Improved schemes for traditional current-based phase selectors in wind power systems

This study shows that traditional current-based phase-selection (TCPS) schemes are not applicable to wind power systems, and subsequently proposes two improved phase-selection schemes. On the basis of the practical TCPS criteria, derivations of the essential conditions for correct phase selection, which show that the positive-to-negative sequence current branch coefficient (PNSCBC) ratio at the relay location should be within a certain range, are presented. Wind power generation systems are weak-infeed systems; consequently, the PNSCBC ratio on the wind power side is approximately equal to the negative-to-positive sequence impedance (NPSI) ratio of the wind power system. However, the authors' theoretical and simulation analysis results show that the NPSI ratio of a wind power system may exceed the range required for reliable phase selection; therefore, TCPS schemes are not applicable to wind power systems. Thus, they propose improved phase-selection schemes based on sequence current fault components and phase current difference fault components. In the proposed schemes, the PNSCBC ratio is obtained using the backside system impedance measured by the relay, and it is then used for compensating the TCPS criteria. Simulation results and field fault test data verify the efficacy of the proposed schemes.