Improved delayed signal cancellation-based SRF-PLL for unbalanced grid

Problems with power quality in the grid have gained a lot of attention recently due to rapid increase in the amount of grid-connected power converters. The converter should produce sinusoidal currents also during abnormal conditions, such as unbalanced grid voltages. Several methods, like the delayed signal cancellation-based method (DSC), have been proposed to alleviate the detrimental effect of unbalance. This paper proposes an improvement to a delayed signal cancellation based synchronization algorithm for unbalanced grids. The proposed PLL structure employs only half of the delay required by the previously reported delay-based methods and has a simple structure, requiring only one delay block. Therefore, higher control bandwidth can be used to achieve fast tracking of the grid frequency and phase angle due to smaller internal delay. The proposed method is compared with conventional DSC-based PLLs implemented in synchronous and stationary reference frames. The proposed PLL is shown to obtain significantly smaller settling time in the estimated grid angle and frequency, after a symmetrical phase angle jump in the grid voltages and sudden appearance of an unbalanced fault. A small-signal model is also proposed, which can be used to design a stable control loop.