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Currently the article equivalent to War of the Currents in Russian Wikipedia section says that direct current is beneficial to pumped-storage hydroelectricity plants and provides no reference to that claim. The English section makes no such claims.

I don't get it. A pumped-storage hydroelectricity plant is connected to the power grid and alternates two processes:

  • feeding water from the higher reservoir and using freed energy for generating electricity and feeding that energy into the grid (during peak consumption) and
  • consuming energy from the grid and using that energy to pump water to the higher reservoir (during off-peak consumption)

Both processes require passing lots of energy between the grid and the plant. The grids aroung the world use high-voltage AC most of the time.

Why would using direct current at a plant be beneficial?

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    \$\begingroup\$ AC is more wasteful than DC. It won the war because transformers allow easy conversion of voltages, but in the modern world, switch-mode DC-DC converters are available, so HVDC becomes more appealing in certain contexts. Not sure if this is relevant to storage plants, though. \$\endgroup\$ – endolith Aug 15 '11 at 14:42
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    \$\begingroup\$ Perhaps they don't want to use induction machines for their motors & generators. Or perhaps they do but they want to use them flexibly - the usual practice today for variable speed operation of an induction motor is to rectify to a DC bus and then use PWM'd IGBT's to synthesize 3 phase at the desired frequency and voltage. \$\endgroup\$ – Chris Stratton Aug 17 '11 at 8:08
  • \$\begingroup\$ @Chris Stratton: Aren't all generators and motors induction machines? \$\endgroup\$ – sharptooth Aug 17 '11 at 8:12
  • \$\begingroup\$ @sharptooth - no, there are many other types of motors and generators, though it may be the other types are not preferred for these power levels. \$\endgroup\$ – Chris Stratton Aug 17 '11 at 16:52
  • \$\begingroup\$ @Chris Stratton: I'd like to know more - what do I Google for? \$\endgroup\$ – sharptooth Aug 18 '11 at 5:18
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Perhaps it would be better to say "direct current is beneficial to hydroelectricity plants".

Wikipedia shows several hydroelectric power plants use HVDC to transmit power, including Volga Hydroelectric Station, Tianshengqiao-I Hydropower Station, Itaipu Dam, Cahora Bassa, Inga Dam, Tianshengqiao-I Hydropower Station, the various hydro plants that feed into the Radisson Substation, the various hydro plants that feed into the Nelson River Bipole, Benmore Dam, Sakuma Dam, Three Gorges Dam, etc.

Why would using direct current at a plant be beneficial?

Wikipedia: HVDC has a good explanation of why HVDC (sometimes) is better than AC. In summary:

With a given long-distance transmission line, HVDC has fewer losses than high-voltage AC. Both have roughly the same parasitic resistance losses, but only AC has losses due to parasitic inductance and parasitic capacitance.

With a given long-distance transmission line, HVDC can transmit more energy per minute. HVDC can transmit at maximum power all the time (the maximum voltage and maximum current supported by the line), while AC has zero crossings where a line is not transmitting any power.

HVDC is immune to some kinds of failures that occasionally hit AC systems, such as loss of synchronization.

The main disadvantage of HVDC is the cost of inverters to convert power to 60 Hz AC. So these HVDC systems are point-to-point, with inverters only at the two ends, rather than a heavily branching structure which would require many inverters, one at each endpoint. As the cost of inverters continues to drop, I expect HVDC to be used more often.

As far as I can tell, all these advantages would also apply to pumped-storage hydroelectricity facilities.

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HVDC is better than AC for transmission, worse for transforming/fault-tolerance, unclear for generation.

Transmission: AC voltages have higher losses because there is parasitic capacitance to earth ground, so even at 60Hz there is a substantial power loss. I'm not sure about the exact loss mechanism, but it's related to currents that have to charge/discharge the parasitic capacitance.

Transforming: stepup/stepdown AC transformers are widespread. DC transformation to lower/higher voltages requires power electronics + is more complicated.

Fault-tolerance: relays/fuses are easier to manage at AC because the current passes through 0 at some point in the cycle and therefore the arc can be stopped, whereas DC fault current is continuous.

Generating: AC can be generated directly with synchronous machines. DC can be generated as well, and requires either an electric machine with a mechanical commutator, or rectifiers, to convert the inherent AC voltages in the electric machine to DC. The disadvantage of AC is that you have to be in lock-step with the grid; the advantage of AC is that almost all the infrastructure is built around it. (Both AC and DC electric machines usually require modulation of rotor fields to match voltages with the grid)

HVDC is used somewhat; we have a HVDC line going through Vermont and New Hampshire to transmit power. The switchyard is rather impressive-looking and I would love to get a technical tour of the place.

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  • \$\begingroup\$ The power-loss in AC transmission is caused by resistive losses induced by reactive current flow. Basically, the Source is an inductor, and the transmission line is a big capacitor. Theoretically, it's perfectly efficient. However, there are considerable reactive currents that flow, and there is resistive losses in the source transformer and the power-lines themselves. Because HVDC does not have to charge and discharge the power-line capacitance 120 times a second, the net current flow is just the load current, rather than the load + reactive current. \$\endgroup\$ – Connor Wolf Sep 4 '11 at 6:26
  • \$\begingroup\$ Exactly -- what I was wondering though was where the resistive losses came from -- is it the transmission lines themselves, or eddy current losses in the earth, or .... ? \$\endgroup\$ – Jason S Sep 4 '11 at 12:45
  • \$\begingroup\$ There is a resistive loss in any conductor that isn't a superconductor. \$\endgroup\$ – pjc50 May 18 '12 at 9:54
  • \$\begingroup\$ @pjc50 -- I know that, but from what I've heard there are other loss mechanisms: one reason the power lines aren't underground (besides the excavation costs) are that capacitive coupling to earth induces higher currents, and that there are also eddy currents. \$\endgroup\$ – Jason S May 18 '12 at 11:38
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As noted in other answers, the capacitance losses make the DC transmisson cheaper over long distances, however you have losses in the converter station to/from DC. According to ABB promotion is suitable for lines longer than 250 km.

The right of way is usually smaller in DC transmisson. The same amount of power can be transmitted with two lines instead of three as in a three-phase AC transmisson. The height of the tower depends only on the safety distance and not making a sutiable capacitance

If one have a sea cable of some length as from the wind power parks offshore Germany the only solution is to have HVDC transmisson due to capacative losses in the sea cables.

you can transmitt power between 50Hz and 60Hz systems with HVDC too.

@Jason: the losses in the DC tranmisson is resistive losses in cable/overhead lines.

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The pumped storage hydroelectricity is used to supply only peak load demand, i.e. at a certain time period of a day, when the rate of consumption is higher then the rate of generation and it's not feasible to erect a power plant just to carter this demand as this irregularity lasts only for a fraction of day time.

So if a power plant is in operation just for a limited time then it is not economical to employ 3-phase system in it as the initial as well as operating cost will be higher, so will be the complexity also the line loss is higher in A.C due to corona (i^2*R) loss, the only demerit in this D.C is the relatively higher rate of maintenance, but as the plant is idle for most of time in day, the maintenance work can be easily carried out in that time.

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The Northfield Mountain Reservoir ( http://www.firstlightpower.com/generation/north.asp) near hear uses AC and connects to the grid.

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