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Those three wires are already brought into most homes. I do agree that the application potential is limited as it has to be a device that needs a 240V circuit but internally does better with about 170V DC. Some apartments use 120/208V (with two phases accessible to a unit) and that will cause compatibility issues. One possible application is a variable speed condensing unit with a relay to select between bridge rectifier and full wave mode, boosting efficiency at low speeds. If it detects that it's actually connected to 120/208, it will just stay in bridge rectifier mode.
The harmonics cancel out inside the transformer, assuming there's a PFC stage.
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Who's transformer are you talking about anyways?
If there's an effective load side harmonic mitigation like used in lighting ballasts, you don't have much reaching the transformer. Harmonics DO NOT cancel out in single phase transformer and they go right into the feeder as well as cause voltage distortion for customers on same transformer. For consumer products, I think the only effective way is the way they do it in Europe and regulate it on supply end, hold manufacturers responsible.
You're still not getting it. Assume resistive load and ideal diodes. You're still increasing the losses and wasting transformer capacity, because half the transformer carries the entire half cycle at full load.
Given a real world transformer and perfect diodes (zero drop voltage) with 240v primary to 240:120 split secondary and 1kW load, the power entering the primary side is higher at the primary side two diode split phase setup than a bridge diode, because the resistive losses is I^2R, which means doubling the RMS current quadruples the loss.
Make everything real life. If the diode loss offset by using two best available diodes compared to using the best available bridge is surpassed by increased losses in transformer and wiring, the result is increased overall loss. I don't have any solid data at this point, but if you're cutting 2W in diodes, but increasing loss by 5W between the primary side input and input of diode, you're doing more harm than good in terms of total conservation not to mention requiring non standard 3 wire branch circuit is required to the unit rather than than standard 2 wire for condensing units.
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The idea of using "HVDC" for residential power distribution is really about new construction, not retrofitting. It is mostly of interest in off grid homes, not so much on grid.
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Please take this off topic matter to your own thread. The thread specifically says the effect on power systems. Not battery powered self-contained devices or "off grid" LVDC utilization applications.
When Con Edison removed the revenue loss leading DC service, it didn't change the customer equipment. It simply relocated the burden of power conversion equipment onto customer owned equipment on customer premises.
MVDC transmission for irrigation and rural use shown in second diagram does have the advantage that power can be supplied directly to customer owned rectifier at 3 phase 7.2/12kV and avoids unbalanced load on the feeder associated with L-N loads. This can be carried over 12kV MVDC line and let the customer deal with the cost of 12kV to utilization voltage DC-DC facility and "PoL" VFD for pumps and homes.
Obviously the huge advantage is that conversion losses in equipment located past the medium voltage feeder including the rectifier loss is billable charge and customers bear the cost and repair bills of fragile solid state static converters. Changing the point of metering doesn't change the energy efficiency. It minimizes the amount everyone else have to subsidize to power up those middle of nowhere rural users.
If there's a windstorm and tree branches hit the MVDC private line and the unreliable solid state static converter goes poof, they get to replace it on their own dime.