Harmonics and unbalance compensation of the generated voltage in a standalone DFIG

The paper describes a variable speed, doubly fed induction generator based, standalone power system supplying unbalanced and nonlinear load. A sensorless control method of the fundamental 50Hz harmonic uses a PLL for synchronization of the measured stator voltage vector with the reference one. Additional part of control includes feedbacks from the components of voltage asymmetry and harmonics, extracted in the adequate frames. The frames rotate at angular speeds corresponding to each harmonics and to a negative sequence of the fundamental one, respectively. A grid side converter control, besides its basic functionality eliminates the stator current asymmetry and harmonics to reduce the electromagnetic torque ripples. An improvement of generated voltage quality achieved by using the proposed extension of the sensorless stator voltage control method and adequate control of the grid side converter is shown based on the results of the laboratory tests on a small power unit.