29.8 : Fault Types

When analyzing a single line-to-ground fault from phase A to ground at a three-phase bus, it is important to consider the fault impedance. This impedance is zero for a bolted fault, equal to the arc impedance for an arcing fault, and represents the total fault impedance for a transmission-line insulator flashover. To derive sequence and phase currents, fault conditions are translated from the phase domain to the sequence domain.

For line-to-line faults occurring between phases B and C, the conditions in the phase domain are similarly converted to the sequence domain. This conversion establishes the relationship between phase and sequence currents. In these fault conditions, the positive-sequence and negative-sequence networks are connected in parallel at the fault terminals through the fault impedance, ensuring the accurate representation of the fault.

In the case of a double line-to-ground fault, which occurs from phase B to phase C via fault impedance to ground, fault conditions must first be determined in the phase domain. These conditions are then transformed to the sequence domain. This process involves connecting the zero-sequence, positive-sequence, and negative-sequence networks in parallel at the fault terminal. By doing so, the sequence fault currents and line-to-ground voltages can be accurately determined.

This approach to fault analysis, by translating conditions between the phase and sequence domains, allows for the comprehensive evaluation of fault currents and voltages. It ensures that all relevant network connections and fault impedances are appropriately considered, providing a detailed understanding of the fault's impact on the system.