Solar and wind energy may stabilise the power grid

Sep 14, 2012
The energy transformation as a supply grid issue: Currently, large central power plants basically supply their immediate surroundings. In future, more small, decentralized wind and solar generators will take on a greater load of the supply. This will lead to a new network architecture that may be less vulnerable to power outages than present-day grids, contrary to the fears of some. Credit: designergold, based on outlines provided by the MPI for Dynamics and Self-Organization

(Phys.org)—Renewable energies such as wind, sun and biogas are set to become increasingly important in generating electricity. If increasing numbers of wind turbines and photovoltaic systems feed electrical energy into the grid, it becomes denser – and more distributed. Therefore, instead of a small number of large power plants, it links a larger number of small, decentralized power plants with the washing machines, computers and industrial machinery of consumers. Such a dense power grid, however may not be as vulnerable to power outages as some experts fear. One might assume that it is much harder to synchronize the many generators and machines of consumers, that is, to align them into one shared grid frequency, just as a conductor guides the musicians of an orchestra into synchronous harmony.

In contrast, scientists at the Max Planck Institute for Dynamics and in Göttingen have now discovered in that consumers and decentralized generators may rather easily self-synchronise. Their results also indicate that a failure of an individual supply line in the decentralized less likely implies an outage in the network as a whole, and that care must be taken when adding new links: paradoxically, additional links can reduce the of the network as a whole.

Synchronization, the coordinated dynamics of many units to the same timing is found throughout the natural world. Neurons in the brain often fire simultaneously, synchronize their , and crickets chirp in shared rhythm. A similar form of harmony is also necessary in , in that all generators and all machines that consume electricity must be tuned to the grid frequency of 50 Hertz. The generators of large are regulated in such a way that they stay in rhythm with the . The grid, in turn, imposes its frequency on the washing machines, vacuum cleaners and fridges at the other end of the line, so that all elements remain in synchrony, avoiding short circuits and emergency shutdowns.

In the course of the energy turnaround, however, the structure of the power grid will change. Today's large power plants that supply energy to the surrounding areas will be largely replaced by multiple photovoltaic panels on roofs, systems on fields, and on hills and offshore. Power lines will no longer form star-like networks and only transmit energy from large power plants to nearby consumers, but will look more like dense fishing nets linking many generators with the consumers. Experts believe it will be very difficult to bring this multiplicity of small generators into synchronous harmony. In effect, it would be like conducting a huge orchestra with thousands of musicians, instead of a chamber orchestra. However, as the Network Dynamics Group, headed by Marc Timme at the Max Planck Institute for Dynamics and Self-Organisation in Göttingen has now discovered, synchronization in a decentralized power grid may actually be easier than previously thought, as a grid with many generators finds its own shared rhythm of alternating current.

In a decentralized grid, power plants and consumers synchronize themselves

The Göttingen-based scientists have simulated a dense network of small generators and consumers. Their computer model calculates the grid for an entire country (for practical reasons, they chose Great Britain) and takes into account the oscillations of all generators and electric motors that are connected to the grid. Combining this level of detail with this grid size is a new departure. Previously, the dynamics of the oscillating 50 Hertz AC current was basically only simulated for small networks. Simulations for larger grids did exist, but they were generally used only to make predictions regarding the static properties of the network, such as how much electricity would be transmitted from A to B. They completely ignored the oscillations of the generators and electric motors. "Our model is sufficiently complex and extensive to simulate collective effects in complex networks and, just as importantly, it is simple enough that we can understand these effects too", says Dirk Witthaut, project leader within the research group.

The scientists simulated a very large number of networks, each with a different structure. The networks consisted of different mixes of large and small generators with lines of varying capacities, a little like country lanes and motorways for electrical current. This enabled them to identify differences between centralized and decentralized power grids.

How robust is the power grid? Using the British grid as an example, the Göttingen-based scientists calculated what happens when individual lines are cut. If one of the blue lines is cut, an outage in the whole network is unlikely. In the case of the dark red lines, however, the likelihood of a total blackout rises to almost 50 per cent. Credit: Dirk Witthaut and Marc Timme / MPI for Dynamics and Self-Organisation

A dense grid can compensate more easily for a line outage

The scientists in Göttingen examined additional aspects that are discussed in relation to the transition from a centralized grid to a decentralized one. What happens, for example, if a single transmission line is damaged or malfunctions? In existing grids, this can have a kind of domino effect, as seen in the 2006 power outage around Europe, caused by the shutting down of a single line in Northern Germany. The simulations of the Göttingen-based team indicate that decentralized grids are much more robust when single lines are cut. This is because a dense grid always more often has neighbouring lines that can take on the extra load of a downed line. Unlike the case of large-meshed networks, they have few indispensable main links with the potential to cripple the whole grid.

