Matras English – Researchers from South Africa have found a new method for non-destructive testing cross-linked polyethylene (XLPE) insulated power cables. These cables operate for decades under voltage underground in conditions of constant fluctuations in temperature, humidity and loads. Over time, their insulation ages and loses its properties, which can lead to failures and accidents in the power grid.
Today, in most countries, XLPE cables are tested according to a destructive pattern: the sample is cut and tested with high voltage. This helps to identify defects, such as water trees — microscopic branches in the insulation caused by moisture. But this approach is expensive and impractical: the cable becomes unusable after testing.
To find a more effective solution, scientists from the Vaal University of Technology of South Africa decided to test non-destructive methods.
To do this, they used 10 identical sections of XLPE cable with a length of 10 meters each. All cables underwent an accelerated aging process: at first they were kept at a temperature of about 50 °C, as during long-term operation, and then they were supplied with an increased voltage of 500 Hz for 3,000 hours. One cable remained intact — it was used as a reference. Every 300 hours, the scientists took out one cable and performed three types of non-destructive measurements on it: Tan δ, IRC and RVM.
The Tan δ (tangent of the dielectric loss angle) method made it possible to quickly and simply assess the overall condition of the cable insulation. Ideally, insulation should behave like a good capacitor and not lose energy, but with aging, moisture ingress or micro-damage, current leaks begin in it, which are expressed in losses. The higher the Tan δ value, the more the insulation is worn out and at risk of failure. This method is particularly useful for primary testing, as it provides an instant indication of the degree of degradation.
The IRC (isothermal relaxation current) method showed how deeply and severely damaged the inside insulation was. When a cable is in operation, charges accumulate in its insulation, especially in micro-defects (so-called “traps”). After the voltage is removed, these charges begin to flow out, and this process can be measured as a decaying current. The longer and stronger this current flows, the more such traps there are, which means that aging and internal changes in the insulation structure are stronger. This method helped not only to record the fact of wear, but to understand its nature and depth.
Finally, the RVM (reverse voltage measurement) method made it possible to determine how many residual charges remained in the insulation and how strongly it was saturated with moisture. After charging and briefly discharging the cable, voltage appears again at its ends (the so-called “reverse voltage”) caused by residual polarization. The higher this voltage is and the longer it lasts, the more moisture or defects in the insulation that hold the charges. By the shape and magnitude of this signal, it is possible to accurately judge the degree of degradation and humidity of the insulation. That is, the method is especially valuable for testing moisture penetration, one of the main factors of cable aging.
In general, the combination of the three methods gave a complete and objective picture of cable degradation. The results showed that with the course of aging, all parameters change predictably: losses increase, the number of charges in traps increases, and more moisture appears. Moreover, these changes are recorded consistently and coherently by all three methods, which proves that a complex of non-destructive measurements can be used to reliably assess the condition of XLPE cables.
Thus, the approach actually opens the way to regular and safe testing of underground cables without disconnecting or damaging them, which is especially important for long and hard-to-reach sections of backbone networks. In practice, this means that energy companies will be able to monitor the condition of cable lines in a timely and accurate manner, without resorting to expensive and destructive tests. This will help to identify wear and hidden defects in advance, reducing the risk of accidents, shutdowns and emergency repairs, as well as optimize the replacement and maintenance schedule.
