Q:Excess solar generation with no neighbors - Page 1
Forum section: electronics / general

author: rea5245
author_rank: Frequent Contributor
timestamp: January 26, 2021, 07:29:48 pm
content: Hi,I'm looking into getting solar panels for my house and I have a question that none of the installers have been able to answer.On sunny days, I'm likely to produce more power than I use. The excess goes back to the grid (and I get credits for that) and the installers talk about how it can be consumed by my neighbors.But I live in a lakefront home where my neighbors aren't around in the winter. Even in the summer, they're only around on weekends.So what will happen to that power? How far can it travel? Does it stop at a transformer that just covers a handful of houses? Or can it go all the way to the center of town, where someone is sure to use it?If it can't travel far enough to get to someone who will use it, does it have to get shunted to ground? Sure, I'd still earn credits from the electric company, but I wouldn't be reducing fossil fuel usage (beyond my own consumption).I would think this would also be an issue in a neighborhood where everyone has solar and everyone is generating excess power.Thanks.- Bob

author: Ian.M
author_rank: Super Contributor
timestamp: January 26, 2021, 07:39:38 pm
content: Distribution transformers aren't inherently directional. Any excess power you generate gets fed back into the (probably) 11.47KV medium voltage local grid the other side of the pole-pig feeding your property.Its a major issue if *EVERTYONE* is generating excess power in rural areas as the distribution transformers are tapped to pre-adjust the voltage to partially compensate for the voltage drop at the end of long feed lines. Revers the power flow in the line, the direction of voltage drop is reversed, and everyone at the remote end can experience mains over-voltage that may exceed the permitted high voltage limit, so all the grid tie inverters are required to 'throttle back' during high line conditions, so cant actually feed in the excess power you generate if everyone else is trying to do the same.

author: jfitzgerald
author_rank: Newbie
timestamp: January 26, 2021, 07:55:48 pm
content: Bob,Any excess power from your panels flows into the distribution line feeding your house. Every watt that flows out can flow to any other consumer of power on that line or indeed other lines. It can flow 100 km or more even across transformers, but of course the farther it flows the more is "lost" as heat to the environment due to resistance in the wires. The power you add to the line reduces the load on other generators in the system and in most cases allows a fossil fuel based generator to throttle back a little, thereby reducing carbon emissions.Of course as more and more solar and wind comes on line utilities need to scramble to figure out how to efficiently store that energy use later since the peaking generators can only throttle back so much. Adapting grids around the world to these new distributed energy resources is a colossal problem that a lot of smart people are working on.

author: rea5245
author_rank: Frequent Contributor
timestamp: January 26, 2021, 07:58:42 pm
content: Sounds good. Thanks!

author: tszaboo
author_rank: Super Contributor
timestamp: January 26, 2021, 08:20:01 pm
content: If you generate more power, than what you use, you will increase the voltage.Current will flow from your solar panels to everything else. That's the only way for currents to go, they always go from the higher voltage to the lower voltage.Your transformer for "your street" wants to generate 110V for example, while your solar panel will make this 111V. Transformers are bidirectional, so the power will go into that, and eventually the power will go somewhere, where it is being used. There is no difference whatsoever between your panel, and the local nuclear power plant (for example), both of them deliver power this way.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 27, 2021, 03:56:15 am
content: Quote from: Ian.M on January 26, 2021, 07:39:38 pmIn another thread discussing using grid tied power systems for local power when the grid goes down several warned that using a generator as the synchronizing signal to fool the inverters safety disconnect would result in generator failure. If all (or most) grid tied inverters have this throttle back feature what is the mechanism that kills the generator?

author: f4eru
author_rank: Super Contributor
timestamp: January 27, 2021, 05:53:05 am
content: Quote from: CatalinaWOW on January 27, 2021, 03:56:15 amIt's not designed for that.GTI are designed for a grid which provides extremely low impedance and an extremely rigid phase you can't influence locally, and which can eat the power you throw at it.A generator is not that at all. It's phase can be influenced quickly, and it has a much higher impedance, and more importantly, what should it do with this excess power coming init has absolutely no load for it, so the voltage will just rise extremely fastThey are both not designed for this use at all, and the closed loops in both of them to regulate phase and voltage can do absolutely anything in this misuse case. (probably it just goes unstable and the whole system pumps and oscillates in and out of protections and regulations until something fails.)Really, connecting 2 generators together, and expecting them to behave? Who does that ??

