the paper presents the modeling and simulation study of a variable speed pump-storage power plant composed of a reversible turbine, a doubly fed induction machine (DFIM), and a multistage-multilevel frequency converter. The DFIM rotor circuit is fed by a 5-level NPC VSI. The inverter DC-link capacitor voltages are kept in balance by means of four multistage PWM rectifiers. The paper considers start-up responses and performance characteristics of the drive system along with its behavior under the stator short-circuit fault conditions. An emphasis is placed upon modifying the PD PWM modulation waveforms in order to level RMS values of currents flowing in PWM rectifiers and related secondary windings of a step-down transformer.

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The structures of systems with high-voltage cascade frequency converters containing multi-winding transformers and low-voltage low-power converters connected in series at each output phase of the load are considered. Low-voltage blocks contain three-phase diode or active rectifiers, DC capacitor filters, single-phase stand-alone voltage inverters and block disconnecting devices in partial modes (in case of failure when part of the blocks are disconnected). The possibilities of operation of cascade converters are determined, equations for correcting tasks to units in partial modes are given, tables of correction of tasks with estimates of achievable load characteristics are proposed. The results of experiments on the model of a powerful installation with a cascade frequency converter are presented, confirming the possibility of ensuring the symmetry of the load currents when disconnecting part of the blocks and the asymmetry of the circuit.