Question about UK distribution transformers and fusing requirements

[Beachcomber]

I am a US resident who is interested in different electric systems
around the world.

I understand the British practice for house-serving power
distribution transformers is to not have a neutral connection in the
primary, that is, the primary is usually is wired to the two hot wires
coming from the regional sub-station.

My question concerns the fusing and protection of the transformer.
How is the primary of the transformer fused? Is there one fuse for
each hot leg and if so, isn't this a bit of a hazard when just one
fuse blows? Or is there some scheme to disconnect both fuses when
just one blows open?

Also, what is the typical (primary) voltage? Is it standardized
throughout the country, city-rural, etc? Do sub-stations typically
run an earthed safety ground along the outgoing mains? ... and for
that matter, is the earth-ground connected to the frame of the (local
distribution) transformer? Is there, in fact, an earth connection (a
driven ground rod, for example) at the transformer?

I don't want to be accused of being one of those
my-country's-system-is-best Americans. That's why I like to learn how
it is done in other countries and what is the reasoning behind
different practices.

Beachcomber

 

[[email protected]]

| I don't want to be accused of being one of those
| my-country's-system-is-best Americans. That's why I like to learn how
| it is done in other countries and what is the reasoning behind
| different practices.

That's a good approach. That way you know _specifically_ what is wrong
with all those other systems

IMHO, everyone got it all wrong.

 

[Andrew Gabriel]

I am a US resident who is interested in different electric systems
around the world.

I understand the British practice for house-serving power
distribution transformers is to not have a neutral connection in the
primary, that is, the primary is usually is wired to the two hot wires
coming from the regional sub-station.

Distribution is 3-phase in the UK (and all of the rest of Europe
that I know of). The final stepdown transformer will be a delta
primary and a star/wye secondary, thus converting a 3-phase 3-wire
medium voltage supply to a 3-phase 4-wire low voltage supply.

My question concerns the fusing and protection of the transformer.
How is the primary of the transformer fused? Is there one fuse for
each hot leg and if so, isn't this a bit of a hazard when just one
fuse blows? Or is there some scheme to disconnect both fuses when
just one blows open?

Sorry, don't know the details, but I'm pretty sure the circuit
breakers are always ganged across all 3 phases.
Low voltage (230/400V) supply lines are separately fused.

Also, what is the typical (primary) voltage? Is it standardized

11kV, standardised.
It's also the standardised supply voltage for industry which
wants higher than the normal 230/400V.

Next higher distribution voltage is 33kV, but I don't know if this
is ever used to directly feed low voltage network.

throughout the country, city-rural, etc? Do sub-stations typically
run an earthed safety ground along the outgoing mains? ... and for

There are 3 types of supply:
TN-S -- a separate safety ground is run
TN-C-S -- safety ground is combined with the neutral
TT -- no safety ground is run (old rural supplies).

that matter, is the earth-ground connected to the frame of the (local
distribution) transformer? Is there, in fact, an earth connection (a
driven ground rod, for example) at the transformer?

Yes.

 

[John Rye]

Hello Beachcomber

You have had some good answers from Andrew Gabriel. I will try and fill the
gaps.

I am a US resident who is interested in different electric systems
around the world.

I understand the British practice for house-serving power
distribution transformers is to not have a neutral connection in the
primary, that is, the primary is usually is wired to the two hot wires
coming from the regional sub-station.

My question concerns the fusing and protection of the transformer.
How is the primary of the transformer fused? Is there one fuse for
each hot leg and if so, isn't this a bit of a hazard when just one
fuse blows? Or is there some scheme to disconnect both fuses when
just one blows open?

Practice varies. In urban areas where transformers are ground mounted and >
200 kVA the transformer will be protected by either a circuit-breaker, or a 3
phase fuse-switch, which will have a trip all phases feature.

In rural areas where the transformers are smaller, and pole mounted there is
frequently no protection local to the transformer on the high voltage side of
it. Transformers tend to be protected in groups using fuses, and
auto-reclosers. There are a variety of policies with different authorities,
and situations, and it is difficult to go further on a news group.