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 27, 2021, 06:16:14 am
content: Quote from: f4eru on January 27, 2021, 05:53:05 amSo what are the specs that would make it reasonable? If the impedance is really that low there is no point to the load dumping feature. Line voltages in the US are allowed to rise by 10-15 volts, which means for a kW backfeed impedance is about an ohm. Or for a more reasonable 10kW backfeed an impedance of 0.1 ohm. Phase error is a different thing, but there are really simple solutions to slow phase wander. Mass in a flywheel for example.It seems that this use case is of wide interest, but no one has addressed it. And we connect generators together all the time. That is what area grids are called. It isn't trivial, but it isn't a stupid thing to do or an unsolved problem. Also Honda and others make provisions to connect generators. So the only stupid thing is connecting two dumb, small generators without thinking.

author: Berni
author_rank: Super Contributor
timestamp: January 27, 2021, 08:23:33 am
content: It is indeed possible to connect multiple generators, its called the grid, but it is far from simple.A careful balancing act is constantly taking place to put the exact amount of energy into the grid as the loads on it are using. The consequence of producing too much power in that case is NOT a rising voltage, instead all the spinning generators on the grid start putting less of a load on whatever is spinning them, causing them to start speeding up and rising the frequency above the 50/60 Hz, storing the excess energy in the form of kinetic energy. The voltage is regulated separately by various means (adjusting rotor magnetizing current in generators, switching transformer taps etc).For solar/wind this causes a bit of an annoyance with there unpredictable nature since extra capacity in coal plants has to be kept on "hot standby" where the generator is spinning synchronized and ready, so that it can start generating at the flick of a switch to cover any dips.Small solar setups don't really care about any of this. They simply watch the rigid low impedance voltage of the grid and push current back into it in sync. They just push out however much current they could extract from the solar panels. Its the grids job to dial down the beans on those coal plants enough to make room for the extra power.If you hook up a garden verity 3HP generator on there then that generator is the only one doing regulation. If the generator is doing full wack running a 2kW load and then the solar inverter contributes 1kW the load on the generator just goes down to 1kW and from its point of view the load simply reduced as some of the current is now coming from the solar inverter. But then say the load drops to 0.5kW, yet the solar inverter is still pushing out its 1kW while the generator can't produce less than 0kW, now there is an excess of power in your DIY grid and things are about to go wrong, the frequency or voltage is going to go out of control and possibly damage something before the solar inverter realizes something is very wrong and shuts down.Then there is also the detail of electronic inverters not being designed to work together being forced to work together, Some small generators have electronics to generate a cleaner 50Hz AC from a DC generator, and these electronics could also get confused and do something bad when suddenly fed backwards.Just like the real power grid such a home grid needs energy storage to stabilize it. So you need a bank of batteries to store excess power and release it again when needed. Hence the DC output solar systems followed by an inverter to make it into AC on demand.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 27, 2021, 09:33:43 am
content: Quote from: Berni on January 27, 2021, 08:23:33 amAn excellent description of the problem. But assumes extreme stupidity on the part of the solar controller. Something I was willing to assume. Even though it is obvious that they aren't completely stupid because they don't raise the voltage to infinity when the grid goes down.My question arose when a comment was made that these controllers actually have some smarts in them to reduce output as voltage rises.Stability of these control loops is a valid concern, but it is not in the nature of engineers to say the problem is difficult and throw their hands up and walk away.I haven't been motivated enough to dive in myself. I recognize that it is a big job and I would have a lot of learning in yet another area of expertise. But I am surprised no one has taken it on. The market for this is not trivial. Those of us who aren't so far out in the boonies to force investment in full off grid systems, but who do endure relatively frequent power outages.

