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Cable Testing and Fault Locating at Wind Farms … Part IV

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Most cable fault locating failures and wasted time can be attributed to not properly interpreting test results, selecting the wrong tools for the job or taking short cuts in the process.  The first step in the process is to understand the basics of the cabling system.  This includes cable type, insulation type, overall circuit length, and type of bonding.  Conduct a preliminary proof test of the cable.  This can be done with a VLF Hipot.  The test results will confirm if there is a failure in the cabling system.  It will provide information to the fault profile such as breakdown voltage.  Once we know the breakdown characteristics, we can begin to profile the fault and select the appropriate fault locating equipment and techniques.  Since many of the modern fault locating techniques utilize a pre-location method such as Time Domain Reflectometry (TDR), Arc Reflection or Impulse Current Reflection, knowledge of the bonding method will be important before proceeding. 

Since many wind farm collector cabling systems span long distances they are often designed with cross bonding joints.  The interchanging of the cable sheath at joint locations helps to reduce losses in the high voltage cable.  For the application of cable fault location methods, as well as for partial discharge diagnosis, the influences of cross bonding have to be kept in mind.  Cable fault pre-location methods that are based on reflectometry techniques are influenced by the cross bonding.  Every cross bond creates a significant change in the characteristic impedance of the cable.  For reflectometry techniques this significant impedance change shows similar characteristics like a cable end.  Therefore, on such arrangements, the cable fault pre-location methods like TDR, Arc Reflection and Impulse Current are highly influenced.   To eliminate the effect of high impedance change, all cross bonding connections would have to be bridged by means of solid short circuit jumpers that can be fixed directly.  If undoing the cross bonds with jumpers is not an option, a fault sectionalizing approach may be required before using reflectometry techniques.  This can be done with a surge generator or “thumper” and an electromagnetic or “ballistic” tracker.

Typical Cross Bond Layout

Another critical question in determining the proper fault locating tools is the overall capacitance of the cable being tested.  The larger the capacitance (long cable runs) the larger (voltage and energy) the thumper needs to be able to supply.  There is a danger in selecting a thumper that is too small.  If the thumper is not powerful enough to break down or arc the fault, the voltage that is applied to the cable will be stored in the cable.  When the cable finally reaches its capacitance, the voltage stored in the cable is reflected off of the far end and back fed into the thumper causing a catastrophic failure in the thumper.

At Protec Equipment Resources we understand cable fault locating.  Talk with our knowledgeable application experts before embarking on your next locate.  Allow our knowledge and vast inventory of cable fault locators to make your next fault locate a great success!