Quick transformer question.

La Poste · Jun 21, 2012

Greetings.

Here are a couple of pictures of our local transformer.

[ElectriciansForums.net] Quick transformer question.

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;