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

"Michael Moroney" wrote in message
...
"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.
Older units do this whole thing with a fancy cam/gear arrangement circa
1940's. Just takes a single reversable motor to drive the unit and some
limit switches to be sure it can only stop at full 'steps' (or 'half
steps' for those capable of running 'half-step')

The one we have here operates with a motor.
Because the system intermittently inserts an additional voltage drop
through the inductor, the control circuits typically have time-delays that
prevent it trying to reverse direction or something while stepping.

As far as zero-crossing and thyristors, I suppose it's certainly possible,
but I haven't run across them for large substations. I have seen such a
setup in power-conditioners for computer complexes and such, but that's
only a few kVA (one unit I know of was rated for 25 kVA).

The mechanical-switch tap changer is well-matured and has the nice
advantage that when they 'fail', they 'fail' at the last 'step' and power
continues to flow (albeit perhaps the wrong voltage).

When I was a kid living in a rather rural area, there would be a pair of
these on poles every few miles, connected open delta. (all transformer
primaries were connected phase-phase then).

Those are smaller than the units I'm thinking of. I'm talking about
multiple MVA rated units.

I had no idea how it really works, but I got the general idea.

--
Tzortzakakis Dimitrios
major in electrical engineering
mechanized infantry reservist
hordad AT otenet DOT gr
NB:I killfile googlegroups.