Our current objectives are to characterize inherent instabilities for small-scale power grids and thereby to derive a control algorithm for avoiding them.
Our final goal is to establish a novel engineering for electrical energy infrastructure. Power grid is a typical networked and safety-critical infrastructure with large number of components (generation unit, transmission line, load, substation, etc.) and complex network topology. Power grid analysis and control are well-estabilished subjects with a long history of research. However, they are now strongly required to change substantially, because it cannot fully consider recent emergent phenomena and trends in power grids. Examples include the occurrence of widespread blackouts (e.g. 2003 grid blackout in North America), the development and penetration of renewable energy sources (solar, wind, geothermal, etc.), the emergence of novel grid designs (e.g. Microgrid and smart grid) integrated with control / information architecture, and the deregulation of electricity markets. We now stand at the phase of these drastic changes that are not considered in the conventional engineering for electrical energy. In order to drive these changes properly and make future society with low energy consumption and high energy efficiency, it is necessary to establish a novel engineering for energy infrastructure that can analyze and control possible emergent phenomena in future power grids.
We currently study an instability phenomenon and its dynamical mechanism for "small-scale" power grids. The notion of "small-scale grid" means that its geographical scale is much smaller than the current nationwide grids. This "small-scale" feature inevitably results in the power grid in which generation plants and loads are closely coupled. Examples of the small-scale grid include windfarm and Microgrid. They are important objects for future power grid, and it is fundamentally important to understand swing dynamics and stability for enhancing their performance. Thus we study stability of short-term swing dynamics for small-scale power grids and uncover a new phenomenon, termed the Coherent Swing Instability (CSI). This is an undesirable and emergent phenomenon of synchronous machines in a power grid, in which a group of machines coherently loses synchronism with the rest of the grid after being subjected to a local and, possibly, finite disturbance (see the movie: Consider swing dynamics of 20 synchronous generators. Each red point on the circle represents the motion of each generator (described by phase angles). The local disturbance, denoted by the left red point, causes bounded swings (close to 60Hz) of all the 20 generators in a while and finally unbounded and coherent swings (far from 60Hz) that are described as the motion of all points rotating on the circle in the counterclockwise direction). We now try to understand a dynamical mechanism underlying the occurrence of CSI using the so-called ergodic partition and resonance theory, thereby to derive a design strategy of small-scale grids for preventing CSI and a control algorithm for avoiding it.