A line in the wrong place: If the British grid operators were to lay a new power line along the broken line (arrow), they would apparently be closing a gap in the grid. Based on Braess's Paradox, however, this new line would disrupt power transmission throughout the network. Credit: Dirk Witthaut and Marc Timme / MPI for Dynamics and Self-Organisation

Nonetheless, the expansion of renewable energy also holds challenges for the stability of the supply network. Another simulation showed the scientists that a highly decentralized grid is more vulnerable to strong fluctuations in consumption, as occurs, for example, when millions of people turn on their washing machines at the same time. Large power plants can buffer these fluctuations in demand more easily than small ones, as their rotating generators store more kinetic energy. The grid can tap into these spinning reserves at short notice to cover supply gaps - an option which is not available in the case of solar cells.

Adding new lines can hinder power transmission

In a second study using the same mathematical model, Marc Timme and Dirk Witthaut revealed another effect that is known from road traffic and is counterintuitive. Building a new road and thereby increasing the capacity of the network does not necessarily improve traffic flow; on the contrary, even more congestion may occur with the same volume of traffic. This is the case when the new stretch of road provides a shortcut for many drivers, but has been poorly chosen in that it links bottlenecks which were previously avoided by most.

Witthaut and Timme have shown that this situation, known as Braess's Paradox, can also be observed in power grids, specifically in decentralized networks. If such a dense network self-synchronizes, it might be assumed that synchronization would become easier with each new link; however, this is not always the case: the addition of a new line may actually disrupt self-synchronization.

In order to understand this paradox, it is helpful to consider two machines in a dense network. Additional machines are located along the line that connects them. The two machines are in synchrony as long as the phases of their oscillations – where phase describes the state of oscillation at a given time and place – remain in a fixed, mathematically defined relationship to one another. This may be visualized as two pendulums. The phase of the pendulum describes its degree of deflection at a given point in time. If the pendulums swing at the same frequency, their oscillations will be in a fixed phase relationship to one another. This does not mean that the two pendulums are swinging exactly parallel to each other, i.e. that they always have the same degree of deflection; in fact, they may even shift out of phase with each other. However, the distance between the swinging pendulums is fixed for every point in time, and the points at which they have the same degrees of deflection recur at regular intervals.

How does the grid react to supply fluctuations?

If two machines in a power grid are to be synchronized, that is, if their fixed phase relationship is to be fulfilled, they must always reach minimum and maximum voltage at the same time. This means that they must not be out of phase, or only by a full wave train. Every line in the network now yields a fixed phase relationship, either directly or indirectly. If a new line is now built to link the two machines directly, their oscillations must conform to a new phase relationship; however, this may not be compatible with the old one. Because the latter is consistent with the other machines on the old line, there is a conflict between the shortcut and the old line, which has the potential to desynchronize the entire network.

"Care must therefore be taken when adding lines to a decentralized grid", cautions Witthaut. Careful consideration should be given to which nodes can be linked without risk. However, Witthaut sees the results of the simulations as encouraging for the construction of decentralized networks. "Until now, concerns rather centred on the possible collective impact that a large number of small generators could have in a dense grid", says the physicist. The fear was more frequent power outages. "But our work shows that the opposite is the case and that collective effects can be very useful."

The Network Dynamics Group based in Göttingen currently starts collaborating with engineers and network operators to ensure that their findings can be put to practical use. Initial contacts have already been made, and, in the meantime, the scientists are improving the model. Their current focus is to integrate weather-related fluctuations in sources into their simulations.

Explore further: Sensitive detection method may help impede illicit nuclear trafficking

More information: Martin Rohden, Andreas Sorge, Marc Timme and Dirk Witthaut. Self-Organized Synchronization in Decentralized Power Grids, Physical Review Letters, 9 August 2012; DOI:10.1103/PhysRevLett.109.064101

Dirk Witthaut and Marc Timme. Braess' paradox in oscillator networks, desynchronization and power outage. New Journal of Physics, 29 August 2012; DOI:10.1088/1367-2630/14/8/083036

