Control of grid connected AC-DC converters with minimized DC link capacitance under unbalanced grid voltage condition

Due to the limited lifetime of electrolytic capacitors, use of film capacitors is gaining popularity in AC-DC converter applications. In order to constrain both cost and converter volume while using film capacitors, small DC link capacitance values must be employed. Two challenges are associated with converters that have small DC link capacitors. First, such converters are highly susceptible to transient over/under voltages on the DC link. Secondly, even minor grid voltage imbalance can cause large 2^nd harmonic DC link voltage ripple to appear, possibly interfering with the load. This paper details a control scheme that overcomes both these problems. A feedforward matrix is employed to generate a set of reference current commands that are processed by high bandwidth stationary frame current controllers. A novel DC link voltage controller combines load feedforward with a resonant term to reduce transient over/under voltages and improve immunity to 2^nd harmonic voltage ripple under unbalanced grid conditions. Experimental results demonstrate the efficacy of the proposed converter controller.