A dual three-phase induction machine based flywheel storage system driven by modular multilevel converters for fault ride through in HVDC systems

One of the main challenges of voltage source converter based high voltage direct current (VSC-HVDC) transmission systems is the AC faults at the grid side. This work introduces the integration of multiphase induction machine (IM) based flywheel energy storage systems (FESS) with VSC-HVDC systems for AC side fault ride through purposes employing modular multilevel converters (MMC). MMCs have become suitable candidates for medium/high power energy conversion systems due to the capability of simply extending the levels of the converter while retaining high levels of reliability. In order to enhance the storage system reliability, a dual three phase IM is used to drive the FESS due to its fault tolerance capability. In this paper, the performance of the FESS is investigated under the operation of a dual three phase IM being driven by two three-phase MMCs. To step-down the DC-link voltage of the HVDC system to a proper voltage level for IMs, the DC-link voltage is divided into two series connected capacitor, and each capacitor voltage is fed as an input DC voltage for each three- phase MMC. The control strategies of the MMCs and the IM are presented, in addition to the IM mathematical model. Simulation case studies are performed using MATLAB/Simulink to validate the proposed system.