- [John] Hi, John here. In this video we're gonna have a look at an electrical bushing. I'll show you all of the
main components, we'll look at some of its design
features, I'll tell you how the bushing works, and
I'll also tell you why we have bushings in the first place. So let's start by taking
a look at a bushing. Here is one such bushing. This is actually an oil
insulated porcelain bushing. There are two main types of bushings that you are going to see. They're classified as
bulk or solid, that is one classification, and condenser or capacitance graded bushings. That is the other type of classification. This type of bushing is
a solid type of bushing, that is to say it's a bulk type bushing. Bulk and solid, you can use
the words interchangeably, they mean the same thing. So we've got a bulk bushing,
and if we spin it around here we can actually see the
internal components. So let's just start at
the top of the bushing. We've got here, upper
terminal, you can see we've got four holes, we can
actually connect a cable or a connector onto that terminal, and we'll clamp it to the
terminal and get a very good contact surface area
between the two connections. And this ensures the
current can flow throughout the system and that we
don't generate too much heat at the terminal. If we go on the top of
the terminal, if we look from this angle, you can see we've got a piece of metal on the inside
here, and that piece of metal actually runs all the way
down through the bushing. You can see it's just
a solid piece of metal, and we call that our center
conductor or central conductor or you can even just
call it the conductor. It's usually gonna be
manufactured from bronze or aluminum or copper. Remember that bronze itself
is a copper based alloy and that's what gives it its copper color. Either way, whatever material
is chosen to be the central conductor is going to be a very good conductor of electricity,
because this conductor is going to be live, and
it may be at 3,000, 6,000, 11,000 volts depending upon
the electrical system served. To the side of the conductor,
we've got this void space, you can see that the void
actually starts at the bottom here and it actually is
all around the conductor. Comes all the way up to
here, and this void space over here and on the other
side, is full of mineral oil. We actually call it
electrical grade mineral oil and we use it as an insulator. I'll come back to that in a moment. Outside of the void space
which is full of mineral oil when the bushing is in
service, we actually have this outer piece here, it wraps all the way around the bushing. You see it's quite shiny. And it's also quite smooth. This entire outer piece is
manufactured from porcelain. Now porcelain is just baked clay. The type of porcelain we use
for this particular bushing is actually mechanically
stronger than the type of porcelain that you have in your home. If you want to see an
example of porcelain, just go and take a look
at a cup or a plate in your home and that is a
basic example of porcelain. The only difference with this porcelain is that it is slightly
stronger mechanically so it's less prone to cracking. The reason it has this
very shiny outer surface is because on the outer
surface of the porcelain we've added a glaze,
and this glaze actually serves two purposes. One purpose is that
when we apply the glaze, it actually puts the
porcelain under compression when it sets, and porcelain
is a bit like concrete. It's very strong under compression. So if you squeeze porcelain,
it can handle a lot of compression stress
before it begins to break. This is much like concrete. However, if you stretch
porcelain, in other words, if you apply tensile stress,
it breaks very easily. So the glaze allows us
to keep some residual compression force on the
porcelain which increases its mechanical strength. As I mentioned before,
concrete is similar in this way to porcelain because it does
not like to be stretched, it's not very strong under tensile stress. The other reason that we
add the glaze on the outside of the porcelain is because
it keeps the porcelain body smooth and that helps it stay clean. But also notice on this particular bushing that we have some arcing horns. That's the upper horn
and the lower horn here. We're not going to discuss
those in this particular video, but we'll discuss them a
little bit later in the course. On the lower side of the bushing
you can see we've got this white item wrapped around
the base of the bushing. This is called a gasket,
and the reason we have that gasket for this particular bushing is because we would install
this electrical bushing on a transformer. So the transformer tank would
be at around this level here and then on the top
side we would have air, so from here going upwards
to the top terminal, that is all the air side. On the lower side we've actually got oil. But why would we need to have
a bushing in the first place? Well the reason we need a
bushing is because once current is flowing through a
conductor, there are several things that's going to happen. One of them is that we
create a magnetic field around the conductor. With direct current, the magnetic
field is fairly constant, with alternating current,
the magnetic field expands and contracts,
and it may do this 50 or 60 times a second,
this is what we refer to as the frequency. Now we're not so interested
in the magnetic field that's created around the conductor. The reason that we require
a bushing is because of the electric field that's
created around the conductor. Every time current flows
through a conductor, we're going to create an electric field. The intensity of the
electric field depends upon the voltage. If we have a higher voltage,
then we will have a more intense electric field. If we have a lower voltage,
then we will have a less intense electric field. As the electric field intensity increases, it ionizes any insulator surrounding it. Typical things surrounding
it would include air and oil, and I'm going to refer to
them as the insulators. As the insulators become
ionized, their ability to conduct electrical current increases. So the problem that we
have with live conductors at very high voltages is
that they tend to ionize any surrounding insulator,
and the ionization of these insulators allows
current to flow to ground, the several ways this may
