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Re: Pole Pig Question
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> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Pole Pig Question
> Date: Saturday, February 19, 2000 11:07 PM
>
> Original Poster: "Ryan Ries" <spud-at-wf-dot-net>
>
> Hey. In my search for a distribution transformer, a couple of questions
> came to my mind. First of all, I have observed that single-phase
> transformers only have one high voltage terminal, while triple-phase
> transformers have two.
This is kind of lengthy, but may help to explain why you see various
configurations of surplus transformers, and where all those weird voltages
come from.
Essentially all bulk distribution of power is done with three phases (it is
most efficient, in terms of $$ of conductor and installation per megawatt
carried, with the exception of very large links running long distances (for
which EHV DC is cheaper and easier to manage)).
With three phase power, only three wires are necessary to carry the power,
with the loads wired from phase to phase (called "Delta" connection).
However, there are a lot of applications where it is handy to have one of
the connections at ground (or near) potential (like household wiring, for
instance)(or, more to the point, building a transformer is cheaper when one
end of the winding can be connected to the case). For this, there is a
three phase connection called "Wye", where the loads for the three phases
are all connected at a central point, which, if the loads are balanced, is
near ground potential and is labelled "Neutral".
Which connection is used depends on the voltages required, the level of
balance, and, to a large part, the whim of the system designer (no
kidding...) and what parts are available. If you have a lot of single phase
loads, the Wye connection is popular, because it has a neutral. For
instance, in light industrial and office, a very popular connection is the
208Y/120V connection. This is three phase power where the phase/neutral
voltage is 120V, so you can hook up conventional 120V appliances, lights,
etc. In this case, the phase to phase voltage is 208 V, which is close
enough to 240V that many 240V appliances will work.
However in factories, and the like, where you have lots of 3 phase motors
(by far the most popular in >1HP sizes, for efficiency reasons), the
standard motor voltages are 230V and 460V. For these kinds of loads, the
delta configuration is used (motors are inherently delta kinds of devices
and nicely balanced), and you'll see 240 Delta (240 V phase to phase) or
480 Delta (480 phase to phase). In factories, they also want to run
lighting loads, etc., but there aren't many lights made to operate at 480V.
(the insulation requirements change dramatically at a circuit voltage of
300V) The choice is either to provide a neutral (wiring the supply side
as Wye, but still giving 480V phase to phase), and connecting the motor
loads in delta, and the lighting loads as phase to neutral and getting 277
Volts. 277 is close enough to 240 that it isn't too tough to design
fluorescent ballasts and the like to work. I haven't figured out yet why
there are so many 277V neon transformers, because I can't imagine that
there are that many big neon installations in connection with 480V
distribution, but certainly, for overhead lighting, it fits a real need.
Another approach to running lower voltage appliances from the 480V
distribution in the factory is to put small "dry" transformers (so called
because they don't have oil in them) that change the 480 into 240 or 120 as
required. There are tons of these transformers available cheaply.
Typically, they have 2 primary windings at 240V and 2 secondary windings at
120V, so you can connect the various windings in series and parallel to
convert just about anything into anything else. Sometimes, they'll also
have taps for 208, etc. These are often ranging from 0.5 kVA up to 10 kVA.
For instance, in that huge rolling mill with 480V Delta everywhere, say
you need power for a foreman's office in the middle of the floor.. you just
put in a suitable dry transformer to create 240/120 just like at home,
ground the centertap/neutral, and you're all set.
Another strategy which you will see (although less popular) is if you have
240Delta wiring, you ground the centertap on one of the transformers,
making the feed 240/120 between those phases, but making the third phase
somewhat higher. This is unpopular because it leads to unbalanced loads
and higher voltages relative to ground on one of the phases, but may be
useful, if you don't have much 120V load to work with.
Why 240V for the transformer vs 230V for the motor? The 240V refers to the
nominal supply voltage, 230V refers to the usual voltage at the motor
terminals (which is lower, because of the IR drop in the wires supplying
the motor). The actual voltage can vary quite widely, depending on the
load. I once worked on a motor control panel supplied by a 75 kVA branch
circuit near a roller coaster that the supply voltage was 510V when the
roller coaster was off, but under load, it came down to 480V. The 30V
change is <10%, and so, is within spec..... Then, by the time the power got
to the motors after passing through several hundred feet of wire, it was
down to about 470V.
Aside from that, what are differences? (I guess
> I'm asking for the theory behind the different phases of AC current.)
> Also, some of the transformers have primary voltages that I can
understand.
> For example, 12470 volts. Others have weird things like 7200/12470Y, or
> 12470GY/7200. Could you please explain this to me also? I have a
feeling
> it has to do with the 1-phase and 3-phase thing, but I don't know.
Look for the ratio of 1.73:1... This is the ratio between the phase to
phase voltage and the phase to neutral voltage in three phase systems.. So,
12470 is 1.73 * 7200. What this is is a 7200 V transformer winding,
designed to be connected in Wye, in a system that is 12470V when measured
phase to phase (Delta).
I suggest the introductory textbook by Wildi, "Electrical Machines, Drives,
and Power Systems", published by Prentice-Hall, for a good explanation of
all this, without getting buried in the math and derivations.