Electrical Engineering - New world power system - Was: US Electrician qualifications for a Brit

Author

New world power system - Was: US Electrician qualifications for a Brit

phil-news-nospam@ipal.net

2006-10-11, 1:25 pm

On 11 Oct 2006 05:49:22 -0700 furles@mail.croydon.ac.uk wrote:

| phil-news-nospam@ipal.net wrote:
|
|> So, if we could scrap the whole system, or better yet, if you invented a
|> time machine and could go back and create a standard for the whole world,
|> how would you do it?
|
| That's a good question, isn't it? If I can have the time machine can I
| take my present-day knowledge back with me? When Edison designed his
| 100V d.c. sytem it made sense in terms of the requirements at the time.
| Small incandescant lamps were about the only load. and generators were
| small, serving a small area, with short transmission distances, and the
| full output current being capable of being taken out via the brush gear
| of a dynamo. God solution at the time. Where he got it wrong, and
| where the Westinghouse/Tesla camp got it right is that they designed a
| system with a future; one that could be developed into the sort of
| power systems we have today, which span countries and continents, and
| connect 500MW generators and multi-MW motors. The Edison system was a
| dead end, but if I couldn't take my knowledge back with me I might
| well design something similar. Even with that knowledge, there were
| applications in the past which might have required a different system,
| but which are obsolete today, and therefore no longer required.

Yes, take back the knowledge of power systems and safety. You can't
really do it better if you don't take something back.

| Firstly, I would try to get a common worldwide standard. Enables
| people to move equipment from place to place, enables manufacturers to
| sell the same product in different markets, reduces the risk of people
| making mistakes with an unfamilar system, and makes life easier for
| people like the OP in this thread, who need to work in different
| locations.

Yes, fully universal would be a good thing.

| Just about every power system in the World is three phase a.c., and I
| see no reason to do otherwise.

Single phase is cheaper in rural areas. But maybe a usable compromise
is to rule out ground return systems, which means you have to have at
least 2 wires, and use 2 line wires out of the 3 for single phase.
Then you can at least get the same ratio between L-N and L-L as you
get with real three phase. But then, three phase is just one more
wire, and allows the 3 wires to be a bit thinner to serve the same
area.

| Voltage. There seem to be two main camps at the moment, Japan, USA,
| Canada, Mexico, I think parts of South Americatoo, all in the 100-130V

And Taiwan.

| range, phase to neutral, and just about everywhere else on 220-240.
| About the only application where the lower range is better is
| incandescant lighting, which is well on the way to extinction in
| industrial and commercial use, and rapidly going the same way in the
| home. Small decorative lamps, and halogens are better run at a much
| lower Voltage anyway, and the transformer for this can work equally
| well on either Voltage, so I'd go for something towards the upper end,
| in the 200-300V range. Considerable savings in copper, less risk of
| connectors running hot, and possible fire risk with the lower current.
| No need for extra insulation; that on existing cables is determined by
| physical requirements, and is quite adequate for either Voltage.

Wall switches for the most part are used for lighting. They like to be
cheaper as single pole, which means the lights need to run L-N. And if
that Edison screw base is to be used, L-N is more important.

But I'd prefer a L-L connection for just about everything else.

If L-N is kept at a low voltage suitable for incandescent lights and
L-L is run at a much higher voltage, these would then have to be
separate systems (e.g. a small transformer in the home to create the
low voltage for the lights). If these are kept separate, then the
different ratio between L-N and L-L for single vs. three phase is no
longer an issue (the L-L would be made from 2 different L-N voltages
depending on whether the distribution source is single or three phase,
but these voltages would not be directly used, and so would not be a
basis for equipment utilization voltage).

For example, 24 volts L-N for lights (fixed and pluggable) and small
loads (shaver, wall warts). Then 288 volts L-L for big loads (this
would be 144 volts relative to ground for single phase source, or 166
volts relative to ground for three phase source). The transformer to
convert 288 to 24 would be a 12:1 winding ratio. 12 volts would also
be lighting option.

| Frequency. Just about everywhere is either 50 or 60 Hz. now.
| Applications for which lower frequencies were used, mainly transport
| related, e.g. rotary converters providing d.c., and a.c. series wound
| traction motors are almost extinct, so I see no reason to go lower. I
| would choose at least 60 Hz., possibly 75. Probably not higher, due to
| domestic equipment with series wound universal motors. Higher
| frequency is a disadvantage with very long transmission lines, but
| these days high Voltage d.c. is a possible solution in this case.

I, too, would go with 60 to 75 Hz.

That would eventually impact TV standards, too :-)

| I would supply three phase to homes; in this country all three phases
| are taken, at low Voltage, down each street, and houses are fed
| alternately from each phase. I believe that in some cases all three
| phases are actually taken into each house, but only one is normally
| actually connected to anything. So near and yet so far.

I would make three phase optional. With my L-L preference, three
phase is just one more wire. But I see relatively little need for
three phase in homes, other than as a means to have the main wires
be 33% smaller.

| Plugs and sockets. To be honest, I can see problems with all of them.
| You said: "Personally, I don't really like the BS1363." What don't you
| like about them? They are painful if one is on the floor, with pins
| up, and you tread on it with bare feet! They are quite large, but not

That's one reason.

| much more so than a Schuko. I like the internal fuse, essential on a
| 30A ring circuits of course, but would have preferred to see different
| sizes for different ratings; that would have required different plugs
| of course, but would prevent fitting the wrong value fuse. I like the
| shuttered outlets, and the shrouded pins. Not perfect, but not a bad

I do think shrouding or recessing, as well as shutters, are good.

| design. There was one strange problem with it; a one penny coin will
| fit exactly between the three pins, and will touch all three. In the
| days before the shrouded pins a practical joke was to place a coin in
| this position, put the plug in a socket, and wait for somebody to
| switch on. Most British sockets have a switch. The pins are nice
| silid lumps of brass, expenive of course, and well capable of carrying
| well over 100A, but they don't bend, have a large gontact area for a
| good, low-resistance connection, and there's plenty of metal to allow
| some to be removed at the base of the pins, to allow the plastic
| shrouding; you couldn't do that with a NEMA, the metal just isn't thick
| enough.

The joke done over here was to fold a piece of solder into the shape of
a staple, with a hook on each end, and slip it over the plug. Then just
leave the plug unplugged. I did try this a few times in college. What
I discovered is that no one actually looks. They are so into having
discovered the cause of the appliance not working, they just react by
plugging it in. DON'T DO THIS AT HOME OR ANYWHERE ELSE. It really was
stupid.

When I was in high school, someone folded up two pieces of foil from gum
wrappers and fit each one carefully into the 120 volt outlet that was
convenient for the movie projector in a class that was starting to fill
up. When the teacher came in to show the movie, she noticed the projector
had been unplugged, then noticed the wrappers in the socket. She remarked
that this was a stupid trick and that someone could get hurt trying to
play with the outlet. Then she proceeded to pull them both out at the
same time. Fortunately, only her pride had any permanent injuries.

As a result of that, which I witnessed, I actually thought up my own
design of outlet shutters. My design had a front face that was rotated
around the ground pin. You plug in ground pin first and when it is in
far enough, the ground pin would unlock the rotating face. Then you
twist the plug some number of degrees and then it can be inserted the
rest of the way. I was wishing the ground pin was in the middle of
the outlet instead of offset.

Today I would have a circular metal shroud around the pins that is also
the grounding connection, and have the shutters unlocked by the shroud,
possibly by the rotating action to minimize the mechanics in the outlet
that could break.

What I might do with the electrical system I've described so far is use
a circular outlet/plug for the high voltage L-L, with 3 openings if three
phase is available, arranged in a triangle, or just 2 of them if single
phase is all that is available. A 2-pin plug would then fit a 3-opening
outlet. The low voltage L-N would have a polarized pair of pins in a
rectangular shroud.

Low voltage outlets would be limited to 10 amps. High voltage outlets
would have versions for 16, 25, and 40 amps. I might make the 16 amp
plug be compatible with the 25 amp socket by using a plug blade that
is wider (not thinner or longer) so that the snugness is always there
regardless of which plug type is used.

The low voltage 24 volts might even be DC, rectified and filtered from
the 288 volts AC.

The tiers of current protection would be taken from every other value in
a logarithmic decade scale: 10 16 25 40 64 100 160 250 400 640 1000, etc.

I'll try to make some drawings of my plug/socket configurations later.

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2006-10-11-0802@ipal.net |
|------------------------------------/-------------------------------------|

Matthew Beasley

2006-10-11, 1:25 pm

<phil-news-nospam@ipal.net> wrote in message
news:egitrp0prq@news2.newsguy.com...
> Single phase is cheaper in rural areas. But maybe a usable compromise
> is to rule out ground return systems, which means you have to have at
> least 2 wires, and use 2 line wires out of the 3 for single phase.
> Then you can at least get the same ratio between L-N and L-L as you
> get with real three phase. But then, three phase is just one more
> wire, and allows the 3 wires to be a bit thinner to serve the same
> area.
>

Y connected transformers are preferred on distribution systems. No problems
with ferroresonance or backfeed.

phil-news-nospam@ipal.net

2006-10-11, 8:25 pm

On Wed, 11 Oct 2006 16:24:20 GMT Matthew Beasley <nobody@spam.com> wrote:
|
| <phil-news-nospam@ipal.net> wrote in message
| news:egitrp0prq@news2.newsguy.com...
|> Single phase is cheaper in rural areas. But maybe a usable compromise
|> is to rule out ground return systems, which means you have to have at
|> least 2 wires, and use 2 line wires out of the 3 for single phase.
|> Then you can at least get the same ratio between L-N and L-L as you
|> get with real three phase. But then, three phase is just one more
|> wire, and allows the 3 wires to be a bit thinner to serve the same
|> area.
|>
|
| Y connected transformers are preferred on distribution systems. No problems
| with ferroresonance or backfeed.

I would definitely have three phase transformer secondaries configured
as Y or star. The primary would be delta. The loads would be L-L or
L-L-L.

