MV cable test & diagnostics: four essential methods
Ensuring the reliability and longevity of power cables requires effective testing and diagnostic techniques. As seen in the first blog in our MV cable test and diagnostics series, Very Low Frequency (VLF) testing plays a crucial role in identifying insulation weaknesses, detecting potential failures, and maintaining the integrity of power systems. However, with multiple excitation voltage options available, choosing the right one for each application is essential for obtaining accurate results and optimizing maintenance strategies.
This blog, the second instalment in our VLF testing series, explores four key excitation voltages used in cable testing and diagnostics: DC voltage, 0.1 Hz VLF Sine voltage, 0.1 Hz VLF cosine-rectangular (50 Hz Slope) voltage, and Damped AC (DAC) voltage. Each method offers unique advantages and is suited to specific testing scenarios. By understanding the strengths and limitations of each voltage form, engineers and technicians can make informed decisions that enhance testing efficiency, reduce downtime, and ensure power system reliability.
DC voltage
For decades, DC excitation voltage was the standard for cable withstand testing. Today, it’s only used for HVDC cables due to the risk of insulation damage and its inability to detect certain type of defects.
So why is DC voltage still relevant? Primarily, it’s essential for sheath testing in compliance with IEC 60229, typically conducted at 5 to 10 kV. If a sheath fault is found, pulsed DC voltage enables precise pinpointing of the fault location, making DC testing an indispensable tool for sheath integrity assessments.
0.1 Hz VLF Sine voltage
With two VLF technologies integrated into the all-in-one TDM systems, which is preferred for cable withstand testing? The answer is clear: VLF cosine-rectangular voltage is the optimal choice for withstand testing. So why use 0.1 Hz VLF Sine voltage? If you’re looking to assess insulation aging, dielectric loss (tan delta) measurement using the 0.1 Hz VLF Sine wave is essential. It also ensures standard-compliant testing for short cables. When tan delta is integrated, a monitored withstand test – tracking dielectric losses during testing – is recommended, similar to how leakage current is measured with VLF cosine rectangular voltage.
While 0.1 Hz VLF Sine can also be used for partial discharge (PD) measurements, it’s not the recommended solution. Due to its limitations in detecting weak spots, Megger recommends the VLF cosine-rectangular technology for more reliable PD testing, as demonstrated in real-world case studies.
0.1 Hz VLF cosine-rectangular voltage / 50 Hz Slope technology
Megger’s VLF cosine-rectangular voltage, also named 50 Hz slope technology, is the ultimate solution for cable withstand testing – whether for short or long cables! Combining near-operational frequency (20-500 Hz) with low power consumption, this innovative test equipment ensures efficient testing at a standard-compliant 0.1 Hz, guaranteeing cable system safety. Its ability to reuse stored power allows testing even the longest cables at a standardized frequency.
Additionally, it excels in partial discharge measurements, closely simulating real operating conditions for accurate results. With leakage current monitoring, insulation quality can be evaluated during testing, making it the ideal choice for commissioning and acceptance tests.
Damped AC/DAC
The fourth test or diagnostic voltage is DAC (Damped AC) voltage. DAC measurements are designed to detect partially discharged defects that may not cause an immediate breakdown during a typical cable test but still pose a long-term risk to the cable’s lifespan. With damped AC, the cable is exposed to voltage for the shortest possible duration, which is especially critical for aging cable sections.
When testing severely aged cables, the goal is to conduct diagnostics without subjecting the cable to unnecessary risk. DAC minimizes the potential for breakdowns during or after testing by applying a time-limited voltage stress. The DAC voltage form and frequency are highly effective in identifying defects that are prone to partial discharge. The principle of voltage generation is based on a free oscillating circuit, ensuring minimal interference, making DAC an ideal voltage form for partial discharge diagnostics in both new and aging cables.
Summary
This comparison highlights the importance of selecting the right diagnostic approach based on the cable type, test objective, and operational conditions. By leveraging Megger’s advanced testing solutions, engineers can ensure accurate fault detection, enhanced system reliability, and proactive maintenance—helping to prevent failures before they occur.
In our next blog in the MV cable test and diagnostic series, we’ll look at a real-world case study that shows how implementation of these methods was crucial to locating and resolving a critical cable defect.
