As offshore wind projects move towards higher voltages, larger turbines and longer cable routes, cable testing is becoming an increasingly important part of project reliability. For HIGHVOLT, its role now extends beyond confirming installation quality at commissioning to identifying potential weaknesses before they result in costly in-service failures.
Inter array grid designs have moved from 36 kV towards 66 kV and an operating voltage class of 72.5 kV, helping to reduce current and thermal losses. This brings cable systems closer to high-voltage practice and changes expectations around commissioning, diagnostics and documentation.
Why resonant testing matters offshore
For primary distribution cables rated above 36 kV, standards and industry practice point towards resonant AC testing as the preferred method.
A resonant test system (RTS) uses cable capacitance and a resonant reactor to form an oscillating circuit. The source only needs to compensate for circuit losses, making it possible to energise long cable lengths without an impractically large power supply.
HIGHVOLT’s WRV O system was developed for testing array cables once laid onsite. Key specifications include:
For offshore wind, practical considerations are as important as electrical performance. The equipment must be transported, lifted and operated within restricted offshore substations while accounting for weather, deck access and other work taking place nearby.
Adding partial discharge insight
A withstand test determines whether a cable system can tolerate the required voltage for a defined period. Partial discharge (PD) measurement can go further by helping engineers identify and locate localised defects.
This is particularly important for long cable systems, where joints, terminations and installation-related deviations can become weak points.
Conventional PD measurement can be limited by signal attenuation over longer distances. HIGHVOLT’s TruePD methodology instead analyses lower-frequency signal components and applies signal processing to distinguish PD activity from background noise.
The company says its HiMON TruePD approach can detect and locate PD-active faults over distances of up to 12 km between two sensors. It also supports remote evaluation, expert diagnosis and early detection of ageing signs.
These are company-specific performance claims, but they illustrate the wider move towards condition assessment and earlier fault detection.
Designing testing around offshore operations
The development of HIGHVOLT’s offshore RTS also demonstrated that the test source is only one part of the process.
Equipment must move from onshore mobilisation through marine transport, offshore lifting, installation, operation and eventual demobilisation. The original offshore system therefore limited individual units to a maximum weight of 3.7 tonnes and used a modular arrangement intended for setup by a small qualified workforce.
This modular approach uses separate units connected to form the offshore high-voltage testing arrangement.
Practical design can influence:
Reducing complexity at these stages can help prevent cable commissioning from becoming a project bottleneck.
From commissioning to lifecycle reliability
Demand for testing capacity is also increasing. In 2025, Kinectrics signed a contract with HIGHVOLT for ten additional mobile resonant test systems, with an option for four more, linked to expanding renewable energy construction and high-voltage cable commissioning requirements.
For offshore wind operators, the wider issue is lifecycle reliability. An unplanned cable failure can result in lost generation alongside vessel and weather-related delays.
Developers need reliable commissioning evidence, while owners want earlier warning of developing faults and operators need information that supports planned maintenance.
As offshore wind becomes more important to electricity networks, cable testing is therefore moving beyond a final installation check. Resonant testing and advanced PD monitoring can provide greater insight into installation quality and developing defects, helping operators manage critical cable assets throughout their operating lives.
For the full Industrial Insight feature from HIGHVOLT on resonant AC testing, partial discharge monitoring and offshore cable reliability, visit: https://pes.eu.com/exclusive-articles/cable-testing-moves-into-the-reliability-era