Control strategies for grid connected PWM-VSI systems

This paper evaluates the current control methods for three-phase voltage-source-inverters (VSIs) connected to the grid via LCL-filters. Current feedback methods from both line and converter sides are compared and theoretical analysis is stated. The analysis is based on the existence of a critical resonant frequency dividing the controlling frequency interval into two regions; namely low and high resonant frequency regions. The feedback types and damping methods are evaluated for both low and high resonant frequency regions with regard to resonant and low frequency poles of the system. In the low resonant frequency region, damping of LCL-filter resonance is necessary for line current feedback; while the current controller provides inherent damping when converter current feedback is utilized. Moreover, total harmonic distortion (THD) and power factor performance of the designed system employing either line or converter current feedback is assessed under varying load conditions. Besides, thorough theoretical approaches supported with simulation results are presented.