Calculating Intermediate Faults in Underground Cables

Short-circuit calculations are extremely important in the application and setting of protective relays and the analysis of power system operations. This paper illustrates the calculation of fault currents and voltages during intermediate faults in underground cables, taking into consideration the cable sheath bonding and grounding method. The compensated ground loop impedance for faults along the cable is nonlinear with respect to fault distance when the cable sheaths are cross-bonded and grounded at both cable ends. This makes the application of ground distance relay elements for cable protection complex and undesirable. This paper provides numerical examples of intermediate fault analysis in underground cables to help protection engineers study similar or more advanced power system faults. This paper demonstrates the importance of phase frame of reference calculations in fault analysis, relay settings considerations, and an overall understanding of power system behavior. In addition, this paper shows the comparison of fault calculation results between Mathcad® and an Electromagnetic Transients Program steady-state solution.