The dielectric condition of medium voltage cables and their accessories can be easily evaluated by means of partial discharge measurement PD.
While VLF cable testing provides just a “Go/No Go” statement regarding the functional status of a cable, interpretation of PD measurements results allows an assessment of the operational reliability.
Hence categorized assessment of cables as “new”, “strongly aged” and “faulty” is possible and appropriate maintenance and repair measures may be planned and organized in advance.
Imperfections of the insulation, such as cavities, inclusions in the synthetic material, mechanical irregularities at the boundary layer or contaminants in the conducting layers, may lead to lasting discharge channels.
These discharge channels in the insulation are referred to as electrical trees. Find more information under treeing in the insulation.
Along electrical trees and defects in the insulation, partial electrical flash over occurs – referred to as partial discharge.
Such a partial discharge short-circuits over just a restricted distance of the available insulation system.
PD systems with a nominal frequency of 50 Hz are large, heavy and expensive and are usually employed only by cable manufacturers.
More economical and more practical for mobile diagnosis is the use of a sinusoidal 0.1 Hz VLF (very low frequency) alternating voltage.
At this frequency partial discharge occurrences can be measured cost-effectively with a simple test system.
Furthermore this prevents the cable or device under test from being damaged during testing and guarantees damage-free partial discharge measurement.
Comparison of the gathered measurement values with the data from the same cable collected during the acceptance-test allows classification of the dielectric condition (new, strongly aged, faulty) of the cable.
The test method is applicable for medium voltage cables with various insulation types, such as:
Besides cable diagnostics, partial discharge measurement is also used for testing plant such as:
As well as VLF cable testing, PD measurement has been included in many common testing standards worldwide. (DIN VDE 0276, IEEE 400.2 and IEEE 400.3, CENELEC, HD620 S1, NEN 3620, SANS 10198, IEC 60060.3)
Components of the b2hv series HVA and PD can form partial discharge measurement systems providing voltages up to 90 kV peak.
may be combined with PD devices
So the user can retrofit their hipot tester with a PD system or a number of HVA devices may be expanded with just one PD-system.
The portable cable diagnostics systems are compact and due to their low weight are specifically designed for field operation.
The PD-systems by b2hv meet all respective testing standards. Hence they are suited for use worldwide.