A practical and fast sequence components detection scheme for three-phase unbalanced grid voltage

Obtaining the sequence components (i.e. positive, negative and zero sequences) of three-phase unbalanced grid voltage (TUGV) is a prerequisite for realizing the high-performance grid-tied converter controls. To address slow response problems of the traditional closed-loop methods, this paper presents a practical open-loop scheme for sequence components detection, which is characterized by fast detection and easy implementation. Firstly, TUGV is treated as three independent single-phase voltages, and the virtual TUGV vectors can be obtained by their virtual orthogonal signals and the single phase-locked operation. And then, according to these vector expressions, the sequence components of TUGV are calculated by the symmetrical component method and the single phase-locked inverse operation. Based on the above steps, the formulas for solving the voltage sequence components are derived. Moreover, the real-time phases and magnitudes of voltage sequence components can be obtained in this scheme. Within the standard frequency range of grid operation, the scheme can use a fixed frequency to capture sequence components without the real-time frequency of the grid. The proposed scheme can extract the sequence components within 3 ms or 11 ms for the application scenario without or with harmonics. The correctness of the proposed scheme is verified by simulation results. The scheme is free from the parameter debugging of the traditional closed-loop methods and achieves the fast acquisition of sequence components.