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drhoo
1 / 5 (1) Sep 14, 2012
The article doesn't make sense. Sure the phase relationships change with new wiring but this is quickly corrected at each generator so that the power is flowing as desired. Each generator sets it's phase advance relative to the grid in order to specify how much power it delivers.
I believe what they really mean is that power cannot be directed toward a particular customer with many connections as it will flow toward the load of least resistance naturally which would be the nearest.
As far as wind destabilizing the grid this will only be true in incompetent installations or poor management conditions. There are no technical hurdles to implementing wind integration.
Horus
1 / 5 (2) Sep 14, 2012
Over 60% of electrical production is lost through line transmissions, irrespective of the desired destination(s). The next national/regional grid structure being a dense mesh design with circuits in parallel will also rely upon vastly improved material science and reduce the heat loss dramatically while being managed by control systems along the path instead of just following the `path of least resistance.'
NotParker
2.5 / 5 (8) Sep 14, 2012
No Wind Everywhere = No Power Everywhere

Perfect synchronization!

In summer and winter you can get giant high pressure systems with no wind which is peak power need times.

http://www.forbes...-work-2/
Roland
not rated yet Sep 15, 2012
For this to work, power companies are going to have a lot better regulation and a lot more info from distributed generation sources. Power companies should install fiber all over their existing network, and sell the excess bandwidth. It would be cheaper for them to do that than anyone else, and they already have the right-of-way. This would revitalize a business that is strangling on its past.
djr
3.7 / 5 (3) Sep 15, 2012
"Should we shut down our computers, because the wind isn't blowing enough?" No - we should let the engineers do their job - and develop a stable and well integrated grid - using a good mix of energy sources. Oklahoma is about to add 7 GW of wind power - the farms will be built in just a couple of years - we are talking 10,000 temporary construction jobs - and 1,000 ongoing support jobs.
http://cleantechn...klahoma/

Offshore wind could soon provide 300,000 jobs - and $200 billion economic boost. http://cleantechn...re-wind/
alfie_null
5 / 5 (1) Sep 15, 2012
No Wind Everywhere = No Power Everywhere

The article discussed using wind to augment, not replace. Don't imply otherwise.

I read the linked article. It mentioned that same sort of cold/high pressure weather system, but not in a very technical context. I took at look at the author's bio. Seems to be mostly an economics pundit, not a weather expert. I'm not giving it much credence.
VendicarD
2 / 5 (4) Sep 15, 2012
Why not? Maybe you will finally get laid.

"Should we shut down our computers, because the wind isn't blowing enough?" - Natello
VendicarD
1 / 5 (4) Sep 15, 2012
No temperature differential Anywhere = No Wind Everywhere

Probability Zero.

"No Wind Everywhere = No Power Everywhere" - ParkerTard

Next!
VendicarD
2.3 / 5 (3) Sep 15, 2012
If by 60 percent you mean 7 percent, then you are exactly right.

"Over 60% of electrical production is lost through line transmissions" - ZippyVonZipper

http://www.eia.go...&t=3
NotParker
3 / 5 (4) Sep 15, 2012

The article discussed using wind to augment, not replace. Don't imply otherwise.


Really?

"Today's large power plants that supply energy to the surrounding areas will be largely replaced by multiple photovoltaic panels on roofs, biogas systems on fields, and wind turbines on hills and offshore. "

"largely replaced" is not the same as "augment".

Maybe you have trouble with the meaning of augment?
NotParker
3 / 5 (4) Sep 15, 2012
"Should we shut down our computers, because the wind isn't blowing enough?" No - we should let the engineers do their job - and develop a stable and well integrated grid - using a good mix of energy sources. Oklahoma is about to add 7 GW of wind power - the farms will be built in just a couple of years - we are talking 10,000 temporary construction jobs - and 1,000 ongoing support jobs.
http://cleantechn...klahoma/


Oklahoma will end up burning more gas and coal to produce electricity (and generate more Co2) because the fossil fuel plants will be running in a reserve capacity waiting for the wind to die down (which it will).

The money is squandered. The turbines will have been built in some other country.
djr
1 / 5 (1) Sep 15, 2012
"If by 60 percent you mean 7 percent, then you are exactly right." Isn't it bizarre that the anti progress gang can just make shit up - and feel entitled to spam science sights with this crap? The new HVDC lines lose around 2 - 3% per 1,000 kilometers http://www.desert...st-asia/ Meaning you could transmit from coast to coast in the U.S. with only 15% loss.
djr
2.5 / 5 (4) Sep 15, 2012
"Oklahoma will end up burning more gas and coal to produce electricity (and generate more Co2)" As usual - our resident nay sayer - who knows nothing - continues to show the world that he/she knows nothing. C02 emmissions have been declining - in large part because of retirement of old coal plants. Nice quote here - Over the past couple of years, energy companies have
announced the retirement of more than 30,000 MW of coal-fired
generating capacity. From - http://www.reuter...20120424

