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NERC sounds alarm on solar tripping in ‘sobering’ summer reliability report


> But the biggest long-term risk may be the unexpected tripping of solar resources during grid disturbances,

This is entirely the electricity grids own doing. They told everyone who wanted solar that they must use inverters that are super safe and self disconnect if anything unexpected happens. The regulations were written super tight, with frequency limits, harmonic limits, reactive power limits, rate of change of frequency limits, residual current limits, DC injection limits and vector shift detection requirements, presumably because rulemakers were hoping that having a massive thick rulebook would make rooftop solar too expensive. Unfortunately for them, all the complexity is pushed to software, and when that's written once it can be deployed lots almost free.

Fast forward 10 years, and this stuff is widely deployed, meeting all the very strict criteria. Except the grid only stays within those strict limits during the good times. When the grid is stressed, for example low frequency due to insufficient generation, it will go outside the frequency limit, and all rooftop solar generation statewide will disconnect all at once. And now the grid has lost gigawatts of solar production, making a minor incident far far worse. The end result is a cascading failure and within a matter of seconds half the state has a blackout.

Smart rulemakers back in the 2010's would have dropped islanding protection, and vector shift protection (they actively destabilise the network). They would also remove underfrequency protection (again - destabilises the network) and slacken substantially undervoltage protection.

The only protections important to leave in are rate of change of frequency (upwards only), overfrequency, and overvoltage.

What's ironic is there is a more sensible requirement they could have required to make rooftop solar cost more and add substantially to grid stability. They could have required solar systems have a load line equivalent to coal. That pretty much means if the grid frequency decreases, the solar systems must output more power. But solar can't output more power than the sun provides. So in turn, to meet this requirement all solar systems would have to be constantly throwing away say 10% of capacity, so that it could be delivered if and only if the frequency suddenly dropped.


Perfect comment on the topic. With that said, newer inverters do have “grid fault ride through” capabilities where they will continue to operate during excursions from nominal operating windows, and some regulators are more hip to it than others.

Grid forming inverters (versus the traditional grid following) are at the finish line of being proven, and paired with distributed storage providing ancillary services (including frequency response), regulators simply need to be dragged to the present to solve for this (distributed energy orchestration, rapid system segment isolation and recovery during transient events [“self healing” for the marketing folks], etc). This is one of those tug of wars between very conservative regulators and a quickly innovating industry (DER power controls).


The Enphase IQ8 is now in general release, and is grid-forming.

During sun, a site can island with perfect 60Hz AC power from the ASIC-driven IQ8. No battery –– and certainly no grid –– needed.


I spec’d my 21kw system with IQ8+s for this reason. It’s future proofing distributed generation by being software defined (Enphase’s Ensemble system). Highly recommend them to anyone considering residential solar, and go for as big of a system as you can (and your utility will allow, if they limit based on usage history; tell them you’re buying an EV if necessary).


This made me think "What about SMA Solar, they are supposed to be a technology leader?"... and yes, they do have all the grid stabilization features (and an island feature in at least some models, not mentioned there):


Interesting. Can it sense when the grid is present, and can it match phase?


Last year, I had the opportunity to talk to some PG&E-associated contractors [0] who were replacing the distribution transformer serving me. I asked them whether they trusted the anti-islanding protection in everyone’s inverters and whether they would like me to turn off my main breaker. They laughed and said that they couldn’t care less. They were going to intentionally short the secondary circuit if they were doing dangerous work, and if anyone’s inverter was trying to energize it, that was the inverter’s problem.

I’m genuinely unsure what purpose anti-islanding actually serves.

(My inverter is moderately intelligent and formed a one-house microgrid all by itself. That being said, this capability may be at odds with helping the grid survive a major disturbance. When my inverter decides to disconnect from the grid, it cannot support the grid regardless of what its software and the regulators think.)

[0] By which I mean line workers at a company that PG&E contracts with to maintain their distribution network. Apparently PG&E outsources real work. Go figure.


> I’m genuinely unsure what purpose anti-islanding actually serves.

Scroll up to read about the HNer required to carrying $1M in insurance for having more than 10kW in solar on their roof, which is just...insane. Solar is mired in bullshit to make it seem dangerous, make it look ugly, make it as complex and expensive to install as possible. That's how you end up with regulations requiring what looks like an electrical substation on the side of your home, covered in neon-colored labels. Gotta make sure someone knows the system is "RAPID SHUTDOWN EQUIPPED" from 100 feet away!

