Quick transformer question.

La Poste · Jun 21, 2012

Greetings.

Here are a couple of pictures of our local transformer.

As you can see the star point is earthed and then we have an earth cable going down into the ground to stablise the voltage or so I have been taught.

My question is what sort of current flows down this earth cable?

Does the current flow from the star point to the ground vary with the different loads placed on each phase?

Does any current flow down this cable at all?

I have been taught that the star point is earthed to stabilise the voltage but I am just wondering in terms of actual current flow how this is achieved.

This is purely a technical query and not some made up story to see if it's safe to steal some copper cable.

Thanks.

Knobhead · Jun 21, 2012

Normally no current flows. Only under earth fault conditions will current flow.

I’m surprised there’s no capping over the earth.

Silly Sausage · Jun 22, 2012

Do you live a long way from the Police?
I could do with that tranny

La Poste · Jun 22, 2012

Tony said:
Normally no current flows. Only under earth fault conditions will current flow.

If no current flows how does it stabilise the voltage?

This is what i am finding hard to get my head around.

If no current flows down this earth cable then how does earthing the star point stabilise the voltage?

If you took this earth connection away then the star point would rise above zero volts, the degree it would rise would be dependent on the loading of each phase and yet if you earth the star point it brings the star point down to zero volts and keeps it there and yet it manages to do this with no current flowing down this earth cable.

Can anyone suggest some good reading on transformers and earthing?

Archy, this transformer supplies our house, there aren't many coppers around but there is me, and I'd be out there like a shot and so would my little jack Russel ready to bite your ankles.

Knobhead · Jun 22, 2012

Nice bit of weigh in, Archie?

Knobhead · Jun 22, 2012

Imbalance in the phase loading is carried by the neutral. The earthed point is to give a reference and to carry fault currents.
I take it here’s more the just one house near you? That’s a bit of a beast for one house.

La Poste · Jun 22, 2012

I'm not really sure how many houses are supplied by this tranny, probably in the region of thirty or thereabouts.

Tony fault currents from where?

Fault currents from the transformer itself or fault currents from the consumers earthing (lead sheath of cable).

Are we talking an internal problem from the tranny or earth fault currents from a consumers house?

MarkieSparkie · Jun 22, 2012

Archy Styrigg said:
Do you live a long way from the Police?
I could do with that tranny

Deviant practices with Lady-boys mate? Don't want the Police to see, nudge, nudge, wink, wink, know what I mean Squire! :lol:

Knobhead · Jun 22, 2012

La Poste said:
I'm not really sure how many houses are supplied by this tranny, probably in the region of thirty or thereabouts.

Tony fault currents from where?

Fault currents from the transformer itself or fault currents from the consumers earthing (lead sheath of cable).

Are we talking an internal problem from the tranny or earth fault currents from a consumers house?

Sorry, earth faults any where in the local distribution system.

La Poste · Jun 22, 2012

Tony said:
Sorry, earth faults any where in the local distribution system.

Local as in just the secondary side of this transformer and associated premises supplied by it or local as in including the secondary and primary side of this transformer and including the next transformer up the chain?

Cheers Tony.

Knobhead · Jun 22, 2012

The blow-out fuses you can see in the pictures take care of the HV side.
The whole set up is about as subtle as a brick when compared to an industrial transformer.

La Poste · Jun 23, 2012

Here's a few more pictures of our supply transformer.

Look at the the three joints that connect the 11KV overhead lines to the transformer.

In the middle picture you can see a long brown shaft.

What are they for?

Are they simply breakers to isolate the transformer from the overhead supply or do they serve some sort of overload function?

They look like they have a hook on them.

Thanks.

Guest123 · Jun 23, 2012

Those are the 'drop out' fuses Tony mentioned.

La Poste · Jun 23, 2012

Thanks Lenny.

So the transformer frame and star point are earthed via the long cable that disappears into the ground.

The supply neutral is connected to the star point and hence the frame of the transformer.
Our own domestic earthing system is connected to the star point and hence the frame of this transformer.

What would happen if there was a fault within or without of the transformer where one of the 11KV wires touched the frame of the transformer?

I'm sitting at home and all the pipework of this house is bonded back to the MET which in turn is connected to the star point of this transformer which is connected to the frame of the transformer.

So if an 11KV cable came into contact with the frame of the transformer would all our domestic bonded metalwork suddenly shoot up to 11KV?

It's a scary thought, what would stop this happening?

I don't think the fault current would be high enough to blow one of those drop out fuses.

snowhead · Jun 23, 2012

La Poste said:
What would happen if there was a fault within or without of the transformer where one of the 11KV wires touched the frame of the transformer?