Sad that people have to keep talking - when they clearly don't know what they are talking about.
NotParker
2.6 / 5 (5) Sep 15, 2012
"Oklahoma will end up burning more gas and coal to produce electricity (and generate more Co2)" As usual - our resident nay sayer - who knows nothing - continues to show the world that he/she knows nothing. C02 emmissions have been declining - in large part because of retirement of old coal plants. Nice quote here - Over the past couple of years, energy companies have
announced the retirement of more than 30,000 MW of coal-fired
generating capacity. From - http://www.reuter...20120424

Sad that people have to keep talking - when they clearly don't know what they are talking about.


The EPA forced much coal to retire, but the sharp drop in shale gas prices also caused power companies to switch to natural gas.

And those gas plants do not start instantly. So they will be running and burning fuel as spinning reserve.

http://www.aee.ne..._ok1.jpg

djr
3 / 5 (2) Sep 15, 2012
"The EPA forced much coal to retire, but the sharp drop in shale gas prices also caused power companies to switch to natural gas."

Parker is correct on this point - but it does not have any relevance to the benefit of integrating wind into our mix. Cheap natural gas has been a boon to states like Oklahoma - and I think will continue to keep our economy ahead of many other states. Wind power is also an economic benefit to our state. Ideally we will be smart (seems the engineers know what they are talking about - unlike happy Parker) and continue investing in renewables. In time we can shift the balance from fossils - to renewables - to usher in a new era of cheap - non polluting energy. Parker will benefit from the foresight of others who know what they are talking about.

Eikka
1 / 5 (1) Sep 17, 2012
There's one aspect which they didn't touch, which is grid safety and fault recovery.

In a star-grid, when you do get a failure it's relatively easy and fast to pinpoint the location. The automatic systems in place right now simply disconnect every branch and then within two minutes of the failure, try to reconnect them in sequence until the fault trips it again. The faulty branch is disconnected, all the others reconnected and a service team is dispatched.

In a decentralized grid, simply knowing where the fault is can be cumbersome, disconnecting it will be more difficult, and dangerous because you can have multiple routes feeding the same fault with the fault load shared between them so that none will see an alarmingly high current that would trip the automatic switches.
Eikka
1 / 5 (1) Sep 17, 2012
Or to give a more concrete example: suppose there's an electrical fire going on down the end of your street. The power company senses the fault condition by the fault current and disconnects your street from the main grid to stop the arcing.

How does the power company know that the solar panels on your roof, which are on the same circuit, won't still keep powering that fire? When the fire department gets there and finds out that there's still power in the lines, how long does it take for them to check everyone's house and find the malfuntioning breaker that is still feeding the local grid?

And if it should happen that some locale switches out of the bigger grid and into an "island mode", will the local generators deal with the overload gracefully, or simply break down?
drhoo
not rated yet Sep 17, 2012
Eikka

The power company does not monitor systems at the distribution level, the situation you outlined would have to be handled by local safety trip equipment.

As far as disconnecting alt sources from the grid when the grid is down there already exists technology to do this automatically.

Combine with the safety mandate that all lines are to be physically grounded before they are assumed dead.

SatanLover
1 / 5 (1) Sep 17, 2012
human entropy.
Lex Talonis
1 / 5 (1) Sep 17, 2012
They didn't factor in the gravitational effect, for when a power line breaks at the top of a hill or on high ground, how all the electricity runs back down the lines and into the grid, causing an electron surge - throwing all the machines out of phase, as the tsunami like effect of the electrons reaches all the generators at different times, and the effect of the back flow is to blind the synchronisation and over fill the generator.

This is called a melt down of the grid.
Eikka
1 / 5 (1) Sep 21, 2012
The power company does not monitor systems at the distribution level, the situation you outlined would have to be handled by local safety trip equipment.


I've had the pleasure of working with a small power company, and it's true that they don't directly monitor the distribution; they do monitor the safety trip equipment and control them centrally to locate the faults. It used to be done with the power grid itself as a carrier wave, sending pulses down the line, but now it's done through radio for efficiency reasons.

And they highlighted to me the problem of feeding in energy through the branches because their existing circuit breakers wouldn't be able to deal with it, and the complexity and cost of adding so many more automatic breakers to the system which would increase fault rates, since these equipment aren't perfect either.

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