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

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Re: New world power system - Was: US Electrician qualifications for

Tom Horne, Electrician

2006-10-16, 9:25 am

phil-news-nospam@ipal.net wrote:
> 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?
>

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.
--
Tom Horne

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

Member, Takoma Park Volunteer Fire Department

2006-10-27, 3:25 am

Don Kelly wrote:
> <phil-news-nospam@ipal.net> wrote in message
> news:ehoprl11qhv@news1.newsguy.com...
> ----------
> No, I don't share your motivation and may we may share a lack of a solution.
>
> I believe that the problem in the dairy industry was the so called stray
> currents. Considering that such currents, of a higher magnitude, can be
> produced from the LV system, additional, smaller currents from the MV system
> may not be the problem. I once read a book which dealt with this and this
> contained a detailed technical analysis which brought the concept of ground
> currents from MV systems into question. A greater problem was poorly
> maintained LV wiring. However technical opinions and legal opinions are two
> different things.
> If the MV system is grounded properly, there will be unbalance ground
> currents-true. A separately grounded LV system, including one using your
> isolation scheme will have much higher ground currents and, since the earth
> is a common conductor, there will still be coupling between MV and LV ground
> systems. Rather than isolate or try to do so, it might be better to spend
> money on ground rods- even a ground grid and tie everything to this grid.
> Note that in HV substations a grid is used in order to make the ground as
> nea an equipotential as possible and the fence is either outside the grid
> far enough to have a low step potential in case of a fault or inside by a
> distance such that anyone or anything outside and touching the fence is well
> within the grid. This is a bit extreme but the same idea of an equipotential
> grid to which all neutrals, enclosures, etc are connected may be a better
> alternative for locations considered critical. This will mean that the
> ground under a cow is at the same potential as the milking machine hardware
> attached to the udder. Your isolation scheme doesn't do this.
>

Don
I don't claim to have your theoretical expertise but I may have more
practical experience in farm and ranch electrical problems. Here is one
example of that experience. I had a customer who was having production
problems in his dairy herd. I converted all of his feeders to four wire
and megged his neutrals until I cleared all ground faults. There were
only three. One was in the aging well pump, another in a water trough
heater, and a third one in one of the homes light fixtures. Great, I
thought, I should have this whipped. WRONG. As soon as the neutral
connection was restored animal behavior resumed being erratic and
production fell. During the repairs and updates the farm had been
running on its power take off emergency back up generator to supply the
dairy operation and a portable engine generator to power the
refrigeration and like items by cord and plug in the home. While on
this free standing power the production rose markedly when it should
have fallen. The reason that we expected production to fall was the
absence of ventilating fans in the dairy barns outside of the milking
parlor area. Those of us that were capable of sharing our perceptions
were definitely less comfortable in the barn during this period. To our
surprise the production rose without public power and fell as soon as
it's neutral connection was restored. Keep in mind that while I was
chasing ground faults on the LV neutral a sub contractor was redoing the
entire yard pole and building drop system. While the farms electrical
system was disconnected from the Multi Grounded Neutral of the utility's
medium voltage system production rose. When it was reconnected
production fell. Now I was really confused. I went there to cure what
I strongly suspected was a stray current problem. Having megged all
neutrals clear of ground at 300 volts I couldn't see were the stray
current was coming from but when the sub contractor approached to get
his work order signed off I had a clue. With all stock out to pasture
we shut down both generators and measured for current flow on every
feeder conductor. The entire farm was deenergized at the yard pole but
we had measurable current on all of the feeder Equipment Grounding
Conductors. The readings on these EGCs were only slightly less than the
readings obtained on the old feeder neutrals before starting the upgrade
work. I finally came to the conclusion that the current was coming from
outside the farm property via the utility's medium voltage MGN. When
the utility MGM was disconnected from the premise wiring at the service
equipment the stray current disappeared and production rose. I then
purchased a used three pole switch and all milking and barn feeding was
done with the power fully disconnected from the utility while
negotiations with the utility began. Their first reaction was to attack
my competence. I threatened to file suit for defamation and they agreed
to send an engineer. After the engineer saw my readings using three
different clamp on ammeters and a meter shunt he sent for the utility's
measuring equipment. After measuring for three days they claimed the
condition was normal. The rancher filed suit but more importantly he
called his state senator who, as luck would have it, was serving on the
committee that oversaw the states public service commission. When the
Public Service Commission's engineer denounced the claim of normalcy the
utility began to be civil. I then found out that the tariffs included
single phase 480 as a service option to supply long runs of private
overhead wire to feed customer owned transformers. The utility supplied
that power from a different point on the property line. The only main
bonding jumper on the entire premise became the one at the customer
owned dry transformer that is located at the existing yard pole. There
is now no measurable current flow on the main bonding jumper or any of
the Grounding Electrode Conductors at any of the buildings. Production
stabilized at 15% higher than its previous average. In a two hundred
and fifty head herd that is huge.

I said all of that to say that isolation from the utility's MGN is
sometimes a very good idea. All that went down over twenty years ago.
It is true that modern practice calls for an equipotential plane in the
floors of stock barns but it is also true that the stray current does
not always originate in faults in the premise wiring.
--
Tom Horne

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

Tom Horne, Electrician

2006-10-28, 1:25 pm

Don Kelly wrote:
> "Member, Takoma Park Volunteer Fire Department"
> <postmaster@takomaparkfire.org> wrote in message
> news:gBf0h.68370$rx.36366@trnddc04...
> ---------------------
> Thank you for the information. I did not intend to imply that <all> problems
> are due to the customer's own system. There can be and are MV ground
> currents coupling into the customer's LV system as the earth is a common
> conductor.
> Your final solution is a damned good one.
> AC ground currents are fairly localised (and tend to follow the line) so
> having a more remote utility grounding point certainly helps- more so than
> the 4 wire setup which apparently didn't help.
> --
>
> Don Kelly dhky@shawcross.ca
> remove the X to answer
> ----------------------------

Don
The four wire set up did eliminate three + amps of current but breaking
the connection to the MV neutral eliminated over ten amps of stray
current. Point is the motivation for having a truly derived electrical
system is that it puts all of the systems characteristics under the
owners control. The power utilities needs and the customers do not
always coincide in the short run.
--
Tom Horne

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

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