Power cable on-line diagnosis using partial discharges ultra wide band techniques

A noninvasive ultra-wide band (UWB) partial discharge technique was employed in cable diagnosis of Mexico City underground distribution feeders. The technique presented here was applied to over 2000 joints, 400 terminals and thousands of meters of installed paper insulated impregnated cables rated at 23 kV. The technique allowed circuit classification according to electromagnetic field levels produced by partial discharges and recorded on each manhole with a joint along the circuits evaluated in 12 substations. The PD detection technique employs near field sensors working within the 80 MHz range and whose information is digitized and fed to a conventional PD digital detector, where a PD pattern is obtained. This pattern is correlated to the phase current by using an inductive sensor; therefore corona and interference problems could be disregard during measurements. To attain repetitive values, calibration was performed using a very fast pulse (less than 50 ns front) injected in a small cable loop. Results demonstrate a great signal to noise ratio, high selectivity and great noise discrimination. The large amount of measurements allowed the determination of limit values to differentiate areas in acceptable state from those requiring attention. Those results include detection of bad joints, cable deformation due to ducts collapse and damaged terminals, besides aged cable detection caused by overheating operation conditions. The results obtained had been confirmed by subsequent faults following the detection of very high levels of PD.