A different idea from A More Efficient Bridge Rectifier?
in [Design]
|
Prev: 24Hz to 60Hz PLL?
Next: Protecting 3x16 bits with 16 bits ? (crc8 + 8 bit hamming code seems perfect)
From: Hammy on 14 Feb 2010 21:02 On Sun, 14 Feb 2010 17:30:48 -0600, "Tim Williams" <tmoranwms(a)charter.net> wrote: >"Hammy" <spam(a)spam.com> wrote in message >news:sstgn5p9ib16pu4mibl3u72n7vctn3326h(a)4ax.com... >> The reason DC transmission is seldom used is because of DC >> transmission losses. For example how would you get 300Vdc to every >> residence say at +/- 20%? The DC losses in the transmission line make >> that difficult and costly to implement on a wide scale. > >Not too bad. The technology exists today to replace our infrastructure with >DC -- same voltages for transmission efficiency reasons, just use DC >transformers (i.e. converters). Nice part is each 'transformer' can be >regulating and current protected, electronically. I'd say that's filed on under costly and difficult to implement. Are you suggesting that all current pole transformers be replaced with SMPS's like a buck or sepic? >The advantage over the current system would be slight, while retooling the >world as we know it (putting a VFD in front of every motor) would cost >trillions, which is why nobody's doing it, of course. > >Tim
From: Tim Williams on 14 Feb 2010 22:53 "Hammy" <spam(a)spam.com> wrote in message news:ggahn590bbl2lh3ku34a07qitr83ffhlur(a)4ax.com... > I'd say that's filed on under costly and difficult to implement. > Are you suggesting that all current pole transformers be replaced with > SMPS's like a buck or sepic? Exactly! I would guess polyphase forward converters more likely at that power level, but exactly what's used isn't a big deal. Not at all difficult, we have the technology. It would just be really, really expensive. The true expense might not be that big a deal, since pole pigs have their own cost. But they last for 30 or 50 years or more, and asking to replace all of them when they're younger than half their design life (on average), that essentially doubles the cost already. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Paul Keinanen on 15 Feb 2010 03:01 On Sun, 14 Feb 2010 15:53:33 -0500, Spehro Pefhany <speffSNIP(a)interlogDOTyou.knowwhat> wrote: >But if, say, 300 VDC was available world-wide out of the wall, we >could all have notebook computers that didn't require those stupid >brick things. That would be good. The IEC specifies extra-low voltage (ELV) below 120 VDC and low voltage (LV) between 120..1500 V, thus it would make sense to use a voltage close to the top of either range, for a specific level of protection requirements. For instance +/-42 V might be an interesting ELV alternative, provided that the suggested 42 VDC automobile standard (3x14 V battery) is actually going to be common. Due to the low voltage, the voltage would have to be generated in the house, to avoid high losses. If three phase AC is available and a traditional transformer is used, it is easy to generate 6 or 9 phases and use 12 or 18 pulse rectifiers, with minimal need for PFC. However, if only single phase AC is available or if a high frequency switcher is used to generate the ELV, then a PFC is required between the mains rectifier and switcher. A building would need only a single PFC, instead of building the PFC into every load. On the other hand a +/-750 VDC distribution would fit within the LV directive and could carry more power than the current LV AC distribution for longer distances with the same conductors. Since a traditional 50/60 Hz distribution transformer would be needed to convert the HV/MV (say 20 kV) to the LV AC distribution, why not add some extra secondary windings to that transformer and generate extra phases and use 12 or 18 pulse rectifiers to generate the +/-750 VDC LV, without much need for PFC. The +/-750 VDC could be used directly for heating applications and devices needing lower DC voltages could use simple square wave DC/DC converters, while only motors would require a VFD. With individual VFDs, the motor starting current could be better controlled, the efficiency at partial load could be improved and motors designed to work at say 50-400 Hz would be lighter for a given power.
