Adaptive Mho Characteristic-Based Distance Protection for Lines Emanating From Photovoltaic Power Plants Under Unbalanced Faults

The exclusive fault characteristics of photovoltaic (PV) power plants and the existence of fault resistance cause a high probability of failure in operation and unwanted operation of distance relay with conventional mho or quadrilateral characteristic. Different from the fixed operation zones of conventional mho and quadrilateral characteristics, the adaptive mho characteristic presented herein can adaptively adjust its operation zone to solve the aforementioned issues. The basic principle of adaptive mho characteristic-based distance protection is described and the uniform expression of adaptive setting impedance is derived. Subsequently, taking the fault characteristics of PV power plant into consideration, the method of solving the phase angle of fault current is presented, which is the key to calculating adaptive setting impedance. The performances of the proposed protection scheme are evaluated by extensive digital simulations. The proposed protection scheme exhibits satisfactory robustness against the fault resistance and excellent adaptability to different grid codes. It can not only discriminate the internal and external faults occurring in the lines emanating from the PV power plant but also correctly identify the fault direction.