Performance Improvement of AC-DC Power Converters under Unbalanced Conditions

In this paper, a single synchronous reference-frame based control method is proposed to improve the performance of AC-DC interlink converters feeding DC loads under unbalanced AC grid conditions. Unbalanced grid voltages cause undesirable double frequency (2!0) oscillations on the DC link voltage in AC-DC converters. In the medium/high power applications, low switching frequency and the oscillatory power of the input filter considerably degrade the functionality of the existing methods for removing 2!0 ripples from DC link voltage. In this paper, an analytical equation for the terminal active power is derived considering the input filter inductance. Accordingly, suitable current references for the control are proposed to eliminate the undesirable 2!0 ripples from DClink voltage considering low switching frequency. It is shown that the filter inductance adds a non-linear term to the active power equation, which complicates the current reference calculation. Also, a real-time recursive method is proposed to solve the equations and find the current references. To evaluate the performance of the proposed method, different grid unbalanced conditions, including asymmetrical short circuit faults, are applied to a test system in the PSCAD/EMTDC environment. Furthermore, the functionality of the proposed method is compared with that of the existing method for unbalanced conditions.