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Increased risk of grid instability ?
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adam2
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PostPosted: Mon May 29, 2017 4:25 pm    Post subject: Increased risk of grid instability ? Reply with quote

It must be stressed that this is not the same as a shortage of capacity. If you wish to comment on possible capacity shortages, please add to the existing thread here http://www.powerswitch.org.uk/forum/viewtopic.php?t=26255

Some experts believe that that the UK grid is at increased risk of instability, this is not the same as lack of generating capacity, indeed the risk is greatest at times of light load.

The potential problem is the growing amount of generation that is connected via static inverters, and the decline in traditional steam turbine driven alternators.

Consider the "classic" grid system with a large proportion of steam turbine plant. Any modest increase in load resulted in a drop in frequency. In the very short term, of less than a second, the mechanical inertia of the rotating turbine and alternator provided extra power by converting some of the rotational energy into output power.
Within a few seconds, the turbine governor would react to the drop in speed by opening wider the steam valve to the turbine, and thereby restoring normal speed and frequency.
In some cases the extra output was available for but a few minutes until the water level in the boiler dropped to a certain level. That few minutes was however crucial in ensuring stability, it give time to increase output elsewhere or to start OCGT plant.

Consider now a more modern grid system in which a large proportion of energy is from static inverters, typically connected PV arrays or wind turbines.
Such installations have an output absolutely fixed by the wind speed or sunlight intensity, and simply can not react to a drop in frequency by increasing output even slightly or short term.
A drop in frequency will of course increase output from other plant to an extent, but not perhaps enough in the future.

Or put simply, 1GW of steam turbine plant can produce a fair bit more than 1GW short term in response to a drop in frequency.
1GW of wind or solar can not.

I again stress that this not the same as lack of generating capacity in relation to load, indeed instability is more likely at low load since wind and PV are then often a greater percentage.
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vtsnowedin



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PostPosted: Mon May 29, 2017 5:32 pm    Post subject: Reply with quote

This is an example of what I have always said will befall a grid once the percentage of intermittent renewables rises to somewhere in the range of twenty to thirty percent (exact figures would vary based on whether calculating average yearly production or just peak instantaneous production).
At first glance and first cup of coffee I'd suggest the solution might be having a number of combined cycle gas turbine plants scattered around the grid that spend their days running and fully operational but at some low level of say ten to twenty five percent of capacity. These would then be slaved to the grid flow so they immediately opened up to as much as full power if and when necessary to keep the grid stable.
Of course the unit cost for the production of these plants would be much higher then if they ran on a normal demand response schedule and the extra cost would have to be reconciled with the cost of the renewable energy and perhaps the carbon emissions reduction value.
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Pepperman



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PostPosted: Tue May 30, 2017 6:35 pm    Post subject: Reply with quote

I don't see it as a major issue because at the same time as building up variable renewable energy sources we will also be building up very fast responding demand sources.

With millions of EVs plugged in to chargers there will be many GW of very finely controllable demand. You could add millions of water heaters to that too giving yet more GW.

Not only would that demand be fast responding but it would also be at known locations so you could use it to balance more localised problems.

I expect you could even use it to reduce wear and tear and fuel consumption in conventional plant by keeping that plant at a constant load and varying demand in response to fluctuating renewable supply rather than varying fossil supply to fluctuating demand and renewable supply.
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Pepperman



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PostPosted: Tue May 30, 2017 6:54 pm    Post subject: Reply with quote

I should note that smart charging is only found in a small number of test vehicles at the moment.

However as far as I can make out it's the vehicle that is the main charge rate controller, not the charging point. As high spec modern vehicles are now typically connected to the mobile networks and in an increasing number of cases get firmware updates over the air, it might even be possible to introduce this kind of functionality retrospectively.

With any DSR technology, the incentives for the owners need to be pitched right or you won't get the necessary uptake, but I suspect that a combination of low cost off-peak electricity along with a share of the not insignificant grid service revenues would tempt most.
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adam2
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PostPosted: Tue May 30, 2017 7:46 pm    Post subject: Reply with quote

Strictly speaking it is not the inherent variability of wind and PV that is the problem. The wind or PV output in a few minutes to a few hours time may be readily forecast to a good degree of accuracy by a computer program that "looks at" the near term weather forecast and based on previous experience calculates the likely near future output.

If it is forecast that wind will drop from say 6GW to 5GW in the next hour, and then drop further to 4GW in a few hours, it is a relatively simple matter to schedule other generation to run.
A drop of say 1GW in wind over an hour is no different to an increase in load of 1GW over a similar time.

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.
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PS_RalphW



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PostPosted: Tue May 30, 2017 8:11 pm    Post subject: Reply with quote

It takes milliseconds to switch a battery from charging to feeding back into the grid. Anybody who is charging overnight will not mind if it takes five or ten minutes longer. 100,000 electric cars on domestic chargers could balance 1Gw and not even notice.
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BritDownUnder



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PostPosted: Wed May 31, 2017 9:56 am    Post subject: Reply with quote

This has been talked about in Australia also, particularly South Australia because it has a high proportion of wind energy (which Rupert Murdoch's newspapers don't like) and has recently had statewide power cuts (which Rupert Murdoch's newspapers blamed on wind power).

This link explains a bit about the background to this issue http://reneweconomy.com.au/managing-frequency-modern-electricity-system-85447/

The company I work for installs gas turbines, steam turbines and wind turbines and were recently asked to tender for three 40MW gas turbines to be installed in Adelaide with the state government specifically asking for high inertia generators so it does seem that this issue is being taken seriously.

I did not study this issue much at university but over periods of several seconds grid frequency will decline more slowly in response to a increase in demand for power if the generators have a higher inertia. Basically you are extracting some of the rotational energy in the shaft to continue to provide power at the expense of a drop in frequency. Who knows what will happen when wind and solar make a larger contribution to the grid. It could be that solar farms in particular get routinely run at only 90% capacity so that there is something 'left-in-the-tank' to provide a quick response. Not knowing much about control theory I don't know if this will increase instability. I know that some relays can trip on rate of change of frequency being too high.

Going back to solar energy if you look at this website http://anero.id/energy/solar-energy and on the chart tick only the Barcaldine solar farm it looks like this solar plant out put keeps being switched suddenly in steps of about a megawatt very quickly, possibly in response to sudden power demands. Barcaldine is right at the end of a very long power line. On the other hand this plant is very new and may just be being tested i am not sure. I understand that there a twelve utility scale solar farms coming online this year in Australia, mainly in Queensland.
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Pepperman



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PostPosted: Wed May 31, 2017 11:23 am    Post subject: Reply with quote

adam2 wrote:
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.
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PostPosted: Sun Jun 04, 2017 9:55 pm    Post subject: Reply with quote

Pepperman wrote:
adam2 wrote:
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?
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fuzzy



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PostPosted: Mon Jun 05, 2017 8:27 am    Post subject: Reply with quote

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.
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PostPosted: Mon Jun 05, 2017 12:42 pm    Post subject: Reply with quote

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.
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careful_eugene



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PostPosted: Mon Jun 05, 2017 1:14 pm    Post subject: Reply with quote

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.
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adam2
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PostPosted: Mon Jun 05, 2017 1:25 pm    Post subject: Reply with quote

fuzzy wrote:
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.
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PostPosted: Mon Jun 05, 2017 5:15 pm    Post subject: Reply with quote

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.
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Pepperman



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PostPosted: Wed Jun 07, 2017 9:46 am    Post subject: Reply with quote

vtsnowedin wrote:
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.
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