author: f4eru
author_rank: Super Contributor
timestamp: January 27, 2021, 10:35:45 am
content: QuoteWhy do you say nobody has a solution ?There are many solar inverter solutions with inbuilt islanding (most of them with battery storage, which is kind of a must in an island to absorb startup current etc... ).

author: Berni
author_rank: Super Contributor
timestamp: January 27, 2021, 11:08:33 am
content: Quote from: CatalinaWOW on January 27, 2021, 09:33:43 amYep typically there is a voltage window where solar inverters are allowed to operate in, as soon as it goes out of that range it has to shut down. They don't really have any voltage regulation, just a "shit something is wrong" trip point. But when you have a dinky little poorly regulated generator on there you might have weird interactions that confuse the safety systems, or even confuse the generators voltage regulation circuitry to start swinging wildly around. The solar inverters feeding the same line are all the same kind of current source pushing power in, so it reacts predictably when the grid disappears. They are specifically designed and tested to behave correctly in that situation. Two solar inverters on the same line won't falsely see each other as being the grid and continue running.If you do want to run your house from solar, batteries and mains simultaneously you simply buy the correct inverter for it. They are smart enough to store excess power in batteries and dynamically allocate the power between solar, battery and grid as appropriate. If the grid goes out then it simply uses battery power instead. This is a more expensive solution and is not needed in a lot of cases. Its simpler to just sell all excess solar power and then buy it back when needed, effectively using the grid as a free zero maintinance battery. Its only if you are not allowed to sell power, or get very little money for sold power that it becomes financially viable to maintain your own battery bank.If you just want the batteries to have a UPS for your house then you can just go buy a big ass rack sized datacenter UPS on the used market (plus new batteries likely) and wire that into your distribution panel.

author: richard.cs
author_rank: Super Contributor
timestamp: January 27, 2021, 12:44:11 pm
content: Quote from: CatalinaWOW on January 27, 2021, 06:16:14 amA typical 3 kW petrol generator might have an output impedance of tens of Ohms, likely very inductive, with a voltage regulator whose control loop reduces the output impedance to say 100 mOhm. Unfortunately it only reduces the output impedance within its loop bandwidth which might be 10 Hz or less, so for any fast changes the impedance is much higher, but also the control loop has limited range - once it saturates (field winding is at max or minimum current) the impedance will come up again in an exciting non-linear way. How this interacts with the control loops in the solar inverter is anyone's guess, itcouldbe made to work, but a solar inverter that expects an infinite grid with a stable resistive impedance is likely to barf. And this is before we even get to frequency effects.Quote from: Berni on January 27, 2021, 08:23:33 amWell, with the throttle fully closed, as would be expected when the mechanical governor responds to overspeed, the engine could absorb some power., I'd guess 0.5-1 kW could be lost to the various mechanisms - pumping losses would dominate, but valve train losses and any cooling fans might make significant contributions. Still the situation where the power excess is more than the generator can absorb could easily occur, and who knows how the governor response time and even it's nominal frequency accuracy would interact with the frequency limits programmed into the inverter. Most likely the generator would spend little time within the frequency window where the inverter is allowed to operate.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 27, 2021, 09:57:33 pm
content: Quote from: richard.cs on January 27, 2021, 12:44:11 pmAttacking the deficiencies of the generator does not address the fundamental desire. If I could have a battery powered 60 Hz oscillator that could keep the solar setup kicking out power it would meet my needs. With 4-8 hours of power a day I could keep the freezer frozen, the well accumulator pumped full (and maybe even water the garden a bit) and a few other necessities.It would really be nice to have a generator and the solar system play nice together. I could have some amount of power around the clock without twice a day reconfiguration by me and without using more fuel than absolutely necessary. And I don't doubt that it would require some sophistication, though it isn't obvious why one of the inverter based generators wouldn't be able to deal with many of the issues with appropriate software. But that isn't the selling feature. The feature is being able to put the capital investment in solar panels to work when the grid isn't.