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2006-10-11-1716@ipal.net |
|------------------------------------/-------------------------------------|

Matthew Beasley

2006-10-13, 1:25 pm

<phil-news-nospam@ipal.net> wrote in message
news:egjqjs02asv@news2.newsguy.com...
> On Wed, 11 Oct 2006 16:24:20 GMT Matthew Beasley <nobody@spam.com> wrote:
> |
> | <phil-news-nospam@ipal.net> wrote in message
> | news:egitrp0prq@news2.newsguy.com...
> |> Single phase is cheaper in rural areas. But maybe a usable compromise
> |> is to rule out ground return systems, which means you have to have at
> |> least 2 wires, and use 2 line wires out of the 3 for single phase.
> |> Then you can at least get the same ratio between L-N and L-L as you
> |> get with real three phase. But then, three phase is just one more
> |> wire, and allows the 3 wires to be a bit thinner to serve the same
> |> area.
> |>
> |
> | Y connected transformers are preferred on distribution systems. No
> problems
> | with ferroresonance or backfeed.
>
> I would definitely have three phase transformer secondaries configured
> as Y or star. The primary would be delta. The loads would be L-L or
> L-L-L.
>

I guess there aren't any utility engineers reading this thread. I was
holding off hoping one of them would comment.

L-L or delta transformers are _REALLY_ discouraged for new installations.
Most new installations are Y-Y for three phase or L-N for single phase. L-L
can cause all sorts of problems when a phase is lost. First off there is
ferroresonance. That's when the magnetizing inductance of the transformer
resonates with the line capacitance of the open phase. It's a real problem
for underground service because of the higher capacitance of underground
lines. It can still be a problem for open overhead, particularly at higher
voltages, particularly 34.5kV. I have heard of lots of problems when
utilities upgrade to 34.5kV primary and still have delta connected services
they can't change, for example a customer with 240V center tapped delta.
The other problem is with back feed to a faulted phase. Yes, motors can
back feed and add to fault current. But usually the motors can't blow the
line fuses on the transformer. But a delta connected bank will. It's not
uncommon for every delta bank to blow one or two fuses when one of the
primary phases has a hard fault (like crossed over conductors) and the
source is single phase tripped. After the line is repaired and the feed
restored, the line crew must then go around and refuse a bunch of
transformer banks. Three phase tripping eliminates this, but many utilities
like the single phase fault protection out on long lines.

phil-news-nospam@ipal.net

2006-10-13, 5:25 pm

On Fri, 13 Oct 2006 17:35:27 GMT Matthew Beasley <nobody@spam.com> wrote:
|
| <phil-news-nospam@ipal.net> wrote in message
| news:egjqjs02asv@news2.newsguy.com...
|> On Wed, 11 Oct 2006 16:24:20 GMT Matthew Beasley <nobody@spam.com> wrote:
|> |
|> | <phil-news-nospam@ipal.net> wrote in message
|> | news:egitrp0prq@news2.newsguy.com...
|> |> Single phase is cheaper in rural areas. But maybe a usable compromise
|> |> is to rule out ground return systems, which means you have to have at
|> |> least 2 wires, and use 2 line wires out of the 3 for single phase.
|> |> Then you can at least get the same ratio between L-N and L-L as you
|> |> get with real three phase. But then, three phase is just one more
|> |> wire, and allows the 3 wires to be a bit thinner to serve the same
|> |> area.
|> |>
|> |
|> | Y connected transformers are preferred on distribution systems. No
|> problems
|> | with ferroresonance or backfeed.
|>
|> I would definitely have three phase transformer secondaries configured
|> as Y or star. The primary would be delta. The loads would be L-L or
|> L-L-L.
|>
|
| I guess there aren't any utility engineers reading this thread. I was
| holding off hoping one of them would comment.
|
| L-L or delta transformers are _REALLY_ discouraged for new installations.
| Most new installations are Y-Y for three phase or L-N for single phase. L-L

I still see L-L, which I assume to be D-Y, all over the place. And in
dry transformers, it seems to be the only way it is done.

| can cause all sorts of problems when a phase is lost. First off there is
| ferroresonance. That's when the magnetizing inductance of the transformer
| resonates with the line capacitance of the open phase. It's a real problem
| for underground service because of the higher capacitance of underground
| lines. It can still be a problem for open overhead, particularly at higher
| voltages, particularly 34.5kV. I have heard of lots of problems when
| utilities upgrade to 34.5kV primary and still have delta connected services
| they can't change, for example a customer with 240V center tapped delta.
| The other problem is with back feed to a faulted phase. Yes, motors can
| back feed and add to fault current. But usually the motors can't blow the
| line fuses on the transformer. But a delta connected bank will. It's not
| uncommon for every delta bank to blow one or two fuses when one of the
| primary phases has a hard fault (like crossed over conductors) and the
| source is single phase tripped. After the line is repaired and the feed
| restored, the line crew must then go around and refuse a bunch of
| transformer banks. Three phase tripping eliminates this, but many utilities
| like the single phase fault protection out on long lines.

How does a D-Y backfeed (not the motor backfeeds)?

Anyway, if you really really want all the harmonics issues you'll get with
a Y-Y transformer configuration, then you can have your Y primary. The
design of the L-L and L-L-L loads doesn't depend on the transformer primary
that I am aware of.

What do you suggest as a means to prevent primary ground return through the
secondary service drop ground?

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2006-10-13-1500@ipal.net |
|------------------------------------/-------------------------------------|

Matthew Beasley

2006-10-13, 5:25 pm

<phil-news-nospam@ipal.net> wrote in message
news:egort1025n9@news4.newsguy.com...

--snipped a bunch--

> I still see L-L, which I assume to be D-Y, all over the place.

It depends on where you are. Some utilities still have substations set up
for delta connected primaries, hence they continue to add new services delta
or L-L. But for a brand new substation, or for primary voltage upgrades,
it's overwhelmingly set up for Y connected primaries.

> And in
> dry transformers, it seems to be the only way it is done.

Unless special ordered. But as you note, most dry types are are D-Y.
Ferroresonance isn't a problem there since they are pretty much only used
where the primary is feed from a three phase breaker.

--snipped more--

> How does a D-Y backfeed (not the motor backfeeds)?

If a phase is opened on the primary, the opened phase wants to center
between the other two phases. It can backfeed enough current to blow the
primary fuses.

>
> Anyway, if you really really want all the harmonics issues you'll get with
> a Y-Y transformer configuration, then you can have your Y primary. The
> design of the L-L and L-L-L loads doesn't depend on the transformer
> primary
> that I am aware of.

Zero sequence harmonics. The positive and negative phase sequence harmonics
go through the D-Y connection also. But yep, more harmonics. The zero
sequence harmonics go back to the substation, and usually there is a delta
or zig-zag winding somewhere that will eat up them up.

>
> What do you suggest as a means to prevent primary ground return through
> the
> secondary service drop ground?
>

Nothing is done. You trade that off with the voltage rise on a primary to
secondary fault. In a delta system, there is no primary ground present, so
it relies entirely on the secondary ground system. Since that often can be
10 + ohms, it's possible to get a LOT of voltage rise when that happens.
One good thing is that the ground relays can be much more sensitive to trip
quicker, but then there is a loss of selectivity.

phil-news-nospam@ipal.net

2006-10-13, 8:25 pm

On Fri, 13 Oct 2006 20:50:45 GMT Matthew Beasley <nobody@spam.com> wrote:

|> How does a D-Y backfeed (not the motor backfeeds)?
|
| If a phase is opened on the primary, the opened phase wants to center
| between the other two phases. It can backfeed enough current to blow the
| primary fuses.

What are the phase angles of this current with respect to the remaining
phases?

|> What do you suggest as a means to prevent primary ground return through
|> the
|> secondary service drop ground?
|>
|
| Nothing is done. You trade that off with the voltage rise on a primary to
| secondary fault. In a delta system, there is no primary ground present, so
| it relies entirely on the secondary ground system. Since that often can be
| 10 + ohms, it's possible to get a LOT of voltage rise when that happens.
| One good thing is that the ground relays can be much more sensitive to trip
| quicker, but then there is a loss of selectivity.

But the delta system won't have a primary neutral to secondary neutral
path because there is no primary neutral to have a path with. So all
you have is the middle L-L voltage relative to ground (e.g. half of the
L-L voltage) as capacitive coupling. The ground can eat that easily as
the current is just small charging current. But when you have L-N, the
primary N is connected to the secondary N since both are grounded there.
Bad neutral connections don't result in a system failure, but rather,
in ground currents that get picked up elsewhere at better connections.
The end result is part of the primary return current is by way of the
LV service drop neutral and the customer grounding electrodes.

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2006-10-13-1723@ipal.net |
|------------------------------------/-------------------------------------|

Matthew Beasley

2006-10-13, 8:25 pm

<phil-news-nospam@ipal.net> wrote in message
news:egp4cn01ccu@news2.newsguy.com...
> On Fri, 13 Oct 2006 20:50:45 GMT Matthew Beasley <nobody@spam.com> wrote:
>
> |> How does a D-Y backfeed (not the motor backfeeds)?
> |
> | If a phase is opened on the primary, the opened phase wants to center
> | between the other two phases. It can backfeed enough current to blow
> the
> | primary fuses.
>
> What are the phase angles of this current with respect to the remaining
> phases?

Somewhere close to in phase, unless a motor is pulling it out.

>
>
> |> What do you suggest as a means to prevent primary ground return through
> |> the
> |> secondary service drop ground?
> |>
> |
> | Nothing is done. You trade that off with the voltage rise on a primary
> to
> | secondary fault. In a delta system, there is no primary ground present,
> so
> | it relies entirely on the secondary ground system. Since that often can
> be
> | 10 + ohms, it's possible to get a LOT of voltage rise when that happens.
> | One good thing is that the ground relays can be much more sensitive to
> trip
> | quicker, but then there is a loss of selectivity.
>
> But the delta system won't have a primary neutral to secondary neutral
> path because there is no primary neutral to have a path with. So all
> you have is the middle L-L voltage relative to ground (e.g. half of the
> L-L voltage) as capacitive coupling. The ground can eat that easily as
> the current is just small charging current. But when you have L-N, the
> primary N is connected to the secondary N since both are grounded there.
> Bad neutral connections don't result in a system failure, but rather,
> in ground currents that get picked up elsewhere at better connections.
> The end result is part of the primary return current is by way of the
> LV service drop neutral and the customer grounding electrodes.
>

It's a trade off of the all the time problematic neutral current with Y
connected vs. the potential high voltage on the ground under failure with
delta. The ground on a service may not be low enough in impedance to trip
the primary protection, so it could just sit and cook.

