Variable Speed Pumped Hydroelectric Storage

Executive Summary

Globally, there are 270 pumped hydroelectric storage (PHS) stations either operating or under construction. This represents a combined generating capacityThe rate at which equipment can either generate, convert or transfer energy1. Energy is the potential of a physical system to perform work. (A common unit of work is foot-pound—the amount of energy needed to lift one pound up a distance of one foot.) Energy exists in several forms such as electromagnetic radiation .... 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 serviceThose services that are necessary to support the transmissionAn interconnected group of lines and associated equipment for the movement or transfer of electric energy between points of supply and points at which it is transformed for delivery to customers or is delivered to other electric systems. of capacity and energy from resources to loads while maintaining reliable operation of the Transmission ServiceServices provided to the transmission customer by the transmission service providerThe entity that administers the transmission tariff and provides transmission service to transmission customers under applicable transmission service agreements. to move energy from a point of receipt to a point of delivery. Provider’s transmission system in accordance with good utility practice.... 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 powerThe rate at which energy is generated, converted, transmitted, distributed or delivered. 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 loadAn end-use device or an end-use customer receiving electric power and using electric energy from the electrical system (grid). Note: The term load is sometimes treated as a synonym for demand, which is the measure of power that a load receives or....  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 generationThe manner in which electricity is generated. The electricity that flows through California, 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 fieldA description of the magnetic effect that electric currents and magnetic materials have on their surroundings. The magnetic field is a vector field meaning that it is characterized by both the direction and a magnitude of the magnetic influence. 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 powerThe portion of electricity that establishes and sustains the electric and magnetic fields of alternating-current equipment. Reactive power must be supplied to most types of magnetic equipment, such as motors and transformers. It also must supply the..., 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 electricAn adjective meaning “needing electricity to operate” such as electric motor or wire. IEEE: Containing, producing , arising from, actuated by or carrying electricity. grid frequency to provide grid stabilityThe ability of an electric system to maintain a state of equilibrium during normal and abnormal conditions or disturbances. 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.