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 the hydropower generating systems of both fixed and variable speed types. These converters are used in the form of a simple rectifier for the static dc excitation in the fixed speed system and in the form of the ac excitation (ac-ac or ac-dc-ac) in the variable speed system. The 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 article proposes a methodology for effective utilization of a parallel-connected BTB power converter for VSPSU of large capacity by performing the load-dependent activation /deactivation of converters. A vector control strategy for the active and reactive power deliveries of VSPSU is formulated. This article also focusses on the 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 the power converter system. The effectiveness of the proposed method is explained on a real scale of 250-MW doubly fed induction machine with 5 × 5 MW power converters using MATLAB/PLECS software, and experimental validation is performed through a downscaled 2.2-kW laboratory prototype.