Don Kelly

2006-10-14, 3:25 am

Don Kelly

2006-10-14, 3:25 am

"Matthew Beasley" <nobody@spam.com> wrote in message
news:z7QXg.1069$8N1.178@news.cpqcorp.net...
>
> <phil-news-nospam@ipal.net> wrote in message
> news:egjqjs02asv@news2.newsguy.com...
>
> I guess there aren't any utility engineers reading this thread. I was
> holding off hoping one of them would comment.
> ---------------

> L-L or delta transformers are _REALLY_ discouraged for new installations.
> Most new installations are Y-Y for three phase or L-N for single phase.
> L-L can cause all sorts of problems when a phase is lost. First off there
> is ferroresonance. That's when the magnetizing inductance of the
> transformer resonates with the line capacitance of the open phase. It's a
> real problem for underground service because of the higher capacitance of
> underground lines. It can still be a problem for open overhead,
> particularly at higher voltages, particularly 34.5kV. I have heard of
> lots of problems when utilities upgrade to 34.5kV primary and still have
> delta connected services they can't change, for example a customer with
> 240V center tapped delta. The other problem is with back feed to a faulted
> phase. Yes, motors can back feed and add to fault current. But usually
> the motors can't blow the line fuses on the transformer. But a delta
> connected bank will. It's not uncommon for every delta bank to blow one
> or two fuses when one of the primary phases has a hard fault (like crossed
> over conductors) and the source is single phase tripped. After the line
> is repaired and the feed restored, the line crew must then go around and
> refuse a bunch of transformer banks. Three phase tripping eliminates
> this, but many utilities like the single phase fault protection out on
> long lines.

Delta delta was used long ago- capacitance problems could mean that
appreciable fault current could flow in the case of a phase to ground
fault- without being detected. This can occur even where resonance or
ferroresonance is not a factor. In addition, the problem with ungrounded
overhead lines is that the line to ground potential is at the mercy of any
overhead charged cloud- a cause of such phase to ground faults (this was
discovered about 70 years agoso Delta at the HV level went out) .
Hence the idea of a system which is Y grounded. Y-Y transformers have
problems with third harmonics in the voltages while Y-Delta do not. Part of
the problem can be eliminated by Y-Y delta for larger units such as
autotransformers or grounding both Y s to the same point so that triplen
harmonic currents flow through but voltages are normal. Where single phase
supplies are considered there are advantages to a (grounded) Y secondary.
--

Don Kelly dhky@shawcross.ca
remove the X to answer
----------------------------

>
>

phil-news-nospam@ipal.net

2006-10-14, 1:25 pm

On Fri, 13 Oct 2006 23:39:52 GMT Matthew Beasley <nobody@spam.com> wrote:
|
| <phil-news-nospam@ipal.net> wrote in message
| news:egp4cn01ccu@news2.newsguy.com...
|> On Fri, 13 Oct 2006 20:50:45 GMT Matthew Beasley <nobody@spam.com> wrote:
|>
|> |> How does a D-Y backfeed (not the motor backfeeds)?
|> |
|> | If a phase is opened on the primary, the opened phase wants to center
|> | between the other two phases. It can backfeed enough current to blow
|> the
|> | primary fuses.
|>
|> What are the phase angles of this current with respect to the remaining
|> phases?
|
| Somewhere close to in phase, unless a motor is pulling it out.

In phase to which of the 2 remaining phases?

If you have 3 lines running to a transformer primary that is a delta,
where the secondary is wye, and one of those 3 lines loses power, what
is the phase of the voltage being fed back into the dead line by that
delta primary?

Suppose phase B goes out and phases A and C remain powered. The A-C
winding in the delta would be powered at full voltage (minus any drops
due to the extended loading effects). Windings A-B and B-C would be
in series relative to phases A and C, resulting in the corresponding
secondary windings having half voltage. Phase B coming it is tapped
in the center of that A-B and B-C series. I would think that the
voltage being applied to line B would be however far offset from the
center point that the middle of A-C would be at, which would be 28.9%
(1/(2*sqrt(3)) of the L-L voltage (half of the L-G voltage), and at a
90 degree angle from A-C.

Is this the backfeed you are referring to?

| It's a trade off of the all the time problematic neutral current with Y
| connected vs. the potential high voltage on the ground under failure with
| delta. The ground on a service may not be low enough in impedance to trip
| the primary protection, so it could just sit and cook.

Are you referring to a fault directly to earth? I don't see how that would
be any different between delta and wye connected loads where the source is
a wye secondary and the ground is carried on the distribution but does not
normally carry current. The choice here is how to load the circuit, not
how to supply the circuit. The circuit is the same (source is a secondary
in wye configuration). What differs is whether the load put on it to feed
a LV customer is connected D-Y or Y-Y or for single phase, L-L or L-N.

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2006-10-14-1124@ipal.net |
|------------------------------------/-------------------------------------|

phil-news-nospam@ipal.net

2006-10-14, 1:25 pm

On Sat, 14 Oct 2006 03:29:18 GMT Don Kelly <dhky@shaw.ca> wrote:

| Delta delta was used long ago- capacitance problems could mean that
| appreciable fault current could flow in the case of a phase to ground
| fault- without being detected. This can occur even where resonance or
| ferroresonance is not a factor. In addition, the problem with ungrounded
| overhead lines is that the line to ground potential is at the mercy of any
| overhead charged cloud- a cause of such phase to ground faults (this was
| discovered about 70 years agoso Delta at the HV level went out) .
| Hence the idea of a system which is Y grounded. Y-Y transformers have
| problems with third harmonics in the voltages while Y-Delta do not. Part of
| the problem can be eliminated by Y-Y delta for larger units such as
| autotransformers or grounding both Y s to the same point so that triplen
| harmonic currents flow through but voltages are normal. Where single phase
| supplies are considered there are advantages to a (grounded) Y secondary.

So what about every step along the way being a D-Y transformer, with the
star point grounded AND carried along (and earthed at various intervals),
but all loads connect only L-L-L (e.g. a D-Y for each LV service) or L-L?

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2006-10-14-1150@ipal.net |
|------------------------------------/-------------------------------------|

Don Kelly

2006-10-15, 3:25 am

----------------------------
<phil-news-nospam@ipal.net> wrote in message
news:egr4md12o8s@news1.newsguy.com...
> On Sat, 14 Oct 2006 03:29:18 GMT Don Kelly <dhky@shaw.ca> wrote:
>
> | Delta delta was used long ago- capacitance problems could mean that
> | appreciable fault current could flow in the case of a phase to ground
> | fault- without being detected. This can occur even where resonance or
> | ferroresonance is not a factor. In addition, the problem with ungrounded
> | overhead lines is that the line to ground potential is at the mercy of
> any
> | overhead charged cloud- a cause of such phase to ground faults (this was
> | discovered about 70 years agoso Delta at the HV level went out) .
> | Hence the idea of a system which is Y grounded. Y-Y transformers have
> | problems with third harmonics in the voltages while Y-Delta do not. Part
> of
> | the problem can be eliminated by Y-Y delta for larger units such as
> | autotransformers or grounding both Y s to the same point so that triplen
> | harmonic currents flow through but voltages are normal. Where single
> phase
> | supplies are considered there are advantages to a (grounded) Y
> secondary.
>
> So what about every step along the way being a D-Y transformer, with the
> star point grounded AND carried along (and earthed at various intervals),
> but all loads connect only L-L-L (e.g. a D-Y for each LV service) or L-L?
>
> --
> |---------------------------------------/----------------------------------|
> | Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below
> |
> | first name lower case at ipal.net / spamtrap-2006-10-14-1150@ipal.net
> |
> |------------------------------------/-------------------------------------|

But, with the grounded neutral carried along, you also have the option of
loads from line to ground(L-N). This is the usual practice. In many rural
situations a line and the grounded neutral are carried down one road
supplying single phase transformers. while on other roads the other phases
and neutral are used in order to get a rough load balance. This is cheaper
than running either 3 phase or 2 of the 3 phases. If you check your service
in the back alley in a residential area in a town or city, the same thing is
done. For example, in my block, one phase (7200V to ground) and neutral are
brought across and run underground to distribution transformers to get
240/120V single phase. The neutral is common to both the HV and LV sides.
Even if 3 phase is present on the local line in the alley (if overhead), the
typical procedure is as above, using L-N rather than L-L to supply local
distribution transformers. A transformer designed for line to grounded
neutral operation is cheaper than one designed for line to line operation at
the same voltage.

There may be areas where single phase loads are connected line to line but
I haven't lived in any of those.

In the past many small town supplies were D-D with local single phase
transformers connected line to line and when the lines were getting near
capacity, the main substation transformer was either reconnected D-Y or more
likely replaced while the single phase transformers were then connected line
to neutral. This meant that a small town's could be changed with a minimal
disturbance and cost as no new distribution transformers or larger wires
were needed (insulation was more than adequate).
--

Don Kelly dhky@shawcross.ca
remove the X to answer

phil-news-nospam@ipal.net

2006-10-15, 1:25 pm

On Sun, 15 Oct 2006 03:42:25 GMT Don Kelly <dhky@shaw.ca> wrote:

| But, with the grounded neutral carried along, you also have the option of
| loads from line to ground(L-N). This is the usual practice. In many rural
| situations a line and the grounded neutral are carried down one road
| supplying single phase transformers. while on other roads the other phases
| and neutral are used in order to get a rough load balance. This is cheaper
| than running either 3 phase or 2 of the 3 phases. If you check your service
| in the back alley in a residential area in a town or city, the same thing is
| done. For example, in my block, one phase (7200V to ground) and neutral are
| brought across and run underground to distribution transformers to get
| 240/120V single phase. The neutral is common to both the HV and LV sides.
| Even if 3 phase is present on the local line in the alley (if overhead), the
| typical procedure is as above, using L-N rather than L-L to supply local
| distribution transformers. A transformer designed for line to grounded
| neutral operation is cheaper than one designed for line to line operation at
| the same voltage.

However, I still would not do it that way. If cost were the exclusive
reason for all decisions, we could have a much lower cost electrical
system than we have today. But it would also be much less safe.

The inverse of this is that we can have a safer system, but it will cost
some more. And that is how I would design it: safer

Having the same metal wire used for a return of MV circuits _and_ being
connected to the LV customer service drop and the premise electrodes and
EGC does have dangers. Many of those can be avoided, and many technical
problems I believe may be linked to this bad practice, could be avoided by
having a separate grounding wire on the MV distribution that is not used
for return current whatsoever, which can be used for grounding the secondary
side of the MV->LV transformers (along with a grounding electrode).

| There may be areas where single phase loads are connected line to line but
| I haven't lived in any of those.
|
| In the past many small town supplies were D-D with local single phase
| transformers connected line to line and when the lines were getting near
| capacity, the main substation transformer was either reconnected D-Y or more
| likely replaced while the single phase transformers were then connected line
| to neutral. This meant that a small town's could be changed with a minimal
| disturbance and cost as no new distribution transformers or larger wires
| were needed (insulation was more than adequate).

