Synchronization of grid-connected renewable energy sources under highly distorted voltages and unbalanced grid faults

Renewable energy sources require accurate and appropriate performance not only under normal grid operation but also under abnormal and faulty grid conditions according to the modern grid codes. This paper proposes a novel phase-locked loop algorithm (MSHDC-PLL), which can enable the fast and dynamic synchronization of the interconnected renewable energy system under unbalanced grid faults and under highly harmonic distorted voltage. The outstanding performance of the suggested PLL is achieved by implementing an innovative multi-sequence/harmonic decoupling cell in order to dynamically cancel out the oscillation of the positive sequence voltage vector, produced by the existence of unbalanced and harmonic distorted voltage. The fast and accurate response of the proposed PLL under abnormal grid conditions is very important for the appropriate synchronization and control of the interconnected renewable energy systems. Therefore, the performance of the new PLL can increase the quality of the injected power under abnormal conditions and in addition enable the renewable energy systems to provide the appropriate support to the grid under balanced and unbalanced grid faults.