Re: Plug in solar panels - announced

What happens when a consumer buys two of them, and plugs them into the same extension lead? Mains fails, but each will see a live connection on its output. Are they smart enough to tell that is not actually the mains they can see?

If the consumer now unplugs the extension lead where it connects to the house, what happens now (with both PV sets back feeding each other)?

If the householder plugs them into a circuit that is already running at full load, the MCB at the circuit origin won't see the additional current being added after it - you could (although unlikely[1]) get a situation where a cable is overloaded and the MCB does not protect it.

On a more practical level, will they last long enough to repay their cost?

I think that each inverter will try to increase its output frequency. Normally the mains is stiff enough to resist this and each unit detects this and tracks the mains. If two or more are connected together without the mains they will all try to increase the frequency without constraint. This will very quickly trip the over-frequency cutouts in all of them. John

This may well be the case... but since there is no BS document that describes the requirements for the things and how they should be tested, it does at this stage seem to be a bit of a lottery depending on what you buy.

I don't know how they work, but this is what I'd do, at least for a first stab...

You need to lock to the mains phase on startup and maintain this lock so that power can be transferred.

It's not a normal rotating generator, and the power is supplied as an approximate sinusoid comprising pulses of varying width (PWM) with a fixed voltage of maybe +330V or -330V. These get filtered by an inductor.

The mains is normally at a very, very low impedance. If it disappears, that impedance will rise and be readily detectable from the shape and voltage of the pulses. If there's a short instead, ie no mains but low impedance, that could also be readily detected. And as you're doing all this using a microcontroller, you might as well do frequency checks as others have suggested.

That's called a 'hybrid inverter'. It does power transfer solar -> grid, solar -> battery, battery -> grid, grid -> battery depending on conditions.

But you need a DC bus to the battery, and if the battery is 48V as most are that's fairly chunky gauge wire to carry the current. If the battery isn't co-located with the solar (which typically on the roof, but the battery might be in a garage at the bottom of the garden; lofts are not good for either inverters or batteries) then it's better to put an inverter at each end and make the bus AC mains instead. It also means you can reuse an existing connection there might be, eg a buried cable to the garage.

If you end up with two boxes, ideally you want some comms between them - could be wired current transformers, RS485, ethernet, RF, wifi, cloud, ...

Not really, power electronics are cheap nowadays. This is all off-the-shelf stuff.

Theo

This is true and even as an electronics engineer, I'd have to do a fair bit of head scratching to build such a beast. It is extremely non-trivial.

How about one of these?

All it would need, is it's default frequency to be wide of 50Hz, then in the absence of 50Hz, it shuts to say 40 Hz. It then sees 10 Hz difference, and so switches generation to off.

Constant current, not constant voltage. You can supply maximum X amps, so you adjust your output voltage to achieve it. The grid might be an infinitesimal input impedance[*], but a CC supply will happily supply a dead short all day long without breaking.

(in fact, I've been using a bench PSU as a milliohmeter by setting the CC to

If the mains supply changes, the output voltage changes to maintain the CC. If the sun goes in and your achievable amps drops, you reduce the CC. Control loop 101.

Theo

I think you are missing my point... it is not that there are no technical ways of detecting the difference, it is that there are no BS standards specifying the requirements and what the limits are and how the things should be tested.

So you are buying stuff with an unknown feature set, that may or may not be safe and interoperable with other manufacturers kit.

One would like to think that a bit more thought has gone into the introduction of this class of products into the market than a government minister having a knee jerk reaction to unexpected energy supply and price disruption.

There are, G98 specifies anti-islanding disconnect times. Section 9:

These are, however, very generous (seconds/minutes). The proposal is to change this to milliseconds. It is possible that existing inverters already exceed the standard so they will work without changes, especially if they are sold in other markets with less generous standards.

There is. There's a study been done about how 'plug in' solar relates to UK domestic installations (anti-islanding, RCD blinding, wiring system considerations). The contract end date was 27 February so the report should have been submitted by now. I don't know if the report is something that's published or just internal research for DESNZ:

Theo

The existing microinverters (such as Enphase) take the AC mains up to the roof, rather than bring the solar DC down to a single larger inverter. There doesn't seem to be a concern that a string of those will 'fool each other' into thinking the mains is still up, when they're islanded?

