A new hybrid MMC with integrated energy storage

As the penetration level of wind power increases, displacing conventional power generation with wind power will reduce the inertia of power system and decrease the frequency stability. 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 new hybrid modular multilevel converter (HMMC) with integrated battery energy storage. It has the capability of smoothing battery current and reducing battery voltage, while possessing DC fault ride-through capability with no additional costs in comparison with previous work on energy storage MMC. Voltage balancing, control strategy and parameter design are discussed in the paper, while simulation results illustrate three working states (charging, discharging, and idle) of battery and power balancing of submodules. If the proposed new submodule accounts for half of the cells, the new HMMC is able to provide sufficient virtual inertia for the wind farm with capacitor voltage requirement only 5% higher than conventional MMC.