How were the MV->LV transformers grounded before ... and after? I bet
they were grounded with a wire that was NOT carrying current before, and
afterwards, they were grounded with a wire that was carrying current.

They had a safer ground wire before. They took a step backwards in safety
for economic reasons.

So, tell me, how can I wire up an isolation at the end of the service drop
which isolates my grounding wires from the distribution neutral?

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2006-10-15-1005@ipal.net |
|------------------------------------/-------------------------------------|

Don Kelly

2006-10-15, 9:25 pm

<phil-news-nospam@ipal.net> wrote in message
news:egtjl90dej@news3.newsguy.com...
> On Sun, 15 Oct 2006 03:42:25 GMT Don Kelly <dhky@shaw.ca> wrote:
>
> | But, with the grounded neutral carried along, you also have the option
> of
> | loads from line to ground(L-N). This is the usual practice. In many
> rural
> | situations a line and the grounded neutral are carried down one road
> | supplying single phase transformers. while on other roads the other
> phases
> | and neutral are used in order to get a rough load balance. This is
> cheaper
> | than running either 3 phase or 2 of the 3 phases. If you check your
> service
> | in the back alley in a residential area in a town or city, the same
> thing is
> | done. For example, in my block, one phase (7200V to ground) and neutral
> are
> | brought across and run underground to distribution transformers to get
> | 240/120V single phase. The neutral is common to both the HV and LV
> sides.
> | Even if 3 phase is present on the local line in the alley (if overhead),
> the
> | typical procedure is as above, using L-N rather than L-L to supply local
> | distribution transformers. A transformer designed for line to grounded
> | neutral operation is cheaper than one designed for line to line
> operation at
> | the same voltage.
>
> However, I still would not do it that way. If cost were the exclusive
> reason for all decisions, we could have a much lower cost electrical
> system than we have today. But it would also be much less safe.
>
> The inverse of this is that we can have a safer system, but it will cost
> some more. And that is how I would design it: safer
>
> Having the same metal wire used for a return of MV circuits _and_ being
> connected to the LV customer service drop and the premise electrodes and
> EGC does have dangers. Many of those can be avoided, and many technical
> problems I believe may be linked to this bad practice, could be avoided by
> having a separate grounding wire on the MV distribution that is not used
> for return current whatsoever, which can be used for grounding the
> secondary
> side of the MV->LV transformers (along with a grounding electrode).
>
>
> | There may be areas where single phase loads are connected line to line
> but
> | I haven't lived in any of those.
> |
> | In the past many small town supplies were D-D with local single phase
> | transformers connected line to line and when the lines were getting near
> | capacity, the main substation transformer was either reconnected D-Y or
> more
> | likely replaced while the single phase transformers were then connected
> line
> | to neutral. This meant that a small town's could be changed with a
> minimal
> | disturbance and cost as no new distribution transformers or larger wires
> | were needed (insulation was more than adequate).
>
> How were the MV->LV transformers grounded before ... and after? I bet
> they were grounded with a wire that was NOT carrying current before, and
> afterwards, they were grounded with a wire that was carrying current.
>
> They had a safer ground wire before. They took a step backwards in safety
> for economic reasons.
>
> So, tell me, how can I wire up an isolation at the end of the service drop
> which isolates my grounding wires from the distribution neutral?
>
> --
> |---------------------------------------/----------------------------------|
> | Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below
> |
> | first name lower case at ipal.net / spamtrap-2006-10-15-1005@ipal.net
> |
> |------------------------------------/-------------------------------------|

I may not have interpreted your question properly but as I read it, here
goes.

In the situation that I mentioned, a previous step down delta-delta
transformer was changed to a delta-wye transformer. Then single phase loads
referenced to ground could be take from each phase. Increased capacity.
Certainly there will be neutral current in the case of unbalanced loads. Is
this a major factor for safety? Not really, provided that the neutral is
properly grounded and of adequate size. Note that the heaviest currents in
the neutral (about 97-100%) were those due to the 120/240 Edison system
customer loads -these would not change but account for 95-100% of the total
neutral current. In fact, any primary neutral current will actually reduce
the current in the neutrals (admittedly not by much). In your home you are
dealing with 120V/240V loads which are rarely balanced so the neutral
carries current- does this bother you? Yes, equipment is tied to a seperate
ground for good reasons. Note that the current carrying neutrals of a MV LV
or HV-MV system are also very well grounded and the fact that they may carry
current in the case of unbalanced loads is recognised and accounted for.

Now consider the delta with a neutral tap on one side. Will this mean that
the neutral is not carrying current- ideally so but ??? Suppose also that
it was 12.5KV line to line. That means that 2 legs are at 6.25KV with
respect to ground and the other is at 14KV with respect to ground. Is this
better than having all 3 legs at 7.2KV to ground? Zig- zag grounding
transformers were often used to get a neutral point which was equidistant
electrically from all phases. The center tapped leg of a delta is a cheap,
but poorer alternative to this.

Seeing that the user with a single phase supply sees no difference from the
situation where the distribution transformer is connected l-l vs l-n on the
primary- your last question is meaningless. Run a separate ground wire if
you want.
If you are looking at an industrial system taking 3 phase from a delta with
a neutral on one side and single phase to neutral loads on the tapped side-
where while neutral current can't flow, unbalanced voltages can result -then
I would prefer a Grounded wye system. A separate safety ground wire to the
frames of equipment is just as feasible there as with the household single
phase system.
Note also that ground fault protection is a hell of a lot easier with a Y.

Don Kelly dhky@shawcross.ca
remove the X to answer
----------------------------

phil-news-nospam@ipal.net

2006-10-16, 9:25 am

On Mon, 16 Oct 2006 02:13:12 GMT Don Kelly <dhky@shaw.ca> wrote:

| In the situation that I mentioned, a previous step down delta-delta
| transformer was changed to a delta-wye transformer. Then single phase loads
| referenced to ground could be take from each phase. Increased capacity.
| Certainly there will be neutral current in the case of unbalanced loads. Is
| this a major factor for safety? Not really, provided that the neutral is
| properly grounded and of adequate size. Note that the heaviest currents in
| the neutral (about 97-100%) were those due to the 120/240 Edison system
| customer loads -these would not change but account for 95-100% of the total
| neutral current. In fact, any primary neutral current will actually reduce
| the current in the neutrals (admittedly not by much). In your home you are
| dealing with 120V/240V loads which are rarely balanced so the neutral
| carries current- does this bother you? Yes, equipment is tied to a seperate
| ground for good reasons. Note that the current carrying neutrals of a MV LV
| or HV-MV system are also very well grounded and the fact that they may carry
| current in the case of unbalanced loads is recognised and accounted for.

Having the neutral carry current does NOT bother me when there is a
separate grounding conductor. Even in cases where that is not quite
true, such as poor connetions, a fraction of LV is not nearly the
same level of issue as a fraction of MV. There is no separate ground
in MV distribution circuits. That, combined with connection between
the current carrying MV neutral and the customer service drop neutral,
are what I have issue with. Add the 4th (for L-L transformers) or 5th
(for L-N transformers) wire and use it correctly, then I do not have
an issue with a solid metallic path from customer to distribution.

| Now consider the delta with a neutral tap on one side. Will this mean that
| the neutral is not carrying current- ideally so but ??? Suppose also that
| it was 12.5KV line to line. That means that 2 legs are at 6.25KV with
| respect to ground and the other is at 14KV with respect to ground. Is this
| better than having all 3 legs at 7.2KV to ground? Zig- zag grounding
| transformers were often used to get a neutral point which was equidistant
| electrically from all phases. The center tapped leg of a delta is a cheap,
| but poorer alternative to this.

I'm not suggesting a center tapped delta.

| Seeing that the user with a single phase supply sees no difference from the
| situation where the distribution transformer is connected l-l vs l-n on the
| primary- your last question is meaningless. Run a separate ground wire if
| you want.

Please clarify what you mean by "Run a separate ground wire if you want."
There are a number of different ways to accomplish that. But given that
power company practice is to connect the secondary of the transformer to
the primary current carrying conductors, then the first step to running a
separate ground wire is to have another transformer with its primary wired
L-L (240 volts) and its secondary not connected to the primary at all ...
not even to the service drop neutral (which also must not be grounded to
earth anywhere near the points the new separate ground is earthed).

| If you are looking at an industrial system taking 3 phase from a delta with
| a neutral on one side and single phase to neutral loads on the tapped side-
| where while neutral current can't flow, unbalanced voltages can result -then
| I would prefer a Grounded wye system. A separate safety ground wire to the
| frames of equipment is just as feasible there as with the household single
| phase system.
| Note also that ground fault protection is a hell of a lot easier with a Y.

I still think you are misunderstanding me. It seems you are assuming that
when the loads (transformers at customer taps) are L-L or L-L-L, then the
source of the circuit they connect to must have a delta secondary. I do
realize you described the case where a town switched from delta secondary
(for example at 7200 volts) feeding L-L and L-L-L loads, to a wye secondary
(for example at 12470 volts) feedling L-N and L-N*3 loads, to achieve a
73% boost in system capacity.

But this was all in repsonse to my description of how things should be from
the beginning, which would have precluded that down from having the starting
point they had in the first place.

The substation transformer secondary should be WYE. The center point is
earthed at the substation. Now there are two different ways to run that
circuit:

1. Run 5 wires, identified as A,B,C for the phases, N for the neutral,
and G for the grounding wire. Loads can then be connected to any
combination of A,B,C,N as needed.

2. Run 4 wires, identified as A,B,C for the phases and G for the grounding
wire. Loads can then be connected to any combination of A,B,C as needed.

With design #1 you can have L-N taps for customer service transformers.
With design #2 you are limited to L-L taps. In all cases the secondary
would be WYE.

