Study On Grid Connected Inverter Used in High Power Wind Generation System

In high power wind power generation systems, grid connected voltage source inverters (VSI) are essential devices for power transporting and energy converting. Output currents of inverters increase harmonic distortion due to low switching frequencies. A series inductance usually is used to reduce switching harmonics entering the power distribution network. Another kind of filter, LCL filter, can achieve reduced levels of harmonic distortion at lower switching frequencies. However grid connected inverters with an LCL-filter require more complex control methods to make the system stable. This paper proposes a robust strategy to control grid currents entering a distribution network from a three-phase VSI connected via an LCL filter. This grid connected inverter system is used in high power wind generation systems for reduced current harmonics with low switching frequency. Different from previous control strategies integrating an outer loop grid current regulator with inner capacitor current regulation, this control strategy integrates one bridge current regulation loop with grid current and voltage feed forward (dual feed forwards) to stabilize the system. Therefore, the grid currents are controlled indirectly, and the current sensors number can be reduced. Linear analysis, comparisons with previous methods, simulation and experimental results verify the effectiveness of this control strategy across a range of operating conditions