Inreased risk of grid instability, merged topic.

[vtsnowedin]

The problem with most renewables is that the available output at any instant is absolutely fixed by the available wind or sun.
If the renewable input to the grid is say 10GW, then there is no question of increasing this, even slightly or short term, to compensate for a drop in frequency.
10GW of steam turbine plant would easily produce say 11GW short term, and give "thinking time" of at least a few seconds, and probably some minutes during which hydro power output may be increased (run up time of some seconds) or OCGT plant called for (run up time of a few minutes)

Rotating machinery has inertia which promotes very short term stability, static inverters have no such inertia and promote instability if they form too much of the generating capacity.

Sure but supply is only one side of the equation. If you are a grid controller and you start to see frequency drop because you're on your way to a 1GW shortfall then instructing a few hundred thousand EVs to ramp their charging down will have about the same impact.

That assumes the grid controller has the means to throttle back the charging rate of the plugged in EVs.While probably a good idea that will be implemented eventually has any utility designed and built such a system to date?

[fuzzy]

If anyone thought there was going to be grid instability and power outages, why would they want to take longer to charge their, presumably essential, car and not get to work? Are we going to mandate it? Buy an electric car, range 30 - 0 miles.
Basic low power hybrid cars will be getting >100mpg in the tail end of oil, if the decline is slow enough to allow transition.

[vtsnowedin]

I read a piece of click bait yesterday that opined that along with the end of keys we would reach the end of electric grid blackouts due to the advent of Tesla's wall batteries and plugged in EVs using smart chargers linked to grid controllers. I suppose it comes down to whither you are optimistic or pessimistic about the future.

[careful_eugene]

Isn't this why we have Dinorwig powerstation? It can provide a very quick response to fluctuations in the grid. It's well worth a visit if you're in the Llanberis area.

[adam2]

If anyone thought there was going to be grid instability and power outages, why would they want to take longer to charge their, presumably essential, car and not get to work? Are we going to mandate it? Buy an electric car, range 30 - 0 miles.
Basic low power hybrid cars will be getting >100mpg in the tail end of oil, if the decline is slow enough to allow transition.

We are talking here about grid instability over periods from fractions of a second up to a few seconds. This may result from a lack of traditional rotating alternators that possess considerable mechanical inertia, and thereby smooth out very short term variations between supply and demand.

Nothing to do with outages caused by lack of capacity.

Also in years gone by, the vast numbers of large induction motors connected to the grid helped a bit, again due to mechanical inertia, the modern practice is to use inverter drives on motors, thereby isolating the inertia of the motor from the grid, and perhaps adding to instability.

[fuzzy]

If you have transient high voltages, substations may assume a pylon cable fault and trip safety breakers. Then it's an outage. I don't mind if they want to design EV chargers to draw like damping loads on the grid, although that only helps for dips in grid output. You can't tell them to charge more on high supply voltages. I doubt there is much electrical water heating except kettles nowadays..

It would help if we defined what 'instability' is a problem.

The grid is kept at a long term voltage constant so that:
lots of distributed generators can tip power into it. So that motor and resistive loads like bulbs can give constant power output. So that the grid infrastructure is lightly loaded and doesn't dissipate excessive power itself. So that consumer equipment is not damaged by high voltages eg transformers.

After that, it has variations in AC voltage ranging from radio freq noise right down to several cycles of 50Hz eg if you are on the same mains as a large motor switching on. The variations will be different at any time in all locations due to the local demand and propagation delay of the grid. Anything longer is a surge or dip in AC. Surges in voltage fry things like surge protectors [which are only designed to live for a few over voltages] and mains ethernet transmitters.

I think we will see more unstable mains in the future due to renewables and the decline of resistive loads in industry, heating and lighting, and electrical manufacturers will have to design accordingly. There is nothing to stop manufacturers putting floating batteries in eqpt for supply dips so that TV junkies can get their fix. It would seem critical for medical and 'real time' eqpt such as computers, clocks, recording gear etc.

[Pepperman]

That assumes the grid controller has the means to throttle back the charging rate of the plugged in EVs.While probably a good idea that will be implemented eventually has any utility designed and built such a system to date?

Sure, it's not something they have control over right now but it's being tested. The technology should be relatively straightforward as the on board charger (typically 3.3kW or 6.6kW) can ramp the charge rate up or down very rapidly and most if not all EVs are connected to the GSM network.

What is needed are aggregators who will build up networks of EVs which they can then make available to the grid controllers. That will require the right incentives and the right regulatory environment to happen. It's not clear to me how far away we are from that right now and no doubt it varies greatly between the US and EU and probably within the US and EU as well.

[PS_RalphW]

From a warehouse that I am currently sitting in.

https://midsummerwholesale.co.uk/buy/moixa

Made in the UK offering local grid stabilisation

[BritDownUnder]

I think that only Adam and myself are getting this.

