PowerSwitch

Since we have replaced our resistive loads [filament bulbs, heating etc] with switching loads [gadgets, LEDs etc] any supply voltage dips will have a much stronger destabilising effect on the grid, so supply stability is ever more critical. I saw this in action with large backup supplies in datacentres where those paid the big bucks couldn't work out why you couldn't changeover heavy switching loads that were supposedly within the nameplate ratings of the emergency systems.

Edit by admin, this post and those following has been split from an existing thread about a likely shortage of generating capacity. The original thread may be found here http://www.powerswitch.org.uk/forum/vie ... hp?t=22392

We have to watch this with our off grid genny supply and also from the battery/inverter supply. We can't start our saw table when we have a lot of other stuff on, say the washing machine, although if I spin the saw blade first and then switch on the load kick is much less.

Switched mode power supplies (SMPS) are now in very general use and have a number of advantages over older technologies including greater efficiency, lower weight and bulk, and the ability to accept a very wide range of input voltages.
Apart from the obvious application of transforming AC mains supplies into much lower DC voltages for electronics, variable speed motor drives and electronic lamp ballasts are also types of SMPS.

Unfortunately widespread application of SMPSs is liable to contribute to instability of grid systems under fault conditions.
Under steady state conditions, SMPS are a load like any other and all should be well, they are after all, in very general use and the lights have not gone out.
Consider however what happens in the event of say a cable fault in a grid system. A short circuit or gross overload may persist for a second or two before fuses or circuit breakers open.
During the fault, consumers nearby will receive a much reduced voltage for a second or two. In the past, with resistive loads, the current drawn would also reduce until the fault was cleared.
Now however with a lot of SMPS, during a voltage dip the current drawn might double, thereby causing a further drop in the voltage. This will increase the stress on the system, perhaps leading to otherwise unaffected consumers receiving a reduced voltage and drawing more current.

What would once have been a localised cable fault, perhaps blacking out a dozen consumers, and subjecting a few hundred consumers to a brief voltage dip, can now escalate to a wider problem.

This is NOT THE SAME as a shortage of generating capacity, it can occur when margins seem ample.

So you are saying that SMPS provide a positive feedback on current fluctuations, and if the percentage of demand from SMPS type loads on a given circuit gets too large, it can overwhelm the negative feedback provided by resistive loads, leading to runaway over or under voltage.

This is a bit like the problem with grid stability on the supply side caused by intermittent renewable sources, only potentially worse.

The two together are even harder to solve.

PS_RalphW wrote:So you are saying that SMPS provide a positive feedback on current fluctuations,

It's true. SMPS will try to deliver the required output power, if the input voltage drops they will take more current to keep the power constant. Many are rated to run from approx 90 - 260 volts, so they won't just trip out when mains voltage is low.

Surely they'll only try and deliver current up to their maximum rated amps won't they? That's probably typically less than 50% more than their average current and they're found on comparatively low power devices (not washing machines, kettles, showers etc) so I'd be surprised if you'd see major spikes.

Would be interesting to see any research that's out there on this though. Anyone got any links?

Pepperman wrote:Surely they'll only try and deliver current up to their maximum rated amps won't they? That's probably typically less than 50% more than their average current and they're found on comparatively low power devices (not washing machines, kettles, showers etc) so I'd be surprised if you'd see major spikes.

Would be interesting to see any research that's out there on this though. Anyone got any links?

Yes, the SMPws will only try to deliver up to it rated amps, but the input amps will rise substantialy if the supply voltage be low.
Consider as an example a SMPs with a rated output of 10 amps at 12 volts DC. With a 240 volt AC input it will draw about half an amp.
Now suppose that the supply voltage briefly drops to 100 volts as a result of a fault.
The input current will now more than double to well over an amp, and if many such SMPS are in use a severe grid disturbance could result.

Whilst most SMPs are indeed small in wattage, they tend to be long hour loads such as lighting and IT equipment that are used throughout the working day, rather than say a kettle or shower that is used only briefly.
More and more washing machines BTW, DO use a SMPs to drive the motor.

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/vie ... hp?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.

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.

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.

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.

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.

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.

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-fre ... tem-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.

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.