Protection challenges for North America's first combined cable/overhead double-circuit 500 kV transmission line with mutual coupling

In order to reduce congestion and increase reliability, Southern California Edison (SCE) set forth to construct a new 500 kV transmission line. Due to right-of-way concerns, SCE needed to install a section of the new line underground, resulting in a combined cable and overhead transmission line. The finished line consists of two overhead sections separated by an underground section. The inclusion of underground cables causes the line to draw significant charging current, resulting in severe overvoltage conditions when the line is open circuited or lightly loaded. The protection package for this transmission line consists of distance protection complemented by a communications-assisted permissive scheme operating in parallel with a line current differential scheme. Because impedance-based fault locating would not be reliable for a line of this structure, we explored the use of a traveling-wave fault locater. To verify the correct operation of the proposed protection and fault locating scheme, the transmission line and its surrounding power system were modeled in a real-time digital simulator. This paper describes how to calculate line impedance and explores impedance-based and traveling-wave fault locating methods. It also discusses the protection challenges associated with operating a double-circuit transmission line as a single circuit and the challenges of a transmission line containing both overhead and underground conductors. Finally, the paper describes using real-time digital simulation to evaluate the performance of the protection system.