On-grid domestic battery storage

zeupater

Martyn1981 wrote: »

I hope this makes sense, and is correct, as I'm simply regurgitating old news (perhaps 2013ish), but don't pretend to understand it.

So, we all know that inverters now monitor grid voltage, and because the push it up a bit, they should shutdown when it hits 253V. But I recall that in Germany they also have to monitor grid frequency, and if it goes too high, they also have to shutdown to prevent oversupply.

I think I'm right in believing that frequency will go up with over supply and under demand?

Hi

It's pretty likely that a major mitigating factor is to create a slight variance in the 'out of specification' switching voltage & random timer (very short term) on the grid condition monitoring to prevent an 'all or none' switching event involving huge numbers of microgeneration inverters which would impact on over-voltage grid stability leading to blackouts in areas where microgeneration provided a high proportion of supply, which was the initial concern in Germany ...

HTH
Z

orrery

Martyn1981 wrote: »

I think I'm right in believing that frequency will go up with over supply and under demand?

Absolutely. If you have to push a power station to produce more then it tries to rotate faster and "push the waveform forwards" i.e. the phase will try to move forwards (think of cog wheels being pushed forwards) and the frequency will rise.



I suspect that frequency has become a proxy for voltage in this case.


There's nothing to stop inverters doing the same (to mimic the same action), but they don't have to. They can produce whatever behaviour is mandated by the standards (within the laws of physics, of course).


The ideal situation for the grid would be that we all have more short term battery support than we require (power, in kW) so that we can just ride through major events while the grid gets itself sorted out. Which I guess is why we're all here!

mmmmikey

Martyn1981 wrote: »

I think I'm right in believing that frequency will go up with over supply and under demand?

A simple way to look at this is to think of a car driving at a constant speed along a flat road. As long as you stay in the same gear, the engine will rotate at a constant speed. If you start to go uphill, other things being equal the car and engine will start to slow down as the load increases. Similarly, the engine will rotate quicker if you go downhill and the load decreases.

In exactly the same way, if you put some additional load on a generator (for example we all turn our kettles on at half-time) then it will slow down, and similarly if you reduce the load on the generator (for example, I switch off my aluminium smelting plant) then it will decrease.

A fossil fuel generator spinning at 3000 rpm (revs per minute) is spinning at 50 revs per second (i.e. 3000 divided by 60) which is where the 50Hz frequency comes from. So as the load on the grid increase the generators slow down and the frequency decreases and as the load on the grid decreases the generators speed up and the frequency increases.

Going back to the car example, as you start to go up the hill, you can push the accelerator pedal to keep the speed the same, and back off as you go downhill. You are regulating the speed of the car (i.e. keeping it constant) by using the accelerator. In exactly the same way, generators have regulation mechanisms to keep the speed of rotation and the frequency constant.

Of course, there are limits to this. Again, thinking of the car, if you try to go uphill towing a caravan against a strong headwind then it may well be that even with the pedal fully down the car starts to slow down. In the same way, generators get to the point where they reach their limit and the frequency starts to drop off.

So somebody in a control room somewhere can tell if the grid is under pressure by monitoring the frequency. If the frequency starts to drop they can fire up another power station somewhere or do whatever they have to do. The aim is to keep the frequency the same all the time, within fairly close tolerances.

Hope this makes sense.....

Martyn1981

Thanks guys.

orrery wrote: »

I suspect that frequency has become a proxy for voltage in this case.

and yes, as you (and Z) say, the issue here was to find a solution to the potential problem of too much PV.

Long story short, every possible problem with PV was thrown at this board in the past, one of them being that PV would destroy the local grids when it was too sunny, but such comments came at a time when UK (based) inverters had to start cutting out at 253V, and German inverters already had frequency shutdown settings.

Basically, every problem a negative nancy could come up with, had already been identified, solved, and rolled out.

joefizz

Martyn1981 wrote: »

Basically, every problem a negative nancy could come up with, had already been identified, solved, and rolled out.

We'll agree to disagree on this Martyn because problems have arisen, a lot of which were forseen but just ignored for various political and economic reasons (amongst others).


Weve just had a power outage of 4 hours (battery ups fully tested this morning ;-)) and in the course of that Ive found out it there was a power surge the day after I went to america which the dno didnt reveal when I asked. Ive no need to be convinced now of the benefit of batteries, pity my mobile supplier decided not to go with battery backup for their cells, but I digress.


There are places here in NI its still almost impossible to get DNO approval for solar installations because the network wont cope under maximum output conditions. Well the network probably will cope but the restrictions and safety fallbacks wont allow it. Thats a problem, it was forseen, is fixable and can be addressed in various stages from easy (limiting max output - something owners would be serverly peed off about, installing batteries for max output etc right up to digging up the roads and installing thicker cables/building localised storage) but its not.


