Enhanced Grid Fundamental Positive-Sequence Digital Synchronization Structure

The second-order generalized integrator (SOGI) filter and delayed signal cancellation (DSC) structures have been used separately in order to improve the synchronization systems response in adverse grid conditions. Taking into account its characteristics, this paper proposes a new structure based on a combination of SOGI filter, DSC method, and a software phase-locked loop (S). The enhanced synchronization structure is used to estimate the fundamental positive-sequence component of grid voltages (v(s1)(+)) in distorted and asymmetric three-phase systems. DSC and SOGI working in the stationary alpha-beta frame are used to obtain the v(s1)(+) component, while S in a synchronous reference frame (SRF) or the d-q frame is used to obtain v(s1)(+) pulsation and phase. The cutting frequency of the SOGI filter is selected by taking into account the capability of DSC to eliminate determinate order harmonic components. The bandwidth of S is adjusted to achieve a good relation between dynamic response and filtering capability. The SOGI filter has an adaptive structure to modify its cutting frequency when a variation in the fundamental frequency of grid voltages occurs. The performance of the proposed structure is verified through simulations and experimental cases, using a grid set of voltages that include all nonefficient voltage components (distortion, asymmetries, and negative-sequence components).