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USA converting back to Direct Current

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    #16
    DC transmission makes enormous sense, as the power level and distance goes up, DC becomes more economic, as the cost of the equipment required gets dwarfed by the losses on the AC line.

    You guys have several HVDC systems, mostly back-to-back, like the Texas examples, and some over longer distances.

    Just a couple of miles down the road from me passes the line of what was, at the time of comissioning (1965), the longest and biggest HVDC line in the world.

    In terms of polarity: it is a bipole, so both +ve and -ve lines are overhead with the ground taking the balance...

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      #17
      Originally posted by dereckbc
      Simple Dc cannot be distributed/transmitted with any efficiency known to man or physics.
      Not sure what is meant by simple DC, but asdie from that DC can be transmitted at very high voltages, more efficiently than AC, and I don't see much difference in the way distribution is done for the two. It is true however that we have a lot of experience with stepping up and stepping down AC using very efficient transformers. Clearly magentic core transformers do not work with DC, and therefore expensive and less efficient solid state circuitry will have to be used with DC. However, I think the efficiency of DC transmission will make up for the added expense, as has been pointed out in another post. e/m
      A kwh saved is 2 kwhs earned

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        #18
        Don, High voltage direct current (HVDC) is used to transmit large amounts of power over long or short distances or for interconnections between asynchronous grids like TX. DC is way to expensive to regulate and change voltages which would require very expensive electronic converters, invertiers, and rectifiers. The conversion is also be very inefficient, and prone to failures.

        Look at it another way from the POCO plant. The 1200 or 2400 VAC out of the generator would have to stepped up via transformer, rectified, transmitted to distribution, then at distribution converted back to AC, stepped down in voltage, rectified again to dc arriving at you interconnection, be converted to AC again, and stepped down to a usable lower voltage. Keeping it AC all the way is simple and cheap with transformers.

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          #19
          Originally posted by dbuckley
          In terms of polarity: it is a bipole, so both +ve and -ve lines are overhead with the ground taking the balance...
          Wow. Just think of the audio amplifiers we'll be able to build. :smile:
          Master Electrician
          Electrical Contractor
          Richmond, VA

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            #20
            Originally posted by hardworkingstiff
            Or maybe an evolution of the best of both? We seem to be able to convert back and forth between AC and DC without difficulty.
            I don't think either will ever go away, it is only a question of which may become more dominant. Right now almost all electronic devices run on DC. However most of the generation, transmission and distribution is done in AC. There is a lot of conversions from one to the other, which is fairly inefficient. Think of a battery in an RV (a DC source), powering an inverter to generate 60 hz AC, which is then used by the power supply of the computer you plug into it, converting it to DC again, so that it can power the circuitry of the computer. Clearly DC distribution within the RV would make better sense in this case. e/m.
            A kwh saved is 2 kwhs earned

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              #21
              Originally posted by LarryFine
              Wow. Just think of the audio amplifiers we'll be able to build. :smile:
              I'm gonna miss that humming noise, though.

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                #22
                Originally posted by dereckbc
                Don, High voltage direct current (HVDC) is used to transmit large amounts of power over long or short distances or for interconnections between asynchronous grids like TX. DC is way to expensive to regulate and change voltages which would require very expensive electronic converters, invertiers, and rectifiers. The conversion is also be very inefficient, and prone to failures.

                Look at it another way from the POCO plant. The 1200 or 2400 VAC out of the generator would have to stepped up via transformer, rectified, transmitted to distribution, then at distribution converted back to AC, stepped down in voltage, rectified again to dc arriving at you interconnection, be converted to AC again, and stepped down to a usable lower voltage. Keeping it AC all the way is simple and cheap with transformers.
                I don't think you need to go through all the conversions between AC and DC anymore. Certainly you will not be able to use magnetic core transformers with DC, but you can step DC up or down, with power electronics. I will be more expensive, however the tradeoff is that tansmission in HVDC is much less lossy compared to AC. e/m.
                A kwh saved is 2 kwhs earned

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                  #23
                  Originally posted by Energy-Miser
                  I don't think you need to go through all the conversions between AC and DC anymore.
                  Originally posted by Energy-Miser
                  Certainly you will not be able to use magnetic core transformers with DC, but you can step DC up or down, with power electronics. I will be more expensive, however the tradeoff is that tansmission in HVDC is much less lossy compared to AC. e/m.
                  The only place where DC has economic advantage is between transmission and distribution sub-stations. It is not practical, economical, or useful for distribution, for the reasons I stated above

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                    #24
                    I thought that, historically speaking, the whole reason AC won out over DC in the days of Westinghouse and Edison was because AC could travel long distances without a problem unlike DC.

                    Edison had a power plant to cover extremely small areas, unlike today's plants that cover millions of square miles apiece...?

