Offline Testing on Power Cables

High-voltage cables are used for supplying power in densely-populated areas or for example to off-shore installations. Trouble-free operation over decades with little or no maintenance is therefore a requirement to ensure a secure energy supply.

The diagnostic possibilities for assessing the condition of power cables and cable accessories include partial discharge tests for assessing the condition of the cable insulation. Additional diagnostic tests for determining the condition of power cables include capacitance, power/dissipation factor and dielectric response measurements. 

Solutions

Impedance Measurement on Power Cables

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Impedance Measurement on Power Cables

For the determination of parameters for your single circuit line, you inject a test current into several different test loops. Each of the loops represents a possible fault scenario. Thereby, the measured loop impedances equal the loop impedances, which the connected protection device would determine during a real fault on the line. Based on these loop impedances, your required single circuit line's parameters are finally calculated.

Conventional test methods use high-power injection at mains frequency for suppression of interferences. You can freely select the frequency with modern systems: By injecting a signal with a frequency higher or lower than the mains frequency and applying a steep filter, you effectively suppress the interference.

Partial Discharge Measurement and Analysis on Power Cables

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Partial Discharge Measurement and Analysis on Power Cables

Partial discharge (PD) phenomena are local dielectric breakdowns of a small portion of a solid or liquid electrical insulation that is subjected to high voltage stress. PD often precedes insulation breakdown in power cables and cable accessories, such as joints and terminations, which can result in cost-intensive repairs and possibly prolonged outages.

PD can be the result of internal weak spots in power cables, such as voids, cracks or particles. It is also caused by damage to the outer semi-conductive layer of cables or to joints and terminations during installation.

Partial discharge measurement is a critical criterion for inspecting the quality of power cables and cable accessories during factory acceptance testing, on-site cable commissioning and troubleshooting. Accurate, reliable and reproducible PD measurements are performed based on IEC 60270, IEC 60840 and IEC 62067 standards.

Dissipation/Power Factor Measurement on Power Cables

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Dissipation/Power Factor Measurement on Power Cables

Capacitance, power factor and dissipation factor (tan delta) are important measurements to make in cables and cable accessories in order to assess the overall insulation losses of the insulation. The tests are useful in detecting deteriorated insulation and moisture or other contaminants.

A deviation from acceptable levels gives an indication of a change in the insulating material. Aging processes in the form of a structural modification of the insulation or the ingress of moisture, can increase the dissipation factor. A decrease factor can be explained, for example, by impregnating loss.

These measurements are useful in determining overall machine integrity when used in conjunction with other inspection activities. Timely periodic checks can identify problems and prevent the breakdown of cable insulation and eventual cable failure.

Dielectric Response Analysis on Power Cables

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Dielectric Response Analysis on Power Cables

When the insulation ages, the condition and thereby the dielectric strength of cables also deteriorates. In XLPE cables, electrical trees or water trees, caused by partial discharges or moisture, represent a serious problem.

These changes can be detected by changes in the dielectric response. Therefore, this method is ideally suited for performing cable condition diagnostics. Problems can be detected early and corresponding actions can be performed before any major damage or failures can occur.

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Partial discharge testing on power cables

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What is Partial Discharge?

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