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Cabling, wind farms and the details of acceptance testing

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The hot new item on the power equipment agenda may be cable fault locating for wind farms, but Tom Sandri, director of operations and technical support with Protec Equipment Resources, points to acceptance testing for cables as a larger player in the wind farm cable market (by a nearly 6-1 margin).

Traditionally, cable testing falls into three major arenas: installation testing, acceptance testing and maintenance testing. Installation testing occurs after cable installation but before splicing or terminating.  The test is intended to detect shipping, storage, or installation damage.   Acceptance testing happens after cable system installation and after splicing and terminations, but before the cable system is placed in normal service.  These tests are intended to further detect installation damage and to show any gross defects or errors in installation of the various system components.  Finally, maintenance testing is scheduled over the service life of the cabling system. Unfortunately, underground cables are usually “run to failure,” which means maintenance testing isn’t a high priority at most wind farms. (It’s much easier to trace the fault when there’s an issue.)

But, one area of testing that does get a lot of attention these days is acceptance testing, as Sandri noted earlier. There are three specific tests that may help technicians verify cables in the acceptance phase: VLF, tan delta or partial discharge.

According to Sandri, while the more traditional DC testing is still an option, VLF is “probably the most sought after.” And, VLF, once costly, has come down in expense and now compares closer to the expense of DC testing.

The one issue Sandri has noted with the rise of VLF testing is a learning curve for contractors who didn’t originally specialize in cabling issues. Just running from client specifications, these contractors would choose test equipment that wasn’t in line with the testing required. For example, they’d choose an AC hipot set for AC voltage field testing much more easily done with VLF. But, since they were testing AC, they chose the AC test set. Unfortunately, for 60 Hz AC testing on cable, an AC test set would have a large VA output requirement, making the equipment to create that output enormous. Using VLF (or very low frequency) testing, however, offsets this issue.

Lowering the frequency lowers the VA output requirement, which allows large capacitive testing (as with a long cable section) with portable field equipment. No monster AC tester needed.

This learning curve, however, has been a real boom for Protec, as it allowed them to demonstrate knowledge and educate customers, which, Sandri notes, sets Protec apart from the competition. He noted that it is a fine line between consulting and instructing, at times, but that technical expertise can really help with 3 AM phone calls about how to set up equipment in the field. (And, yes, he did note that he’d received a few 3 AM calls.)

To find the right piece of equipment for a client, Sandri talks with the client about various options, along with the application being done and a short history of the project. Sometimes, it takes a bit of finesse to steer a client away from the smallest, lightest equipment that’s easiest to carry to a larger piece that will fulfill testing needs. But, getting the right results quickly always helps that conversation.

And those right results may come from a VLF test or from the other two areas of acceptance testing: tan delta and partial discharge.

While tan delta, ideally, is a diagnostic test trended over time, it can still be valuable at the beginning of the cable’s life. Comparison across phases and comparison across applied voltage can also give a good diagnostic overview of the cable, as VLF does.

Pinpointing issues in that cable up front, however, is best done with partial discharge (PD). Traditionally, this testing was applied to electrical apparatus in three levels: a basic scanning tool that could justify further investigation, the attachment of sensors and monitors for a time-lapse look at things, and then focusing in on the equipment or issue in question. Essentially, these levels merge when applying partial discharge testing to cables.

Sandri labels partial discharge testing as “the Holy Grail” of acceptance testing because it not only reveals a problem but exactly where that problem may be at along the cable. On the other hand, PD for cable acceptance testing has a few downsides: expense, the need for a coupling unit, TDR (time-domain reflectometry) and software and, most importantly, someone to interpret the results. And, unfortunately, someone to interpret those results may be expensive as well, adding to the bottom line.

In the end, while Sandri notes that there is a lot of interest in PD, and that he’s had a number of inquiries, there aren’t numerous folks willing to cover the costs when VLF and tan delta does the basic work cheaper. But, he does believe PD for cable acceptance testing may become cost effective in five years or less, perhaps offering a glimpse into the future of cable acceptance testing for wind farms and beyond.