Active Power Control of Grid-Tied Squirrel-Cage Induction Generators under Subsynchronous Mode

A grid-tied squirrel-cage induction machine can operate at the generating mode if its rotor is driven above the synchronous speed. However, once the mechanical power provided by the prime mover is unable to overdrive the rotor beyond the synchronous speed, the induction machine would switch to the motoring mode and begins to drain the electrical power from the power system. To restore the induction machine back to the generating mode, a series-shunt power conditioner (SSPC) was presented to serve as a controllable voltage source between the stator circuit and the power grid. The SSPC is formed by two power inverters in back-to-back connection through a common dc link. The coupled voltage provided by the series inverter contains not only a fundamental component but also a subsynchronous component. The subsynchronous component at the stator terminal enables the induction machine to generate subsynchronous active power to the power grid. As for the fundamental component, which is designated in phase with the grid voltage in order to alleviate the fundamental active power drained by the induction machine. To avert the current harmonic distortion due to the subsynchronous voltage produced by the series inverter, the shunt inverter was employed at the point of common coupling (PCC) to sink the current harmonic. Experimental results show the effectiveness of the proposed approach.