The basic idea of Variable Speed Pumped Hydroelectric Storage is to use extra generated renewable energy for pumping water uphill to a reservoir, which acts like a “natural battery,” sometimes referred to as a water battery. When needed, the water is released to the hydropower plant downhill, so the gravity is responsible for most of the energy.
Variable speed drive technology provides a full variable speed range that could be optimum for best performance and most economic mode to pump water up to the upper reservoir—when in saving mode—or to release water to a lower reservoir for generating power.
Design of Variable Speed Pumped Hydroelectric Storage
The typical pump turbine equipment design is the reversible single stage Francis pump turbine, which acts as a regular pump in a single direction and as a turbine in the other. Although it is a proven technology and has done well for over six decades, there are several limitations to the performance of this technology. Although design enhancements have improved the unit efficiency and output power, regulation of frequency while in pumping mode is not possible with single speed equipment. In the turbine mode, the unit cannot operate at the peak efficiency while in part load.
Variable speed drives enable the consumed power during pumping mode to be varied over output range. Varying the speed allows the turbine to operate at near its peak efficiency over a larger part of its operating band as well. Because the variable-speed technology is well suited to be integrated into the variable renewable energy generation, most of the new proposed pumped storage projects are considering variable speed drives.
In an ordinary, single speed pump turbine, the magnetic field of the stator and that of the rotor rotate with an equal speed and the two are coupled. In a variable speed system, the two magnetic fields are decoupled. Either a frequency converter between the grid and the stator winding is used to decouple the stator field from the grid, or a multi-phase rotor winding that is fed from the frequency connected to the rotor is used to decouple the rotor field from the rotor body.
The first variable speed technology implemented was the cycloconverter which provides the rotating magnetic field in the rotor. There are certain limitations with this type of variable speed technology as cycloconverters can’t be used to start the machine in the pumping mode, so an extra static frequency converter is needed in the powerhouse for starting the unit. Cycloconverters absorb reactive power as well, which should be compensated by the converters or provided by the generator.
A double fed induction motor generator (DFIG) is the existing standard design for variable speed machines.
Current Use of Variable Speed Pumped Hydroelectric Storage
Only a limited number of the Pumped Hydroelectric Storage systems consist of variable speed machines. All of these units are installed in Europe, China, India and Japan. In spite of the significant advantages of variable speed pump turbines, the majority of new pumped storage projects globally continue to be normal fixed speed pump turbines. There are numerous reasons for this which includes additional equipment costs for the variable speed equipment as well as the lack of recognition for the additional benefits provided by the equipment upgrades.