High Voltage DC Breakthrough

Discussion in 'Environmental Discussion' started by zenMachine, Dec 7, 2012.

  1. zenMachine

    zenMachine Just another Onionhead

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    The Secret To HVDC Grids: ABB Unveils HVDC Circuit Breaker - Seeking Alpha

    ABB, which has been delivering power over high voltage direct current for more than 60 years, has solved an engineering puzzle that has been around as long as HVDC.

    The power automation giant just announced that it has cracked the code on building circuit breakers for HVDC power lines. The development allows for extremely fast mechanics with power electronics. In this case, the enormous amounts of power that are moving over HVDC lines can be interrupted within five milliseconds.

    Currently, HVDC lines are widely used to move power across huge distances, because DC is more efficient across long spans than alternating current. The technology is particularly key for renewable projects, such as offshore wind farms, which are often far from city centers where most of the power is used.

    “This historical breakthrough will make it possible to build the grid of the future,” Joe Hogan, CEO of ABB, said in a statement. “Overlay DC grids will be able to interconnect countries and continents, balance loads and reinforce the existing AC transmission networks."

    China is heavily investing in HVDC, as is South Korea. In China, HVDC will make up 40 percent of the country's 300 gigawatts of new transmission capacity, according to GTM Research in its recent report, The Smart Grid in Asia, 2012-2016: Markets, Technologies and Strategies.
     
  2. icarus

    icarus Senior Member

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    The tricky part of DC current is turning it off at high voltages and high currents. Because of the nature of DC, it s much more able to sustain an arc when connecting and more important, when disconnecting. Because AC is essentially "off" 50% of the time, it is a much smaller arc with similar voltages and currents.

    Icarus
     
  3. fuzzy1

    fuzzy1 Senior Member

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    Another way to describe it: AC current drops to zero once every half cycle, providing an opportunity for traditional circuit breakers to blow out the arc that forms when the mechanical contacts are pulled apart. DC has no such zero crossings.
     
  4. qbee42

    qbee42 My other car is a boat

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    Not DC, but it still gives the general idea:

    Tom
     
  5. richard schumacher

    richard schumacher shortbus driver

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  6. bwilson4web

    bwilson4web BMW i3 and Model 3

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    When I was in high school, one of my fellow students had an interest in X-ray diffraction so he got an old hospital X-ray machine, transformer, and an impressive DC switch. Solenoid operated, it had two sets of contacts. The main contacts opened first to transfer the load to a smaller, second set with a 'massive' current limiting resistor. Then the smaller contacts opened with a much reduced current.

    When I look at this hybrid switch, the arc suppression resistor is now a bank of IGBTs. Unlike the fixed resistor of the 1960s hospital x-ray switch, IGBTs can start low and go high fast without generating an arc. Smarter than a fixed resistor, it is a familiar architecture.

    Still, impressive voltage and current ratings.

    Bob Wilson
     
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  7. austingreen

    austingreen Senior Member

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    Just a quick question - Why are they doing high voltage DC instead of AC? ERCOT is building the newer high voltage long distance interconnects to wind with AC.
     
  8. fuzzy1

    fuzzy1 Senior Member

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    I believe DC has somewhat lower line loss. It doesn't make sense for short lines, but can for longer runs.

    The main example out here is the Pacific Intertie, but this article doesn't get into the efficiency issue.
     
  9. bwilson4web

    bwilson4web BMW i3 and Model 3

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    Correct!

    An AC signal has an RMS (Root Mean Square) voltage that really means how much it would heat a resistor with an equvalent DC voltage. So 120 VAC heating a 120 W incandescent lamp would have a current flow that averaged 1 amp and generates the same amount of heat as 120 VDC feeding the 120 W lamp with 1 amp. But AC is not there all the time.

    Part of each cycle has to pass through 0 volts, briefly, twice. So to average things out, the AC peak voltage is higher than 120 V. For a sine wave, this is 1.414 * RMS value, or a peak voltage of ~170 V. This is the voltage threshold insulators must handle. So how does this relate to efficiency?

    A power line has to be built for the maximum voltage and the peak AC voltage sizes the insulators. But the same lines will work just fine with a DC voltage equal to the peak AC voltage. So a 100 KV AC power line can handle 141 KV DC, a 41% increase in power for the same wire and same RMS and DC current.

    Bob Wilson
     
  10. austingreen

    austingreen Senior Member

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    The new high power lines are 345 KV AC within ERCOT. Aren't there losses converting back to AC from that high voltage? I think they are boosting the voltage up here, and building lines for it.

    I know they use some DC lines underground because of capacitance.
    There are DC interconnects to mexico and OK for higher control. In the summer of 2011 they did use the interconnects to mexico to import power.
     
  11. bwilson4web

    bwilson4web BMW i3 and Model 3

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    There are losses on both ends which means it only makes sense for very long and very high voltage lines. Places where the transmission losses are significant to justify the conversion losses. But now you're getting into the limits of my understanding. I'm OK with synchronous rectifiers and boost switching power supplies but going the other way . . . is a black art to me. I know the limits of my knowledge and would recommend an afternoon with Mr. Google. <grins>

    Bob Wilson
     
  12. fuzzy1

    fuzzy1 Senior Member

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    There is more than that. While the increased resistance caused by AC skin depth isn't a significant factor in house wiring, it does add up on these big long transmission lines. And DC doesn't have any reactive current component that can increase line losses in AC.

    When I was in school, another mentioned benefit of DC on the Pacific Intertie was that a fault on a single conductor didn't shut down the whole line. They could run it at half capacity by using an earth return. The main roadblocks to HVDC were lack of DC transformers and circuit breakers. The only breakers on that line where on the AC sides of the conversion stations.
     
  13. qbee42

    qbee42 My other car is a boat

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    AC has the advantage of being able to use conventional transformers to boost voltages for transmission and reduce it at the point of load. This is why AC won the battle back when Edison was pushing DC. You couldn't make an efficient inverter back in those days, and without transformers there was no way to send electricity across long distances.

    Because of efficient power electronics, DC transmission lines can now achieve higher efficiencies than AC, but at the cost of more complex and expensive electronics. This currently limits its use to bulk transmission lines.

    AC has resistive losses, reactive losses, and transformer losses. DC avoids the reactive losses and transformer losses but adds conversion losses. DC also has galvanic issues, since the conductors stay energized at the same potential without reversing polarity.

    Tom
     
  14. hill

    hill High Fiber Member

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    Efficiency in both line loss as well as conversion from DC generation to AC use.
     
  15. austingreen

    austingreen Senior Member

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    I thought it was less efficient to convert low voltage DC to high voltage DC then back to various low voltage DC devices need- than it was to covert low voltage DC to high voltage AC to less high voltage AC to low voltage AC to low voltage DC. Power electronics may have changed since I was in school though.
     
  16. hill

    hill High Fiber Member

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    I don't recall any formula to determine where the efficiency cut off is ... factoring in distance of transmission, versus step up / step down variables. I think I got a headache just thinking about it though. :eek:

    SGH-I717R ? 2
     
  17. qbee42

    qbee42 My other car is a boat

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    Each conversion extracts a cost. With AC it is easy to change voltage, as simple magnetic transformers do the job. Not so with DC.

    Tom
     
  18. NiHaoMike

    NiHaoMike Member

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    Another benefit of using DC is that the connected parts do not have to be synchronized. That theoretically lets you play tricks like V/Hz control in order to reduce the load on the grid during peak times without putting extra strain on motors.
     
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