Electrical grid operators want you to buy electricity, not make it for them. Their nightmare is becoming "just" a grid.

Their biggest nightmare, however, is you realizing that you no longer need them at all. Lots and lots of people, especially those out in suburban or rural areas, could easily go off-grid these days. Homes are better insulated, heatpumps are quite common, solar costs a fraction of what it used to, lithium ion battery prices are crashing and LiFePo batteries are getting commonplace, etc. So what's a utility to do? Push microinverters that require a grid connection and so on.


> I’m genuinely unsure what purpose anti-islanding actually serves.

I think it mostly prevents inverters from trying to destroy themselves feeding into a dead short, or creating weird instabilities - the grid segment is either up or down from the inverter's point of view. Any sane inverter will detect it's trying to feed a dead short and shut down. And if you're grid tied and your grid segment is down, it looks like a dead short.

It's a useful enough filter for people who understand power systems and power system work, because, as your linemen pointed out, if safety is a question, the workers simply create a bolted short across all the phases and neutral, and you're not simply not going to make that ring.

And, yes, migratory line workers are a thing. I don't get it, but relatively few people I've talked to out here (Idaho Power territory) actually work for Idaho Power - it's mostly migratory contract workers doing the power pole inspections and such.


> I’m genuinely unsure what purpose anti-islanding actually serves.

Because a typical solar inverter can't handle transient loads. It just spits out whatever power is coming in from the panels. It can't shed if too much is coming in and it can't magically create power to fix a shortfall. The power has gotta go or come from somewhere.

If you have a battery you can soak the transient load and disconnect from the grid and operate as an island but without the grid to act as a buffer for the load the inverter is useless on its own.


Shedding excess power is very, very easy for solar, especially as compared to any other power source. The MPPT can move away from the maximum power point. (Compare to, say, wind or hydro where moving a turbine away from its optimal speed and torque can be quite destructive without extreme care. I visited a small hydro installation with a monstrous space heater to dump power in the interval between when a load disappears and when valves can adjust to reduce the flow of water.)

I am curious how SolarEdge’s inverters reduce output, though. It’s not fundamentally hard, but the inverter does not have appear to have a particularly high speed data connection to the MPPTs, and I haven’t found the underlying mechanics documented anywhere. I’m guessing that the inverter pulls the incoming voltage down such that the MPPTs hit their preprogrammed output current limits and curtail production.

(SolarEdge’s system can’t just dump excess power into a battery — their common configurations have the battery behind a DC-DC converter with considerably lower capacity than that of the inverter’s output. The DC-DC can’t fully absorb the solar string’s output if the sun is shining and an island’s load goes away. The battery itself likely could, at least for a little while, but there isn’t any way for the power to get there.)


Indeed, came here to write basically this. Solar tripping off is solar obeying the rules set on them by the utility that is "You will trip if anything so much as looks funny."

And then a few decades later, they figured out this was a terrible idea and the newer 1741-SA/CA Rule 21/etc stuff with far greater ridethrough for reasonable disturbances showed up, but I don't think it's the majority of solar installs.

I should probably get a Grid Guard code for my SMA inverters and use the revised ridethrough rules - my utility handwaved at it when I installed stuff, and it's using the CA Rule 21 stuff, but they now have far more explicit guidelines on what they want programmed in the inverters.


It’s such an own goal to take massively distributed and resilient power generation with no inherent single point of failure and just give it one anyway.


The generation isn't naturally massively distributed and resilient, that's the thing. Assuming that the grid will always be there and stable simplifies the problem quite a bit: all the inverters have to do is sync up with it and then dump all the power they've got into the grid. If you want the solar generation to be the grid, rather than just boosting it, suddenly you have to deal with all the hairy control theory problems involved in grid operation and stability, except that you have to do it using millions of generators rather than thousands - and the behaviour of those generators is closely correlated, so you still have to deal with problems like a large proportion of generation going offline at once as well as the extra complexity of a much bigger system.

Old-fashioned spinning generators also naturally act to stablilize the grid by pulling energy out and dumping energy back in through their inertia as necessary to counteract changes due to varying demand and supply. Solar and wind don't inherently do that: if you want similar behaviour from them, that requires additional software and sensing, and obviously for home solar that would have to be done in a distributed fashion which complicates matters further. (The grid is not in fact at a single frequency and phase everywhere, and can develop all kinds of interesting and undesirable oscillations.)


With modern inverters the ability to provide reactive power for voltage support could be signal driven. It doesn’t inherently require overbuilding or sacrificing production all the time, just when needed. Further, it could be compensated thus justifying the lost production.




