Compound synchronous reference frame PLL and unbalance control strategy for power conditioning system in weak grids

An unbalanced control strategy is proposed to eliminate the 2^nd harmonic at the DC bus, based on a compound synchronous reference frame phase-locked loop (CSRF-PLL) system, which composed of a T/4 delay positive sequence PLL (PS-PLL) and a T/3&T/6 delay negative sequence PLL (NS-PLL). The proposed PLL system can detect the phase angle and frequency of the utility voltage without errors, and both of the positive-sequence and negative-sequence components in dq-coordinate system can be figured out concurrently. By interaction between the original signal and the delayed one, effect of some characteristic harmonics, e.g. -5^th and +7^th harmonics, will be obliterated. According to the principle of simultaneous power balance, a negative sequence current is injected into the positive sequence reference current via a negative-sequence-to-positive-sequence transformation (NS-PST). And a regulator composed of a PI link parallel with 2^nd and 6^th quasi-resonant links is designed. Finally, the simulation and experimental results demonstrate that the proposed PLL system can work well even when the input signals have severe asymmetry in both amplitude and phase angle, its advantage at handling with harmonics is verified as well. Furthermore, the unbalanced control strategy can suppress the 2^nd harmonic at the DC port evidently, which would reduce the power loss and lengthen the cycle-life of the batteries installed in the power conditioning system (PCS).