Im less than 100m away from a substation, which is in turn less than 100m away from a new substation, in between is a school which would have been an ideal place for a large solar array or wind turbine because its beyond the edge of metropolitan planning. It would have made more sense to incentivise them than say our RHI scheme ;-)



Ive only been monitoring this thread since I installed my own battery and had enough data to talk about it with confidence/history/experience and you saw how that went down! ;-)



There are bigger issues coming up (as mentioned in my frequency comments) when renewables reach a critical mass as far as the current grid infrastructure/design is concerned and they are addressable. Addressable in that in some cases unpalatable decisions will have to be made re time of use, smart meters, cancelling of fit contracts etc etc. Most of these are political decisions and as with privatising networks and successive underfunding and running very close to the wire they will be found out and have consequences. Those consequences may include the type of price rises we have seen in the last while (one of the other factors in my deciding to go pv and then pv/battery - borderline economical at the time but prices here have risen over 25% in real terms in that period).


Theres very little that cant be fixed, its all a question of time and money ;-)


Going back to the outage this morning, I had the UPS power up the kitchen ring main and then took an extension lead from the garden wall outlet out to the shed to separately plug in the two freezers out there. Now, I have other methods of doing this but wanted to see how easy/quick that was as Ive decided to not go for an automatic cutover procedure, I want to be in control of it all.
Id set the UPS cutover time for 2 mins so when the power went out it would take 2 mins for the UPS 'plug' to go live. It took more than 2 mins to check the circuit breakers to see why my computer suddenly died in the middle of typing and for my neighbour to reply about if their power was out as well.

Battery was at 51% when it happened (9.6kwh nominal). Wired it all up and let it run. The power came back on... briefly, PV came back online, battery inverter, went into standby for 2 mins before returning to charge from PV (heavy rain so only about 400W being produced). Switched kitchen circuit back to mains power, power goes off again, 2 mins standby, repeat x3 as they switched power back on and off again. Battery down to about 40% with 3 freezers, 1 fridge, cordless phones back on (why I still have one corded phone attached) and one cup of tea made.

The switching back on and off again 5 times was something I hadnt really thought about and to be fair thoughts of going to auto system for some of the circuits did come to mind walking back from the shed with extension lead in the rain and seeing the kitchen lights go off again....;-)


Ive put off thinking about how to charge the battery from generators if the power is off , so will have to start looking at that seriously soon. Thats over 6 power outages/surges this year alone, more than in probably the previous 20 years IIRC! The reason for using the generator for the battery is rather than leaving the generator on for long periods with a load below its minimal/optimal, use the generator(s) to charge the battery fully and tailor usage to that (power going off at 50% battery was a reminder that power going off isnt likely to occur at 100% battery!)

Martyn1981

joefizz wrote: »

We'll agree to disagree on this Martyn because problems have arisen, a lot of which were forseen but just ignored for various political and economic reasons (amongst others).

Sorry Joe, both for your troubles and my poorly written comment. I didn't mean that the solutions had been rolled out to locations, I meant rolled out as in actual production, such as frequency/voltage 'rules' in inverters etc.. Rolled out is clearly a poor choice of words by me.

You are absolutely right that the technology that, shall we say, exists, hasn't necessarily been rolled out in to our communities, but all the solutions exist, whereas the negative nancies liked to point out that nobody had thought of these things, and everything would fail, or go boom!


Bit of a diversion, but long ago I came up with (yet another) one of my imaginary solutions. Basically, we all no that storage will be needed, and that it can help in multiple roles, so I theorized that if all parties with a financial interest shared the costs, then perhaps batteries then (and still today sadly) would become cost effective.

The government has a financial stake, since we face fines for missing CO2 targets, or simply from wanting more demand side investment in RE.

The National Grid (company) has a stake, since limiting supply peaks will benefit them on investment and maintenance.

The leccy supply companies have a stake as they charge households an average cost, but 'suffer' high rates at times, and don't necessarily want PV export at times when supply prices are very low. I'm sure they'd much rather supply more leccy at low price times (wholesale price), and less at peak price times.

The DNO's have a stake, since they too have to build out and maintain to a peak load, so reducing that peak would save monies.

Households/businesses have a stake given the large difference between import and export rates.

So if all parties contributed to the cost of a demand side battery, or even localised DNO battery, then what might not be economical for any individual from that list, might be economical for them all as a whole.


As I'm typing this, I've realised that to an extent this is happening, with DNO subsidised battery trials, and the introduction of leccy supplier deals where they use the batts to balance out peaks and troughs.

Perhaps we are on our way to a much more balanced system, but also perhaps, not soon enough for you. Sorry.

orrery

Martyn1981 wrote: »

Perhaps we are on our way to a much more balanced system, but also perhaps, not soon enough for you. Sorry.

There is no doubt that the government is sitting on its hands and doing very little.


I went on a tour of the Flitwick solar farm (the 1st unsubsidised, plus with a battery stack) and they managed to get a government official to come along to explain the significance.


