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.
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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.
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