Detection and Location of Electrical Trees
Electrical trees can be detected and located by means of partial discharge measurement.
As the measurement values of this method allow no absolute interpretation, data collected during the procedure is compared to measurement values of the same cable gathered during the acceptance-test. This allows simple and quick classification of the dielectric condition (new, strongly aged, faulty) of the cable under test.
To measure the level of partial discharges, a sinusoidal 0.1 Hz VLF (very low frequency) voltage is used, just as it is used with VLF cable testing. Modern PD-detection systems employ personal computer-based software for analysis and display of measurement results, and a PD-detector. By the so called time domain reflectometry (TDR) method the PD-detector measures partial discharge activity. The partial discharge intensity is measured in picoCoulombs (pC) and displayed versus time.
A fully automatic analysis of the reflectograms collected during the measurement allows the location of insulation irregularities and electrical trees. Usually they are displayed in a partial discharge mapping format. Additional useful information about the device under test can be derived from a phase related depiction of the partial discharges.
A sufficient measurement report contains:
- Calibration Pulse (in accordance with IEC 60270) and end detection
- Background noise of the measurement arrangement
- Partial discharge inception voltage PDIV
- Partial discharge level at 1.7 Vo
- Partial discharge extinction voltage PDEV
Read more about this topic: Electrical Treeing
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