How is excess electricity generated dissipated? I know that the grid is balanced, etc. But given that there will be momentary over-generation at some point over the whole grid where does any excess electricity go? For example, in the event of a significant sudden power outage across the Northeastern US (or any other large geographical section of the grid), how does the grid shed the sudden excess electric flow? Or is there a gigantic copper wire running into the ground somewhere? I am not an engineer, but I am a very curious person and would appreciate a focused answer. Thank you.
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1\$\begingroup\$ If a generator runs slightly faster than needed,voltage will slightly increase. Collectively all of the resistive elements on the grid will then dissipate infinitesimally more power. \$\endgroup\$– user1850479Commented Dec 4, 2020 at 18:16
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1\$\begingroup\$ There is no excess. The grid has to be balanced at all times. \$\endgroup\$– user57037Commented Dec 4, 2020 at 18:55
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13\$\begingroup\$ This is a good question, but what it really comes down to is not so much where does the excess energy go, but rather, how does the generation stay matched to consumption, and what is required to keep the match stable over time? I am not enough of an expert to give a good answer. \$\endgroup\$– user57037Commented Dec 4, 2020 at 19:00
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3\$\begingroup\$ I just wrote a rather detailed answer to a similar question over on engineering.SE which you may find relevant. \$\endgroup\$– LShaverCommented Dec 4, 2020 at 19:06
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1\$\begingroup\$ @Alex Energy is not purposefully dissipated wastefully except as an extremely short term emergency measure where response time of the main systems is too slow. Modern methods make this even less likely. Battery storage is increasingly common. Peaking control flywheel systems, such as you suggest, already exist. Large pumped hydro systems that use a water reservoir as a battery are increasingly common. \$\endgroup\$– Russell McMahon ♦Commented Dec 7, 2020 at 1:48
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3 Answers
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A good question. The excess of generator drive power over generator load will cause all of the generators on the grid to start speeding up.
For a small over-power, there will be time for the mechanical steam valves and water valves to start closing, and reducing power to the generators, which will slow them back to nominal speed.
For a large over-power event, let's say there's a generating station at the end of a long feeder, and the feeder opens for some reason, then a set of large (very large) resistors will be switched onto the station as a safety load, until the power input to the generators can be throttled down.
A few years ago, I read the report of a test of such a bank of resistors, where the old cast-iron ones (which absorb the energy by heating up) were replaced by sheet stainless steel ones (which were much less massive and had to dissipate the power to the air). I'll see if I can find it again and link to it. The test went on for 30 seconds, which sounded like the length of time they expected could pass before they shut down the steam input to the turbines.
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\$\begingroup\$ Newer grid-tie inverters are designed to help stabilize the grid a bit by backing off when the frequency increases. Obviously that only helps when solar generation is significant, which is the case in some states during sunny days. \$\endgroup\$ Commented Dec 4, 2020 at 18:57
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\$\begingroup\$ Neil: Thanks!! I now started to understand the question. If you can find the report and send it I would be grateful. I'd love to find some pictures of the very large resistors. I'll try on line. This is very interesting and something I never knew about. Thank you! \$\endgroup\$– BMA1200Commented Dec 4, 2020 at 19:24
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\$\begingroup\$ Hi Again Neil. I tried to look on line for photos of such resistors but cannot find any. Mostly I have found manufacturers and most have to do with small electronic circuits. Any idea where I can find a couple of photos. Its interesting that this question has been in my mind for a couple of years but just retired I can now follow up on stuff. \$\endgroup\$– BMA1200Commented Dec 4, 2020 at 19:39
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3\$\begingroup\$ Better images if you have IEEE access: ieeexplore.ieee.org/document/1601490 \$\endgroup\$– esilkCommented Dec 4, 2020 at 21:16
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1\$\begingroup\$ Synchronous Condensers are also used to balance the supply of reactive power into the grid. Doesn't warrant a complete answer, but thought I'd mention in case you're interested.en.wikipedia.org/wiki/Synchronous_condenser \$\endgroup\$ Commented Dec 25, 2020 at 3:01
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Further to Neil's answer:
For tiny amounts of over-power (e.g. when a machine is switched off) the excess power is consumed by the remaining connected loads.
During over-power, all loads are exposed to a small amount of excessive voltage, and thus they generally draw more current and dissipate more power. These tiny fluctuations are usually not regulated out. Rather, grid statistics provide for a generally constant power consumption over short terms, and thus constant voltage.
With a sufficiently high number of loads (large neighbourhoods...) the fluctuation is a statistical effect with small variance. The voltage variation is small enough so as not to cause damage. With a low number of loads (a portable generator), controlling and maintaining the output voltage for fluctuating loads is paramount.
As an aside, active loads (regulated loads, like LED, controlled motors...) generally respond differently and do not draw proportionately more current. Their power-draw rigidness does not ameliorate the over-voltage.
The duration of the over voltage is determined by the lag in controlling the primary power (up or down), and/or the lag in detecting this and shunting-in dummy loads.
The primary power lag is small for non-mechanical power supplies like solar cells. Switching off power is as fast as switching an array of power transistors or relays.
For mechanical supplies, like impeller-based generators (steam, diesel, hydro) the lag is a matter of mechanical momentum: how rapidly can you accelerate or decelerate the rotation of a heavy lump of metal.
A whole different question is maintaining the efficiency of power generation. Where does excess generated primary power go (heat, steam, water)?
Primary power delivery is controlled by the primary power consumption (e.g. the amount of fuel burn) times the efficiency or efficacy in transferring the mechanical power to electrical power.
The primary power consumption is relatively more difficult to regulate. For rapid momentary excess of power, primary power is "let off" through by-pass vents in the generator. This however results in an immediate loss of efficiency. For long term power reduction, generators are operated at lower power or they are "taken off the grid".
So back to the question, where does the excess power go?
- into existing loads (connected appliances, esp. passive loads)
- into dummy loads (shunted-in as needed)
- bled off (by-pass of mechanical power)
- regulated out (reduced primary power)
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1\$\begingroup\$ Neil: Thanks. I will search the internet as you suggested. I appreciate the other answers, but they are not as focused on my question - When a major geographic area like the Northeast goes suddenly dark - a massive "local" failure - where does the huge mass of inbound electricity go? I think you have answered my question and will look on line for the search terms you suggested. I am grateful that you understood the heart of my question. For other answers I am grateful but I "get" how the grid evens out daily demand. \$\endgroup\$– BMA1200Commented Dec 6, 2020 at 18:17
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I'll add that often the power companies make deals with customers to make some devices (eg, air conditioners) shut off during periods of "peak demand" (ie, the opposite of what you are discussing). And I suspect that there are cases where devices (industrial water heaters and the like) can be made to come on in periods of "excess power", to balance things.
And note that while "peak demand" periods can last hours, "excess power" can usually be controlled within seconds or minutes, by taking some power sources off-line (as discussed in other answers), so special arrangements to "drain" power are not as critical.
