Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Featured China Boldly Goes ...

  1. Nov 25, 2016 #1


    Staff: Mentor

    Here is an interesting item I would like to share. It seems that China has drastically pushed the boundaries of power transmission grid operations. They boldly go where no man has gone before. :wink:

    How and why?
    • The combination of geography and demographics leads them to place large generation resources very far away from the loads where consumers live.
    • Excessively long transmission lines make HVDC attractive as a competition for AC.
    • Excessive number of HVDC lines brings new operational challenges and opportunities, which they seem to be managing just fine.
    They are doing things that old timers like me never imagined. Specifically, they challenged the catechism that increased interconnectivity of the AC grid is more reliable; a principle we used for more than 100 years. Hats off to them.

    Last edited by a moderator: May 8, 2017
  2. jcsd
  3. Nov 25, 2016 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    How "High?"
  4. Nov 25, 2016 #3


    Staff: Mentor

    There is no hard rule, but 100 kV up to 1100 Kv.
  5. Nov 25, 2016 #4
    Fascinating, can someone in the field demonstrate with simple calculations how the transmission losses compare to HVAC.
  6. Nov 25, 2016 #5


    User Avatar
    Science Advisor
    Gold Member

    Been "my experience", having worked with a lot of Chinese people, ... they're a smart people. My daughter is visiting with her Chinese friend for the Thanksgiving holiday. I'll pass this along him. Thanks @anorlunda. :approve:
  7. Nov 26, 2016 #6


    Staff: Mentor

    It is far simpler than you imagine. For a given conductor, there is a resistance R per foot of length. Power losses are ##P_{loss}=I^2R##. Power transmitted is ##P_{tran}=VI##. Those equations are the same for AC and DC as long as we use RMS measurements for the AC. So the first answer to your question is that power losses for the same power transmitted are identically equal for AC and DC.

    If you want to reduce losses for the same size conductors, just use higher voltage. If you double the voltage (and halve the current for the same power) losses are reduced by a factor of 4. Ten times more voltage provides a 99% reduction in losses. But there are limits to how high we can make the voltage. In AC transmission, the combination of high voltages and long distances results in unmanageable high capacitance to ground.

    See the PF Insights article, AC Power Analysis: Part 2, Network Analysis. Below is an equivalent circuit for an AC power line from the article. The R, L, and C parameters in the circuit are all per unit length of the line, so for a 1000 km line those values are 10 times higher than for a 100 km line. The distributed capacitance is approximated by putting half at each end. The same article discusses VARs and Voltage Control problems, and why too much C causes big problems.

    Since DC has no capacitance to ground, the capacitance limit does not apply. The operating voltages over long distances can be pushed higher and higher. That is why HVDC becomes more attractive than AC for long distances, the HV for high voltage is the key part of the phrase. For example:
    p.s. I know that there are different numbers of wires in three-phase AC and DC, and that there are complexities regarding power factor, and so on. Those effects are secondary. The primary effects are what I described above.
    Last edited by a moderator: May 8, 2017
  8. Nov 26, 2016 #7
    I cant tell in the diagram if C is between lines or between line and the ground?

    Because P = VI the tiny currents in HVAC are going to be less lossy than HVDC. Where is HVDC advantage?

    If it is in all the other frequency related losses I would have to see the calculations to believe it. Will view link.

    I thought this was settled in the 1800's hence HVAC now.

    The HVDC currents must be frikkin huge at kilovolts.
    Last edited: Nov 26, 2016
  9. Nov 26, 2016 #8


    Staff: Mentor

    Sorry, the line at the bottom of the diagram is meant to represent ground. The capacitances are to ground.

    You got it backwards. P=VI, therefore I=P/V. If V is twice as large for the same P, then I is half as large. So HVDC currents are not frikking huge, they are lower.

    Yes it was settled in the 1890s, or so we thought. The huge advantage of AC was the ability to change the voltage using transformers. But the problem with too high C discussed in #6 didn't come up until nearly 100 years later as we tried to make really high voltage AC lines for very long distances.

    HVDC can't be used to completely replace AC. We still need many voltages for transmission, distribution and household uses. Therefore, AC and transformers will never go away, and most of the 1890 conclusions still apply. But AC no longer has a monopoly for the longest lines.

    There are numerous other AC versus HVDC advantages and disadvantages. The wiki article is pretty good.
  10. Nov 26, 2016 #9
    I get it, the DC is being stepped up an down now like AC which they could not do in the old days.
  11. Nov 26, 2016 #10


    User Avatar
    2017 Award

    Staff: Mentor

    Interesting news. Do you expect that to become a common trend as HVDC gets more common?
    That is the key point. Power semiconductors give a high conversion efficiency, but they are a quite recent development. The demand for power transmission over 1000+ km didn't exist 100 years ago either.
  12. Nov 26, 2016 #11
    My town still has its two functioning power generators (only started for tourists now). Every other town had the same. We are 5 minutes drive from a modern generator that supplies 25 percent of the states power thru the national grid. Every year we have power failures and some of our power comes from 1000's of km's away.

