Effective Utilization of Parallel Connected Megawatt Three-Level Back-to-Back Power Converters in Variable Speed Pumped Storage Units

Power electronic converter plays a significant role in hydropower generating systems of both fixed and variable speed types. These converters are used in the form of simple rectifier for static dc excitation in fixed speed system and in the form of ac excitation (AC-AC or AC-DC-AC) in variable speed system. Parallel connected multiple three-level neutral-point-clamped (3L-NPC) megawatt back-to-back (BTB) converters have been adopted in recently commissioned variable speed pumped storage units (VSPSU). For such configurations, reliability and efficiency are considered to be prominent. This paper proposes a methodology for effective utilization of parallel-connected BTB power converter for VSPSU of large capacity by performing load dependent activation /deactivation of converters. Vector control strategy for active and reactive power deliveries of VSPSU is formulated. This paper also focusses on losses and thermal behavior of converters at different speeds of generator in accordance with water head variations. The proposed control system is designed in such a way to decrease overall losses in power converter system. The effectiveness of proposed method is explained on a real scale of 250 MW doubly fed induction machine (DFIM) with 5x5 MW power converters using MATLAB/PLECS software and experimental validation is performed through a downscaled 2.2 kW laboratory prototype.