A New Hybrid Modular Multilevel Converter With Integrated Energy Storage

The penetration level of wind power is increasing in power system. Replacing conventional energy with wind energy will reduce the inertia of power system and decrease the frequency stability because wind turbines are unable to naturally contribute to system frequency response. Studies have shown that energy storage devices with only 5% energy storage of wind farm's rated capacity can produce the same virtual inertia as a same capacity synchronous generator. To reduce the additional costs of individual energy storage devices, this paper proposes a novel hybrid modular multilevel converter (HMMC) with integrated battery energy storage. It has the capacity of smoothing the battery current and reducing the battery voltage, while possessing DC fault ride-through capability. Steady-state analysis, operation modes and control strategy for battery current control and capacitor voltage balancing control are discussed in the paper. The viability of the HMMC, as well as the effectiveness of the control strategy, is confirmed by the simulation and experiment.