A signal reforming algorithm based three-phase PLL under unbalanced grid conditions

This paper proposes a signal reforming algorithm for a three-phase phase-locked loop (PLL) based technique to estimate the phase angles of an unbalanced three-phase voltage system. The proposed technique works on three-phase voltage system suffering from simultaneous amplitude and phase unbalances by transforming it into a balanced three-phase voltage system and keeping the information of phase angles of all the phases. This makes the conventional synchronous reference frame PLL (SRF-PLL) suited for accurate estimation of phase angle of one phase. The phase angles of other two unbalanced phases are also calculated accurately without using additional PLLs. The proposed PLL works perfectly even when the frequency of the system is time varying. The proposed technique not only solves the phase estimation difficulties of the conventional SRF-PLL working under unbalanced grid conditions in a relatively simple way, but also it avoids the use of three single-phase PLLs to estimate the phase angles of a three-phase voltage system under both amplitude and phase angle unbalanced conditions. All the simulations have been carried out using MATLAB/Simulink software to verify the performance of the proposed technique.