Dynamic performance improvement of MMC inverter with STATCOM capability interfacing PMSG wind turbines with grid

Abstract

Most of the wind turbines recently used are of PMSG technology due to its high power efficiency, low cost, high power density and its direct power ejection. The technology comes usually in a lower voltage level (less than 1KV) that is not compatible to the low loss medium voltage distribution girds, another way of upgrading the voltage level is through use of step up transformers which adds to cost and loss. This paper suggests a series connection to raise the voltage level for compatibility through modular multilevel converter (MMC) inverter. As with a regular wind energy inverters (WEI), this inverter can be placed between the wind turbine(s) and the grid. The function of the proposed inverter is to convert dc power coming from the dc link (from dc collection point made of series connected wind turbines) to a suitable AC power for the main grid, and to fix the power factor (PF) of the grid at a target PF during normal operation and dc fault contingencies. This is achieved by controlling enough reactive power to the grid. The MMC inverter is for the permanent magnet wind generators connected through a buck converter that not regulates but tracks the turbines fluctuating voltages. It also serves as a bypass control in case of an open circuit from any of the connected wind turbines. The PMSGs have optimized power tracking for each independent variable voltages which depend on individual wind speeds for maximum power extraction. This proposed MMC interface has a d.c link adjustable voltage level control that has served as part of the MMC inverter. From the results, this control has maintained the inverter ac output voltage and upgraded the power factor despite the voltage variations. The control is achieved by STATCOM constant dc-modulation index control for variable dc voltage from different wind speeds or failure contingencies.