A study on fault arc and its influence on digital fault locator performance

Many fault locator algorithms were developed to be operated on digital relay data. The methods were derived from frequency domain equations and were established by a phasor simulation. The algorithms were based on the assumption that a fault arc had a constant impedance and did not show a nonlinear effect. But in practice, a fault arc in air is known to show nonlinearity due to its physical characteristic. To study the influence of the nonlinearity on the accuracy of a digital fault locator, a time domain simulation must be performed considering the characteristic of the fault arc. In the present paper, a time domain model of a fault locator and that of a fault arc are represented using the MODELS language in the ATP-EMTP. Various fault types are modeled considering constant and nonlinear fault resistances. The simulation results have shown that the impedance relay type method using one terminal voltage and currents is influenced by the nonlinear characteristic of the fault arc, while the current diversion ratio method using one terminal currents for two circuits is not. A sensitivity analysis of the locating error with respect to the model parameters of the nonlinear arc was performed for the impedance relay type method. It has been confirmed that the greater the degree of nonlinearity, the greater the location error.