We usually think of the power grid in terms
of its visible parts: power plants, high-voltage lines, and substations. But, much of the complexity
of power grid comes in how we protect it when things go wrong. Because of the importance
of electricity in our modern world, it’s critical that we be able to prevent damage
to equipment and perform repairs quickly when they’re needed. The grid got its name for
a reason, it’s an interconnected system, which means that, if we’re not careful,
small problems can sometimes ripple out and impact much larger areas. So its protective
systems are thoughtfully designed to work together and minimize the number of people
affected when faults occur. Hey I’m Grady and this is Practical Engineering. Today we’re talking about power system protection and how blackouts work. This video is sponsored
by HelloFresh, America’s #1 Meal Kit. Get started with 9 free meals at HelloFresh.com
by using code Practical9 at checkout. More on that later. Things go wrong on the grid all the time.
Just like a car or the device you’re watching this video on right now, the grid is a machine.
It’s a big machine that sits out in all kinds of weather, exposed to a wide variety of
meddling and destructive animal species and just the general wear and tear that comes
from providing humanity with an absolutely essential yet extremely dangerous amenity:
electricity. It shouldn’t come as a surprise that faults happen from time to time. One
common type of fault on transmission lines comes from sagging. During peak demands, these
lines move tremendous amounts of energy as electrical current. Well, no wire is a perfect
conductor; they all have some resistance. So, the more current you try to pass through
a wire, the less efficiently it works. That energy that doesn’t make it to the end of
the line is instead lost as heat. And what does heat do to metal? It causes it to expand.
So the lines get longer, which means they sag lower, and occasionally that brings them
into contact with tree limbs, creating a path to ground and shorting out the line. So what happens during a short circuit? Electricity
will take any path to ground that it can find and the lower the resistance of the path,
the more current that will flow. A short circuit is when a low resistance path to ground happens
where it’s not supposed to, bypassing the customers and literally shortening the circuit.
This has a number or unwanted consequences. All that energy is being wasted, for one.
Arcs created by short circuits can start fires for two. But more importantly, faults create
massive spikes in current that can overload and damage equipment on the grid. I probably
don’t need to mention that most pieces of the power grid are expensive, they take a
long time to install and repair, and they’re important, they’re providing an essential
utility, so we don’t want them to get damaged. "Easy enough" you might be thinking “Just
make them strong.” Put all the power lines underground where they’re protected from
weather and animals. Make them as big as a bridge suspension cables and use indestructible
alloys. Put the substations in big concrete buildings. Hide the solar panels under the
ocean. You see what I’m getting at. I don't know how much a car that never breaks down
would cost, but I’m sure I wouldn’t want to pay for it, and the same is generally true
for the power grid. Resiliency doesn’t just mean durability. It’s a balancing act between
making our infrastructure strong enough to resist threats, keeping faults from creating
further damage, and making it easy to diagnose and repair problems so that equipment can
be brought back online with minimal downtime. Those last two items are the job of power
system protection engineers and can be summed up pretty easily in one word: isolate. Engineers
establish zones of protection around each major piece of the power grid to isolate faults
and make them easy to find and repair. You can trace these zones of protection from your
house all the way to the power plant. A short circuit in your coffee maker isn’t going
to overload the service transformer because there’s a fuse or breaker in between. If
a car knocks down a pole and grounds out a line, it’s not going to take out the entire
substation, again because it’s isolated with a fuse or breaker. If a transformer has
a fault in a substation, it’s not going to melt the transmission lines feeding it
because it can be isolated using breakers. And if a transmission lines sags into a tree
limb, the resulting surge in current is not going to destroy the generator at the power
plant because it has its own zone of protection. Of course, this is a super simplified explanation.
These zones of protection are thoughtfully considered to balance the complexity and resiliency
of the grid. But, how do they actually work? There are a wide variety of types of electrical
faults. Identifying and differentiating them can be a major challenge. The fundamentals
of electrical devices can be boiled down pretty easily. Electrical current travels from a
source, through a series of components, and back through a return path that is referenced
to ground. There really isn’t that much information that protective devices can use
to identify problems. For example, there’s very little difference between what’s happening
in your toaster and what happens when you take the live and neutral lines from a socket
and short them together. The circuit breakers in your house identify faults primarily based
on electrical current. If you get too many amps moving through the breaker, it assumes
that something is wrong and shuts off the circuit. That makes sense for a lot of cases,
since high current can seriously damage equipment and conductors, leading to all sorts of issues.
But, it’s not the only kind of electrical fault. On the grid, protection is primarily done
through relays that can measure all kinds of parameters to identify faults and activate
circuit breakers to isolate equipment and notify utilities of the problem. These relays
are measuring voltage, current, and power on the lines, like you’d expect. They also
measure differential current. Even if the current isn’t too high, you want to make
sure that as much current is going out as is coming in, otherwise you’re losing it
somewhere else which can be signal of a fault. This is the same principle that GFCI outlets
in your house use. Relays also keep an eye on the frequency of the grid to make sure
different components don’t lose synchronization. Certain breakers can also be manually activated,
like during rolling blackouts, where utilities are forced to shed non-critical electrical
loads due to lack of generation capacity. These are all types of “managed failures”
where you have some loss of service at the cost of protecting the rest of the system.
The goal is that isolating equipment when things go wrong speeds up the process and
reduces the cost of making repairs to get customers back online. But, there are cases when isolation of equipment
can actually make things worse. And I’ve built a little demo to show how this works.
Imagine a series of interconnected transmission lines, all feeding their own service areas,
represented by the power resistor and LED light in the model. During peak demand, these
lines might be operating at nearly their maximum capacity. If one line experiences a fault,
for example shorting out against a tree branch, protective relays will isolate the line. In
my case, when I short out a line, the fuse blows. But, if not handled correctly, that
can mean that the entire electrical load gets automatically distributed between the remaining
transmission lines, pushing them beyond their limit. All of a sudden, you have a cascading
failure. Much of our grid is designed to avoid this type of failure, but occasionally you
get the perfect alignment of faults, communication errors, and human factors that lead to massive
outages, like the one in 2003 that took out much of the U.S. northeast and Ontario. Starting back up from a major blackout like
this can be really complicated. Even just choosing which equipment to unisolate and
in what order takes a lot of consideration and engineering. There’s a chicken and egg
situation because most large power plants actually need some power to operate, so it
can be difficult to start back up during a wide area outage, also called a black start.
But, it’s still better than the alternative of having to perform major equipment replacements
because things spiraled out of control. When your power goes out, it’s easy to be frustrated
at the inconvenience, but consider also being thankful that it probably means things are
working as designed to protect the grid as a whole and ensure a speedy and cost-effective
repair to the fault. Thanks to HelloFresh for sponsoring this video
and asking us to film ourselves making dinner. “Hold it straight, babe.” Spending time
in the kitchen is one of our favorite things, and HelloFresh makes it super easy to cook
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for watching, and let me know what you think!
Grady's videos are the best!