His answers to questions revealed that the government were not in any way convinced that solar and wind was the answer - their policy seemed to be stuck years in the past. No live plans to introduce contracts to sink excess power at peak times to create (e.g.) hydrogen (etc) for long term storage, so we can get to a point where we have excess generation at peak times. Hopeless.


Par for the course with this government - sit around and wait for the market to sort it out.

1961Nick

joefizz wrote: »

Ive put off thinking about how to charge the battery from generators if the power is off , so will have to start looking at that seriously soon. Thats over 6 power outages/surges this year alone, more than in probably the previous 20 years IIRC! The reason for using the generator for the battery is rather than leaving the generator on for long periods with a load below its minimal/optimal, use the generator(s) to charge the battery fully and tailor usage to that (power going off at 50% battery was a reminder that power going off isnt likely to occur at 100% battery!)

How about buying another battery & keeping it as a standby? You'd probably only have to charge it every couple of months & 2kWh would last you a few hours.

mmmmikey

joefizz wrote: »

Ive put off thinking about how to charge the battery from generators if the power is off , so will have to start looking at that seriously soon.

The low frequency inverter/charger I've just bought for my second battery system supports just this kind of use. Essentially you can simply switch it into "crappy input from a generator mode" alongside "nice clean output from the batteries and inverter" mode and it will do exctly what I think you're after.

I'm not actually doing that, I'm using it in "nice clean imput from the mains mode" with the incoming supply on a timeswitch that aligns to E7 cheap rate hours. The output is set to AC priority mode, so if there's an AC input it passes that straight through to the output as well as charging the batteries, or when the input is off (during the day) it seemlessly switches to battery/inverter mode for the output. It's really just a UPS with batteries, charger and inverter sized to charge overnight and discharge during the day on a planned cycle.

A couple of minor teething issues, like erring too much on the side of caution with the input MCB, which tripped when the charger started at 01:30 this morning. Complicated by the fact it also tripped the garage roller shutter door circuit so I had to manually wind up the door to get to the consumer unit and reset the MCB. Problem solved with separate circuits and a more appropriate MCB - we live and learn

Currently playing with splitting the load in the house between the high-power regualr circuits and the UPS circuits, ensuring I don't overload the latter whilst doing enough to utilise the new UPS batteries and making sure there's enough background load on the regular circuits to keep the PowerVault working.

Plenty to keep me happy for a few days

joefizz

1961Nick wrote: »

How about buying another battery & keeping it as a standby? You'd probably only have to charge it every couple of months & 2kWh would last you a few hours.

I have the 12v lead acid bank topped up by small scale solar, its disconnected from the house but in a proper emergency (if I am about) I can attach an inverter to it and run the freezers etc.


Its the knock on effects of the panels going offline that interests me now. I know we spoke about this earlier in the year but it was theoretical and I didnt want to knock the system off for a day just to test it out ;-) One of the things I mentioned in the past didnt work today so thats that ticked off the list.



As it stands its been a crappy rainy day and with the solar inverter being off for a peak part of the day the batteries are now at below 30%. Tomorrows weather doesnt look great (or the next or the next) so its unlikely now that the batteries will be fully charged again for the next few days at least. Didnt get the chance today to connect the 12v inverter up to the solar input to see if it would still charge the batteries as at the time of the cut there was severe heavy rain and output from them was pretty poor anyway.


The lessons from today are that even with a half charged battery it would probably power the freezers, oil heating etc for a couple of days, certainly enough time to sort out some other solution (or eat all the food ;-)) Time for me at least to start sorting out that other solution (spent this morning researching hacking old bluetooth dongles to set up cordless rear speakers for showing movies in the garden with projector at night without trailing cables everywhere but funnily enough last thing looked at on ebay was low power alarms ;-))


Going back to the other folks posts, couldnt agree more, I used to work in telecoms and IIRC each of the street boxes used to have backup batteries in, probably still do for voice but today the lot went. On mobile could text and probably call but 4g went out the window (and when it did come on it looked like overloaded by everyone looking to get on). So laptop on backup 4g dongle was a waste of time ;-) Only 1000 people affected and a sunday morning but with the traffic lights out as well you could see the failings in infrastructure investment for the last few years, everything run right up to the wires (apologies for that particular pun!).
The upcoming rumoured qe for infrastructure spending might end up addressing all of this, then again, pigs might fly.. ;-)



Another interesting bit was the push to get people to tweet faults (twitter account not manned at weekends) and not registering a fault until 10 people in the same area do an online report or call in. Of course to do an online report you have to have net access, to call in you have to have an old phone book handy or have net access to look up the number... According to talking to my neighbours the fault wasnt registered for the first half hour or so.... ...thats just hearsay though, luckily the local councillors were also cut off and they called in...


Still, all in all a successful test of the ups system so pleased with that. Also pleased that it made me break out the old camping stove to find out the gas canister was empty, would hate to be poking around in the garage at night, in winter, by torchlight trying to find the spare (at least that was full).
Probably time to test the generators as well ;-)