A new approach for power quality improvement of DFIG based wind farms connected to weak utility grid

Most of power quality problems for grid connected doubly fed induction generators (DFIGs) with wind turbine include flicker, variations of voltage RMS profile, and injected harmonics due to switching in DFIG converters. Flicker phenomenon is the most important problem in wind power systems. This paper described an effective method for mitigating flicker emission and power quality improvement for a fairly weak grid connected to a wind farm with DFIGs. The method was applied in the rotor side converter (RSC) of the DFIG to control the output reactive power. q axis reference current was directly derived according to the mathematical relation between rotor q axis current and DFIG output reactive power without using PI controller. To extract the reference reactive power, the stator voltage control loop with the droop coefficient was proposed to regulate the grid voltage level in each operational condition. The DFIG output active power was separately controlled in d axis considering the stator voltage orientation control (SVOC). Different simulations were carried out on the test system and the flicker short term severity index (Pst) was calculated for each case study using the discrete flickermeter model according to IEC 61400 standard. The obtained results validated flicker mitigation and power quality enhancement for the grid.