Answer to Erin’s question – Cable Testing and Fault Locating at Wind Farms … Part III

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QUESTION:  What are the effects of tan delta cable testing when there are numerous splices installed? Will the splices cause the results to be worse or identify weaknesses that are not necessarily part of the cable?

ANSWER:  What we need to keep in mind is that the goal of tan delta testing is to provide a quality/reliability indication of an insulating system.  In a cabling system the splices are part of that system.  If the insulation of that system (cable and splices) is free from defects, like water trees, electrical trees, moisture and air pockets, etc., the cabling system approaches the properties of a perfect capacitor. It is very similar to a parallel plate capacitor with the conductor and the neutral being the two plates separated by the insulation material.  In a perfect capacitor, the voltage and current are phase shifted 90 degrees and the current through the insulation is capacitive. If there are impurities in the insulation, like those mentioned prior, the resistance of the insulation decreases, resulting in an increase in resistive current through the insulation. It is no longer a perfect capacitor.  The extent to which the phase shift is less than 90 degrees is indicative of the level of insulation contamination in the cabling system, hence quality/reliability. This “Loss Angle” is measured and analyzed.  Tan delta does not “locate” defects in cabling systems, it simply gives an indication of the insulation quality between point A and B in the system.