Advanced cable test and diagnostics in action: partial discharge diagnostics

Accurate and reliable partial discharge (PD) measurements are essential for identifying weaknesses in power cables before they lead to costly failures. However, inconsistencies in testing results can create uncertainty, making it difficult for asset managers to make informed maintenance decisions. This case study, part of our MV cable test and diagnostics series, highlights how using the right testing methodology is crucial for obtaining dependable results.

At a critical customer site, multiple service providers conducted PD measurements on an essential cable but produced conflicting findings. To resolve this, Megger was brought in to perform a repeat assessment using our state-of-the-art, all-in-one solution. By leveraging a full range of excitation voltages—Damped AC (DAC), Very Low Frequency Cosine-Rectangular (VLF CR/Slope), and 0.1 Hz VLF Sine wave—we provided a comprehensive and consistent evaluation of the cable’s condition. 

The results not only exposed critical weak spots but also demonstrated the limitations of certain testing methods, reinforcing the importance of selecting the right diagnostic approach.

Partial discharge diagnostics with 3 excitation voltages

The tested cable was a 12/20 kV XLPE cable, installed in 2004, with a total length of 1,200 meters. The exact number of joints and their positions were unknown. For the assessment, a van-mounted version of the TDM4540, equipped with an internal partial discharge coupler, was used as the test voltage source to ensure precise and reliable diagnostics.

Three comprehensive measurements were conducted on this cable system, utilising all available excitation voltages. For aging cables, damped AC voltage (DAC) is recommended for its superior performance, while VLF CR/Slope voltage is the optimal choice for new cables, such as during commissioning tests.

Although the 0.1 Hz VLF Sine wave is a basic option for beginners, it lacks the in-depth insights provided by DAC and VLF slope voltage, as demonstrated in this case.

Graph series 1 illustrates the partial discharge mapping at operating voltage, highlighting the cable’s condition during normal operation. Notably, two vulnerabilities – located at approximately 175 m and 650 m – were clearly identified using both the DAC and VLF CR/Slope excitation voltage. These weak points are active during normal operation, degrade the insulation, and will eventually cause cable failure.

However, with the 0.1 Hz VLF Sine wave, these critical issues were not detected.

With an increased test voltage of 1.7 U0, the differences become even clearer, as can be seen in the following series of graphs. A total of seven critical defects were identified with DAC and VLF CR/Slope, while only four were identified with the 0.1 Hz VLF Sine wave.

What is particularly important, however, is that the inception voltage (the voltage at which partial discharges begin) with 0.1 Hz VLF Sine wave is not comparable to the results obtained from DAC or slope voltage in any of the identified cases.

Based on Megger’s recommendation, the customer removed and inspected the joint at 650 m.

Several critical assembly issues were uncovered, including contamination (sand) within the insulating shrinkable body and incorrect assembly dimensions of the connector. Additionally, the brown discoloration was a clear sign of partial discharge activity during normal operation.

Conclusion

This case study underscores the significant impact of choosing the appropriate excitation voltage for partial discharge measurements. The ability of DAC and VLF CR/Slope to detect multiple critical defects—including those missed by the 0.1 Hz VLF Sine wave—highlights the superior sensitivity and reliability of these advanced testing methods.

By following Megger’s recommendations, the customer was able to identify and inspect a faulty joint, uncovering serious assembly issues that would have led to insulation failure. The clear visual evidence of partial discharge activity confirmed the accuracy of our testing, giving the customer the confidence to take proactive maintenance action. 

This real-world example reaffirms the importance of using advanced PD testing techniques to ensure accurate fault detection, enhance system reliability, and prevent unexpected cable failures.