In an attempt to moderate the cost of a micro-SMES voltage sag compensating system the minimum energy injection algorithm has been adopted. To protect the load voltage from sudden phase angle shift when the source voltage phase jump occurs simultaneously with the voltage sag, a phase-invariant voltage injection scheme is applied. A method for fast detection of the fundamental positive sequence components and a minimum energy injection algorithm are proposed and described respectively. Experiments of voltage sag compensation were carried out to confirm the validity of the proposed methods.
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