Advanced cable test and diagnostics in action: cable condition assessment

Effective power cable maintenance is essential for minimizing failures, extending asset life, and ensuring grid reliability. In our last blog in the MV cable test and diagnostics series, we highlighted how utilizing the complete range of excitation voltages provides a significant advantage in testing and diagnostics. This blog presents a real-world case study in which a condition assessment was conducted on a service-aged medium-voltage cable, utilising these methods.

By applying DAC, VLF CR/Slope, and 0.1 Hz VLF Sine wave excitation voltages, technicians evaluated insulation integrity and localized a critical defect. The findings highlight key differences in detection sensitivity between test methods and emphasize the importance of selecting the right diagnostic approach to maintain cable system reliability.

Tan Delta and partial discharge diagnostics with 3 excitation voltages

As part of this company’s maintenance strategy, a condition assessment was conducted on a service aged power cable. The tested cable was a 12/20 kV XLPE cable, installed in 2007, with a total length of 995 meters. The exact number and position of joints were unknown. For the assessment, a van-mounted version of the TDM4540 equipped with an internal PD coupler was used as the test voltage source.

The assessment included Dielectric Loss Measurement (Tan Delta) to evaluate insulation health and detect potential degradation. Partial Discharge (PD) testing was also performed, utilizing the full range of excitation voltages – DAC, VLF CR/Slope and 0.1Hz VLF Sine wave – to identify insulation defects under different stress conditions.

These diagnostic tests provide critical insights into the cable’s aging and potential failure risks. The data supports informed maintenance planning, helping to extend asset life, minimize downtime, and ensure system reliability.

Dielectric loss measurement

The dielectric loss measurement revealed elevated losses across all phases, with results deemed critical per IEEE 400.2-2014 Annex I, indicating the need for further investigation. Notably, phase L2 exhibited significantly higher losses compared to the other two phases, suggesting a localized issue, such as a fault in a joint or termination.

Since dielectric loss measurement provides only a global assessment of the cable’s
condition, it cannot precisely locate the issue. To pinpoint the fault, either a Partial Discharge (PD) measurement or a withstand test is recommended. It should be noted that a withstand test will only identify the problem if the defect is severe enough to cause immediate failure during testing.
 

Partial discharge measurement

The utility decided to conduct a PD measurement on the cable, utilizing three excitation voltages to fully leverage the capabilities of their TDM unit. In graph series 1, the PD mappings at nominal voltage Uo are shown for DAC, VLF CR/Slope, and 0.1 Hz VLF Sine wave excitation voltage. Both DAC and VLF CR/Slope identified a defect in phase L2 at approximately 280 m, aligning with the previously observed anomaly from the dielectric loss measurement, likely indicating an issue in a joint.

Interestingly, this weak spot was not detected using the 0.1 Hz VLF Sine wave excitation voltage, highlighting the sensitivity differences between the individual testing methods.

In graph series 2, the PD mappings are displayed at a test voltage of 1.7 times the nominal voltage Uo, the standard maximum test voltage for PD measurements on medium-voltage cables. As expected, the PD concentration and intensity at the localized weak spot (~280 m) increased with both DAC and VLF CR/Slope compared to the measurements at nominal voltage.

However, even at 1.7U0, the weak spot identified by DAC and VLF CR/Slope remains undetected using the 0.1 Hz VLF Sine wave, highlighting a significant difference in detection capabilities between the testing methods.

Following the results of the dielectric loss and PD measurements, the utility made the decision to address the weak spot located at approximately 280 m, which was identified as a faulty joint. Upon dissection, workmanship-related issues were discovered, specifically insufficient use of assembly paste and mastic tape to fill cavities, as shown in the images.

After replacing the joint, a follow-up PD measurement was conducted, confirming that the weak spot had been successfully eliminated and the cable is now in optimal condition.

Conclusion

This case study highlights the effectiveness of combining dielectric loss and PD measurements to identify and localize critical cable issues, such as the defective joint discovered in this instance. By utilizing DAC and VLF CR/Slope, the PD measurement successfully pinpointed the weak spot showing PD activity at nominal voltage Uo already, leading to the proactive decision to replace the joint before a failure occurred.

Crucially, the 0.1 Hz VLF Sine wave excitation voltage failed to detect this critical defect, highlighting the limitations of relying on this method. This case underscores the importance of selecting the right diagnostic tools to ensure accurate fault detection and maintain the integrity of cable systems.

In the next blog of our MV cable test and diagnostic series, we’ll explore another case study showcasing how Megger’s solutions, leveraging the full range of excitation voltages, effectively identified a previously overlooked defect - ultimately preventing a major failure.