Adaptive loss of field protection based on phasor measurements

Loss of field protection is of great importance for the security of large-scale generators. With the development of large-scale generators and corresponding loss of field protection, the complicated structure leads to the high possibility of loss of field faults. According to the statistical data, more than sixty percentages of the synchronous generator faults are the excitation system faults, which might cause loss of load, tie-line tripping, instability, collapse and etc. Therefore, it is crucial to identify loss of field as early as possible and take countermeasures. In loss of field protection, the impedance boundary criterion of steady state stability limit is widely used to distinguish loss of field and normal condition. However, the steady state stability limit circle is affected by the system operating modes, which might result in mis-operation of the relay. In this paper, an approach for on-line identifying system equivalent impedance is presented based on PMU measurements. The steady state stability limit circle can be adaptively fit for operating modes. The computer simulation results demonstrate that the proposed method works well.