from transmission to distribution.

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