From: Paul E. Schoen on 15 Feb 2010 04:21 "Paul Keinanen" <keinanen(a)sci.fi> wrote in message news:uethn5dtjsb29fq6ua08lb9023ii6egunt(a)4ax.com... > On Sun, 14 Feb 2010 15:53:33 -0500, Spehro Pefhany > <speffSNIP(a)interlogDOTyou.knowwhat> wrote: > >>But if, say, 300 VDC was available world-wide out of the wall, we >>could all have notebook computers that didn't require those stupid >>brick things. That would be good. > > The IEC specifies extra-low voltage (ELV) below 120 VDC and low > voltage (LV) between 120..1500 V, thus it would make sense to use a > voltage close to the top of either range, for a specific level of > protection requirements. > > For instance +/-42 V might be an interesting ELV alternative, provided > that the suggested 42 VDC automobile standard (3x14 V battery) is > actually going to be common. Due to the low voltage, the voltage would > have to be generated in the house, to avoid high losses. > > If three phase AC is available and a traditional transformer is used, > it is easy to generate 6 or 9 phases and use 12 or 18 pulse > rectifiers, with minimal need for PFC. > > However, if only single phase AC is available or if a high frequency > switcher is used to generate the ELV, then a PFC is required between > the mains rectifier and switcher. A building would need only a single > PFC, instead of building the PFC into every load. > > On the other hand a +/-750 VDC distribution would fit within the LV > directive and could carry more power than the current LV AC > distribution for longer distances with the same conductors. > > Since a traditional 50/60 Hz distribution transformer would be needed > to convert the HV/MV (say 20 kV) to the LV AC distribution, why not > add some extra secondary windings to that transformer and generate > extra phases and use 12 or 18 pulse rectifiers to generate the +/-750 > VDC LV, without much need for PFC. Why have LV AC distribution at all? Just make a DC-DC converter operating at high frequency. If anyone really needs 60 Hz AC, just supply an inverter. The automotive types are less than $100/KW. > The +/-750 VDC could be used directly for heating applications and > devices needing lower DC voltages could use simple square wave DC/DC > converters, while only motors would require a VFD. With individual > VFDs, the motor starting current could be better controlled, the > efficiency at partial load could be improved and motors designed to > work at say 50-400 Hz would be lighter for a given power. I agree with pretty much all of that. But there may be safety issues with such high DC voltage, and even 120 VDC and 300 VDC can be deadly. Not from V-Fib as caused by 60 Hz AC, but the fact that is causes muscles to contract and hold tightly which increases the current and stops the heart. Maybe if the DC supply was floating, it might be safer, but capacitance would cause some shock hazard. Also, DC might be more difficult for a GFCI circuit. But it could be done with a Hall effect sensor - just a bit more expensive and finicky than a simple dual primary transformer or two wires through a CT. Paul
From: Jan Panteltje on 15 Feb 2010 06:38
On a sunny day (Mon, 15 Feb 2010 10:01:55 +0200) it happened Paul Keinanen <keinanen(a)sci.fi> wrote in <uethn5dtjsb29fq6ua08lb9023ii6egunt(a)4ax.com>: >The IEC specifies extra-low voltage (ELV) below 120 VDC and low >voltage (LV) between 120..1500 V, thus it would make sense to use a >voltage close to the top of either range, for a specific level of >protection requirements. > >For instance +/-42 V might be an interesting ELV alternative, provided >that the suggested 42 VDC automobile standard (3x14 V battery) is >actually going to be common. Due to the low voltage, the voltage would >have to be generated in the house, to avoid high losses. > >If three phase AC is available and a traditional transformer is used, >it is easy to generate 6 or 9 phases and use 12 or 18 pulse >rectifiers, with minimal need for PFC. > >However, if only single phase AC is available or if a high frequency >switcher is used to generate the ELV, then a PFC is required between >the mains rectifier and switcher. A building would need only a single >PFC, instead of building the PFC into every load. > >On the other hand a +/-750 VDC distribution would fit within the LV >directive and could carry more power than the current LV AC >distribution for longer distances with the same conductors. > >Since a traditional 50/60 Hz distribution transformer would be needed >to convert the HV/MV (say 20 kV) to the LV AC distribution, why not >add some extra secondary windings to that transformer and generate >extra phases and use 12 or 18 pulse rectifiers to generate the +/-750 >VDC LV, without much need for PFC. > >The +/-750 VDC could be used directly for heating applications and >devices needing lower DC voltages could use simple square wave DC/DC >converters, while only motors would require a VFD. With individual >VFDs, the motor starting current could be better controlled, the >efficiency at partial load could be improved and motors designed to >work at say 50-400 Hz would be lighter for a given power. In my view DC for in the house would cause problems with switches. The modern switches are based on arc extinction from zero crossing, So you would need solid state switches with EMF feedback protection everywhere perhaps. The other problem with DC is chemical reactions, say electrolysis, causing conductors to be eaten away over time. And just as with AC, switching on loads can create big surges, so your solid state converters will have to be able to deliver all that surge power (if everybody switches on at the same time sort of thing). DC very high voltage systems make sense for long trajectories, say hundreds of kilometres (or miles if you live that far away), but induction losses are low over shorter trajectories, and transformers are reliable. Also now all electric clocks need a crystal, GPS time, or DCF77 :-) |