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Unusual electrical service wiring

Discussion in 'Electrical Engineering' started by Michael Moroney, Sep 18, 2003.

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  1. I was looking at a webpage that showed typical power transformer
    connections for different types of services. In addition to
    the usual 120V/240V single phase and 120V/208V three phase
    configurations, they showed a few odd ones One was 3 240V
    transformers in a delta configuration with the center tap
    of one of them grounded. The leg not connected to the
    grounded transformer was referred to as the 'wild' leg
    and apparently carries a voltage of 208V to ground.
    What sort of devices would one have that would cause
    one to request such a service from the electric company?
    3 phase motors with a requirement for 240V delta seem
    like they'd be kind of oddball, esp with 208V equipment
    being common. A motor connected in a Y would be even
    odder, it would get 138V (if my math is correct) and its
    neutral would be 70 volts above ground.

    Also in the examples shown sometimes one of the
    3 transformers (not the one with the ground) was

    Even weirder was a "Scott" arrangement, two
    transformers fed from 3 phases so to produce
    two phases at 90 degrees. What would want to be
    fed with _that_?

    I've also wondered about a small industrial
    building. It has 3 individual heavy wires from
    the pole to the building, the pole has 3 "cans".
    I found this odd, there should be 4 wires. Looking
    closer the secondaries are in a delta configuration
    and none are grounded as far as I could see.
    The same building has a second feed which appears
    to be a standard Y setup, 3 hots and a neutral.
    So the first must be some special equipment.

    Also what are the advantages of Y and delta
    configurations for power wiring? Y provides a
    natural neutral for one. For secondary wiring
    (the 11,000 volts or whatever that run down your
    street) it appears most of it is Y connected but
    older is delta. Why? From what I can see the
    very high voltage power lines are always delta.
    Why? Also, if a power line is described as
    carrying 345kV, is that phase to phase or
    345kV phase to ground?
  2. That is a 240V power bank. It provides a three phase 240V delta service as
    well as 120V from the center tapped transformer for "lighting". That
    transformer is usually larger than the other two and is used to serve all
    single phase load in the facility. Nothing odd about it. It is a quite
    common service for commercial facilities.
    Open delta. Provides three phase service with only two transformers. Works
    well when the three phase load is well balanced and there is little, or no,
    single phase load. Again, not unusual at all.
    Scott T was developed to supply three phase loads from a two phase
    distribution system. This still happens from time to time. It is rather
    rare. I know of a few radio/television broadcasting locations with this
    type of service since it was cheaper than running all three phases the
    multiple miles to the top of the mountain.
    Delta service is not for "special equipment". Motor drives run on three
    phases, no need for a neutral in most cases.
    You sure have a lot of questions. The comparison of the
    advantages/disadvantages of delta vs wye could keep this group hopping for
    months. One advantage of delta is a savings in the amount of wire that must
    be run.

    The "11,000 volts that runs down your street" is not secondary, it is
    primary and is referred to as medium voltage. It is not 11kV, but usually
    12.47, 13.2, 13.8, 25, or 34.5kV. Some 4kV also exists. Most of the US
    uses a 4 wire multigrounded wye setup. There are many reasons; two of which
    are that most load is single phase and the other being that it is easier to
    detect single line to ground faults (these are the most common faults).

    345kV refers to the line to line voltage.

    Charles Perry P.E.
  3. You certainly *do* have a lot of questions.. :)
    It depends which country you are talking about (silly Americans think they
    run the whole world! ;-) :)

    Down under, the most common sight around the back blocks certainly is 11kV
    Delta (no neutral or ground). As you get further out of town you find 22kV,
    33kV, 66kV and finally 132kV. The easiest way to check the voltage in use
    is to count the number of disks on the insulators.

    Pole-mounted transformers (D/Y) convert the high-voltage primary to
    low-voltage (415V) secondary with a neutral and an earth at each pole. The
    main reason for doing this is to save on cable costs.

    Oh, and in the country you might find a 22kV SWER line, but that's another

    Or phase to phase if you prefer ;-)
  4. One was 3 240V
    We call this 240 - 4 wire, the wild leg has no reference to ground and can
    be most any voltage. I usually see 140-160 volts to ground, tho it varies.
    Actually, there is not a lot of 208 equipment, many, many 230 volt motors
    are in use at 208...which is often about 215, thanks to the boys at the
    power company. They usually work fine and the voltage is within the
    tolerance of the motor. 208 is becoming more common in motors due to
    hi-efficiency design, but 208 shines in small offices and factories where
    both lighting and power can be balanced from one source. It is also often
    desired where owners/managers do not feel their maintenance personnel should
    deal with higher voltages.
  7. Nukie Poo

    Nukie Poo Guest

    Please tell me what SWER is. Thanx
  8. Single Wire Earth Return. It's a system for supplying single phase power
    over very long distances using only one wire. :)

  9. Dave Dahle

    Dave Dahle Guest

    You forgot one older service voltage - 2400V delta :) Not a factor though -
    these systems are probably as old as if not older than the 4kV wye systems
    mentioned by Charles...

    Our municipal utility finally made the push last year to upgrade the last
    remaining 2400V lines to 13.8 kV (apparently culminating a 20+ years of
    transition)... yet the IOU that *also* operates here in town still has miles
    and miles of 4kV in use.

