? "daestrom" ?????? ??? ??????
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"Tzortzakakis Dimitrios" wrote in message
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? "daestrom" ?????? ??? ??????
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"Michael Moroney" wrote in message
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"daestrom" writes:
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.
Are the load tap generators configured make-before-break?
Break-before-make would mean a (very short) power outage every
activation
but make-before-break would mean a momentarily short-circuited winding
and
the break would involve interrupting a large short circuit current.
Certainly modern ones likely use thyristors and zero crossing
detectors.
I figured someone would 'bite' :-)
Typical large power load-tap-changers have a primary winding and two
secondaries.
You mean a secondary and a tetriary? The transformer for the hotel load
of a 300 MW unit is powered directly from the turbo alternator (21 kV)
and has a secondary of 6.6 kV and a tetriary of again 6.6 kV. This is
done because it has wye-wye-wye connection (IIRC). The hotel load of such
a unit is 10%, also 30 MW, including 7 brown coal mills. Typical size of
a 6.6 kV motor is 1 MW.
One secondary produces about 100% of 'rated' secondary voltage. The
second secondary produces about 15% to 20% of the rated voltage, but has
numerous taps from end to end, about 2.5% 'steps'. (for a total of about
eight taps). The cental tap of the boost/buck winding is tied to one end
of the main secondary. The boost/buck can be used to step from 90% to
110% of the 'design' output. I suppose some can step over a wider range,
but I haven't run across them.
*TWO* rotary switches have each tap tied to one of the positions of each
rotory switch, and each 'wiper' is tied to single heavier contacts that
are opened in the operating sequence. The output side of these two
interrupting contacts are tied to each end of a large center-tapped
inductor.
So, normally both rotary switches are aligned to the same transformer
tap, both interrupting contacts are shut, and load current flows from
the boost/buck winding tap, splits and flows through both rotary
switches, both interrupting contacts, enters both ends of the inductor
and out the inductor center tap. Because the current flows into both
ends of the inductor and the mutual inductance of the two parts cancel,
there is little voltage drop in the inductor.
Begin step sequence:
1) Open one interrupting contactor. Now load current doubles through
half the inductor and is zero in the other half, so the voltage drop
across the inductor actually makes output voltage drop, even if trying
to step 'up'.
2) Move associated rotary switch to next step of transformer bank.
3) Close interrupting contactor. Now, the two rotary switches are
across different taps. The inductor prevents a excessive current,
otherwise you have a direct short of the two winding taps. Some tap
changers can stop at this point and are called 'half-step' units.
Obviously, the inductor has to be rated for sustained operation across a
step of the boost/buck winding plus load current in order to survive
sustained 'half step' operation.
4) But for tap changers that can't operate 'half-step', the sequence
continues. And opens the other interrupting contactor. Now the other
half of the inductor has full load current.
5) Move second rotary switch to next step (now both switches are on the
new step)
6) Close the second interrupting contactor. You're back in the initial
configuration, but with both rotary switches on a new transformer tap.
Quite the same principle is done with diesel locomotives and is called
diesel-electric transmission, and also in pure electric locomotives
(E-Lok in german, for Elektrische Lokomotive). The diesel engine,
2-stroke and usually 600 to 900 rpm at full throttle, is coupled to a
generator. The generator has small windings, connected in series for the
last notch, higher voltage and relatively smaller current, and in
parallel for start, higher amperage and smaller voltage. The traction
motors are directly coupled on the wheel shaft, and are air cooled. An
E-Lok has a trasformer, with the primary directly supplied by the
cetenary, 15 kV 16 2/3 Hz in Germany, and 25 kV 50 Hz in Greece, The
secondary uses the same principle. The typical size of a traction motor
is 1 MW, 4 (one each shaft) and maximum voltage 700 volts, and are series
wound motors with special construction to operate at 16 2/3 Hz (or 50 Hz
with today's technology). Typical power of a diesel locomotive is 2850
HP, while an electric is 6000 HP. with 1500 HP at each shaft, also ~1MW.
There is a heavy duty 12,000 HP diesel engine in USA(with 6 shafts, also
2000 HP at each shaft). The high speed ICE train (InterCityExpress) in
germany is 13,000 HP, has a normal travelling speed of 200 km/h, 2
locomotives, 3-phase induction motors, electronic drive.