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                      #25
                      Originally posted by georgestolz
                      I thought that, historically speaking, the whole reason AC won out over DC in the days of Westinghouse and Edison was because AC could travel long distances without a problem unlike DC.

                      Edison had a power plant to cover extremely small areas, unlike today's plants that cover millions of square miles apiece...?
                      I think the reason AC won over DC back then was, mainly two things: one is that power generation via mechanical motion (rotation), is going to naturally be sinusoidal in waveform, and you can leave it like that and not worry about things like rectifiers or commutators, etc., making it a simpler and cheaper technology. The second big reason is the ease with which sinusoidal AC (or any other waveform of high enough frequency for that matter) can be stepped up / down using a transformer. Of course high voltage is necessary for long distance transmission efficiency, and this gave AC an advantage, because again, it was easy to boost it up, transmit it and then bring it back down at the distribution end. I think the inefficiency of DC back in those days may have stemmed from the simple fact that it was not easy to attain very high votage DC, so transmission over long distances remained a problem. e/m
                      Last edited by Energy-Miser; 12-18-07, 08:50 AM.
                      A kwh saved is 2 kwhs earned

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                        #26
                        I think some of you may have missed the perspective of my question, I was not really asking about HVDC or even distribution although I learned alot from your imput and it will be apart of what happens in the future. I was asking more along the lines of 600V DC and down. Maybe less than 100V. Starting with more like a home owner who gets a renewable energy system on his home or smal buisness, (which is how I see this stating out)and he has solar, wind, thermal,and a few other ways of generting DC current under 600v his home is 5 star rated with high efficiency appliances so that he is self efficiant and the entire system is on or around his home or small buisness so that there is no reason to ever step any thing up or convert it he would just use low voltage (600v-0v DC) for everything. No transformers, rectifiers, converters just generate DC and use DC.

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                          #27
                          Originally posted by No Show
                          I was asking more along the lines of 600V DC and down. Maybe less than 100V. Starting with more like a home owner who gets a renewable energy system on his home or smal buisness, (which is how I see this stating out)and he has solar, wind, thermal,and a few other ways of generting DC current under 600v his home is 5 star rated with high efficiency appliances so that he is self efficiant and the entire system is on or around his home or small buisness so that there is no reason to ever step any thing up or convert it he would just use low voltage (600v-0v DC) for everything. No transformers, rectifiers, converters just generate DC and use DC.
                          I think maybe you are missing something, the renewables you mentioned are not all DC. Wind is AC, no way around that, it can be converted at the turbine with diodes however. What you might also be overlooking even if you do have something like Solar PV, you still have to be connected to the grid. Solar PV systems cannot power high-wattage items like dryers, HVAC, ovens, your blow dryer, etc. Well I guess you could if you are foolish enough to spend 6 or 7 figures on the system and had a acre or two to devote to it.

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                            #28
                            A question came up on why we don't use DC for traffic signal cabinets, as most signal heads are DC leds. The answer was there is a significant corrosion issue with DC, and there is a voltage drop issue that you don't have with AC.
                            Moderator-Washington State
                            Ancora Imparo

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                              #29
                              Originally posted by tom baker
                              A question came up on why we don't use DC for traffic signal cabinets, as most signal heads are DC leds. The answer was there is a significant corrosion issue with DC, and there is a voltage drop issue that you don't have with AC.
                              Bingo Tom, I was just going to bring up the corrosion issue. Would be a nightmare for homeowners. Imagine sleeping in bed and hearing a hissing sound. Upon investgation you discover a water pipe leak in your foundation or walls. Or throw a switch and it burst into flames.:cool:

                              Tom however I will say the VD problem is more pronounced with AC because of the added reactance in addition to resistance. Could it be the lower voltage natue of traffic light bulb reuirements? I can then see VD as a problem as with the Telecom industry using 48 and 24 VDC systems. We have to upsize conductors to compensate for VD.
                              Last edited by dereckbc; 12-18-07, 12:40 PM.

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                                #30
                                Originally posted by dereckbc
                                I think maybe you are missing something, the renewables you mentioned are not all DC. Wind is AC, no way around that, it can be converted at the turbine with diodes however. What you might also be overlooking even if you do have something like Solar PV, you still have to be connected to the grid. Solar PV systems cannot power high-wattage items like dryers, HVAC, ovens, your blow dryer, etc. Well I guess you could if you are foolish enough to spend 6 or 7 figures on the system and had a acre or two to devote to it.
                                You do not "have to be connected to the grid" you could have a battery back up system.(ups)With the advances in battery technology it may not have to be all that big either. And remember I am talking about a house or buisness that is 5 star rated, you do not have the demand on the heatig or cooling or the rest of your appliances like your traditional home would and as for the turbines, do your home work look at the ones that tie in with the pv systems.

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