Also, what is the typical (primary) voltage? Is it standardized
throughout the country, city-rural, etc? Do sub-stations typically
run an earthed safety ground along the outgoing mains? ... and for
that matter, is the earth-ground connected to the frame of the (local
distribution) transformer? Is there, in fact, an earth connection (a
driven ground rod, for example) at the transformer?

 

[Fads]

Distribution is 3-phase in the UK (and all of the rest of Europe
that I know of). The final stepdown transformer will be a delta
primary and a star/wye secondary, thus converting a 3-phase 3-wire
medium voltage supply to a 3-phase 4-wire low voltage supply.


Sorry, don't know the details, but I'm pretty sure the circuit
breakers are always ganged across all 3 phases.
Low voltage (230/400V) supply lines are separately fused.


11kV, standardised.
It's also the standardised supply voltage for industry which
wants higher than the normal 230/400V.

Manchester City Centre is 6.6kv (norweb)

 

[Beachcomber]

Practice varies. In urban areas where transformers are ground mounted and >
200 kVA the transformer will be protected by either a circuit-breaker, or a 3
phase fuse-switch, which will have a trip all phases feature.

In rural areas where the transformers are smaller, and pole mounted there is
frequently no protection local to the transformer on the high voltage side of
it. Transformers tend to be protected in groups using fuses, and
auto-reclosers. There are a variety of policies with different authorities,
and situations, and it is difficult to go further on a news group.

John:

Thanks for some good information. I was just wondering if the
overseas transformer connections were anything like of seen here in
North America. The typical pole mounted transformer is fed from a
high-side primary and a neutral (always mounted in a lower position on
the pole). Usually I will see a lightning arrestor and a
fused-disconnect in the primary lead and a single HV bushing going
into the transformer. Also, in many cases there is a Ground (Earth)
wire running down the pole either to a rod or some other sort of
earthing system.

I was wondering specifically how the primary fusing was handled at the
transformer in the British installations with the two hot leads. Are
you saying the transformers are not fused locally?

Beachcomber

 

[John Rye]

Practice varies. In urban areas where transformers are ground mounted and
Less than 200 kVA the transformer will be protected by either a
circuit-breaker, or a 3 phase fuse-switch, which will have a trip all
phases feature.

In rural areas where the transformers are smaller, and pole mounted there
is frequently no protection local to the transformer on the high voltage
side of it. Transformers tend to be protected in groups using fuses, and
auto-reclosers. There are a variety of policies with different
authorities, and situations, and it is difficult to go further on a news
group.

John:

Thanks for some good information. I was just wondering if the
overseas transformer connections were anything like of seen here in
North America. The typical pole mounted transformer is fed from a
high-side primary and a neutral (always mounted in a lower position on
the pole).[/QUOTE]

Never an HV neutral

Usually I will see a lightning arrestor and a fused-disconnect in the
primary lead and a single HV bushing going into the transformer. Also, in
many cases there is a Ground (Earth) wire running down the pole either to a
rod or some other sort of earthing system.

There may be lightning arrestors fitted on each phase at the pole
particularly if it is at the end of an overhead line. All pole mounted
transformers are fitted with arc gaps, which will flashover if there is an
overvoltage on the line. When they flash over it is important that the
circuit-breaker or auto-recloser protecting the line trips quickly to
interrupt the power follow current before the bushings are damaged.

I was wondering specifically how the primary fusing was handled at the
transformer in the British installations with the two hot leads. Are
you saying the transformers are not fused locally?

Yes !

Many years ago they used to be, but the fuses often blew in large numbers
during lightning storms. So a policy of "Group Fusing" and auto-reclosing was
developed, and this works very well.

John Rye

 

[John G]

Thanks for some good information. I was just wondering if the
overseas transformer connections were anything like of seen here in
North America. The typical pole mounted transformer is fed from a
high-side primary and a neutral (always mounted in a lower position
on
the pole).

Never an HV neutral[/QUOTE]

If UK practice is anything like Australia, then there are not any pole
mounted transformers fed with a hot and a neautral.
They are all 3 phase delta input, lots at about 40KVA size. With 3 phase
Wye output with grounded neautral and all 3 phase continued to houses in
many, not all, cases for Aircon and Ranges.

