Improved Control of an Active-Front-End Adjustable Speed Drive with a Small dc-link Capacitor under Real Grid Conditions

Active front-end topologies will be widely used in the future and among them especially the two-level PWM rectifier, due to the need to improve the quality of the input currents and the robustness against grid disturbances of Adjustable Speed Drives (ASDs). Another expectation is that electrolytic capacitors will be replaced by film capacitors in order to increase the ASD lifetime, but as this has lower energy density, the dc-link capacitance is expected to decrease. In these circumstances, operation under unbalanced and distorted supply voltage as well as high dynamic operation of the ASD makes the control task more challenging. This paper discusses problems related to the ASD dc-link both in respect to its stability (under regeneration as well as in the case of constant power absorbed by the inverter stage) both also in respect to the dc voltage ripple generated by the grid unbalance and made more severe due to the small dc-link storage. To mitigate the latter problem it is proposed to pre-compensate the dc-link voltage reference signal considering the real grid voltage. Experiments show that the system handles successfully unbalance situation even with a small dc-link capacitor.