| What you are attempting to create is a twelve pulse system. The part
| you didn't mention is that there must be a phase shift of 30? between
| the two secondaries. The phase shift is achievable by using either a
| delta and a wye winding, or by using zig-zag windings. The choice
The secondaries are simply isolated (6 leads), so they are neither in
delta nor wye configuration.
I didn't mention a phase shift, because it seems not relevant.
The phase angle of concern is what comes from the transformer.
| between series and parallel is a matter of economics. For high
| voltages, series is used. (X-Ray, e bombardment, electrostatic
| precipitators & HVDC transmission). For more reasonable voltages,
| parallel is used because it is cheaper in terms of diode cost. When
| parallel is used, there must be some form of forced current sharing
| between bridges to achieve good waveforms (both for the AC & DC sides).
| One common method is to use a balancing transformer, and the other is to
| use separate filter inductor for each six pulse bridge. It is not
| practically possible to produce to matched voltages with a 30? shift.
Again, I don't see where the 30 degree phase shift relates. Yes, I do
know a delta-wye transformer will do a 30 degree shift. But in this
case, I'm only dealing with the secondary side and the windings are
isolated from each other (not wired in either delta or wye).
| When parallel is used, this voltage imbalance will cause the entire load
| to be carried on one bridge at light load. At full load, the impedance
| of the two windings can be chosen to get a reasonable balance. The high
| THD at light load really isn't a problem when you consider that it's
| total harmonic current that causes problems.
Is that all on one bridge during the time that bridge is the highest of
the three voltages? That would be the harmonics I would want to avoid.
But this goes away at high load?
| As you mention, SCRs could be used to balance the current in parallel
| rectifiers. In the case where SCRs are already required for voltage
| control, there usually is an additional control loop to help balance the
| current. Where voltage control isn't necessary, the additional cost
| isn't justified.
|
| To achieve 5% THD 18 pulse is required. This is achieve by having a
| shift of 0?, -20? and +20?. For applications like drives where no
| isolation is required, one bridge can be connected direct to the line
| and the other two phase shifted using a autotransformer. In this case
| the rating of the transformer is something like 30% of the total power.
| I believe this method applied to VFDs is patented.
I can clean up the DC with ordninary filtering, so I don't think I need
to go to 18 pulse.
| A method used where there are a lot of drives at a location is to direct
| connect ~ 1/3 of the HP of drives, run 1/3 of the HP through a +20?
| autotransformer, and the remainder through a -20? autotransformer.
| Again, 5% THD is achievable if the drives have a optimal value of DC
| link inductor. (Cheap drives don't have DC link inductors and have real
| bad THD. Some cheap drives bring the terminals out so a optional
| inductor can be added.)
FYI, this isn't for a motor application (although that might be fun to
play around with at some point, too). What I am looking at doing is just
powering an arc lamp with DC from a generator, and thinking about how to
scale that up.
| One reference is:
|
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| For more detail:
|
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Thanks.