So noone was bothered to run a simulation to see the second-order effects of their regulations on the grid?


The conversation went something like this: "well as long as all the solar disconnects when we have an emergency, then we are left with a grid without solar. And we already know we can handle that fine".

The rulemakers were expecting solar to be a failure and there to never be anything more than a handful of geeks with it installed. The problems only become major when 1+% of statewide generation is solar. 1% is (very approximately) the amount of generation an electricity grid can suddenly lose without failure.


I'm having trouble with the power industry terminology.

I /think/ they mean there's going to be more demand, which could overload parts of the distribution grid, causing its overload protections to trip. Is that right? The solar connection is not clear aside from it contributing more power on the grid when it's sunny.


Solar cells generate DC, but this is converted to AC with power electronics: the inverter is the main one. These are all just very high wattage semiconductors, but like all semiconductors, they can be destroyed by a transient event like a lightning strike, or a high fossil-fuel power plant dropping offline. To prevent the power electronics from being destroyed when this happens, they are protected by safety systems which "trip," and isolate them from the grid.

There are loads of designs for grid isolation, but most are big circuit breakers, and break the connection by rotating the conductor out of contact, powered by gravity or a spring.

So I think you're basically right, and right to be confused. The solar connection of the risk isn't really clear. But as far as I can tell, they're concerned about a positive feedback loop, where a thermal plant dropping offline will cause a solar plant to trip, which might cause other solar plants to trip. Perhaps solar plants' power electronics are more delicate, and more likely to trip, than thermal plants' switchyard gear. Again, it's not totally clear.


A lot of the rules for disconnecting solar were written when solar wasn't expected to be a major part of the grid, so the logic has them trip because that was the sane default.

That's no longer the sane default and the utilities have been slow walking software and hardware updates to fix it.

...and now that might be a problem.


Worse still are areas where laws were enacted that _required_ you to get your solar from your utility vs your roof. Or there's a "minimum service fee" even though you don't consume any energy. In the early days you could "sell" your excess energy back to the grid. Now you can't and worse still, they don't know how to handle the infrastructure they have to meet the ever changing climates of the western United States.


Not exactly. The concern raised in the article is that rooftop solar can disconnect off the grid automatically causing a small problem to be cascade to a larger one. has a ton of good info on this


> I'm having trouble with the power industry terminology.

:-) In the UK, the term "prosumer" is the official term for an electrical system which produces to and consumes power from the supply. (example )

(In the past it was widely taken to mean "a customer who wants to buy very high-quality technical products or equipment", )


First context: prosumer: Portmanteau of "producer" and "consumer".

Second context: prosumer: Portmanteau of "professional" and "consumer".


i fail to see what your comment adds


The tricky part is not much "power production" but "keeping the grid at a certain frequency" witch means balanced between consumers and producers.

A classic large-enough but not too large grid is sized like that to achieve a somewhat constant load so their generator have to supply a certain amount of power without sudden high or low loads spike. A small personal example, I have a small domestic p.v. system for self-consumption, when my dishwasher start heating water my micro-grid load suddenly spike +~2kW my inverter take time to produce more, like few seconds, so without a grid or a battery my micro-grid will simply collapse because the frequency goes too low. Similarly when water heating phase is ended there is a sudden -~2kW load and the micro-grid frequency skyrocket. Again without a grid or a battery my micro-grid will simply collapse due to a too high frequency.

Instead a small sudden load change, like a light that turn on or off, have essentially no impact since such so small amount of power vary the frequency of the micro-grid so little that the solar inverter have no issue to keep anything in place.

At a classic grid scale ±2kW are just background noise on the frequency, like for my microgrid the led light turned on and off.

However at a classic grid scale with massive p.v. things start to be far less constant, sudden spike for so many microgrids means sudden spikes projected to the main grid causing much more frequency perturbations. Also Sun power tend to peak really fast witch means that a grid with an actual production of let's say 1GW at 10 a.m. this morning around my latitude suffer a sudden drop in demand, witch means a high frequency peak because that when most p.v. systems in sunny days quickly goes from few hundreds W to few thousands W in a very little time. So the grid need to reduce power quickly, let's say a quick perturbation pass varying much the p.v. production, again a load peak for them, a big drop in frequency they need to boost production in very few seconds to keep up the frequency. The alternative is rolling blackouts to cut most perturbation and keep the overall grid up, something that do not touch much those how have a p.v. system with battery storage since actual inverters are fast enough to keep homes powered on most load cases BUT touch all other citizens...

grid-connected EVs can help quickly absorbing loads and quickly releasing energy BUT for far such applications are just on paper and I doubt we can allow private EVs to talk with the public grid since any vehicle is a mass of proprietary crapware crap built by countless of private parties, many outside national Govs. control, in unknown health shapes, response time, ... such grid is theoretically possible, but practically probably as unstable as without such system, with many smaller rolling blackouts and much higher risks of large scale ones...

