Implementation of a Converter in Sequence Domain to Counter Voltage Imbalances
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This paper demonstrates the basic problem of the conventional d-axis and q-axis control applied to a voltage source converter (VSC) during a system imbalance. The specific imbalances studied are a balanced fault and an unbalanced fault at the terminals of the converter. The dc link voltage of the VSC is disturbed during the unbalanced fault due to negative sequence components in the system. The sequence characteristics of the system during an imbalance are further analyzed. A novel controller in the sequence domain is proposed. The positive sequence components of the control are implemented in the positive synchronous reference frame and the negative sequence components of the control are implemented in the negative synchronous reference frame. The negative sequence components are commanded to zero in the control. The approach demonstrates the stabilization of the voltage to a greater extent. The control also points out the need for a storage element when the VSC is to be used for application with that of a Doubly-Fed Induction Generators (DFIG) in wind turbines.