Solar and wind energy may stabilise the grid

Author:
Dirk Witthaut and Marc Timme, Dynamics Group Researchers, Max Planck Institute for Dynamics and Self-Organization

Date
01/20/2013

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Power grids with many small power plants could suffer fewer outages, but the new lines must be planned with care.

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 model simulations, scientists at the Max Planck Institute for Dynamics and Self-Organization in Göttingen have now discovered that consumers and decentralized generators may self-synchronise easily. Their results also indicate that a failure of an individual supply line in the decentralized grid is less likely to imply an outage in the network as a whole, and that while care must be taken when adding new links, paradoxically, additional links can reduce the transmission capacity of the network as a whole. Synchronization, or the coordinated dynamics of many units to the same timing is found throughout the natural world. A similar form of harmony is also necessary in electricity networks, in that all generators and all machines that consume electricity must be tuned to the grid frequency of 50 Hz. The generators of large power plants are regulated in such a way that they stay in rhythm with the power grid. 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 current 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, biogas systems on fields, and wind turbines 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 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 finding their own, shared rhythm of alternating current. In a decentralized grid, power plants and consumers synchronize themselves The Göttingen-based scientists Dirk Witthaut and Marc Timme 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 Hz 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 research project leader Dirk Witthaut. 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. Dense grid compensates more easily for 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 by the Göttingen-based team indicate that decentralized grids are much more robust when single lines are cut. This is because a dense grid has neighbouring lines that can take on the extra load of a downed line. In the case of large-meshed networks, they have few indispensable main links with the potential to cripple the whole grid. But the expansion of renewable energy does hold challenges for the stability of the supply network. Simulation shows 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 reserves at short notice to cover supply gaps—an option not available in the case of solar cells. New lines can hinder power transmission In a second study with the same mathematical model, Marc Timme and Dirk Witthaut discover another effect, known with road traffic, that is counterintuitive. Building a new road and increasing the network capacity does not necessarily improve traffic flow; on the contrary, even more congestion may occur with the same volume of traffic. This is when the new road provides a shortcut for many drivers, but has been poorly chosen in linking potential bottlenecks that were previously mostly avoided. Braess's Paradox can 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, consider two machines in a dense network. Additional machines are located along the line that connects them. The two machines are synchronous as long as the phases of their oscillations (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. Grid reaction 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. Careful consideration should be given to which nodes can be linked without risk. However, the results of the simulations are seen 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 renewable energy sources into their simulations. Max Planck Institute for Dynamics and Self-Organization

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