6 ms·

Some time ago, I worked for a defunct energy company that provided demand-side relief in exactly this scenario - when total system load outstrips total system c

Some time ago, I worked for a defunct energy company that provided demand-side relief in exactly this scenario - when total system load outstrips total system capacity, we'd begin taking actions on behalf of end users, not the utility. Lights would turn off, generators would spin up, fridge temperatures would be raised. ERCOT (Texas) is an interesting case in that it can't import power from other states as easily - it has its "own grid" at 60Hz, but not necessarily in phase with either of the other two major grids in the country (there's eastern US, western US, and "most of Texas"). So to import power, it first has to convert grid-scale amounts of energy from AC to DC, then back to AC, which incurs a pretty significant loss.

cameronh90 6y ago
Preface: I know very little about power grids, but had a couple thoughts. In the UK, there is a system of load shedding that is tied to the grid frequency. My understanding is that some industrial users agree terms where they get cheaper rates but if the demand outstrips supply, they are first to be cut. This load shedding is implemented by components that monitor the grid frequency, and if it drops below a fixed value, it disconnects. It seemed a nice system to me as in theory it operates just based on the principles of an AC power supply. Secondly, the UK has its own grid but is linked up to the EU grid with asynchronous HVDC connectors. We get a lot of our energy though those. Is this inefficient?
- martinald 6y ago
  No, not really. As far as I'm aware HVDC links are more efficient than AC links (even with AC/DC/AC conversion).
  - jcampbell1 6y ago
    It looks like a tradeoff. The only place in the world doing a significant amount of long distance transmission is China. Their wind and coal resources are in the west, and demand is in the east, and transporting coal by rail just further increases air quality problems in the east. Based on what they are implementing, HVDC make sense at 800km+, and AC for shorter runs. I think it comes down to HVDC requires less conductor due to the AC skin effect, weighed against cost/loss for DC->AC. https://en.wikipedia.org/wiki/Ultra-high-voltage_electricity_transmission_in_China https://en.wikipedia.org/wiki/Ultra-high-voltage_electricity...
    - frozentoad 6y ago
      Manitoba Hydro's total DC transmission distance is +3,000km over 3 runs. https://www.hydro.mb.ca/corporate/facilities/bipole_lines/ https://www.hydro.mb.ca/corporate/facilities/bipole_lines/
    - themoop 6y ago
      Hydro-Quebec is also doing at least one 1500km hvdc to the US and multiple 1000+km lines to the dams in the north. https://en.m.wikipedia.org/wiki/Hydro-Qu%C3%A9bec%27s_electricity_transmission_system https://en.m.wikipedia.org/wiki/Hydro-Qu%C3%A9bec%27s_electr...
      - nikisweeting 6y ago
        If anyone is curious about this HVDC line and the Quebec grid in general I gave a small talk about it ;) https://github.com/pirate/quebec-power-grid-talk https://github.com/pirate/quebec-power-grid-talk It's one of the only multi-terminal HVDC lines in the world, most of the others are just point-to-point. They also literally short out the power lines with DC to warm them up and melt the ice off during ice storms, pretty crazy stuff.
    - coryrc 6y ago
      You also have reactive losses with AC.
    - bsder 6y ago
      The Pacific DC Intertie was the biggest in the world until the Three Gorges stuff. https://en.wikipedia.org/wiki/Pacific_DC_Intertie https://en.wikipedia.org/wiki/Pacific_DC_Intertie More interestingly--it dates to the Kennedy Administration.
    - martinald 6y ago
      Yes, exactly. But for example interconnecting Houston with AZ or NM power plants, I think HVDC will have fewer losses than AC. Could be wrong thoguh.
      - 6y ago
  - rocqua 6y ago
    AC transmission has worse power loss per unit distance. But has very little 'base' cost. HVDC has better marginal loss , but you pay a loss up front for conversion
- bombcar 6y ago
  Load shedding exists in the USA - however industrial customers who can shut off probably already have given the rise in spot prices. I know the Datacenter we were in years ago would be asked to go to generators during CA power crisis.
  - gonzo 6y ago
    Zayo just sent notice that they’re cutting over to generators in the Texas area
- don-code 6y ago