occur, if we have a small amount of current flowing
to ground, then we will call this leakage current. All live conductors have
some leakage current. It's not possible to perfectly
insulate a live conductor. If we have a significant
amount of leakage current, then we will get flashover,
which is effectively a short circuit where large
magnitude current flows from a high potential to a low potential. For example, from my live
conductor through our insulator and straight to ground. So how does that relate to our bushing? Well with our bushing we
have our live conductor here, and as you can see, the live conductor, let's just say for a moment
this is gonna be 11,000 volts, and we can see that our
transformer tank would end right about here, and
the conductor is actually gonna be quite close to the
top of the transformer tank. That means we're gonna have
significant electrical stress from the center conductor
here where it tries to flow to ground to our transformer tank which will be around this height here. In order to stop that
occurring, we fill this space here with mineral oil, and
we manufacture the bushing from porcelain which also has
good insulation properties. So that's how we can deal
with some of the radial electrical stresses that
are placed on the bushing due to this electrical field
generated by high voltage. But there are also axial
stresses placed on the bushing because there is another
path that current can use to flow to ground. So we have a high voltage
potential top terminal here and then we have a
grounded transformer tank right about here, and if
it was possible for leakage current to flow along
here and across and down and along and across and
down, we can see that it's actually possible for the
leakage current to reach the transformer tank. The path that the leakage current takes is actually referred
to as the leakage path or the creepage path, and
anything on those paths is referred to as leakage
current or creepage current. In order to minimize leakage
current, we want to actually increase the resistance
along the leakage path as much as possible, as this will reduce the likelihood of
flashover, that is to say, reduce the likelihood of an
electric arc between the upper high potential terminal and
the grounded transformer tank. So how are we going to
maximize the resistance on the leakage path as much
as possible without building a very very long bushing. Well one way for us to do
that is to change the geometry of the bushing. You can see that if we
had a straight bushing, the leakage path would
be considerably shorter than if we had the bushing
shaped as we do here. These extra bits sticking out the side which we call rain sheds or
sheds are there to increase the length of the leakage path. Not only that, but you'll
notice the shape of the bushing is quite unique, and that's
because when it rains we actually want some of
the bushing to remain dry. These sheds shield part
of the bushing in order that it can remain dry. For example, the under side
here and this section here would normally be dry
even if it should rain. Obviously it depends upon
the direction of the rain and how intense the storm
is, but the general idea is that some parts of the
porcelain body remain dry. If all of the bushing was to become wet, or if it gets for example
covered in fog or moisture, then the resistance on the leakage path is going to be far less,
and we're going to increase the probability of getting a short circuit from the upper terminal,
which is that high voltage potential to our grounded
electrical transformer tank. There are other factors that can also increase the likelihood of flashover. This include any breakdown
of the insulation, but they may also be related
to things such as pollution, and the accumulation of
dirt on the bushing surface which again will reduce
the resistance along the leakage path and increase
the likelihood of flashover. You may also have
noticed that the top side of the bushing is quite
long and the lower side is quite short. When we connect the bushing
to our electrical transformer, the electrical transformer
is actually full of oil and some of that oil is going to travel up into the void space of the bushing, and it's going to fill
up the entire cavity or void space within the bushing. This mineral oil then
acts as an insulator. Because the transformer
tank is actually lower down than the bushing, we
actually have another tank called the conservator
tank which is higher up than the main transformer
tank, and that allows us to keep a positive head
of pressure on the oil inside the bushing. Because oil acts as an
insulator, it's also possible for us to have a much shorter bushing body on the oil side. Remember that it's also
possible for leakage current to come from the underside
of the transformer tank, to travel along this leakage path here. But because the mineral oil
is such as good insulator, the resistance along the
leakage path is far greater compared to over here
where the only insulator that we have is air. So on the air side, the
bushing is a lot longer and on the oil side it's a lot shorter. Other reasons for the
bushing being a lot longer on the air side, are
simply that on the air side it's open to environmental
conditions, salt spray, rain, fog, dirt, sand, and on the oil side this section here is sitting
inside the transformer tank and it's not exposed to pollution or other environmental conditions. That means that the resistance
along the leakage path is relatively stable and that
we can actually calculate that and then determine
the length of the bushing. There are many other types
of electrical bushing in use today, especially in
the power generation industry. If you'd like to learn
more about bushings, then please do check out our website. You'll find an electrical bushings course along with an electrical
transformer course, and we have many other
types of engineering courses related to pumps, valves, heat
exchanges, diesel engines, and other common machinery items. So if that interests
you, then check out some of the links in the
video description area. If you like this video,
then please do like or share it on social media,
it really does help us out, and if you haven't done so,
then please do subscribe to the YouTube channel. Thank you very much for your time.