That's no different from an overhead power cable coming down to a street, where several of the houses have a Classic Inverter. There's nothing special about microinverters, and I've have expected reports of the lights staying on for such streets and zapping linesmen if them holding up in the absence of a grid was a problem.

Theo

Might be easier to use the DC to spin a motor to drive an AC generator :) Or use a mechanical component to achieve the waveform ?

Don't worry - I know my limits

It will be interesting to see how they adapt continental spur mains design to UK ring main design.

Also how they prevent plugging multiple inverters into a single ring main. [Had a random drive by thought that they could use mains Ethernet protocols to identify if there are any other active units in play.]

Also realised that power to my loft uses an old electric shower cable repurposed when the shower was removed. This could give some more headroom if a solar unit was connected through there.]

It will be tempting to play.

I am also intrigued that the pay back period is supposedly around 4-5 years. This seems a lot shorter than a roof mounted solar array, even when there are FIT payments involved.

Noting that the benefit is supposedly much greater if a battery is involved, although I assume that this would make the installation much more complex, including safely siting the battery.

Cheers

DaveR

Those already exist, as a motor-generator set.

One format, is as a phase converter (single phase input, three phase output). Some people, perhaps in the USA, have those in their shop (when three phase power is not available, and the shop has three phase motors in it). When you set up a shop, some retired machinists and the like, buy used equipment that runs off three phase and put it in their home shop.

A phase converter, lasts for around ten years of continuous usage, and you oil the bearings once a year if you own one. It's not clear whether a home user can replace a bearing set in those.

There is also a "lab bench" setup that engineering students use for gaining experience regarding mains. That's around 5 horse or so, motor and generator coupled together. And the university would buy those as a "unit" for the lab. Those have a couple potential failure modes. One unit was ruined in my year in the motors lab, but it wasn't someone in my class that did it.

For example, all the engineers with an Electrical parchment, should have "synchronized a generator to the grid" at least once. And if those students are literate and can read the instructions, they can do that without ruining the equipment :-) With engineering students, this is not always a given (<eye roll>).

The only person to receive an electrical shock in motors lab, he works for Ontario Hydro :-) Not many graduates work in the power industry, they're always looking for people.

If you want to do DC to single phase or DC to three phase, there is nothing wrong with doing it that way. It's clumsy. It isn't 100% efficient (but neither is the inverter solution 100% efficient). The inverter doesn't have any moving parts. If you're running the motor at 12V, the input current will be rather large. A higher DC voltage may be more practical.

With a brushless DC motor, you would not have brush or commutator servicing to worry about. Those are available up to a couple horsepower or so. I don't know if anyone has made any "stupidly large" brushless DC motors :-)

In the diagrams for brushless DC, you might notice they have semiconductors to switch the current flow, which is why very large currents would lead to significant heat output.

Paul

The reason the devices remain energized for "seconds" interval is not a lack of awareness. It was done for the concept of "ride-through" during disruption. In other words, not making transient grid conditions any worse than they already are.

Certain kinds of UPS, in home situations, can make grid-side transients worse. The UPS can in effect, "amplify" the amplitude of a transient during such events. This can blow equipment in your home, when you thought you were protected. Depending on how you design line-interactive equipment, they can do naughty things with the right shape of incoming transient.

If you remove the ride-through "feature", then of course you can have response times that are a lot shorter. It is a question for grid stability, if having a million of those go offline in the same millisecond is a good idea.

Paul

I'm not entirely jesting. Supposing we installed solar panels, then closed up the house and went for a 6 months trip round the world. How feasible would it be to buy a secondhand mining rig off ebay and join a bitcoin mining group? (It doesn't have to be bitcoin, ofc.)

Just to answer my own question:

Mining generates about 5p per KWh.

Suppose we installed a 1KW panel (plug in) at a cost of say £1,000.

That will produce say 900 KWh per year, which at 5p will be worth £45. That's not a great return on £1000, and that's before you add in the cost of the computer equipment, the cost of replacing the solar panels when they get damaged, etc.