Repairing Underground Power Cables Is Nearly Impossible

Summary by Krater

This video examines the history and engineering of an underground high-voltage transmission line built in Los Angeles in 1989, detailing how it works, why high-pressure oil-filled and liquid nitrogen freezing were used to manage thermal and electrical challenges, and how it was eventually replaced in 2018.

From the video

Answers: How do underground high-voltage power lines work and how are they repaired?

• High-voltage electrical transmission
• Underground power lines
• High Pressure Fluid Filled (HPFF) cables
• Thermal mechanical bending
• Liquid nitrogen cable freezing
• Cable fault location and repair

What it concludes

• High-voltage transmission lines use high voltages to reduce current, minimize energy waste, and make electrical transmission more efficient.
• Running high-voltage lines underground requires insulating three phases inside a single pipe filled with pressurised dielectric oil to prevent electrical arcs.
• High-pressure oil-filled cables circulate oil to act as a heat sink, carrying away heat generated by electrical resistance in the conductors.
• Thermal-mechanical bending can cause paper insulation around underground cable conductors to soften, tear, and experience dielectric breakdown over hundreds or thousands of cycles.
• Liquid nitrogen freezing is used to create solid plugs on either end of a damaged underground high-voltage oil-filled cable section so it can be repaired under pressure.
• The Los Angeles Department of Water and Power replaced the original 1989 high-pressure oil-filled transmission line with newer polyethylene-insulated cables in 2018 due to ongoing degradation issues.