Synchronization in highly distorted three-phase grids using selective notch filters

Robust methods able to extract the magnitude and frequency of the positive sequence voltage in highly distorted grids is of paramount importance in distributed power generation (DPG) systems. Phase locked loop (PLL) synchronization methods are commonly used for this purpose. PLLs can be grouped into pre-filter stage methods and filter in the loop methods. Both provide similar performance and can be designed to reject one or more disturbing harmonic components. However, compared to pre-filter stage methods, filter in the loop methods are conceptually simpler and easier to tune. Conversely, filter in the loop methods can show an unstable behavior if they are not properly designed and do not include disturbance rejection strategies when the magnitude is distorted, their use not being therefore advisable e.g. when power calculation, droop control or magnitude synchronization are needed. This paper proposes a filter in the loop synchronization technique, able to isolate the magnitude and phase of the positive sequence voltage of the microgrid/grid even under high distorted conditions. The method combines an angle-tracking observer, to obtain the positive sequence phase angle, and a scalar product to obtain the positive sequence magnitude, enhancing the disturbance rejection capability for both phase and magnitude estimation.