An introduction to VLF cable testing
Ensuring the reliability and efficiency of power networks is critical in modern infrastructure management. One of the most established techniques for assessing cable health is Very Low Frequency (VLF) testing, a key method for detecting insulation weaknesses, preventing unexpected failures, and extending the operational lifespan of cable systems.
Traditionally, power cables were tested using standard 50/60 Hz AC voltage, but the strong capacitive nature of cables made this approach impractical for on-site diagnostics. Over the past 30 years, VLF testing has emerged as a widely accepted alternative, offering a more efficient means of assessing cable integrity with lower power requirements and improved portability. Beyond simple fault detection, VLF testing is now a core component of cable diagnostics, enabling utilities to make informed maintenance decisions that enhance network resilience.
In this blog, the first in our MV cable testing and diagnostics series, we’ll explore the history of VLF testing, its role in insulation diagnostics, and how advanced techniques like partial discharge (PD) measurement and tan delta analysis complement this method. By understanding the evolution of VLF testing, you’ll gain insight into how it supports proactive maintenance strategies and contributes to a more reliable energy supply.
The origins of VLF testing
Cable testing using modern 0.1 Hz VLF testing became widely established in the early 1990s. The primary objective of this testing was to identify operationally hazardous defects caused by “electrical trees,” which are triggered by “water-treeing” within plastic-insulated cable systems. The first generation of cross-linked polyethylene (XLPE) cables had significant issues due to water molecules trapped in the insulation during the manufacturing process.
Under the influence of an electric field, heat, and other by-products, these water molecules led to the formation of “water trees” in the insulation. Over time, these water trees degraded the insulating properties of the material, eventually turning into “electrical trees.” Electrical trees could cause rapid breakdowns in cable insulation, leading to unplanned failures of cable sections.
In the early 1990s, as failures caused by these phenomena became more frequent, academic research explored ways to prevent treeing issues in the future. At the time, sensitive diagnostic measurements on-site were not yet available, making cable testing the only viable method to ensure a cable system’s operational readiness. The testing process allowed defects to break down during testing rather than during normal operation, preventing unexpected outages.
Today, manufacturing processes for XLPE cables have improved, significantly reducing the risk of water molecules being trapped in the insulation. As a result, the formation of water trees is now either non-existent or negligible. However, VLF withstand testing is still used on newly installed cables to detect workmanship-related issues and ensure the safe energization of the cable system.
From testing to diagnostics
While the primary goal of cable testing is to identify operationally hazardous defects and safely break them down, cable diagnostics focus on detecting issues without risking damage to the cable system. Diagnostics aim to uncover and locate potential problems within the cable system while ensuring the insulation remains intact.
Research over the years has shown that assembly errors, which do not cause immediate electrical breakdowns, are often the root cause of cable failures. These faults take time to develop and cannot be detected by standard cable testing. This is where partial discharge (PD) diagnostics become essential.
Assembly errors in cable accessories can lead to partial discharges, which cause the accessories to age prematurely and eventually fail. Advanced PD measurement techniques can efficiently detect and pinpoint these discharges, allowing you to identify which accessories are likely to fail in the future—without pushing the cable to the point of breakdown or requiring immediate repairs.
Another powerful diagnostic tool is tan delta measurement, which assesses the overall ageing of cable insulation by measuring its dielectric losses. Elevated losses often indicate insulation deterioration or moisture ingress, which can potentially lead to cascading failures.
Tan delta measurements provide valuable insights into the cable’s ageing process, enabling you to make better-informed asset management decisions and helping to prevent future operational failures.
The evolution of cable asset management
Cable asset management has evolved from basic withstand testing to advanced diagnostic techniques that offer more detailed insights into cable condition. With improvements in cable manufacturing reducing traditional risks, modern diagnostics such as partial discharge and tan delta measurements play a crucial role in identifying emerging faults before they lead to failures. By implementing a combination of testing and diagnostics, you can enhance reliability, minimize outages, and extend the lifespan of your cable infrastructure.
Look out for the next instalment in our MV cable test and diagnostics series, where we will explore four key methods for cable testing and diagnostics, and how they can help you keep your networks running smoothly.