The 11kv would flash over on it's way down before it actually touched.
The protection the other end for the 11kv would drop out, remember it's 11kv so the currents are relatively low.
The fault current would go to ground not to your earth.

The transformer serving just my Son's house and mounted in his front garden, the earth is capped on it's way down to the ground.

Drop outs are on the main 11kv run across his fields before it "T's" off to this;

Knobhead · Jun 23, 2012

Their proper name is an expulsion fuse.

La Poste · Jun 23, 2012

Ok I had a little walk up to the other end of the overhead cables.

You can see in the photo's the cable goes underground and into an enclosure.

Within the enclosure are three metal "boxes" for want of a better word.

Does anyone know what they are and what they do, what's inside the boxes?

Each box has a different label, you can just about read the names on the labels from my pictures.

They are called "Tee's"

Knobhead · Jun 23, 2012

Merlin Gerin, 630A fused load break switches. Obviously the fuse rating can be lowered. If one fuse blows the switch operates, breaking all three phases. They can be used as ring main units or to split a feed in to two.
The little white boxes are earth fault direction indicators.

La Poste · Jun 23, 2012

Thanks Tony.

Right we have three boxes and three underground cables.

Those three underground cables are three phase cables right, I mean the three cables are not single phase cables they are three phase cables in their own right.

In this enclosure we have three separate underground three phase cables coming up and entering three separate enclosures.

The little white boxes you mention are only apparent on two of the boxes, the third box has no earth fault direction indicator.

Would it be correct to say that one of those cables coming up from the ground is the supply cable which then splits into two separate supply cables that go off in different directions or have I got this totally wrong?

La Poste · Jun 23, 2012

One thing I'm having trouble understanding is the earthing on a delta delta transformer.

I mean big network transformers are delta delta right?
The star formation transformer only comes into play at the lower end of the voltage spectrum and the star type of transformer has its neutral or centre tap directly connected to earth through a cable so that if you get an earth fault at a consumers installation there is a direct path for the earth fault current to flow as in from the consumers installation, through the ground to the transformers earth rod/mat and then back to the star point.

Now if you have a network transformer that is wired delta delta there is no where that a cable is connected directly to earth so how can you have earth fault currents?
I mean to have an earth fault current you need to make a circuit, you need a place where the current is injected into the earth as in the consumers premises and a place where the earth fault current is picked up and returned to the network circuit as in through the earth cable that is connected to the star point of the supply transformer, a circuit has been made and so an earth fault current will flow through the ground.

With a delta delta transformer non of the windings are directly connected to the ground so if for instance one of the phases touched the ground or indeed you touch one of the phases how can you get an electric shock, it isn't possible for you to become part of the circuit because there is no return path for the fault current to rejoin the three phase supply.

I can understand if you were to touch between two phases, you would instantly become part of the circuit but if non of the delta delta windings are directly connected to ground how can you get a shock if you were to touch one of the cables whilst standing on the ground?
There would be no flow and return of current.

Anyone got any suggestions?

Thanks.

Knobhead · Jun 24, 2012

Some have the secondary earthed via an auxiliary Zig-Zig transformer. This is in fact a choke with its star point connected to earth. Any earth fault can be monitored via this star point.

View attachment 13077

La Poste · Jun 24, 2012

Right I did a little more investigation and took some pictures with the help of a good telephoto lens.

So am I right in saying this:

These metal enclosures are the end of the line, they are at the end of the three phase supply and are a type of fusing.
So starting from the furthest point from the transformer, one of these boxes is our supply cable or tee, the supply cable starts/ends here then goes overhead until it reaches the transformer in the picture in post 1 of this thread.

If you look at the transformer in post 1 you will see that the overhead lines not only supply the transformer but also become an underground cable that disappears somewhere off under the field.

The underground cable will eventually join a transformer possibly a 33KV/11KV transformer somewhere in our local vicinity.

Is this correct, are these fusing boxes always placed at the end of the three phase supply line, farthest away from the transformer or am I barking up the wrong tree?

The three boxes have labels on them like the above and each label is a different geographical location but still local so either all the supply cables from the three geographical locations end here or this is some sort of splitter where a supply cable comes in and is then split off in different directions.

The supply seems to go from a transformer somewhere, crosses over meadows and often goes underground, 230V/415V three phase star transformers are placed at various stages along the route to supply houses etc and eventually the 11KV supply cables are terminated in the fused units that you see in this post.

Does that make sense, any thoughts?
I'm just trying to get the bigger picture.

Thanks.