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Index of Subjects --Apple-Mail-209-805571042 Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes Content-Transfer-Encoding: 7bit Hi Steve, Perhaps my understanding of High Voltage DC (HVDC) transmission lines is flawed. It was some time ago that I was looking at this and I can't quite recall what lead me to this conclusion. Looking at the Wikipedia entry on high voltage DC I see it says: "Long undersea / underground high voltage cables have a high electrical capacitance compared with overhead transmission lines, since the live conductors within the cable are surrounded by a relatively thin layer of insulation (the dielectric), and a metal sheath. The geometry is that of a long co-axial capacitor. The total capacitance increases with the length of the cable. This capacitance appears in parallel with the load." Does this indicate that the capacitance of cable itself can store electricity or am I completely off-base? ;~> One interesting thing that Monbiot highlights in relation to HVDC is, "But most importantly, though the initial electricity loss on a DC line is higher, it does not increase with distance. On AC systems, by contrast, the longer the line the more you lose." The wikipedia entry elaborates this further by saying, "Depending on voltage level and construction details, losses are quoted as about 3% per 1,000 km. ... Where alternating current is used for cable transmission, additional current must flow in the cable to charge the cable capacitance. This current flow causes energy loss via dissipation of heat in the conductors of the cable. Additional energy losses occur as a result of dielectric loss in the cable insulation. However, when direct current is used, the cable capacitance is charged only when the cable is first energized or when the voltage is changed; there is no steady-state additional current required. For a long AC undersea cable, the entire current-carrying capacity of the conductor could be used to supply the charging current alone. The cable capacitance issue limits the length and power carrying capacity of AC cables. DC cables have no such limitation, and are essentially bound by only Ohm's Law." In relation to the development of offshore wind energy -- a very promising area, I gather, since wind blow more strongly and consistently over the ocean (not encountering terrestrial obstacles) and there is little NIMBY effect -- Monbiot argues that HVDC lines could therefore efficiently open up pretty much any area on the continental shelf (irrespective of how far it is from land) to the installation of wind turbines. Cheers! Chris On 30-Aug-12, at 2:21 AM, Stephen R. Shaw wrote: > Hi Chris, > Very interesting, never heard of this before. But how is it supposed > to work as some sort of "battery"? > > Suppose you have a 'perfectly' insulated DC high voltage conductor > line made of copper (except perfect insulation is not possible), > that is suspended in air and goes for 1700 km or more but doesn't > connect to anything. When you connect this line to a power source, > no current will flow into it from the power source if there is no > external load and no leakage. I don't see how you can store any > charge/energy at all in a network like this no matter how large. > Actually the network would lose some charge (DC current) from the > source through the small amount of inevitable leakage that would > occur, especially operating at very high voltage. > > Something therefore seems to be missing from the argument -- have > the proponents of this approach found some practical way of making, > attaching and insulating huge capacitors that can store large > amounts of charge at high voltages? Recollecting bygone times with > tube circuits that operated up to fairly high voltages (say 250 > volts DC rating before electrolytic breakdown): a practical > electrolytic capacitor of ~20,000 microfarads capacity (not that > much) already had reached the size of ~4 inches square by ~1 inch > thick. Scaling this up to a 100,000 volts DC rating would probably > make it have to be ~400-fold thicker, which sounds impractical. I > thought that this was the long-recognized problem with storing > electricity, the impracticability of constructing capacitors with > enough storage at non-colossal sizes. > > So what's the actual storage mechanism, and is there a useful place > to look this up on-line? > Steve (Halifax) Christopher Majka - columnist, Rabble.ca Halifax, Nova Scotia, Canada Email: c.majka@ns.sympatico.ca http://rabble.ca/blog/26142 "The significant problems of our time cannot be solved by the same level of thinking that created them." - Albert Einstein --Apple-Mail-209-805571042 Content-Type: text/html; charset=US-ASCII Content-Transfer-Encoding: quoted-printable <html><body style=3D"word-wrap: break-word; -webkit-nbsp-mode: space; = -webkit-line-break: after-white-space; ">Hi = Steve,<div><br></div><div>Perhaps my understanding of High Voltage DC = (HVDC) transmission lines is flawed. It was some time ago that I was = looking at this and I can't quite recall what lead me to this = conclusion. Looking at the Wikipedia entry on high voltage DC I see it = says:</div><div><br></div><div>"Long undersea / underground high = voltage cables have a high = electrical capacitance compared with overhead transmission = lines, since the live conductors within the cable are surrounded by a = relatively thin layer of insulation (the dielectric), and a metal = sheath. The geometry is that of a long co-axial capacitor. The = total capacitance increases with the length of the cable. This = capacitance appears in parallel with the = load."</div><div><br></div><div>Does this indicate that the capacitance = of cable itself can store electricity or am I completely off-base? = ;~></div><div><br></div><div>One interesting thing that Monbiot = highlights in relation to HVDC is, "But most importantly, though the = initial electricity loss on a DC line is higher, it does not increase = with distance. On AC systems, by contrast, the longer the line the more = you lose." </div><div><br></div><div>The wikipedia entry elaborates = this further by saying, "Depending on voltage level and construction = details, losses are quoted as about 3% per 1,000 km. ... Where = alternating current is used for cable transmission, additional current = must flow in the cable to charge th