? "daestrom" ?????? ??? ??????
...

"Tzortzakakis Dimitrios" wrote in message
...

? "Andrew Gabriel" ?????? ??? ??????
...
snip
The transformers are small in comparison, which gives poor
regulation in comparison (and as I said before, it's the
regulation at 120V which is the primary concern -- regulation
of 240V across 2 hots doesn't matter much for typical US 240V
loads).
The regulation, at least in Europe, is done at 150/15 kV substations and
at the HV side of the transformers, thus at 150 kV. Typical current for 2
x 25 MVA transformers is 150 A, 150 kV and of course secondary at 15 kV,
1500 A. The regulation is done automatically with tap changers, live. The
local transformers at your neighborhood are fixed tap, 15 kV (they intend
to change everything to 20 kV).



You're confusing two uses of the term 'regulation'. Tap changers and
voltage regulators actively sense the terminal voltage and adjust
'something' to maintain the voltage within some design limit. That's a
'regulator' and provides 'regulation' of the sensed voltage.

I know that, but it was a temptation to post this:-)
But 'regulation' also is a term used to describe the inherent voltage drop
in some devices. For example, if you review DC generators, you'll find
that simple shunt-wound generators have fairly good 'regulation' and their
output voltage only drops a few percent from no-load to full-load when
supplied with a fixed field. A cumulatively-compound DC generator (which
has a series field and a shunt field), can have a nearly flat voltage
curve from no-load to full-load with just a fixed shunt excitation, or
even have a voltage rise depending on the degree of compounding. (of
course, an active voltage regulator can counteract whatever inherent
regulation a machine may have)

A shame that Tesla won the infamous "battle" and we don't have DC:-() But
then, we would be having a power plant at each neighborhood, instead of the
300 MW ones.
In the case of simple fixed-tap transformers, the term 'regulation' can be
used to describe how much the output terminal voltage changes from no-load
to full-load if the primary voltage is held constant. This use is less
than perfect as it is much better to use the transformer's impedance along
with the load's power factor to get a more precise answer.

In the US, voltage regulation is accomplished with load-tap-changers,
capacitor banks, and other 'voltage support services'.
We have here capacitor banks, too, connected at the LV side of the
substation, 15 kV line-to-line voltage.
But just like in
Europe, it is done at the substation or higher level and not done at the
typical distribution transformer. There are exceptions for rural areas
though where the line length of the primary leads to some issues.

daestrom
P.S. In the US, a 'tap-changer' may be built for either for unloaded or
loaded operation. The 'unloaded' type can not be stepped to another tap
while there is load on the unit (although it can still be energized).
It's switch contacts cannot interrupt load though, so if you try to move
it while loaded, you can burn up the tap-changer. The classic
'load-tap-changer' is actually several switches that are controlled in a
precise sequence to shift the load from one tap of the transformer to
another while not interrupting the load current.

P.P.S. Load tap changers typically have a significant time-delay built
into the controls so they do not 'hunt' or respond to short drops in
voltage such as starting a large load. 15 seconds to several minutes is
typical. So even with load-tap-changers, starting a single load that is a
high percentage of the system capacity will *still* result in a voltage
dip.

Yeah, the ones we have here are automatic, live and even have a shaft for
manual control.

I know, I know, my answer was a bit provocative:-) And of course there are
DC regulators.... You're talking about DC generators;the one a 300 MW uses
for excitation is 220 V, 1000 A DC and probably shunt field. I have seen
here in some machine shops the old type welding generator, which is a 3
phase induction motor coupled to (usually) a compound field DC generator,
which provides the welding current. The modern ones are, maybe, not larger
than a shoe box and powered by a higher wattage 230 V 16 A receptacle.
(Usual receptacles are 230 V 10 A;16 A for washing machines, dryers and the
like).



--
Tzortzakakis Dimitrios
major in electrical engineering
mechanized infantry reservist
hordad AT otenet DOT gr