  I don't know that ERCOT has any similar program - here in the US, for areas that run a load-shedding system, load is typically dropped after 0.2 seconds of 59.5hz or lower (nominally 60hz). Some consumers may choose to cut themselves off prior to that point, to prevent damage to equipment at the lower frequencies. ERCOT (and others) also implement peak demand charges, where some portion of a large user's overal yearly rate is determined by their usage on days when the grid is under the most stress. Usage for that particular day ends up setting rates for the entire year, which makes it not only risky but also very costly.
  - Scoundreller 6y ago
    The UK had that too for demand charges. https://en.wikipedia.org/wiki/National_Grid_(Great_Britain)#Triad_demand https://en.wikipedia.org/wiki/National_Grid_(Great_Britain)#... A small industry popped up predicting when the 3 half-hour snapshots would be taken so their customers could minimize their use during those periods to minimize their costs for the year. https://www.havenpower.com/news/what-you-need-to-know-about-triad-season/ https://www.havenpower.com/news/what-you-need-to-know-about-...
- aidenn0 6y ago
  HVDC is the only reasonable way to transmit power underwater. There's loss due to the conversion, but less than a short underwater run or a long (500+ miles) high tension AC run.
- eigenvector 6y ago
  The technical term for what you're describing is under-frequency load shedding (UFLS) and it is part of most modern power systems around the world, including all of the North American grids. It is an important safeguard against system collapse, however, the goal of system operators is to avoid reaching this stage. In fact, resolving contingency events without interruption of "firm load" (that is, loads that have not previously agreed to be interrupted) is an important performance metric for any grid operator. UFLS is mostly intended to buffer transient loss of power supply (for instance, tripping of several large generators). If the grid operator knows they are facing an inadequacy of generation supply and all resources have already been called in, they will start to shed load under manual operator action to avoid UFLS activation. Before doing this they will declare an emergency which, generally speaking, requires all generators to make best efforts to supply as much energy as they can to the system. Think of it like automated emergency braking on a car vs driver braking. AEB is great, but if you can already see that you're gonna hit something, just hit the brakes right now instead of waiting for AEB. By the time UFLS kicks in, you're already in dire straits and have only moments before reaching an unrecoverable state.
  - namibj 6y ago
    In the central european grid, there is actually a market where companies can offer frequency stabilization service to the grid, which exists in both directions. It doesn't matter if the combat a lack of power in the grid with shedding loads or spinning up another generator. This exists on a few different timescales, and the faster-responding contracts trigger at higher frequency deviations. Primary operating reserve triggers at +-200 mHz and needs to be fully available within 30 s and for up to 15 min, secondary operating reserve has to take over (and be fully available within 5 min), and the tertiary operating reserve has to be fully available within 15 min. The entso-e map: https://www.entsoe.eu/data/map/ https://www.entsoe.eu/data/map/
    - mncharity 6y ago
      Curious that the map doesn't match its legend. Powerplants are drawn as substations. Ah, a PDF matches the legend.
- NicoJuicy 6y ago
  They are not "cut", those industrial users ( mostly in chemistry) get paid to power down. They could receive more money to power down than to work, so they do that when it's interesting enough.. The electricity net is really fragile. You need to guess averages to the system as a whole, so there's not much flunctuation. Renewable energy is an extra variable too. Some additional reserves can be powered on/off to make the system stable. Ps. If you cross those averages by 5% for example, you get a hefty fine, so monitoring your power outage is really important. Source: worked at a company monitoring this for multiple countries.
Aloha 6y ago
ERCOT does have several HVDC links, which when I checked a couple hours ago were exporting power. Also, I believe that with modern technology HVDC is more efficient than synchronous AC connections (largely because of higher voltage interconnects)
- Scoundreller 6y ago
  Did ERCOT build them bidirectionally though?