In US, diesel-electric used to always be DC machines, but modern ones are
now AC generators with thyristers to regulate the power flow to the
traction motors. Traction motors are still DC however to allow for their
use in dynamic braking.
I suppose in Europe the better way to go would be regenerative braking,
putting the braking power back into the overhead line, but that would need
a static inverter. Probably the transformer secondary has a four-quadrant
converter to allow reversal of power flow ??
This is for sure in ICE, where they get 15 kV 16 2/3 Hz AC from the
cetenary, and they convert it to 3 -phase AC for traction motors (3 phase
induction), and they also use regenerative breaking.There's also the french
TGV (Tren de Grand Vitesse) and the just new by Alstom (
www.alstom.com) AGV
(Autometrisse de Grand Vitesse). Classic E-Loks have regular breaking, and
AC motors with series excitation, designed to work at 16 2/3 Hz. (Just like
the ones you'll find in a drill, but much larger, at 1 MW or more). They are
called universal motors, in the small scale, because they can work both in
AC and DC. I'm wondering, how large their brushes are... In the 300 MW turbo
generator, the brushes that suplly the excitation current, are as large as
bricks. Newer type of turbo generators are brushless. The speed record for a
classic E-Lok is held by Siemens' Taurus, IIRC 180 km/h with 12,000 HP.
Nice thing about the newer solid-state control systems (AC-Generator/
DC-Traction) is the ability to control wheel-slip. In the old days it
took a skilled engineer (the train-driving kind) to get maximum power
without slipping a lot (and wasting a lot of sand). Now modern units have
speed sensors on each individual wheel set and control the power flow to
individual traction motors. As soon as a wheel set starts to slip it can
redirect power flow to other traction motors to prevent the slipping set
from 'polishing the rail'. This prolongs life of the wheels and rail and
actually improves the maximum tractive effort a locomotive can deliver.
And when hauling 100+ cars of coal in a unit train up grade, tractive
effort is what keeps you moving.
I have no idea about train driving, but in Germany I got a local train from
a small city to Mannheim, and the Lokfuehrer (train driver) was driving it
like a race car... He accelerated fully to 130 km/h, and when he was close
to the next stop, he braked fully, too. It had one E-Lok, and two cars.
Also, the ICE starts like a race car. It's longer than 500 m, 12 cars, and I
think it accelerates to 100 km/h in 10 seconds.
You forgot to mention that traction motors often have separately powered
blower motors for air-cooling. This is because the motor may spend hours
operating at low speeds and shaft-mounted cooling fans are not enough.
The motor blower is usually mounted up inside the engine house and
connects to the traction motor via a large flexible duct.
Yeah, right, and the transformer is cooled by active oil cooling (that means
that the oil cools the trasformer, and there's a separate oil cooler. Like
the intercooler used in the tanks where I served at army, but that's a
differrent story).
Some diesel-electric unitl have six axles and six traction motors. The
trade-off is between how much power you can get to the traction motors and
how much weight you can keep on the wheels to keep them from slipping.
Sand is okay for starting and some special situations, but you can't carry
enough to use it for an entire run. But of course too much weight and you
need more axles to protect the rail from damage (depending on the size of
the rail being used).
But isn't a locomotive by itself heavy enough? Like 120 tons and above, with
fuel and all?
(Check at
www.wartsila.com some large diesels). In our new power station,
they have installed two 50 MW, 70,000 HP two-stroke diesels. To see how
2-stroke diesels work, look in
www.howstuffworks.com.. The ships that travel
from Iraklion to Piraeus (the harbour of Athens) the new ones, have 4
Wartsila 12 V 46 4-stroke diesels. 12 is number of cylinders, in V, and with
a diameter of piston, 46 cm. When they travel normally at night, they fire
up 2 engines. But, when they make a day trip, they fire up all 4 engines at
full throttle, and the whole ship vibrates. A ship is the only place you can
get free electricity. In my last trip, I saw young students plugging their
laptops to the ship's receptacles. A free lunch, after all:-)
daestrom
P.S. As you can see, I've seen a few railroad locomotives as well.
Mostly just the older EMD's though, not GE's newer 'green' units.
I have no idea what they are doing in continental Greece, they *should* have
electrified all routes.
--
Tzortzakakis Dimitrios
major in electrical engineering
mechanized infantry reservist
hordad AT otenet DOT gr
NB:I killfile googlegroups.