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2006-10-16-0750@ipal.net |
|------------------------------------/-------------------------------------|

phil-news-nospam@ipal.net

2006-10-16, 1:25 pm

On Mon, 16 Oct 2006 03:27:03 GMT Tom Horne, Electrician <hornetd@mindspring.com> wrote:

| You can wire up an isolation transformer but you will almost inevitably
| have to violate one or more code rules to achieve full isolation. I
| wired up a dry transformer on a dairy farm to separate the farms wiring
| from the Multi Grounded Neutral (MGN) of the medium voltage distribution
| system. The power company threatened to cut off service until the
| public service commission of the state government got involved and
| pulled out an old tariff for ungrounded delta service. The rule is that
| if the service is tariffed then the Utility must provide it. When they
| tried to have this "outdated" tariff rescinded the entire dairy coop
| system apposed it. So far that is the only NEC compliant way I have
| found to actually break the earthing pathway between the MGN and the
| customers premise. The US NEC requires that the grounded conductor of
| any wye connected transformer be brought to the service equipment
| enclosure (Customer Service Unit) and bonded to it. That conductor must
| also be grounded / earthed at the customers end. This means that it is
| inevitable that stray MGN currents will be flowing over the customer
| premise grounding electrode system. The reason that that practice
| continues is that the cost of enlarging the utility MGN or installing a
| separate insulated neutral in the medium voltage distribution system
| gives the utilities' management nightmares. With the increasing
| population density the inadequacy of the present neutral system will
| become more and more apparent as the MGN becomes more heavily loaded and
| stray currents increase.
|
| There is a special type of utility transformer that is specifically
| designed to supply dairy farms that accomplishes the prevention of
| utility neutral current flow on the secondary grounded conductor without
| violating the National Electrical Safety Code that governs there work.
| I have no idea how it works.

It breaks the connection between the primary and secondary, but has a
"spark gap" between them to allow a lightning strike discharge to go
to ground. That supposedly prevents internal damage to the transformer
when there is a surge from a lightning strike.

I believe it would be adequate to have the primary and secondary of the
transformer separately grounded at some distance between the electrodes.
The distance needs to be sufficient to effectively block MV circuit
voltages from entering the LV drop. On either side of a single pole
is probably not sufficient.

The correct title for the code that governs their work should be:
National Electrical Economic Code. It tends to focus more on the
"safety" of equipment than of people and animals. Someone obviously
put "Safety" in the title as a selling point.

| Edison may turn out to have been right about the dangers of AC current
| after all. Edison abandoned ground return for electric current fairly
| early in the development of the Edison electric system.

Ground return is certainly bad. Except in a few isolated cases, utilities
don't use that. But the problem is, they effectively have ground return
to some extent due to the way they try to cut their costs.

I do have one "design" for a safer system. In a separate building well
distant from where the ground currents can be an issue, put in a large
motor driven pump system. This pump will then drive a circulating loop
of non-conductive fluid through a non-conductive piping system located
underground running a significant distance to a generator.

Otherwise the only safe electrical system is independent of the grid,
neither drawing from it, nor supplying to it.

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2006-10-16-0859@ipal.net |
|------------------------------------/-------------------------------------|

Matthew Beasley

2006-10-16, 1:25 pm

<phil-news-nospam@ipal.net> wrote in message
news:egr4dd02o8s@news1.newsguy.com...
> On Fri, 13 Oct 2006 23:39:52 GMT Matthew Beasley <nobody@spam.com> wrote:
> |
> | <phil-news-nospam@ipal.net> wrote in message
> | news:egp4cn01ccu@news2.newsguy.com...
> |> On Fri, 13 Oct 2006 20:50:45 GMT Matthew Beasley <nobody@spam.com>
> wrote:
> |>
> |> |> How does a D-Y backfeed (not the motor backfeeds)?
> |> |
> |> | If a phase is opened on the primary, the opened phase wants to center
> |> | between the other two phases. It can backfeed enough current to blow
> |> the
> |> | primary fuses.
> |>
> |> What are the phase angles of this current with respect to the remaining
> |> phases?
> |
> | Somewhere close to in phase, unless a motor is pulling it out.
>
> In phase to which of the 2 remaining phases?
>
> If you have 3 lines running to a transformer primary that is a delta,
> where the secondary is wye, and one of those 3 lines loses power, what
> is the phase of the voltage being fed back into the dead line by that
> delta primary?
>
> Suppose phase B goes out and phases A and C remain powered. The A-C
> winding in the delta would be powered at full voltage (minus any drops
> due to the extended loading effects). Windings A-B and B-C would be
> in series relative to phases A and C, resulting in the corresponding
> secondary windings having half voltage. Phase B coming it is tapped
> in the center of that A-B and B-C series. I would think that the
> voltage being applied to line B would be however far offset from the
> center point that the middle of A-C would be at, which would be 28.9%
> (1/(2*sqrt(3)) of the L-L voltage (half of the L-G voltage), and at a
> 90 degree angle from A-C.
>
> Is this the backfeed you are referring to?

Yep. It's enough to blow fuses on the primary when one phase is grounded in
some cases, guaranteed to blow fuses in a phase crossover (phase lines
looped over each other, quite a common occurrence in overhead).

>
>
> | It's a trade off of the all the time problematic neutral current with Y
> | connected vs. the potential high voltage on the ground under failure
> with
> | delta. The ground on a service may not be low enough in impedance to
> trip
> | the primary protection, so it could just sit and cook.
>
> Are you referring to a fault directly to earth? I don't see how that
> would
> be any different between delta and wye connected loads where the source is
> a wye secondary and the ground is carried on the distribution but does not
> normally carry current. The choice here is how to load the circuit, not
> how to supply the circuit. The circuit is the same (source is a secondary
> in wye configuration). What differs is whether the load put on it to feed
> a LV customer is connected D-Y or Y-Y or for single phase, L-L or L-N.
>

As you note in other posts, the utilities don't run separate grounds. When
they have L-L or L-L-L connections, no neutral / ground is run. If they ran
the ground, voltage rise on the secondary would be less of a problem. But
it's the absence of a connection to the much lower impedance ground present
in a Y connected system where the utility neutral is run along pole to pole.

I have an anecdotal story about just this kind of failure. One of my in
laws farm irrigation systems suffered a primary to case or secondary failure
in one of the three transformers feeding the service. The primary voltage
is 12.5/7.2kV, with most services connected Y. The irrigation service is
connected Y to center tapped delta. The Y center point is not connected to
anything else. The branch line to the irrigation service does not have a
neutral run along with it - just the three line conductors. Two irrigation
services are powered from the tank. The transformer pole has one ground
rod, one of the irrigation services has one ground rod, and my in laws
service has two ground rods. The second ground rod was added when we
replaced the panel to upgrade to a larger pump. A owl tangled up in the
phase conductors several poles away. We know it was a owl because his
smoking carcass was on the ground right aftarwards. He tangled two of the
phase conductors around each other. One fuse at the lateral dropped right
away. After a couple of minutes, one of the transformer cans on the pole
popped and one more fuse dropped. When the can blew, the whole irrigation
system went hot. There was arcing, hissing and sizzling along the
irrigation pipe throughout the farm. I'm sure anyone in the river near the
suction line was would have been fried. This continued for about 15 minutes
until the third fuse finally dropped out. The only damage to my inlaws
equipment was to a float control that ran a small pump to keep a cattle
trough full. There pump motor was definetly saved by proper ground.

phil-news-nospam@ipal.net

2006-10-16, 5:25 pm

On Mon, 16 Oct 2006 16:35:02 GMT Matthew Beasley <nobody@spam.com> wrote:

| As you note in other posts, the utilities don't run separate grounds. When
| they have L-L or L-L-L connections, no neutral / ground is run. If they ran
| the ground, voltage rise on the secondary would be less of a problem. But
| it's the absence of a connection to the much lower impedance ground present
| in a Y connected system where the utility neutral is run along pole to pole.

When no neutral / ground is run, then what all does the LV secondary
get connected to? Just the winding center and the electrode at the
pole?

| I have an anecdotal story about just this kind of failure. One of my in
| laws farm irrigation systems suffered a primary to case or secondary failure
| in one of the three transformers feeding the service. The primary voltage
| is 12.5/7.2kV, with most services connected Y. The irrigation service is
| connected Y to center tapped delta. The Y center point is not connected to
| anything else. The branch line to the irrigation service does not have a
| neutral run along with it - just the three line conductors. Two irrigation
| services are powered from the tank. The transformer pole has one ground
| rod, one of the irrigation services has one ground rod, and my in laws
| service has two ground rods. The second ground rod was added when we
| replaced the panel to upgrade to a larger pump. A owl tangled up in the
| phase conductors several poles away. We know it was a owl because his
| smoking carcass was on the ground right aftarwards. He tangled two of the
| phase conductors around each other. One fuse at the lateral dropped right
| away. After a couple of minutes, one of the transformer cans on the pole
| popped and one more fuse dropped. When the can blew, the whole irrigation
| system went hot. There was arcing, hissing and sizzling along the
| irrigation pipe throughout the farm. I'm sure anyone in the river near the
| suction line was would have been fried. This continued for about 15 minutes
| until the third fuse finally dropped out. The only damage to my inlaws
| equipment was to a float control that ran a small pump to keep a cattle
| trough full. There pump motor was definetly saved by proper ground.

Would this have happened if the system was designed as I suggested, where:

1. The distribution supply secondary is WYE, with the center point
solidly earthed.

2. A grounding-only wire, NOT used as a neutral runs along the poles
of the distribution, originating at the WYE center point, and is
also earthed at periodic intervals.

3. An optional neutral may be run, which may also be earthed only at
poles where the grounding-only wire is NOT earthed.

4. Each LV customer tap primary is connected L-L for single phase or
delta for three phase, if there is no neutral.

5. If there is a neutral, LV customer tap primary may be connected L-N
for single phase and WYE for three phase, but may also be connected
L-L or delta. The neutral is NOT earthed at this pole or within
10 meters of any ground wire earthing.

6. All three phase LV customer tap secondaries are always WYE with the
center point connected to the grounding-only wire described in #2
and is also solidly earthed with one or more electrodes at that pole.

7. All single phase LV customer tap secondaries are always center tapped
with that tap connected to the grounding-only wire described in #2
and is also solidly earthed with one or more electrodes at that pole.