  10. Dave Dahle

    Dave Dahle Guest

    Whattya smoking?

    The largest of the 3 (or 2 in the open delta setup) transformers is the
    singlephase (or "lighting") side, and it is the center leg of this
    transformer that is grounded. Going between any 2 phases, you will see 240V,
    but using the grounded leg as a reference, the 2 phases on the lighting side
    will measure 120V, while the wild leg WILL show 208V to GROUND.

    Whattya smoking?

  11. John Gilmer

    John Gilmer Guest

    Except that there usually are only TWO transformers used to provide the
    service. One BIG transformer for the 120/240 "lighting" and one "dogleg"
    to provide the third phase node.

    It is very uncommon for secondaries to be connected in delta. A delta load
    is usually driven by a Y transformer bank.
    With two transformers, it is VERY common for the service for larger
    restaurants. The distribution utilities seem to be pushing the 120/208
    service over the 120/240/240/240 service described. They want the
    "balance" but the customers end up with inferior power for their heavy loads
    (208 v 240 volts).
  12. Nukie Poo

    Nukie Poo Guest

    Are you serious? That sounds like an old telegraph setup. That doesn't sound
    too safe with the "return" current running through the ground like that. I
    remember reading in an old IEEE transaction about stray currents near the
    Brooklyn bridge causing severe corrosion problems (there's a power plant
    nearby on the Long Island side in Ravenswood). About 20yrs ago, I had a
    customer that complained about getting zinged when his foot was immersed in
    his swimming pool whilst the other was still on the ground. To make along
    story short, I wound up temporarily disconnecting his service drops to prove
    that his problem was caused by stray current and a utility issue (he had a
    private well so it wasn't coming in on any water main). SWER sounds like an
    invitation for such problems.
  13. Beachcomber

    Beachcomber Guest

  14. It is very uncommon for secondaries to be connected in delta. A delta
    It is very common in the coalfields of West Virginia and Kentucky to have
    480 delta, often with a "corner ground", but not always. Then they run the
    ground thru a resistor and monitor the current, if it exceeds a
    predetermined level, an alarm sounds and maintenance takes over.

    Some mines even take 4160 into the mine where a unit substation reduces it
    to utilization voltage.
  15. The high voltage stuff where (around here, northeast US) the wires hang
    Substation to subtation = Transmission lines.....usually 120KV and
    Substation to transformer poles = Distribution lines....12,470 volts up to
    35KV, depending on your utility.
    Don't, you will never get close enough to measure the voltage before you are
    You may be looking at a capacitor bank, used for power factor correction on
    long lines. Common in rural areas. These do not provide power to any user.
  16. Those are not stepdown transformers, they are voltage regulators. They
    adjust the voltage on the conductors.

    Most lines with 2 poles per structure are either subtransmission or
    transmission. Most lines with a single pole are distribution but could be
    subtransmission or transmission.

    Go here for a decent explanation:

    Charles Perry P.E.
  17. Interesting stuff.. stray currents can be a hassle and IIRC most old
    telegraph setups were two-wire DC (at least the ones here were)..

    But to answer your question - yes, I'm quite serious. For more information,
    try a Google search on "Single Wire Earth Return". Happy reading!!

  18. Quite correct. BTW, it was not uncommon to see mist rising from pools of
    rainwater around the base of the transformer pole early on a winter's
    morning.. It was a great place to warm the feet when a long way from the

  19. daestrom

    daestrom Guest

    I have seen some distribution networks 'upgraded' by reconnecting the
    transformers from delta primary to wye primary, then changing the
    transformers feeding into the distribution from delta secondary to wye
    secondary. This simply raises the voltage by sqrt(3) on the distribution
    lines. Often insulators/fuses/etc... are rated to take the higher voltage
    anyway. Or they can be changed out to higher voltage rating equipment
    slowly over a period of time before reconnecting the transformers. If this
    can be done, then the same conductors can now carry more power without

  20. Peter Fisher

    Peter Fisher Guest

    The Scott Transformer system was also used on UK Distribution Networks, but
    you have to be aware of the history. Back in the days of old, electricicl
    energy was supplied from the DC power station in the town during the day,
    and at night, when the load was negligible, by a large battery. The battery
    was centre earthed, so running along the high street was a three wire cable.
    (Positive, Earth, Negative). And the towns residents were happy

    As the electricity networks developed, Three-phase equipment was installed
    at the power station, and connections were made to an embryonic 33kV Grid.

    The customers were connected to the AC system, using the old cables as two
    phases + neutral. The towns residents weren't too keen about having their
    High Street dug up just to install a different cable. Also these old DC
    cables were usually quite large cross sectional area, and in fairly good
    condition, being installed in an age when good engineering was over
    engineering. In order to achieve a balanced three-phase load on the high
    voltage system, the Scott Transformer was installed.

    Having said the above, I'm not aware of any remaining in service. I believe
    that in the Croydon area, south of London we used to have such a network,
    but I think it was decommissioned back in the 1970's, before I joined the
    company. I do know of quiet residential areas where the old three core
    cables are still in use, but they have been cut up into short lengths, and
    used for cross road services and such like.

    Peter Fisher - EDFenergy Networks Branch - CDM co-ordinator
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