Optimization of the start-up time of a variable speed pump-turbine unit in pumping mode

This paper presents the start-up and synchronization procedures of large variable-speed pump-turbine units in pumping mode. These procedures applied to a doubly-fed induction motor-generator with three-level voltage source inverters VSI cascade in the rotor side, can be achieved without any supplementary equipment, for instance without an autotransformer connected to the rotor. The control strategy used in the start-up procedure and based on a stator flux oriented control is summarized as well as different steps in order to reach the minimal speed required for synchronization. An optimization of the start-up time can be achieved by a modification of the modulation type of the converters. Simulated results obtained for two variable-speed pump-turbine units of 340 MW and 250 MW are presented and discussed.