Wind power integration in isolated grids enabled by variable speed pumped storage hydropower plant

This paper presents aspects of control and operation of a variable speed pumped storage power plant for integration of wind power in an isolated grid. A topology based on a synchronous machine and a full scale back-to-back voltage source converter is suggested for variable speed operation of the pump-turbine. With this topology, variable speed operation can be obtained in both pumping mode and generating mode, and reactive current can be controlled independently of the active power flow and the operation of the pump turbine. By utilizing the controllability of the variable speed system, power fluctuations from wind turbines can be compensated to limit the influence on the rest of the grid. At the same time the pumped storage can be controlled to take part in the frequency control of the system and also to control grid voltage or flow of reactive power. The operation of the proposed system is illustrated by simulations based on the situation on the Faroe Islands, where controllable energy storage could allow for higher penetration of renewable energy in the power system and by that reduced dependency on power generation based on fossil fuels.