8. No center tapped delta.

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2006-10-16-1608@ipal.net |
|------------------------------------/-------------------------------------|

Matthew Beasley

2006-10-16, 5:25 pm

<phil-news-nospam@ipal.net> wrote in message
news:eh0tsg01dop@news3.newsguy.com...
> On Mon, 16 Oct 2006 16:35:02 GMT Matthew Beasley <nobody@spam.com> wrote:
>
> | As you note in other posts, the utilities don't run separate grounds.
> When
> | they have L-L or L-L-L connections, no neutral / ground is run. If they
> ran
> | the ground, voltage rise on the secondary would be less of a problem.
> But
> | it's the absence of a connection to the much lower impedance ground
> present
> | in a Y connected system where the utility neutral is run along pole to
> pole.
>
> When no neutral / ground is run, then what all does the LV secondary
> get connected to? Just the winding center and the electrode at the
> pole?

Yes.

>
>
> | I have an anecdotal story about just this kind of failure. One of my in
> | laws farm irrigation systems suffered a primary to case or secondary
> failure
> | in one of the three transformers feeding the service. The primary
> voltage
> | is 12.5/7.2kV, with most services connected Y. The irrigation service
> is
> | connected Y to center tapped delta. The Y center point is not connected
> to
> | anything else. The branch line to the irrigation service does not have
> a
> | neutral run along with it - just the three line conductors. Two
> irrigation
> | services are powered from the tank. The transformer pole has one ground
> | rod, one of the irrigation services has one ground rod, and my in laws
> | service has two ground rods. The second ground rod was added when we
> | replaced the panel to upgrade to a larger pump. A owl tangled up in the
> | phase conductors several poles away. We know it was a owl because his
> | smoking carcass was on the ground right aftarwards. He tangled two of
> the
> | phase conductors around each other. One fuse at the lateral dropped
> right
> | away. After a couple of minutes, one of the transformer cans on the
> pole
> | popped and one more fuse dropped. When the can blew, the whole
> irrigation
> | system went hot. There was arcing, hissing and sizzling along the
> | irrigation pipe throughout the farm. I'm sure anyone in the river near
> the
> | suction line was would have been fried. This continued for about 15
> minutes
> | until the third fuse finally dropped out. The only damage to my inlaws
> | equipment was to a float control that ran a small pump to keep a cattle
> | trough full. There pump motor was definetly saved by proper ground.
>
> Would this have happened if the system was designed as I suggested, where:
>
> 1. The distribution supply secondary is WYE, with the center point
> solidly earthed.
>
> 2. A grounding-only wire, NOT used as a neutral runs along the poles
> of the distribution, originating at the WYE center point, and is
> also earthed at periodic intervals.
>
> 3. An optional neutral may be run, which may also be earthed only at
> poles where the grounding-only wire is NOT earthed.

It would need to have at least arresters so that it couldn't develop high
voltage with respect to local ground.

>
> 4. Each LV customer tap primary is connected L-L for single phase or
> delta for three phase, if there is no neutral.
>
> 5. If there is a neutral, LV customer tap primary may be connected L-N
> for single phase and WYE for three phase, but may also be connected
> L-L or delta. The neutral is NOT earthed at this pole or within
> 10 meters of any ground wire earthing.

For both 4&5, there still is the problem of ferroresonance & backfeed.

>
> 6. All three phase LV customer tap secondaries are always WYE with the
> center point connected to the grounding-only wire described in #2
> and is also solidly earthed with one or more electrodes at that pole.
>
> 7. All single phase LV customer tap secondaries are always center tapped
> with that tap connected to the grounding-only wire described in #2
> and is also solidly earthed with one or more electrodes at that pole.

#6 & #7 You still have secondary neutral currents going through the ground.
This will eliminate some of the problems, but not all.

>
> 8. No center tapped delta.
>

What's the problem with delta secondaries if grounded?

Don Kelly

2006-10-17, 3:25 am

"Tom Horne, Electrician" <hornetd@mindspring.com> wrote in message
news:b_CYg.10014$Y24.8574@newsread4.news.pas.earthlink.net...
> phil-news-nospam@ipal.net wrote:
------------
Some confusion here- you speak of a separate ground wire -not carrying
current -on the MV system and which can be uses for grounding the secondary
side of the MV-LV transformers. Sorry, the picture is not clear.
Lets consider the system that is used as above -where one leg and neutral
of the MV (taken as the primary-say 7200 L-N ) is taken down an alley (or
goes underground) to feed one or more single phase transformers. The neutral
is tied to ground at the transformer. Service tap neutrals are also tied to
ground at that point. They will also be tied to ground at the premises.
However the service and the MV neutrals are tied together at one point only-
at the transformer. If the LV secondary (240/120V) is run along parallel to
the MV for some distance (to feed more than one home as is sometimes the
case, there is the choice between using a common wire or using a separate LV
neutral. I have no problem with the latter if that is what you propose.
A separate LV neutral grounded only at the transformer carrying only
secondary neutral current. and not common to several transformers "may" be
safer but other factors enter the picture -hence the maybe. However, in
that case the LV is grounded at the premises as well so there is a loop
involving a parallel ground path which is common to both the MV and LV sides
and there is no guarantee that some of the MV current won't prefer the LV
neutral to the earth path.
-----------------------------[color=darkred]
>
> You can wire up an isolation transformer but you will almost inevitably
> have to violate one or more code rules to achieve full isolation. I wired
> up a dry transformer on a dairy farm to separate the farms wiring from the
> Multi Grounded Neutral (MGN) of the medium voltage distribution system.
> The power company threatened to cut off service until the public service
> commission of the state government got involved and pulled out an old
> tariff for ungrounded delta service. The rule is that if the service is
> tariffed then the Utility must provide it. When they tried to have this
> "outdated" tariff rescinded the entire dairy coop system apposed it. So
> far that is the only NEC compliant way I have found to actually break the
> earthing pathway between the MGN and the customers premise. The US NEC
> requires that the grounded conductor of any wye connected transformer be
> brought to the service equipment enclosure (Customer Service Unit) and
> bonded to it. That conductor must also be grounded / earthed at the
> customers end. This means that it is inevitable that stray MGN currents
> will be flowing over the customer premise grounding electrode system. The
> reason that that practice continues is that the cost of enlarging the
> utility MGN or installing a separate insulated neutral in the medium
> voltage distribution system gives the utilities' management nightmares.
> With the increasing population density the inadequacy of the present
> neutral system will become more and more apparent as the MGN becomes more
> heavily loaded and stray currents increase.
>
> There is a special type of utility transformer that is specifically
> designed to supply dairy farms that accomplishes the prevention of utility
> neutral current flow on the secondary grounded conductor without violating
> the National Electrical Safety Code that governs there work. I have no
> idea how it works.
>
> Edison may turn out to have been right about the dangers of AC current
> after all. Edison abandoned ground return for electric current fairly
> early in the development of the Edison electric system.
-----
Ground return on DC is more problematic than on AC and the current
distribution in the ground is quite different. However, Steinmetz was
against the use of grounded systems. Practical problems that came about as
systems grew, dictated that the benefits of a grounded system outweighed the
costs.
--

Don Kelly dhky@shawcross.ca
remove the X to answer
----------------------------

> --
> Tom Horne
>
> "This alternating current stuff is just a fad. It is much too dangerous
> for general use." Thomas Alva Edison

phil-news-nospam@ipal.net

2006-10-17, 3:25 am

On Mon, 16 Oct 2006 22:04:00 GMT Matthew Beasley <nobody@spam.com> wrote:

|> When no neutral / ground is run, then what all does the LV secondary
|> get connected to? Just the winding center and the electrode at the
|> pole?
|
| Yes.

Sounds like what I want ... in single phase.

|> Would this have happened if the system was designed as I suggested, where:
|>
|> 1. The distribution supply secondary is WYE, with the center point
|> solidly earthed.
|>
|> 2. A grounding-only wire, NOT used as a neutral runs along the poles
|> of the distribution, originating at the WYE center point, and is
|> also earthed at periodic intervals.
|>
|> 3. An optional neutral may be run, which may also be earthed only at
|> poles where the grounding-only wire is NOT earthed.
|
| It would need to have at least arresters so that it couldn't develop high
| voltage with respect to local ground.

That's why it is earthed. Would it be better if it were not earthed?

|> 4. Each LV customer tap primary is connected L-L for single phase or
|> delta for three phase, if there is no neutral.
|>
|> 5. If there is a neutral, LV customer tap primary may be connected L-N
|> for single phase and WYE for three phase, but may also be connected
|> L-L or delta. The neutral is NOT earthed at this pole or within
|> 10 meters of any ground wire earthing.
|
| For both 4&5, there still is the problem of ferroresonance & backfeed.

These are common for dry transformers. Why wouldn't it be a problem
with them, too?

Sounds like it will be necessary to always have a 5 wire distribution to
be safe.

And how would single phase backfeed?

And what if, instead of one of the phases going out, the neutral does?

|> 6. All three phase LV customer tap secondaries are always WYE with the
|> center point connected to the grounding-only wire described in #2
|> and is also solidly earthed with one or more electrodes at that pole.
|>
|> 7. All single phase LV customer tap secondaries are always center tapped
|> with that tap connected to the grounding-only wire described in #2
|> and is also solidly earthed with one or more electrodes at that pole.
|
| #6 & #7 You still have secondary neutral currents going through the ground.
| This will eliminate some of the problems, but not all.

But not very much at all feeding into the customer ground.

|> 8. No center tapped delta.
|>
|
| What's the problem with delta secondaries if grounded?

Perhaps none if D-D. Y-D would have backfeed. In any case, it's not
part of the design I gave because the intent of it is to have a smaller
L-G voltage.

