A positive and negative sequence component separation method for grid voltage based on the phase locked loop with an adaptive notch filter

In order to meet the unbalanced control demand of the grid-connected power converters, the fast and accurate extraction of the fundamental positive and negative sequence components of the grid voltage is necessary. Under unbalanced grid voltage condition, the negative sequence component appears as the double frequency oscillations in the d-q axes components of the phase locked loop based on synchronization reference frame (SRF-PLL), which affects the extraction of the fundamental components and the phase angle signal. By applying the adaptive notch filter (ANF) to the structure of SRF-PLL, this paper proposed a grid voltage synchronization method based on phase locked loop with adaptive notch filter (ANF-PLL), which is able to achieve the separation of the fundamental positive and negative sequence components. This method employs the two orthogonal output signals of ANF to cancel out the double frequency oscillations in d-q axes components of grid voltage caused by negative sequence component and extract the fundamental positive and negative sequence components at the same time. In this way, the influence of unbalanced grid voltage on the extraction of synchronization signals is eliminated. Without the need of instantaneous symmetrical calculation or decoupling unit, the method can achieve the separating and extracting of the positive and negative sequence components of the fundamental grid voltage under single synchronous reference frame. Compared with the other synchronization schemes with the similar purpose, this method has simpler structure and less computation cost. The experimental results indicate that the proposed method can make a precise extraction of the amplitude and phase angle of the fundamental positive and negative sequence components under unbalanced and frequency-variation grid voltage conditions with good dynamic characteristic. © 2013 Chinese Society for Electrical Engineering.