We are not talking about the ability to put more power into the grid on a seconds to hours scale but about a millisecond scale. It's like a flywheel, and unfortunately coal, gas and nuclear plants are like big flywheels that have a lot of momentum and don't slow down very quickly on a millisecond scale. Anything where the generator is synchronous, has a high mechanical moment of intertia and is 'locked' to the grid can do this.

Unfortunately solar and and to a lesser extent wind can't replicate this stabilizing influence as well. The reason being where there are generators which are absent in solar grid-tie inverters they are asynchronous and only follow the grid and can go faster or slower and are not locked to the grid frequency. It could be that pumped storage, tidal or even giant flywheels themselves will cover the problem in the future or something new gets invented that will.

At the moment it is not a big problem in the UK but in South Australia which is very wind, solar and import dependent it is becoming a big problem at certain times and when then synchronous generation suddenly trips.

[adam2]

I think that only Adam and myself are getting this.

We are not talking about the ability to put more power into the grid on a seconds to hours scale but about a millisecond scale. It's like a flywheel, and unfortunately coal, gas and nuclear plants are like big flywheels that have a lot of momentum and don't slow down very quickly on a millisecond scale. Anything where the generator is synchronous, has a high mechanical moment of intertia and is 'locked' to the grid can do this.

Unfortunately solar and and to a lesser extent wind can't replicate this stabilizing influence as well. The reason being where there are generators which are absent in solar grid-tie inverters they are asynchronous and only follow the grid and can go faster or slower and are not locked to the grid frequency. It could be that pumped storage, tidal or even giant flywheels themselves will cover the problem in the future or something new gets invented that will.

At the moment it is not a big problem in the UK but in South Australia which is very wind, solar and import dependent it is becoming a big problem at certain times and when then synchronous generation suddenly trips.

Yes, you are correct.
Sorry if this seems disrespectful to other members, but it does seem to me that most contributors to this thread have not fully understood that we are talking about very short term instability.

The only likely cure is more rotating machinery, flywheels are a distinct possibility.

The behaviour of large grid systems is still not fully understood, perhaps surprisingly. They are too complex for truly accurate computer modelling, and much too big and expensive to build a spare one for experiments.

[fuzzy]

Let's ask a different question then; what problems can you see as a consequence of a less stiff mains supply?

[vtsnowedin]

I think that only Adam and myself are getting this.
.....
Yes, you are correct.
Sorry if this seems disrespectful to other members, but it does seem to me that most contributors to this thread have not fully understood that we are talking about very short term instability.

The only likely cure is more rotating machinery, flywheels are a distinct possibility.

The behaviour of large grid systems is still not fully understood, perhaps surprisingly. They are too complex for truly accurate computer modelling, and much too big and expensive to build a spare one for experiments.

Perhaps you are correct in that. I consider instability to be anything that can knock the lights off line long enough for me to notice or to make all the clocks reset. Of course millisecond flickers here and there if not corrected might well lead to a full outage so it is a matter of degree and duration. What could be more stabilizing then having fifty percent or more of your generation be from fossil fuel powered steam turbines?

[Pepperman]

In the op there's a reference to conventional plant responding in seconds rather than milliseconds but I'm also keen to know what these problems are. Is there any literature out there that you can link to?

I don't know to what extent they're used in the UK but in Germany solar inverters are far from dumb devices and can provide quite sophisticated grid stabilisation services. I would expect that they can respond to frequency fluctuations extremely quickly.

[fuzzy]

The good old mains we know is only generated to a close voltage and frequency range with a low impedance, because of the practical problems of joining ac generators together - not because it's good for the consumer. It has allowed filament bulbs to have a long life and motors to give a predictable power output. Electricians are obsessed with earth resistance, because they have this concept that everything will be OK if you can trip a fuse [except that very few faults involve a short to earth], but I can't recall much about live-neutral impedance. This site:

http://www.acoustica.org.uk/other/mains_Z.html

reckons mains is 0.25 ohms resistive and 0.23 ohms inductive . Combining these gives 0.34 ohms total at 50Hz and 2.31 ohms total at 500Hz if you want millisecond current changes. A high powered shower is 6-7 ohms resistance, and low powered devices will be 100s or 1000s of ohms load.

I can't see them allowing the mains to vary much because the wear and tear on suppliers eqpt would increase [variable tap transformers at power stations, rotors on generators etc]. I think there will be safety disconnects when supplies become less stable.

[adam2]

In the op there's a reference to conventional plant responding in seconds rather than milliseconds but I'm also keen to know what these problems are. Is there any literature out there that you can link to?

I don't know to what extent they're used in the UK but in Germany solar inverters are far from dumb devices and can provide quite sophisticated grid stabilisation services. I would expect that they can respond to frequency fluctuations extremely quickly.

PV inverters don't help, indeed they are part of the problem.
No matter how clever may be the electronics in an inverter, they simply CAN NOT supply more power in the very short term than is being provided by the PV array.
A short term dip in the frequency requires that more power be supplied, rotating machinery can supply extra power instantaneously by utilising inertia.
A million grid tie inverters supplying a similar amount of power cant. From where is this to come from ?