Wind farm 35 kV cable system design: testing and fault locating

[VLFit]

thread626-354845

Before retiring this subject, I thought I would revive the above thread to try to get some to answer the questions posed.

PROBLEM: Are the engineering companies that design the 35 kV electrical distribution systems for wind farms going to start designing the systems so that proper high voltage Acceptance/Commissioning testing can be performed after installation? As most are now designed, the cable runs are many miles in length without available break points and many even have direct buried splice and/or cross bonding boxes. This makes it very difficult to perform proper power frequency or VLF AC withstand tests and makes it impossible to perform proper Tan Delta and Partial Discharge tests due to the excessive lengths of cable. Cable fault locating also becomes more difficult. Often the same engineering company that designs a system with 10 mile long cable runs also specifies that the cable be hipoted and/or partial discharge and tan delta tested. You can't have both.

QUESTION: Has this problem area been recognized as a design weakness and are changes being made? Lowest cost design and using the lowest cost bidders to do this work is not the lowest cost solution to installation when the repair to cable and accessory failures for the few years following installation are factored in.

 

[fnizbit]

A couple questions from the other side of the coin:

What standards and practices exist for other power generating equipment that would avoid these problems?

What are companies that are cognizant of the problems with the faults and risks in the lowest cost approach doing to educate the clients (and have them consider higher bids)?

Or is it unique to wind turbines because of the installation locations?


(PS: VLFit, a CL user [neoMatrix] posted your post without attribution and later claimed you are a colleague. Was wondering if that was true.)

 

[VLFit]

Hello fnizbit,

Thanks for writing.

I do not know neoMatrix, at least not by that alias.

The problem I speak of is generally found with wind farms that are unique in that their "home runs" can be very long (with no splice box access unless you dig them up, and that assumes they have radio marker balls in them to precisely find them ) and the lowest cost installation is critical to return profit and this is where the problems occur. Utilities don't usually have 10 and 15 mile cable runs that have no access points to the cables. They can be interrupted at several locations. Large industrial and commercial applications usually don't have cable runs more than 1000'. This issue is also in the spotlight now because of the great number of wind farm installations that have been done in the past ten years and the many more yet to come. Also, medium and high voltage cable testing is a big issue now with the common use of VLF AC supplies. In addition to just a basic AC withstand test, many wish to perform diagnostic testing like tan delta and partial discharge. These methods are great, whether VLF or power frequency methods are used, but not effective for cables miles long. A test of a cable maybe 2 miles long is the all that can be done. Another matter that makes wind farms unique and more difficult to test and fault locate is the fact that since the runs are very very long, cross bonding of neutrals is done. This makes the testing even more difficult if you cant easily access the bonding boxes to reconnect the neutrals properly to do the test. You mention locations. Yes, the locations have something to do with it, as many are in places where it is possible to direct bury, sometimes without even a cement trench to put the cable in. The very method of direct burying a cable leads to the greater possibility of installation damage yet it's designed so you can't easily test for it.

Anyway, the problem is there with the wind farms and also with long underwater runs, wind power or not. I just wanted to throw the topic out there again to see if any engineering companies that design for wind farms might jump in with some info. I hope so.

Thanks