Contribution to control strategies for converter connected to unbalanced grid

The paper deals with developed and experimentally tested effective synchronization and power control techniques for grid-connected converter. A new FLL (Frequency Locked Loop) technique is developed that incorporates advantages of both the cascaded delayed signal cancellation and dual second order generalized integrator schemes. Experimental results are evaluated in terms of quality of the positive voltage sequence, frequency and phase angle identification for the grid voltage disturbed by harmonic, subharmonic, and negative sequence components as well as for amplitude, frequency, and phase angle jumps in the voltage. A novel power control technique for grid-connected converter under unbalanced voltage conditions is referred as well. The current positive and negative sequences during grid voltage sags are controlled to assure a proper exchange of active and reactive powers without substantial power ripples, and especially, without current harmonics.