Variable speed pumped storage hydropower plants for integration of wind power in isolated power systems

Energy storage is a key issue when integrating large amounts of intermittent and nondispatchable renewable energy sources into electric power systems. To be able to maintain the instantaneous power balance and to compensate for the influence of power fluctuations that might result from renewable sources, flexible capability for power control is needed. Therefore, sufficient energy storage with suitable interface to the electricity grid is considered to be a necessity for development towards sustainable energy systems based on renewable energy sources (Dell & Rand 2001). Energy storage is also an important issue with respect to power system stability, load balancing and frequency control in power systems with large share of nuclear or thermal power plants that need to run at almost constant power (Shimada & Mukai, 2007; Faias et al., 2007). Many types of energy storage have been proposed for power system applications, and the different technologies have large variety in storage capacity, power rating and cycling capability. There is continuous development going on to increase the performance, reduce the cost and improve the interface to the electric power system for different technologies like battery storage, hydrogen storage, capacitors, flywheels, superconducting magnetic energy storage (SMES), compressed air energy storage (CAES) and pumped storage hydropower. There also exist large scale test facilities, and a few commercial applications, with most types of relevant energy storage technologies, but until now only pumped storage hydropower is considered to be a mature technology with long history of large scale commercial application (Chen et al., 2009; Dell & Rand, 2001; Hammerschlag & Schaber, 2007; Ibrahim et al., 2008; Kondoh et al., 2000; Ribeiro et al., 2001). Even if pumped storage hydropower systems have been used for more than hundred years, new improvements are still being introduced and development is going on with respect to rating, application areas and control. One of the most important advances during the last decades has been the development of variable speed systems to allow for controllable power in pumping mode (McClear & Meisel, 1984; Scherer, 2005; Terence & Schäfer, 1993). Such systems have been constructed on commercial basis in Japan since the beginning of the 1990s for load levelling in the power system, with more recent introduction also in Europe, and are usually based on a reversible Francis-turbine with a power electronically controlled 27