All houshold aplliances are 230 (was 240) volts and all domestic 3 phase
are 400 (was 415) volts
There is no confusion like the US 240/208 mixture.
The 240 to 230 change is like the UK, a work in progress to harmonise
with the rest of the 50 hertz world.
In remote areas there is some Single Wire Earth Return distribution.

There may be lightning arrestors fitted on each phase at the pole
particularly if it is at the end of an overhead line. All pole mounted
transformers are fitted with arc gaps, which will flashover if there
is an
overvoltage on the line. When they flash over it is important that the
circuit-breaker or auto-recloser protecting the line trips quickly to
interrupt the power follow current before the bushings are damaged.


Yes !

With Delta input of course there are 3 wires, and in Australia in many
cases they are fused at the pole with Drop fuses that can be replaced
from the ground by a man with a fibre glass pole.

 

[Ted Squires]

To start at the top end, high Voltage transmission lines are normally
132kV, 275kV and 400kV. These are normally run as overhead lines
suspended from steel pylons (towers), except in urban areas, where
underground cables are used. There is an underwater cable to France,
which is d.c., and I think is 200kV, but I'm not certain about that
Voltage.

Medium Voltage distribution systems where carried overhead are normally
suspended from wooden poles. Again, in urban areas, underground cables
are normally used. The most common Voltages are 11kV and 33kV; 66kV is
sometimes used for very large systems. There are two other Voltages
which are less commonly used; 22kV is sometimes used, I can think of a
couple of substations in London where that Voltage was used, though one
of them was re-equipped a couple of years ago, and I think may have
been converted to 33kV. This Voltage is also used to suply parts of
the London Underground (subway) system. That is, it is used to supply
the substations; the railway itself is a low-Voltage d.c. third and
fourth rail system. 22kV is also more widely used on the European
mainland than it is here. 6.6kV was used in the past, and there are
still places supplied at that Voltage, but it is a pretty much obsolete
Voltage now, and would be unlikely to be used in a new installation.


This is something which interests me as well. Something which does
strike me is just how different American and European practice is; I'm
talking more about the low-Voltage supply side than medium-Voltage
distribution. Also, the American way of doing things does seem to be
much more complicated than the European way. I can see certain
advantages to both.

don't forget the UK 480 single phase centre tapped installation.. working
with
one of these currently... PITA cos you have to use double pole breakers on
a 3ph board...looks a right mess.

 

[Beachcomber]

This is something which interests me as well. Something which does
strike me is just how different American and European practice is; I'm
talking more about the low-Voltage supply side than medium-Voltage
distribution. Also, the American way of doing things does seem to be
much more complicated than the European way. I can see certain
advantages to both.

Complex technological systems evolve from the local needs and culture.
No one system is best...

Here is another thought...

Maybe it's just my imagination, but in the recollection of my travels
in Switzerland and France, it seems they do a better job of hiding the
power poles when compared with rural areas of the USA. I was also
wondering if the same techniques apply in the UK.

For example, pick any state in the US, say Kansas or Nebraska in the
Midwest. Along every road of any size you will see power lines
running along the side of the road and frequently come across high
voltage transmission and sub-transmission lines going every which way.
In the US, it's pretty much like that in every state. To some it is
ugly, to others; it's just part of the American scene, just like an
Edward Hopper painting.

During my travels in the more rural parts of France about 10 years
ago, I was amazed to see, more often then not, that the power lines
seem to blend into the landscape. Occasionally one sees a concrete
set of poles with 3 super-thin wires serving some remote farm. The
practice of running long lines parallel with the roads seems to be
avoided. France is a modern country with 70% nuclear power, yet they
seem to do a better job in hiding their electrical infrastructure.

Beachcomber

 

[Andrew Gabriel]

Somebody was asking about a source for 480V motors in the UK a few days
ago, but I've never seen anything like this; where is it used?

480 (as 240-0-240V) was used on some remote farms which are fed
with only 2 wires from a 3-phase 11kV distribution supply,
where a long run for just one customer of a 3-phase 3-wire 11kV
distribution would have made the installation significantly more
expensive. I don't believe it's installed nowadays, but there
are some still around.