    The locals would prefer the old generators were still uaed.

    What is the logic of a power grid? The infrastructure is insanely expensive and is subject to environmental disasters and price manipulation.

    What is the argument for a grid, many homes here generate their own power and put it into the grid and get paid by the energy supplier who in turn charge people relying on the grid more to make up for lost revenue from homes putting power into the grid.

    Tear the grid down so the minority don't have to subsidise the rest and increase reliability and local economy.

    IMO we should be planning for independent, isolated power generation like what worked in the past. Renewables make local power generation even more logical.
    Last edited: Nov 26, 2016
  13. Nov 26, 2016 #12


    User Avatar
    2017 Award

    Staff: Mentor

    You would have more power failures with a purely local grid. I don't know where you live: Here in Europe we have a well-connected grid, and power outages are basically nonexistent. A few local short-term ones after severe storms or whatever, but nothing large and nothing long-lasting.

    A local grid doesn't work well together with large power plants - but those are the most efficient and cheapest ones. Burning stuff locally is worse for the environment, is more expensive, and can get a logistics nightmare quickly.
    It is exactly the opposite.
    They need the grid as well, and they need a better grid to handle the fluctuating power. You don't want power only if the sun is shining in your town.
  14. Nov 26, 2016 #13
    Well that's the corporate line. Not sure if its the physics. Euro sure, millions of people all stacked on top of each other you need lots of instantaneous power. Isolated rural communities of a few thousand people not so much.
  15. Nov 26, 2016 #14


    User Avatar
    Gold Member

    I don't see any mention of unit cost for the Chinese HVDC projects in the IEEE article.

    One of the most recently completed HVDC projects in N. America (2015) was the Western Alberta Transmission line, 1 GW over 217 miles for $1.65B, or $7.6 million per GW-mile. That seems cost prohibitive for moving something like enough wind generation from the US midwest over a thousand miles to displace a new east coast nuclear plant.

  16. Nov 26, 2016 #15


    User Avatar
    Science Advisor
    Gold Member

    You may like this .pdf: Five keys to wind farm bankability
  17. Nov 27, 2016 #16


    Staff: Mentor

    Not necessarily HVDC, but power electronics yes. There is plenty of room to rethink grid operations given the new abilities of power electronics, but two points.
    1. In the USA and Europe, the bulk transmission is already highly reliable. Improvements in local power distribution are where gains can be made.
    2. The power industry is very conservative, thank God. Replacing a section of bare wire with active components and software :nb) is anti-KISS and the first approximation is that it decreases reliability; especially in a world full of cyber attacks. That sets the bar high for the new things to prove that their benefits outweigh the disadvantages. Advocates and salesmen are motivated to put the bar lower, "Spend the money first and hope for benefits later. Heck, if government pays we don't even care if the benefits never materialize; it stimulates jobs."
    The frustration in your post comes through loud and clear. It is likely that the outages you are experiencing are because of the local power distribution, and not the cross country transmission grid. Here's how to tell. If the outage blacks out a whole city or region, it is a transmission grid failure. If it blacks out a neighborhood or two, it is local distribution.

    There is already a strong trend for distributed generation. Rooftop solar is leading that, but neighborhood level mini-grids are actively investigated. That may improve reliability. But for the millions of people who live in high-rise apartments, there is no room for them to do their own solar. The city of Niagara Falls can't consume all that power themselves, we need a grid to ship it elsewhere. Ditto for North Sea wind farms (unless you are willing to live on a houseboat moored to the wind tower.)

    But you did put your finger on a social problem that might play a bigger role in the future -- groups subsidizing other groups. The unit costs of electric power are lowest with the highest population density. That puts inner city customers in the position of subsidizing everyone else. In the USA, we cherish and celebrate those subsidies. The Rural Electric Adminstration still exists. Someday, that might blow up politically. Europe already leads the USA in control of population density. For example, Jacksonville Florida 370 ##people/km^2## versus Dusseldorf Germany 61000 ##people/km^2## (if I did the arithmetic right).

    However, if the power line on your street is knocked down by a falling tree or chewed up by a squirrel, it doesn't matter where the power is generated. (Squirrels are the #1 problems in parts of the USA.) Before advocating tearing things down, you owe it to yourself to learn whether that would improve or worsen your service..

    I agree if the reference case is Midwest wind delivered to East Coast loads.

    In the OP, I said that the motivation in China is a combination of geography and demographics, so naturally China is not directly comparable to North America or Europe. A better analogy would be if 65% of US power generation was located in Alaska. In Europe, visualize Norway powering the whole continent from Scandinavia down to Italy. Then America and Europe would have a much bigger interest in HVDC. But we don't have that so China will probably continue to lead in HVDC.
  18. Nov 27, 2016 #17


    User Avatar
    Gold Member

    That seems really cheap, considering how long it will last, based on my recent googling, and calculations: $0.0031/kwh

    longevity of the system: 60 years
    power: 1,000,000,000 watts
    energy: 60,000,000,000 watt years = 525,948,768,000 kwh
    dollars: 1,650,000,000
    $/kwh: 0.0031

    What I pay for electricity: $0.11

    That looks like 2 orders of magnitude, to me.