author: Berni
author_rank: Super Contributor
timestamp: January 28, 2021, 06:21:17 am
content: The reason direct solar to AC power without batteries is not done is because this puts you at the mercy of the unstable solar panel production.As soon as a cloud comes by the panel output might drop dramatically, possibly to under what the inverter needs to maintain proper mains voltage, so in that case it would be forced to shut off. And how does it turn on again? Do you have to annoyingly press a button? Or does it make an automatic attempt to restart after a certain delay, only to shut off again if the load is still too large for the panels to support. There are also loads that take very large starting currents such as motors (universal motors being particularly violent here) and since inverters only really have enough energy storage in its capacitors to last it a mains cycle or two (to let it do PFC) means that will also likely trip it into overload.All of this can be solved by simply giving it a battery to provide the energy storage for dips and peak power for surges in load. As i said inverters that do this already exist, if the goal is to just provide a bit of fallback in case of the grid being out (Rather than minimizing grid consumption to a minimum) then you can simply hook up a much smaller battery to the inverter. Instead of a whole rack of batteries just give it one or two lead acid truck batteries and set the priority in the inverter to be solar > grid > battery.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 29, 2021, 02:44:17 am
content: A few "daytime backup without batteries" inverters are available, they use capacitors for energy storage. Presumably, it would make sense to make smart controllers for large loads to automatically switch them on/off as available solar production varies.

author: gnuarm
author_rank: Super Contributor
timestamp: January 29, 2021, 09:19:08 am
content: Quote from: jfitzgerald on January 26, 2021, 07:55:48 pmExcept that it isn't actually flowing back up the various levels of distribution. It flows away from his house, but is immediately used on the first level unless the situation Ian mentioned actually occurs and there is an excess of generation locally. Other than in that rare case the electricity generated simply displaces electricity from the higher levels of distribution rather than feeding back into them.QuoteYeah, it's not really so big a problem since distributed generation generally requires less distribution than concentrated generation. The "hot" generators Berni mentioned aren't for issues with the availability of solar or wind which don't stop or start on a dime, rather they fluctuate much more gradually. The "hot' spare generating capacity is about failures in the grid which is required no matter what the power source is.Even better is localized storage. Batteries are getting better and new types specifically suited for grid storage are coming on line. Google Vanadium Flow batteries. One amazing aspect is while the watts are determined by the size of the battery, the Whr are determined by the size of the electrolyte storage tank! That's a lot like a fuel fired engine! In fact, the main reason they installed battery backup in Australia a couple of years ago was to prevent so many people being impacted from grid failures. Now if they have a problem the 100 MW battery kicks in and the grid stays up! In addition they use it to arbitrage electricity, buying at times it is cheap and supplying it when the demand is high, helping prevent the grid failures while lowering the cost of electricity.

author: Berni
author_rank: Super Contributor
timestamp: January 29, 2021, 10:15:19 am
content: Well hot standby generators are not just there for failures. On the rare occasion something really bad happens you can still quickly shed some load by causing a intentional blackout in less critical areas. The demand itself can be very dynamic.For example the UK has a particularly pronounced issue with it. They are a smaller power grid while its population loves there standardized tea time, so as soon as the clock ticks over people everywhere plug in there kettles. Then later in the day a large potion of the population watches the same TV show or the same sports event, as soon as the commercials come on everyone goes to the toilet, the synchronized flushing putting a sudden huge demand on the water supply causing pumps everywhere to kick on. To combat this they already start up pumped hydro plants into "discharge mode" before these known events hit.