cameronh90 6y ago

Preface: I know very little about power grids, but had a couple thoughts. In the UK, there is a system of load shedding that is tied to the grid frequency. My understanding is that some industrial users agree terms where they get cheaper rates but if the demand outstrips supply, they are first to be cut. This load shedding is implemented by components that monitor the grid frequency, and if it drops below a fixed value, it disconnects. It seemed a nice system to me as in theory it operates just based on the principles of an AC power supply. Secondly, the UK has its own grid but is linked up to the EU grid with asynchronous HVDC connectors. We get a lot of our energy though those. Is this inefficient?

martinald 6y ago
No, not really. As far as I'm aware HVDC links are more efficient than AC links (even with AC/DC/AC conversion).
- jcampbell1 6y ago
  It looks like a tradeoff. The only place in the world doing a significant amount of long distance transmission is China. Their wind and coal resources are in the west, and demand is in the east, and transporting coal by rail just further increases air quality problems in the east. Based on what they are implementing, HVDC make sense at 800km+, and AC for shorter runs. I think it comes down to HVDC requires less conductor due to the AC skin effect, weighed against cost/loss for DC->AC. https://en.wikipedia.org/wiki/Ultra-high-voltage_electricity_transmission_in_China https://en.wikipedia.org/wiki/Ultra-high-voltage_electricity...
  - frozentoad 6y ago
    Manitoba Hydro's total DC transmission distance is +3,000km over 3 runs. https://www.hydro.mb.ca/corporate/facilities/bipole_lines/ https://www.hydro.mb.ca/corporate/facilities/bipole_lines/
  - themoop 6y ago
    Hydro-Quebec is also doing at least one 1500km hvdc to the US and multiple 1000+km lines to the dams in the north. https://en.m.wikipedia.org/wiki/Hydro-Qu%C3%A9bec%27s_electricity_transmission_system https://en.m.wikipedia.org/wiki/Hydro-Qu%C3%A9bec%27s_electr...
    - nikisweeting 6y ago
      If anyone is curious about this HVDC line and the Quebec grid in general I gave a small talk about it ;) https://github.com/pirate/quebec-power-grid-talk https://github.com/pirate/quebec-power-grid-talk It's one of the only multi-terminal HVDC lines in the world, most of the others are just point-to-point. They also literally short out the power lines with DC to warm them up and melt the ice off during ice storms, pretty crazy stuff.
  - coryrc 6y ago
    You also have reactive losses with AC.
  - bsder 6y ago
    The Pacific DC Intertie was the biggest in the world until the Three Gorges stuff. https://en.wikipedia.org/wiki/Pacific_DC_Intertie https://en.wikipedia.org/wiki/Pacific_DC_Intertie More interestingly--it dates to the Kennedy Administration.
  - martinald 6y ago
    Yes, exactly. But for example interconnecting Houston with AZ or NM power plants, I think HVDC will have fewer losses than AC. Could be wrong thoguh.
    - 6y ago
- rocqua 6y ago
  AC transmission has worse power loss per unit distance. But has very little 'base' cost. HVDC has better marginal loss , but you pay a loss up front for conversion
bombcar 6y ago
Load shedding exists in the USA - however industrial customers who can shut off probably already have given the rise in spot prices. I know the Datacenter we were in years ago would be asked to go to generators during CA power crisis.
- gonzo 6y ago
  Zayo just sent notice that they’re cutting over to generators in the Texas area
don-code 6y ago
I don't know that ERCOT has any similar program - here in the US, for areas that run a load-shedding system, load is typically dropped after 0.2 seconds of 59.5hz or lower (nominally 60hz). Some consumers may choose to cut themselves off prior to that point, to prevent damage to equipment at the lower frequencies. ERCOT (and others) also implement peak demand charges, where some portion of a large user's overal yearly rate is determined by their usage on days when the grid is under the most stress. Usage for that particular day ends up setting rates for the entire year, which makes it not only risky but also very costly.
- Scoundreller 6y ago
  The UK had that too for demand charges. https://en.wikipedia.org/wiki/National_Grid_(Great_Britain)#Triad_demand https://en.wikipedia.org/wiki/National_Grid_(Great_Britain)#... A small industry popped up predicting when the 3 half-hour snapshots would be taken so their customers could minimize their use during those periods to minimize their costs for the year. https://www.havenpower.com/news/what-you-need-to-know-about-triad-season/ https://www.havenpower.com/news/what-you-need-to-know-about-...
aidenn0 6y ago
HVDC is the only reasonable way to transmit power underwater. There's loss due to the conversion, but less than a short underwater run or a long (500+ miles) high tension AC run.
eigenvector 6y ago
The technical term for what you're describing is under-frequency load shedding (UFLS) and it is part of most modern power systems around the world, including all of the North American grids. It is an important safeguard against system collapse, however, the goal of system operators is to avoid reaching this stage. In fact, resolving contingency events without interruption of "firm load" (that is, loads that have not previously agreed to be interrupted) is an important performance metric for any grid operator. UFLS is mostly intended to buffer transient loss of power supply (for instance, tripping of several large generators). If the grid operator knows they are facing an inadequacy of generation supply and all resources have already been called in, they will start to shed load under manual operator action to avoid UFLS activation. Before doing this they will declare an emergency which, generally speaking, requires all generators to make best efforts to supply as much energy as they can to the system. Think of it like automated emergency braking on a car vs driver braking. AEB is great, but if you can already see that you're gonna hit something, just hit the brakes right now instead of waiting for AEB. By the time UFLS kicks in, you're already in dire straits and have only moments before reaching an unrecoverable state.
- namibj 6y ago
  In the central european grid, there is actually a market where companies can offer frequency stabilization service to the grid, which exists in both directions. It doesn't matter if the combat a lack of power in the grid with shedding loads or spinning up another generator. This exists on a few different timescales, and the faster-responding contracts trigger at higher frequency deviations. Primary operating reserve triggers at +-200 mHz and needs to be fully available within 30 s and for up to 15 min, secondary operating reserve has to take over (and be fully available within 5 min), and the tertiary operating reserve has to be fully available within 15 min. The entso-e map: https://www.entsoe.eu/data/map/ https://www.entsoe.eu/data/map/
  - mncharity 6y ago
    Curious that the map doesn't match its legend. Powerplants are drawn as substations. Ah, a PDF matches the legend.
NicoJuicy 6y ago
They are not "cut", those industrial users ( mostly in chemistry) get paid to power down. They could receive more money to power down than to work, so they do that when it's interesting enough.. The electricity net is really fragile. You need to guess averages to the system as a whole, so there's not much flunctuation. Renewable energy is an extra variable too. Some additional reserves can be powered on/off to make the system stable. Ps. If you cross those averages by 5% for example, you get a hefty fine, so monitoring your power outage is really important. Source: worked at a company monitoring this for multiple countries.

Aloha 6y ago

ERCOT does have several HVDC links, which when I checked a couple hours ago were exporting power. Also, I believe that with modern technology HVDC is more efficient than synchronous AC connections (largely because of higher voltage interconnects)

Scoundreller 6y ago
Did ERCOT build them bidirectionally though?