    A grand example of "power" vs "energy" vs "time".

    Thank you.

    ps. A brief foray of my "stream of consciousness" morning's googlings:

    The line capacity is 3,100 megawatts, which is enough to serve two to three million Los Angeles households and represents almost half (48.7%) of the Los Angeles Department of Water and Power (LADWP) electrical system's peak capacity.​

    The improvements, estimated to cost approximately $428 million, would modernize equipment that was cutting edge when installed more than 40 years ago but has aged to where BPA now in extreme circumstances locates parts on Ebay.

    "Some of the equipment at our Celilo Substation is so old, finding replacement parts has become a challenge. Some of this equipment should be in the Smithsonian," Silverstein said.

    Increased costs for the upgrade will only be charged to users of the California Intertie.​

    One of the most recently completed HVDC projects in N. America (2015) was the Western Alberta Transmission line, 1 GW over 217 miles for $1.65B, or $7.6 million per GW-mile. That seems cost prohibitive for moving something like enough wind generation from the US midwest over a thousand miles to displace a new east coast nuclear plant.

    time 2015 year
    cap 1 gigawatt
    length 217 miles
    cost $1.65 $billion

    my calculations:
    see above​

    5/26/2010 12:00 AM

    "When you help build a region, you help build your nation."

    That's what President John F. Kennedy said in a letter to the Bonneville Power Administration on its 25th anniversary. It was 1962.

    The DC line came to life May 21, 1970

    The colossal DC line spans 846 miles across 4,200 towers from Celilo near The Dalles, Ore., to the Sylmar Converter Station in Los Angeles.

    Thanks to the intertie, electricity consumers on the West Coast enjoy a unique power-sharing arrangement that takes advantage of seasonal weather differences.

    In spring and early summer, Northwest rivers usually provide more water for power generation than the region needs. At the same time, temperatures – and air-conditioning needs – in the Southwest climb. That is when the DC intertie sends power south. (It is offered in the region first and only sent south if it is surplus to regional needs.) The revenues from sales of this surplus help keep Northwest rates lower.

    At other times, such as in winter and at night, California power plants generate more electricity than local consumers need. When temperatures in the Northwest are low and heating needs rise, the power flows north.

    The Pacific Northwest Consumer Power Preference Act of 1964 concerned Northwest governors that the Intertie would not siphon Northwest power. The Preference Act required that BPA sell firm energy first to electric utilities in the Northwest and that only power surplus to Northwest needs would be offered for sale outside the region.

    Because the two regions can back each other up, thanks to the intertie, less power has been generated at fossil-fuel power plants. And the regions do not have to build expensive power plants to meet peaking needs: plants that would be idled much of the time. Fish benefit from the intertie too. Money that southwestern utilities pay for power from BPA helps finance fish and wildlife restoration projects in the Columbia River Basin.​

    May 2001

    California enjoys the greatest benefit from the Intertie.
    In the last 15 years, for example, the Northwest
    has sent more electricity to the Southwest -- mostly to
    California -- every year than it has received. Power
    generated in the Northwest has saved Southwest utilities
    more than 2.8 trillion cubic feet of natural gas that would
    have been burned in power plants.

    Earlier that August, on the 14th, Congress approved
    the Public Works Appropriations Bill of 1965 -- the
    coming fiscal year -- with $42.2 million for Bonneville
    and $3.3 million for the Bureau of Reclamation to construct
    the federal portion of the Intertie, which is in
    Oregon. The California portion would be built by California
    utilities, primarily the Los Angeles Department of
    Water and Power. In all, the construction cost would top
    $700 million.

  19. Nov 27, 2016 #18


    User Avatar
    Gold Member

    My midwest to east coast scenario: five times greater distance ie 1000 miles, not across the sparse plains of Canada but across the US, fed by 35% capacity factor wind, 5% discount rate, maintenance of 1000 miles of line.
  20. Nov 27, 2016 #19


    User Avatar
    Gold Member

    LCOE is a cost to deploy caculation. An investor needs to know both cost *and* return on investment. For wind generation, after reaching 10% of load or so, as in Texas, the answer to the investor is increasingly, 'zero, or maybe you must pay people to take your wind power'. The reason investors continue to invest is that the government pays them $23 per MWh, for now, which covers the losses.

    Last edited: Nov 27, 2016
  21. Nov 27, 2016 #20


    User Avatar
    Science Advisor
    Gold Member

    I know this thread is about DC ties but to be sure there's no misconceptions when talking about "power flow directions"; power will "flow" where it's consumed through what ever wire paths that are available. Cost of this consumed energy is all done by regulators.
    Market Admin.jpg

    See how these regulators work from United States Electricity Industry Primer - Department of Energy
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted