6 ms·
DC-DC is fine at lower amperages, once you go into a few kW, then it gets more difficult. DC - AC - AC - DC has the advantage that you can use a transformer to
by zaarn 6y ago
DC-DC is fine at lower amperages, once you go into a few kW, then it gets more difficult.
DC - AC - AC - DC has the advantage that you can use a transformer to shift voltage levels, which does this very efficiently, usually between 95% and 99% of the energy is passed along.
A DC-DC converter on the other hand usually will top out at 90% in deal conditions, thyristors seem to be able to do 95 maybe 98%, but those only become economical in the megawatt range. You loose more if you want this to work bidirectional (ie, you can feed energy from both sides of the converter).
There is also some other issues that DC brings along, if you were doing it on a household scale, you'd have to worry about magnetic fields in the house significantly shifting, DC cables can't be safely cut from power as easily as DC (it will ark and it will not stop arking until interrupted, you need special fuses, AC stops arking on the 0 crossing), switching heavy DC loads can cause cables to jump at much lower wattages, etc.
Additionally, DC only grid would not be smooth. Every device that joins or leaves the grid will cause voltage fluctuations. It'll maybe be smooth within 10V or so. Within that it'll be moving fast.
For bigger grids there is also the disadvantage that it's rather difficult to sync up a DC connection. Worst case you get a lot of current until the grid normalizes. On AC grids you only really need to synchronize the frequency, then you can connect and any currents should seize over the next zero crossing.
- hutzlibu 6y ago"if you were doing it on a household scale, you'd have to worry about magnetic fields in the house significantly shifting" You mean every time gets switched on or off? (or changes in device energy demand) But with AC the magnetic field changes constantly all the time. That sounds not better. Or is it, that the constant change is pretty much constant, so the effective influence on other devices is lower?
- zaarn 6y agoAC causes radio emissions (which is lost power), it doesn't affect the magnetic field unless it forms coils. And most coils on AC will be put into iron cores to contain them nice and close to where they belong. Even then an AC coil doesn't really behave magnetic very much since the field reverses polarity so often. A DC wire will constantly produce a static magnetic field. With only a few kilowatts, you can probably measurably shift a compass needle within your house, since all your wires will necessarily have to form a loop (positive and ground). Magnetic fields aren't created by shifting the DC voltage or current, they're created by current flowing through a wire, AC just has the advantage that 20ms later, the field goes the other direction so few things care. What will happen when DC load shifts is that the magnetic field changes and it'll dissipate or absorb power from the DC line to do so (Relays require you to put in a diode so that when they are switched off, they can dissipate the stored field energy safely without damaging electronics. This dissipation happens at much higher voltage levels than what induced it and is of opposite polarity to your input voltage.
- dreamcompiler 6y ago> thyristors seem to be able to do 95 maybe 98%, but those only become economical in the megawatt range. IGBTs and SiC are plenty economical for boosting and bucking in the kilowatt range. For very light loads (like inside the home), switched supercapacitors would work fine and are quite efficient. The need for a big spool of copper is over.
- zaarn 6y agoLast I checked, they are economical but not terribly efficient if you're using them in an environment with the power usage pattern of a household.