Long story short:

- with climate change we know we have to adapt and adapt means change, changes we can't much design up front due to too many variables, witch means we can't build large/complex systems because they solve a PRESENT issue, taking years to be built, and we do not know if such issue will be solved in the same way once we finished such large systems.

- the ideas some from Green New Deal push is: we need to cut such complexity. Witch means making autonomous homes who can be powered locally without a grid, smaller cities with their own local small networks etc instead of nation-wide systems. The very same happen for transportation networks where we start preferring small/medium water and air born systems because we can re-organize them fast instead of rails and roads that take decades to be build and changed.

- the issue here is that PERHAPS we can do so, but very unlikely for anyone, probably only of less than a third of the whole humanity and of course those who are excluded for a reason or another would not be happy of that, and not "being happy" means unrest, revolutions, violence, instability etc that beside the carnage and suffering also impede to build pretty anything for anyone.

Energy grids operators are just some of the party who start understanding the amplitude of such issue and start blowing all whistle they have alerting: "we can't do that, we do not know alternatives to put on the table".

Probably the sole viable alternative we have is PUBLIC only nuclear waiting for tech progress, but so far build such massive nuclear move takes decades.




This is what happens when you have insufficient or no spinning reserve


The Economist had an article on in the May 7th issue with a beautiful picture of inertia storage being delivered.[1]



If you expect your solar panels to power your house when the grid goes down, make sure you check that. They most likely will not, unless you've added a battery and switches.


And an inverter!


The battery is largely optional. The critical bits are the switch, aka “microgrid interconnection device”, and the inverter’s ability to make a decent sine wave without grid assistance.


Only kind of. Without a battery your power will go out every time a cloud covers the sun, unless you have a hugely oversized array.


This sounds like what utility scale batteries should solve, stabilizing the grid frequency for short periods of time.


Why is solar thermal so rare?


Because photovoltaic got so good & cheap, with no moving parts.

In most locales it’s now more cost effective to heat your water with photovoltaic panels & an electric water heater.


Specifically, a heat pump water heater.


So I looked into them because of your comment, and they have a very big drawback of taking a very long time to heat the water. That wouldn't work for me, our water usage tends to be very "peaky" - my system can heat the water almost as fast as you can use it in a shower.

Maybe with a very complicated system with an extra large storage tank, and perhaps integrated with the home heat (so a single heat pump). I feel like the tech isn't quite ready yet, and I don't like being a pioneer.

It seems like a no brainier if you heat with electricity, but not quite ready yet for those who heat with natural gas. Right now it seems better suited for southern climates.


That usually is true if your electricity is generated by natural gas too and you're comparing it against a gas HWH

Rough math is

NG -> Electricty is 50% efficient thermally.

A heat pump can have a COP of 4.2.

100 * .5 * 4.2 = 210% efficient, thermally

A hot water heater powered directly by gas is ~100% efficient thermally.


Not at industry scale, I don't think. At least according to these guys:


Because nowadays p.v. is cheap enough to heat enough water with far less maintenance costs and potential issues, at least that's my choice: I was undecided for a bit of time if going p.v. only or p.v. + thermal and in the end:

- with p.v. I'm formally less efficient BUT in winter/cold but still sunny days my p.v. peak at a certain time for a certain time and my water heater start full power when there is enough energy, a solar thermal system take time to reach a sufficient temperature to make water (glicole) flow;

- almost no external maintenance, just panels and wires, no leaks or freeze risks, no pipes etc;

- no extra energy to make heat transfer fluid flow, witch means no pump, actually my heater is just an isolated "bowl" with a heat pump + classic electrical resistance and a big spiral pipe inside, so I have even no pressurized tank issues (water expansion) nor legionella problems etc;

- for my needs 300l of waters heat also in the winter enough 100% on p.v. except if more than two days without Sun, overall cost is very limited, potentially I can use the optional thermal solar integration with a classic wood burning stove to heat water in emergency conditions.

Why bother for thermal solar? With a big pool I understand but except for such case in witch solar anyway can't suffice for more or less large part of the year...