What Are Underground Power Cables?

Underground power cables are insulated cable systems buried below grade for the transmission and distribution of electrical energy, carrying alternating current (AC) or direct current (DC) at voltages ranging from a few hundred volts in low-voltage service laterals to 500 kV and above in bulk-power transmission corridors. They serve as the subsurface alternative to overhead transmission lines, offering immunity from wind, ice, and lightning at the cost of higher installation expense and more complex fault management. The technology is central to urban grid modernization, offshore renewable energy integration, and the electrification of transportation networks.

Underground power cable engineering draws on dielectrics, thermal science, power systems analysis, and corrosion engineering. Standards governing design, testing, and installation include IEC 60840 for high-voltage AC cables, IEC 62067 for extra-high-voltage AC cables, and IEEE 400, which covers field testing of shielded power cable systems in service.

Insulation and Thermal Design

Cross-linked polyethylene (XLPE) dominates modern underground power cable insulation for AC systems above about 5 kV. Cross-linking converts thermoplastic polyethylene into a thermoset material with a continuous operating temperature of 90°C at the conductor surface, a short-circuit temperature limit of 250°C, and dielectric properties that remain stable over decades of service. The insulation is co-extruded with inner and outer semiconducting screens in a single triple-extrusion process, eliminating voids at the interfaces that would otherwise initiate partial discharge. Older paper-insulated lead-covered (PILC) cables, insulated with oil-impregnated paper, are still found in urban networks installed before the 1980s but are no longer specified for new construction. The cable's current-carrying capacity (ampacity) is determined by the thermal resistivity of the surrounding soil, the installation depth, and the cable's own thermal resistance; these parameters are calculated using methods in IEC 60287, which provides the standard thermal model for buried cable systems.

High-Voltage and HVDC Systems

At transmission voltages above 150 kV, cable circuits carry bulk power between substations in densely built environments where overhead towers are not viable. Extra-high-voltage XLPE cables at 400–500 kV have been installed in metropolitan grids across Europe and Asia, replacing ageing overhead circuits that can no longer be upgraded within constrained rights-of-way. High-voltage direct current (HVDC) underground and undersea cables connect offshore wind farms to onshore grids and link asynchronous AC networks. Mass-impregnated non-draining (MIND) cables and, more recently, extruded HVDC XLPE cables are the dominant technologies for long HVDC land routes. HVDC extruded cables present different insulation challenges from their AC counterparts because space-charge accumulation under a steady DC field can produce localized field enhancement that accelerates insulation ageing, a phenomenon that remains an active area of IEEE-published cable research.

Cable Monitoring and Protection

Because underground power cables are inaccessible after installation, continuous monitoring and precise fault location are essential for reliable operation. Partial discharge (PD) monitoring detects the small internal discharges that precede insulation breakdown, providing an early indication of degradation before a service interruption occurs. Distributed temperature sensing (DTS) uses optical fibers embedded in or alongside the cable to measure temperature along the full cable length in real time, identifying hot spots caused by soil drying or excessive loading that could accelerate insulation ageing. When faults do occur, time-domain reflectometry and decay voltage methods locate the fault point, typically to within a few meters on a cable many kilometers long. The NIST Physical Measurement Laboratory supports calibration standards for the high-voltage and partial-discharge instrumentation used in these diagnostics.

Applications

Underground power cables have applications across a wide range of sectors, including:

• Urban power distribution networks and substation interconnections
• Offshore wind farm export cables connecting turbines to onshore grids
• High-voltage direct current (HVDC) long-distance transmission links
• Railway and transit electrification in tunnels and urban corridors
• Industrial plant power distribution requiring high reliability