author: gnuarm
author_rank: Super Contributor
timestamp: February 01, 2021, 01:30:11 am
content: Quote from: Berni on January 29, 2021, 10:15:19 amI don't know the specifics of how or why power is generated the way it is, but I've never heard anyone suggest there is such abrupt changes in demand that require having generators on "hot" standby for second level time responses. Generation capacity is already capable of responding in a sufficient time simply by the normal controls on all generating equipment. They don't run every piece of generating gear at 100% all the time. That would not match supply to demand. Each generator matches supply by matching the frequency. If the load increases the frequency will start to drop and the generators respond by ramping up power output. Likewise a drop in power demand causes the frequency to rise so the generators respond by lowering output.When the various generators are reaching their 100% capacity additional generating capacity is brought online to maintain spare capacity. When the usage drops below some percentage of capacity some units are taken offline.I do recall someone mentioning that the tea kettles in the UK are a lot more power hungry than in the US and there is some synchronization to commercials on TV. It is hard to imagine handling this requires specific generators already running at essentially zero generating levels. I think what was being described was simply the way the entire network is normally running at below 100% capacity. While the actual cost of the system would be the same in either case, the flow of money would be entirely different to have 95% of your generating billing at nominal rates and to have the remaining 5% of capacity billing at 2 minutes every 15 minutes of use rates or however often commercials are.It would be a great application for battery backup. I've been following vanadium flow batteries and they are the up and coming battery technology for grid storage with the added benefit of providing exactly this type of demand leveling capability.

author: Ian.M
author_rank: Super Contributor
timestamp: February 01, 2021, 02:30:40 am
content: https://en.wikipedia.org/wiki/Dinorwig_Power_StationSpinning & synched: 0 to 1320 MW in 12 seconds, 1728MW in 16 seconds, initial response 5 seconds. 5 hours reserve at full output.And yes, the 12 second response is use to cope with the 3KW tea kettle problem. Its not exceptional for over 6 million households to be watching a particular 'soap', and Dinorwig is only sufficient for approx 9% of them to put the kettle on when the commercials or closing titles roll.

author: Berni
author_rank: Super Contributor
timestamp: February 01, 2021, 07:34:00 am
content: Not all power plants are so agile to quickly ramp up and down.These pumped hydro plants are specially designed for fast response since they are an excellent way of regulating the grid. Not only can they quickly step in to fill gaps in production but also suck up any excess production back into storage.The grid itself can store a little bit of energy in the form of 'flywheels' that the spinning generators themselves are, but beyond that its up to active regulation of production to keep things balanced. All the power plants indeed get throttled up and down with demand, but most of the large high power plants are very slow to do so. Yes the generator does just magically become "harder to turn" as load goes up, but the thing that is doing the turning has to also start producing more torque. So you need to open the valves to let more steam into the steam turbines at the good ol coal plant, but there is only so much extra steam stored in the piping and boilers. They have to throw more coal in to actually produce more of it and this is a pretty slow loop as the boiler has to build temperature. Similar thing with nuclear plants, they ramp the power up and down slowly due to safety. So the grid operators try to keep these big but cheep plants running at the highest capacity possible. Rather throttling down more expensive types of plants such as natural gas, those plants are basically giant stationary jet engines with generators strapped to them, so they can throttle them up much quicker by simply squirting more gas in there. Hydro plants can also throttle quickly, but are still slower than pumped hydro due to not being designed for that sort of agility.But in general the grid gets its stability from simply being very very large so that load fluctuations average out quite a bit. It also means that if one generation block of a power plant unexpectedly goes offline due to a fault (Entire plants don't tend to go offline all at once unless something really bad happens) this is only a tiny part of the grids total production so it is easier to handle. These days we also have DC links between grids, so a grid can ask other grids around to help them out during bad transients.Europes largest grid had a bit of trouble some years ago where some countries ware slacking off on production, causing the grids frequency to be under the spec for something like over a month. There are also occasional disturbances on this gid:https://notrickszone.com/2021/01/10/european-power-grid-narrowly-misses-widespread-blackout-as-frequency-drops-suddenly/https://www.next-kraftwerke.com/energy-blog/who-is-disrupting-the-utility-frequencyThese are the sort of events where blackouts are employed as a lifeline to quickly drop load and recover it. Parts of the grid can even split off to prevent most of Europe going dark if something really bad happens.An example of how wrong this can go is the huge multi-state blackout that happened in the US in 2003. A transmission line failure ended up eventually causing a cascading failure as other lines got overloaded and opened breakers, causing power plants to shutdown..etc