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 Self-Organization in Göttingen have now discovered in model simulations 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 grid 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 transmission capacity 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, fireflies synchronize their blinking lights, and crickets chirp in shared rhythm. 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 Hertz. 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 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 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.