Globally, there are 270 pumped hydroelectric storage (PHS) stations either operating or under construction. This represents a combined generating capacity of over 120,000 megawatts (MW). Of these total installations, 36 units consist of variable-speed machines, 17 of which are currently in operation (totaling 3,569 MW) and 19 of which are under construction (totaling 4,558 MW). All of these units are located in Europe, China, India, or Japan. While there are significant advantages with variable-speed pump-turbines, the majority of pumped storage projects under development around the world continue to be fixed-speed pump-turbines. There are various reasons for this including additional equipment costs for variable speed as well as the lack of recognition for the additional services provided by the equipment upgrades (i.e., ancillary service market development).
The traditional pump-turbine equipment design in the United States is the reversible single-stage Francis pump-turbine, which acts as a pump in one direction and as a turbine in the other. Although this technology is proven and has worked well for six decades, there are limitations to its performance. While design enhancements over the years have improved unit efficiency and power output, frequency regulation while in the pump mode is not possible with single-speed equipment. In the turbine mode, the unit cannot operate at peak efficiency during part load. Variable-speed machines enable the power consumed in the pumping mode to be varied over a range of outputs. Modifying the speed also allows the turbine to operate at peak efficiency over a larger portion of its operating band. Because variable-speed technology is well suited to integration of variable renewable generation, many of the proposed new pumped storage projects are considering variable-speed machines.
Variable-speed pump-turbines have been used since the early to mid-1990s in Japan and the late 1990s in Europe (where the term “adjustable speed” is common). In a conventional, single-speed pump-turbine, the magnetic field of the stator and the magnetic field of the rotor always rotate with the same speed and the two are coupled. In a variable-speed machine, those magnetic fields are decoupled. Either the stator field is decoupled from the grid using a frequency converter between the grid and the stator winding, or the rotor field is decoupled from the rotor body by a multi-phase rotor winding fed from a frequency converter connected to the rotor.
A cycloconverter was the first variable-speed technology implemented and provides the rotating magnetic field in the rotor. There are some limitations with this type of variable-speed machine. Cycloconverters cannot be used to start the unit in the pumping mode, which means that an additional static frequency converter is required in the powerhouse to start the unit. Cycloconverters also absorb reactive power, which needs to be compensated by converters or provided by the generator. Recently there have been improvements in large voltage source inverters that enable the stator magnetic field to be decoupled from the grid. This type of conversion is often more popular than the Cycloconverter, as this method does not absorb reactive power and the inverters can be used to start the project in the pumping mode.
A double-fed induction motor-generator is the current standard design for variable-speed machines. Generally, generator-motors are larger in size and have smaller air gaps than conventional machines. The stator is similar to that of a conventional generator-motor. The rotor requires additional features including at least one slip ring per phase (for three phases) and additional protection from mechanical stresses. This protection is in reinforcement of the rotor winding overhang and rotor rim. The rotor rim of a variable-speed machine carries an alternating magnetic field which may require additional design considerations.
Conclusions and Observations
A major benefit of variable-speed pump-storage technology is the tuning of the electric grid frequency to provide grid stability and frequency regulation. This new key ancillary service opportunity is needed to accommodate variable renewable energy inputs, typically for wind at night or during large ramping periods. The installation of new variable-speed PHS must be considered on a case-by-case basis; market conditions must exist to make the technology economically feasible, and its benefits must be weighed against a more complicated controls scheme and higher parasitic loads. For the purpose of variable renewable generation, however, variable speed machines can provide a wider operating range and faster start and turnaround times than conventional PHS, making this alternative an attractive option.