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2006-10-16-2200@ipal.net |
|------------------------------------/-------------------------------------|

Don Kelly

2006-10-17, 3:25 am

<phil-news-nospam@ipal.net> wrote in message
news:eh00m801h93@news1.newsguy.com...
> On Mon, 16 Oct 2006 02:13:12 GMT Don Kelly <dhky@shaw.ca> wrote:
>
> | In the situation that I mentioned, a previous step down delta-delta
> | transformer was changed to a delta-wye transformer. Then single phase
> loads
> | referenced to ground could be take from each phase. Increased capacity.
> | Certainly there will be neutral current in the case of unbalanced loads.
> Is
> | this a major factor for safety? Not really, provided that the neutral is
> | properly grounded and of adequate size. Note that the heaviest currents
> in
> | the neutral (about 97-100%) were those due to the 120/240 Edison system
> | customer loads -these would not change but account for 95-100% of the
> total
> | neutral current. In fact, any primary neutral current will actually
> reduce
> | the current in the neutrals (admittedly not by much). In your home you
> are
> | dealing with 120V/240V loads which are rarely balanced so the neutral
> | carries current- does this bother you? Yes, equipment is tied to a
> seperate
> | ground for good reasons. Note that the current carrying neutrals of a MV
> LV
> | or HV-MV system are also very well grounded and the fact that they may
> carry
> | current in the case of unbalanced loads is recognised and accounted for.
>
> Having the neutral carry current does NOT bother me when there is a
> separate grounding conductor. Even in cases where that is not quite
> true, such as poor connetions, a fraction of LV is not nearly the
> same level of issue as a fraction of MV. There is no separate ground
> in MV distribution circuits. That, combined with connection between
> the current carrying MV neutral and the customer service drop neutral,
> are what I have issue with. Add the 4th (for L-L transformers) or 5th
> (for L-N transformers) wire and use it correctly, then I do not have
> an issue with a solid metallic path from customer to distribution.
>
>
> | Now consider the delta with a neutral tap on one side. Will this mean
> that
> | the neutral is not carrying current- ideally so but ??? Suppose also
> that
> | it was 12.5KV line to line. That means that 2 legs are at 6.25KV with
> | respect to ground and the other is at 14KV with respect to ground. Is
> this
> | better than having all 3 legs at 7.2KV to ground? Zig- zag grounding
> | transformers were often used to get a neutral point which was
> equidistant
> | electrically from all phases. The center tapped leg of a delta is a
> cheap,
> | but poorer alternative to this.
>
> I'm not suggesting a center tapped delta.
>
>
> | Seeing that the user with a single phase supply sees no difference from
> the
> | situation where the distribution transformer is connected l-l vs l-n on
> the
> | primary- your last question is meaningless. Run a separate ground wire
> if
> | you want.
>
> Please clarify what you mean by "Run a separate ground wire if you want."
> There are a number of different ways to accomplish that. But given that
> power company practice is to connect the secondary of the transformer to
> the primary current carrying conductors, then the first step to running a
> separate ground wire is to have another transformer with its primary wired
> L-L (240 volts) and its secondary not connected to the primary at all ...
> not even to the service drop neutral (which also must not be grounded to
> earth anywhere near the points the new separate ground is earthed).
>
>
> | If you are looking at an industrial system taking 3 phase from a delta
> with
> | a neutral on one side and single phase to neutral loads on the tapped
> side-
> | where while neutral current can't flow, unbalanced voltages can
> result -then
> | I would prefer a Grounded wye system. A separate safety ground wire to
> the
> | frames of equipment is just as feasible there as with the household
> single
> | phase system.
> | Note also that ground fault protection is a hell of a lot easier with a
> Y.
>
> I still think you are misunderstanding me. It seems you are assuming that
> when the loads (transformers at customer taps) are L-L or L-L-L, then the
> source of the circuit they connect to must have a delta secondary. I do
> realize you described the case where a town switched from delta secondary
> (for example at 7200 volts) feeding L-L and L-L-L loads, to a wye
> secondary
> (for example at 12470 volts) feedling L-N and L-N*3 loads, to achieve a
> 73% boost in system capacity.
>
> But this was all in repsonse to my description of how things should be
> from
> the beginning, which would have precluded that down from having the
> starting
> point they had in the first place.
>
> The substation transformer secondary should be WYE. The center point is
> earthed at the substation. Now there are two different ways to run that
> circuit:
>
> 1. Run 5 wires, identified as A,B,C for the phases, N for the neutral,
> and G for the grounding wire. Loads can then be connected to any
> combination of A,B,C,N as needed.
>
> 2. Run 4 wires, identified as A,B,C for the phases and G for the
> grounding
> wire. Loads can then be connected to any combination of A,B,C as
> needed.
>
> With design #1 you can have L-N taps for customer service transformers.
> With design #2 you are limited to L-L taps. In all cases the secondary
> would be WYE.
------------
The present practice is to use (2) with taps being line to neutral. This
means that 3 phase services don't have to be run everywhere and there is a
cost savings. Transformers are also cheaper. As for safety- it is
questionable whether there is any real safety penalty compared to (1). Now
using (1), in effect, you are running a ground wire back to the substation
and grounding it there. This wire will be coupled to the primary and since
loads can be quite unbalanced, there can be appreciable voltages induced in
the wire. These can be much higher than those due to neutral current. This
can be avoided by use of multiple ground points but in the case of a poor
ground at a customer entrance there is the possibility (however rare) that
induced voltages from hundreds to thousands can occur- particularly in the
case of a fault on the primary. Such voltages could exceed the voltages
produced by a high common neutral current (which is actually much less of a
problem) . There are safety concerns with both systems but how many safety
problems have been caused by this considering the extensive use of the
common HV-LV neutral point/ conductor throughout North America for the last
80 years? There are the "stray-voltage" situations met in some dairy barns
but, the cause claimed, has, as far as I know not been proven technically
(although proof in court is easier).
--

Don Kelly dhky@shawcross.ca
remove the X to answer
----------------------------

>
> --
> |---------------------------------------/----------------------------------|
> | Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below
> |
> | first name lower case at ipal.net / spamtrap-2006-10-16-0750@ipal.net
> |
> |------------------------------------/-------------------------------------|

phil-news-nospam@ipal.net

2006-10-17, 1:25 pm

On Tue, 17 Oct 2006 03:45:05 GMT Don Kelly <dhky@shaw.ca> wrote:

|> The substation transformer secondary should be WYE. The center point is
|> earthed at the substation. Now there are two different ways to run that
|> circuit:
|>
|> 1. Run 5 wires, identified as A,B,C for the phases, N for the neutral,
|> and G for the grounding wire. Loads can then be connected to any
|> combination of A,B,C,N as needed.
|>
|> 2. Run 4 wires, identified as A,B,C for the phases and G for the
|> grounding
|> wire. Loads can then be connected to any combination of A,B,C as
|> needed.
|>
|> With design #1 you can have L-N taps for customer service transformers.
|> With design #2 you are limited to L-L taps. In all cases the secondary
|> would be WYE.
| ------------
| The present practice is to use (2) with taps being line to neutral. This
| means that 3 phase services don't have to be run everywhere and there is a
| cost savings. Transformers are also cheaper. As for safety- it is
| questionable whether there is any real safety penalty compared to (1). Now

The difference between #1 and #2 is not as you describe. #2 has no neutral.
For what you describe we need a new entry, #3, with A,B,C,N. Still 4 wires,
but also electrically different.

| using (1), in effect, you are running a ground wire back to the substation
| and grounding it there. This wire will be coupled to the primary and since
| loads can be quite unbalanced, there can be appreciable voltages induced in
| the wire. These can be much higher than those due to neutral current. This
| can be avoided by use of multiple ground points but in the case of a poor
| ground at a customer entrance there is the possibility (however rare) that
| induced voltages from hundreds to thousands can occur- particularly in the
| case of a fault on the primary. Such voltages could exceed the voltages
| produced by a high common neutral current (which is actually much less of a
| problem) . There are safety concerns with both systems but how many safety
| problems have been caused by this considering the extensive use of the
| common HV-LV neutral point/ conductor throughout North America for the last
| 80 years? There are the "stray-voltage" situations met in some dairy barns
| but, the cause claimed, has, as far as I know not been proven technically
| (although proof in court is easier).

Based on what I have seen, the "stray-voltage" is a proven and real
problem. Break the connection between primary and secondary neutral
at the transformer and the problem goes away.

Electricity takes every return path it can find, in inverse proportion to
resistance. The neutral wire running down the pole is best, and most of
the current will go that way (on single phase branches) or via the other
phases (as soon as balancing loads on other phases are reached). But some
will take the earth path back. It's not much, but the higher the voltage,
the more significant it is.

Just how much this will affect things will also depend on the quality of
the distribution circuit design and condition of maintenance. Some power
companies do better than others. Some do quite horrible.

Ham radio operators searching out RFI problem have found a great many of
these problems are on the power lines, so they are usually checking them
in the course of tracking them. Even when the actual RFI cause is
elsewhere, they do frequently find multiple unrelated problems on power
lines. One very common one is a broken ground connection. I found two
of these in my neighborhood when I was a kid. At least I knew not to
touch them. My dad called the power company to report them, and they
never, ever got fixed. Numerous ham radio RFI reports I've read include
the hams coming across the very same thing.

There are many unexplained technical problems with equipment like
computers that I'm wondering if this could be related to. Transients
one the MV lines could translate to transients on the ground wires at
levels too short for people to see or realize what they are feeling,
but could be disasterous to a tiny solid state component. I don't
have the resources to acquire and operate the test equipment to verify
this. But from a technical understanding, it is very plausible.

The key thing is, these problems don't result in power outages, at least
not right away, and as such, there is very little incentive for the power
company to address these issues. They give lip service to safety, and
you see that in actual spent dollars on things like TV ads telling kids
not to touch downed power lines. But the shabby maintenance of some
power lines tells a lot more to me about where their real incentives and
motivations are.

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2006-10-17-0826@ipal.net |
|------------------------------------/-------------------------------------|

phil-news-nospam@ipal.net

2006-10-17, 1:25 pm

On Tue, 17 Oct 2006 03:18:12 GMT Don Kelly <dhky@shaw.ca> wrote:

| Some confusion here- you speak of a separate ground wire -not carrying
| current -on the MV system and which can be uses for grounding the secondary
| side of the MV-LV transformers. Sorry, the picture is not clear.

Which part is confusing?

| Lets consider the system that is used as above -where one leg and neutral
| of the MV (taken as the primary-say 7200 L-N ) is taken down an alley (or
| goes underground) to feed one or more single phase transformers. The neutral
| is tied to ground at the transformer. Service tap neutrals are also tied to
| ground at that point. They will also be tied to ground at the premises.
| However the service and the MV neutrals are tied together at one point only-
| at the transformer. If the LV secondary (240/120V) is run along parallel to
| the MV for some distance (to feed more than one home as is sometimes the
| case, there is the choice between using a common wire or using a separate LV
| neutral. I have no problem with the latter if that is what you propose.

It is not, but only because I didn't address that specifically.

| A separate LV neutral grounded only at the transformer carrying only
| secondary neutral current. and not common to several transformers "may" be
| safer but other factors enter the picture -hence the maybe. However, in
| that case the LV is grounded at the premises as well so there is a loop
| involving a parallel ground path which is common to both the MV and LV sides
| and there is no guarantee that some of the MV current won't prefer the LV
| neutral to the earth path.

