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.