A fast and robust PLL of MCFC PCS under unbalanced grid voltages

In grid-interconnection system, a fast, robust and precise phase angle detector is most important to the active power control and grid synchronization. The phase angle can be easily estimated by synchronous dq PLL system. On the other hand under unbalanced voltage condition, synchronous dq PLL system has problem that harmonics occur to phase angle or magnitude of grid voltage because of the effect of the negative sequence components. So, To eliminate the negative sequence components, The PLL method using APF (all pass filter) in a stationery reference frame to extract positive sequence components under unbalanced voltage condition is researched. In this paper, we propose a new PLL method with decoupling network using APF in a synchronous reference frame to extract the positive sequence components of the grid voltage under unbalanced grid. The cut-off frequency of APF in a synchronous reference frame can be set to twice of the fundamental frequency comparing with that of APF in a stationery reference frame which is the fundamental frequency. The proposed PLL strategy can detect positive sequence voltage quickly and accurately. Simulation and experimental results are presented to verify the proposed strategy under different kind of voltage dips.