35kV cable distribution design for Wind Farms

[VLFit]

Wind Farm Distribution Designers. I understand that low cost drives most wind farm installations, but, how can a common design used, where cables are buried, sometimes with splice boxes for cross bonding, for many miles with no access points available. Ten miles of buried 35kV cable with no accessible splice boxes prohibits the ability to use several of the most popular and effective means of testing the cables. The only test that can be performed, as a practical matter, is a VLF Withstand test to check the integrity of the system after installation. Even that test is impeded and made more expensive by the very long runs since the cable capacitance is so high. The design eliminates the ability to perform important diagnostic testing like Tan Delta/Power Factor and Partial Discharge. Also, the lengths designed make cable fault locating more difficult. Cross bonding neutrals with the boxes accessible and links to connect the neutrals in a phase continuous manner, so some tests can be done, is often not available.

In a nut shell, the typical design gives no consideration for the users ability to test or fault locate in an effective manner? How can this be changed to aid the owners/users of these systems? At least offer a way to access the cable every 3 - 4 miles.

 

[cranky108]

I think you answered your own question when you stated that low cost drives most wind farm installations.

Most of the money is made from the goverment subsities that come with putting the whole thing in service.

 

[VLFit]

You are right. I see many that are installed by the lowest bidder with specs that call for direct bury,some in a trench with only the dirt back filled, no radio markers in the splice boxes to find them when needed, and cable runs 15 miles long. You would think the designer of the system would give some consideration to the Acceptance testing and future fault finding needs and alter the design. Another problem is the same engineer that designs this type of system then specs TD and PD testing after install, not even knowing enough to realize both are impossible with the long cable runs speced. We will keep working with what's out there and push for design changes.

 

[dpc]

cranky is exactly right. They could care less about future testing. It's all about tax credits and other government subsidies. I don't think any of these developers expect this equipment to operate for 30 years. This stuff is the power industry equivalent of the sub-prime mortgages.

 

[VLFit]

So I have learned. Maybe if the Acceptance Test expenses were considered in the bids for the job, the owners would take notice and put some pressure on the design people. I've seen build contracts where an Acceptance test after installation isn't required, at least not a legitimate over voltage Withstand test. The job is turned over to the owners and six months later splices and terminations start popping and the contractor is long gone. I guess that happens all over though. Moving on ....

 

[rterickson]

Slightly off-topic (wind turbine generator step-up transformers vs. feeder cables), but still well worth the read:

http://www.pddnet.com/articles/2010/03/weakest-link-wind-energy-supply

 

[VLFit]

Thanks for the article. Hopefully after many of these wind farms have been operating for 10 years and show a high failure rate and maintenance cost, the designs will become more responsible and conservative. For now, we'll just wait for the next call asking if we have a thumper that can find a fault on a 35 kV cable 15 miles long or someone needing to conform to a specification requiring a PD test on a direct buried un-jacketed cable 10 miles in length?

Any design engineering firms out there that care to jump in here?