Armor Losses in High Voltage Cables in Metrology

Armor Losses

Armor is used in high voltage cables to prevent the cable from being damaged by impact or abrasion and to act as a grounding path. Yet, armor can also contribute to cable losses because alternating current (AC) passing through the cable induces eddy currents in the armor. Armor losses are one reason why cables may not be as effective as they may be.

Armor losses in high voltage cables vary in intensity depending on a number of variables, such as the armor’s material and thickness, the strength and frequency of the AC, and the distance between the armor and the cable’s conductor. Many methods exist for determining armor losses in high voltage cables.

The inductive approach includes wrapping a coil around the armor and measuring the voltage induced in the coil as a result of the eddy currents. The magnitude of the induced voltage, which may be measured with a voltmeter or oscilloscope, is inversely proportional to the armor loss.

To prevent interference from the electromagnetic fields caused by the eddy currents, a shield can be wrapped around the cable as part of the shielding approach. Using a clamp-on ammeter, the amount of the shielded current is measured, and the armor losses are determined by subtracting the shielded and unshielded currents.

The current and voltage in the cable are measured as the AC frequency is swept across a range of values; this is the frequency sweep method. Armor losses are found by subtracting the observed current from the theoretical current.

The heat dissipation method involves taking readings from thermocouples or infrared cameras to determine the armor’s internal temperature. It is possible to compute the heat created by the eddy currents using the measured temperature and the thermal conductivity of the armor material.

Testing and maintaining high voltage cables includes taking measurements of armor losses to guarantee the cable will function effectively and reliably for its intended lifespan.