The system I described for my design would have 3 wires going do that alley.
Those would be the line wire Lp, the neutral wire Np, and the ground wire Gp.
If they were insulated according to color standards used in buildings for LV
circuits, they would be black, white, and green.

The transformer primary would be connected to Lp and Np. Thus Lp and Np
would be carrying current. Gp would not be carrying this current, though
it can acquire a proximity charge. But that charge can bleed off to ground
with very small levels of current.

On the secondary side of the transformer, we have a center tapped winding,
and two line wires. I'll call these As, Bs, Ns. Ns is the center tap.
There is also a wire running between the transformer and an earth electrode
I'll call E. Ns, Gp, E, and the transformer case, are all connect together.

The path between Ns and Np is solidly metallic only all the way back to the
substation where the transmission to distribution transformer connects to
both of them at it's wye secondary center point. The other path between
Ns and Np is through earth between separate electrodes space distances
apart (for example alternating between whether Np or Gp is earthed on even
and odd poles).

Suppose you have a building fed with 480Y/277. On each floor of the building
is a dry transformer stepping that down to either 208Y/120 three phase or
120/240 single phase (different phase taps per floor to keep balance). To
support any possible use, that 480Y/277 riser carries A,B,C,N,G up to each
floor. The elevator system at the top uses 480 volts. Outside lights around
the building are wired at 277 volts. How would you ground the secondary
neutral of each of those dry transformers? Would you connect it to the
gray neutral wire of the 480Y/277 system, or the green grounding/EGC wire?

|> Edison may turn out to have been right about the dangers of AC current
|> after all. Edison abandoned ground return for electric current fairly
|> early in the development of the Edison electric system.
| -----
| Ground return on DC is more problematic than on AC and the current
| distribution in the ground is quite different. However, Steinmetz was
| against the use of grounded systems. Practical problems that came about as
| systems grew, dictated that the benefits of a grounded system outweighed the
| costs.

There is a difference between a groundED system, and a ground return system.
That difference is a bit fuzzy in practice considering that the neutral wire
is not zero resistance, and maintenance is imperfect.

At the voltage Edison was working at (110/220), using earth to return the
neutral would have not performed very well, even over the short distances
in the part of New York he was serving. Ground return on a 2 wire system,
AC or DC, can be done at higher voltage and managed despite the ground
resistance by adjusting the voltage to match the load. But having taps
on that ground return line _and_ having them be not quite in balance over
two opposite polarities or phases, would be more challenging even today.
Edison had no chance of making a ground return successful with existing
technology of his day.

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2006-10-17-0900@ipal.net |
|------------------------------------/-------------------------------------|

Matthew Beasley

2006-10-17, 1:25 pm

<phil-news-nospam@ipal.net> wrote in message
news:eh1hu501e1j@news4.newsguy.com...

> On Mon, 16 Oct 2006 22:04:00 GMT Matthew Beasley <nobody@spam.com> wrote:
> |> 3. An optional neutral may be run, which may also be earthed only at
> |> poles where the grounding-only wire is NOT earthed.
> |
> | It would need to have at least arresters so that it couldn't develop
> high
> | voltage with respect to local ground.
>
> That's why it is earthed. Would it be better if it were not earthed?

If the neutral is grounded only back at the substation and lightning hits
the line, it would develop high voltage with respect to local ground. Even
induced voltage from a nearby strike could result in equipment failure. By
placing an arrester neutral to ground, it would limit the neutral to ground
voltage rise.

>
>
> |> 4. Each LV customer tap primary is connected L-L for single phase or
> |> delta for three phase, if there is no neutral.
> |>
> |> 5. If there is a neutral, LV customer tap primary may be connected L-N
> |> for single phase and WYE for three phase, but may also be connected
> |> L-L or delta. The neutral is NOT earthed at this pole or within
> |> 10 meters of any ground wire earthing.
> |
> | For both 4&5, there still is the problem of ferroresonance & backfeed.
>
> These are common for dry transformers. Why wouldn't it be a problem
> with them, too?

Usually they are fed with three phase protection. Phase loss between the
source and transformer is rare. Single phase protection is prefered by many
utilities for distribution, and loss of a single phase - either open or
grounded is more common.

>
> Sounds like it will be necessary to always have a 5 wire distribution to
> be safe.
>
> And how would single phase backfeed?

The impedance of backfeed on single phase would be small, just the load
current. In single phase the major danger would be ferroresonance.

>
> And what if, instead of one of the phases going out, the neutral does?
>

With the current multipoint grounded wye systems, it becomes SWER (Single
Wire Earth Return).
With no grounding except back at the substation, the neutral could be
subject to ferroresonance.

>
> |> 6. All three phase LV customer tap secondaries are always WYE with the
> |> center point connected to the grounding-only wire described in #2
> |> and is also solidly earthed with one or more electrodes at that
> pole.
> |>
> |> 7. All single phase LV customer tap secondaries are always center
> tapped
> |> with that tap connected to the grounding-only wire described in #2
> |> and is also solidly earthed with one or more electrodes at that
> pole.
> |
> | #6 & #7 You still have secondary neutral currents going through the
> ground.
> | This will eliminate some of the problems, but not all.
>
> But not very much at all feeding into the customer ground.

In most cases, but not always. The voltages are lower but currents higher
in secondary systems. Neutral drop can be significant if there is
significant neutral current. In a properly designed customer's system there
should be, but that's not always the case.

>
>
> |> 8. No center tapped delta.
> |>
> |
> | What's the problem with delta secondaries if grounded?
>
> Perhaps none if D-D. Y-D would have backfeed. In any case, it's not
> part of the design I gave because the intent of it is to have a smaller
> L-G voltage.
>

D-D and Y-D with no connection to center point are about the same. I
absolutely agree on the standardization point - the delta services just add
to the number of "standard" voltages.

phil-news-nospam@ipal.net

2006-10-17, 5:25 pm

On Tue, 17 Oct 2006 16:32:06 GMT Matthew Beasley <nobody@spam.com> wrote:
|
| <phil-news-nospam@ipal.net> wrote in message
| news:eh1hu501e1j@news4.newsguy.com...
|
|> On Mon, 16 Oct 2006 22:04:00 GMT Matthew Beasley <nobody@spam.com> wrote:
|> |> 3. An optional neutral may be run, which may also be earthed only at
|> |> poles where the grounding-only wire is NOT earthed.
|> |
|> | It would need to have at least arresters so that it couldn't develop
|> high
|> | voltage with respect to local ground.
|>
|> That's why it is earthed. Would it be better if it were not earthed?
|
| If the neutral is grounded only back at the substation and lightning hits
| the line, it would develop high voltage with respect to local ground. Even
| induced voltage from a nearby strike could result in equipment failure. By
| placing an arrester neutral to ground, it would limit the neutral to ground
| voltage rise.

If the neutral is not grounded, I can believe that. But consider two points:

1. In my design I do suggest grounding the neutral to earth at as many as
every other pole. The other poles would be where the non-conducting
groundING wire is earthed.

2. What happens to the line wires that are not earthed? They are not
grounded anywhere. Would they not develop that high voltage?

|> |> 4. Each LV customer tap primary is connected L-L for single phase or
|> |> delta for three phase, if there is no neutral.
|> |>
|> |> 5. If there is a neutral, LV customer tap primary may be connected L-N
|> |> for single phase and WYE for three phase, but may also be connected
|> |> L-L or delta. The neutral is NOT earthed at this pole or within
|> |> 10 meters of any ground wire earthing.
|> |
|> | For both 4&5, there still is the problem of ferroresonance & backfeed.
|>
|> These are common for dry transformers. Why wouldn't it be a problem
|> with them, too?
|
| Usually they are fed with three phase protection. Phase loss between the
| source and transformer is rare. Single phase protection is prefered by many
| utilities for distribution, and loss of a single phase - either open or
| grounded is more common.

But that loss of a single phase does not propogate into the premise dry
transformer?

|> |> 6. All three phase LV customer tap secondaries are always WYE with the
|> |> center point connected to the grounding-only wire described in #2
|> |> and is also solidly earthed with one or more electrodes at that
|> pole.
|> |>
|> |> 7. All single phase LV customer tap secondaries are always center
|> tapped
|> |> with that tap connected to the grounding-only wire described in #2
|> |> and is also solidly earthed with one or more electrodes at that
|> pole.
|> |
|> | #6 & #7 You still have secondary neutral currents going through the
|> ground.
|> | This will eliminate some of the problems, but not all.
|>
|> But not very much at all feeding into the customer ground.
|
| In most cases, but not always. The voltages are lower but currents higher
| in secondary systems. Neutral drop can be significant if there is
| significant neutral current. In a properly designed customer's system there
| should be, but that's not always the case.

What about this setup:

The single phase customer drop has 2 phase lines at 240 or 480 volts.
It reaches the service meter placed on a pole just a short distance
away from the customer building, then runs over to the building where
it enters through a disconnect and immediately to a dry transformer
inside. The secondary of the dry transformer derives 120/240 and its
center tap is earthed by 2 paths going out either side of the point
where the power comes in.

The service drop neutral comes to that pole with the meter, and is
earthed there. But it is not extended across to the building.

What I've been told before is that the transformer in the building
would be subject to lightning induced ground gradient voltages by a
nearby strike. But I would think that to be small if the distance
between the pole and building is not too great (for example 3 meters).

|> |> 8. No center tapped delta.
|> |>
|> |
|> | What's the problem with delta secondaries if grounded?
|>
|> Perhaps none if D-D. Y-D would have backfeed. In any case, it's not
|> part of the design I gave because the intent of it is to have a smaller
|> L-G voltage.
|>
|
| D-D and Y-D with no connection to center point are about the same. I
| absolutely agree on the standardization point - the delta services just add
| to the number of "standard" voltages.

It seems all the dispute with my design was how I described it could be
connected at the MV distribution side. But I think my design still has
merit (other than for the fact that there is no chance in hell the world
would make any changes today). Light (as in incandescent illumination
_and_ small appliances) loads would be connected to 24 volts wired L-N
(maybe 24-0-24) and everything else would be connected to 288 volts wired
L-L coming from single phase 144-0-144 or three phase 288Y/166. The 24
volt (24-0 or 24-0-24) system would be separately derived from a 288 volt
branch circuit. There might be more than one such system if there is a
lot more incadescent lighting. I think I would go with 72 Hz.

I do have a few different designs for the ultimate receptacle.

Now I just need to get my time machine working :-)

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at i