Increasing Conductor Cross-Sections

• Over the last few decades, the conductor cross-sections of power lines have been steadily increasing due to the growing demand for improved electric power transportation capacities.
• This increase impacts the sensitivity of cable linear resistance to the skin effect present in alternating current (AC) conditions.

DC vs. AC Linear Resistance:

• Linear conductor resistance is typically specified by its direct current (DC) value (as per IEC 60228), which is used to rate the maximal transmission ampacity.
• However, for large cross-section conductors (typically > 1,000 mm²), the skin effect becomes non-negligible at power frequencies (50 Hz or 60 Hz) and affects the maximum allowed transmitted power.
• The transmitted power is also limited by the cable’s maximum operating temperature, directly related to Joule losses.

Measurement Methods:

• Traditionally, a four-point bridge (Kelvin) method is used to measure the DC resistance, accounting for temperature effects and parasitic voltages.
• For AC measurements, no standardised experimental method exists yet. A new CIGRE working group is preparing recommendations for AC linear resistance measurement.

Challenges in AC Measurement:

While similarities exist with DC linear resistance measurements (such as achieving steady current distribution), specific considerations are necessary for accurate AC measurement: 

• Voltage Pickup: Simply picking up the voltage at the surface of the conductor is insufficient because the generated magnetic fields around the conductor contribute to the measured voltage drop.
• Correction Factor: The technical brochure of CIGRE TB272 suggests potential approaches and defines a correction factor, ks, for estimating the AC resistance based on the DC value and design.