Adjustable-speed operation of doubly fed machines in pumped storage power plants

Pumped storage plants are commonly utilised in order to improve the controllability and stability of power networks. Such power plants have usually been equipped with synchronous hydrogenerators operating at constant speed. The application of doubly fed machines (DFM) enables variable-speed operation, which improves the turbine efficiency, particularly at lower load. The increase in the energy efficiency can be as high as 10% for the control of the mechanical speed in the range of 10% around the synchronous speed. The use of the DFM is also suitable for maintaining the stability of the power network due to its faster control response in comparison to synchronous generators. The independent changes in speed and rotor currents provide more possibilities to control the active and reactive power outputs of the machine both in motor and generator operation. The design of the DFM is similar to that of a wound-rotor induction machine, but whereas the power outputs of existing induction machines are typically up to 30 MVA, some manufacturers design and produce DFMs with power output of up to 500 MVA