Cable Testing – Is Resonance Testing The Way Forward For Offshore Wind?

Published 15 Jun 2021

• Reproduced with kind permission of Calum Galvin from Mane - Worldwide Recruitment Specialists

Offshore Wind

Cable Testing of Medium Voltage Power Systems & Accessories 

Imagine this: Right before the winter season, something goes wrong at an offshore wind farm. There’s a fault somewhere in one of the long submarine cables that carry the generated electricity to land.

But with storms raging and the testing season almost over, nobody can get out there to run a high-voltage test on the cables. There’ll be no way to find the fault until at least April – which means no electricity. The turbines spin all winter for nothing, while the power company bleeds money.

Conventional cable testing systems aren’t designed to be used in the middle of the ocean.

Salty air, rain, snow, and stormy seas make it impossible to operate them reliably. And you can’t exactly pitch a tent over them. Then there’s the difficulty of getting people to the site to carry out the tests. It’s only possible for a few months of the year, and even then it takes considerable advance planning.

This means offshore wind companies have resigned themselves to the use of SOAK tests (a 24-hour test with only nominal voltage) to test cables. But with cables getting longer, and often being made without joints, these tests are becoming increasingly inadequate. Practical and weatherproof testing systems for offshore cables are urgently needed.

Offshore wind is the leading form of renewable energy, and wind turbines are constantly increasing in power. For new wind farms, the operating voltage of the submarine cables that deliver the electricity is being increased to 66kV.

The International Electrotechnical Commission (IEC) has therefore greenlit a second form of testing for operating voltages over 36kV: resonance testing.

Rather than using nominal voltage like the SOAK test, resonance testing actually reproduces the load on a cable operating normally at normal voltage and grid frequency. The test is carried out using a resonant circuit made up of a test inductor (inductance) and a test cable (capacitance). The test system supply has a variable frequency which is automatically set to the test circuit’s resonance frequency, minimising the feed-in power needed.

Resonance testing can also reliably detect installation errors – which are the cause of over half of all problems on offshore wind farms.

The resonance testing system offers wind farm operators, testing service providers, cable manufacturers the flexibility they need to keep complex offshore wind infrastructure functioning continuously, all year round.

And if something does go wrong just before the end of the testing season, resonance testing will enable them to quickly test the systems, fix the problem and keep the winter wind generating power.

• More By Mane Contract Services – Why Large Offshore Wind Farms Should Double Their Intra-Array Voltage

See how Thorne & Derrick support, supply and service the Renewable Energy industry.

Insight

by Ross Piercey | Asset Manager at JDR Cable Systems

Insurers want confidence in power systems – the weakest part of the inter array system is the cable accessories. The resource pool for Competent Jointers is shrinking due to high demand meaning quality of Jointing & Terminating workmanship can fluctuate.

The conditions they are fitted in are not that of labs.

Given over 50% of failures are cable accessories, a PD monitored, resonance test at power frequency does give early indications of potentially critical issues with cable preparation.

Furthermore, understanding PD inception voltages when testing at power frequency gives a much better indication of a “hard working” array cable. Resonance may practically be an issue, but we are an industry and business who thrives on a challenge. The Carbon Trust and others are looking at testing options and this issue needs addressing at an industry level, the cost of failure, dictates we must.

VLF /DAC are great tools but some feel they do not excite failure modes enough to be picked up but PD analysis is helping this, but offshore this is not without its issues. JDR are already bidding resonance for numerous clients, before the year is out we will resonance test with a state of the art purpose built system offshore, watch this space.

Global energy companies depend on high performance subsea umbilicals and cables to provide power and control in the world’s harshest offshore environments. JDR is a leading provider of technology connecting the global offshore energy industry.

Thorne & Derrick are Specialist Distributors to the UK and international Offshore Wind & Renewable industry to provide safe and reliable LV HV Electrical Cable & Power Distribution Systems up to 66kV – we are highly customer responsive and absolutely committed to providing a world-class service.

Contact our UK Power Team for competitive quotations, fast delivery from stock and technical support or training on all LV-HV products.

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