Partial discharge measurement for cables and switchgear in wind farms
Partial discharge measurement on cables, joints and switchgear: early diagnosis instead of costly repairs, higher availability and less fire risk.
Partial discharges are small electrical discharges in the insulation of equipment. They often occur long before equipment fails. Regular partial discharge measurements therefore reduce the fire risk, protect against total failure and increase the availability and thus the yield of your plants. Partial discharges occur where the insulation has defects, such as voids, cracks or inclusions. At these points, the electric field strength is higher than in the surrounding material. Partial discharges can progressively damage the insulation until, in solid polymer insulation, discharge channels form (so-called treeing) and the insulation breaks down. The discharges release energy as heat, sound and light, which accelerates ageing further. In paper-insulated cables, too, progressive partial discharges can lead to total failure.
The size of the discharges is measured as apparent charge in picocoulombs, as described by the standard IEC 60270. For medium-voltage cables in the internal grid of a wind farm, the insulation is usually subjected to a test voltage for the measurement so that existing defects ignite. If the voltage is varied, the inception and extinction voltage can be determined, and with suitable measurement methods, the transit time of the partial discharge signals can be used to narrow down the defect along the cable route. How serious an anomaly is shows up best in the trend of repeated, comparable measurements. This is important because faults in the cable system quickly lead to yield losses: if a cable section fails, the energy generated can no longer be evacuated.
If an anomaly is detected early, the affected section can be repaired as planned instead of during a fault. In addition, there are systems that monitor partial discharges during live operation and report when thresholds are exceeded. Partial discharge measurement complements other diagnostic methods: the dissipation factor (tan delta) can give indications of moisture and the ageing condition of the insulation, a sheath test reveals sheath faults, and reflectometry and fault location narrow down defects along the route. Complete documentation of the measurements is particularly important vis-à-vis insurers in the event of damage.
We measure cable sections up to a length of approx. 2 km. Depending on the measurement method and test setup, cable joints, terminations and connected switchgear parts can be assessed as well; test voltage and possible additional stress on the equipment depend on the method. Our experts inspect the plant and infrastructure with modern measuring instruments. Comprehensive evaluations and comparison with earlier measurements show developments over time and possible deterioration. If anomalies appear, we develop practical and economical solutions. Partial discharge measurement makes sense for regular condition assessment, as additional security for plant takeover and warranty, and to meet expert requirements and permit conditions. Depending on the plant, test concept and permit conditions, it may be part of the scope of testing in the technical acceptance of wind turbines; when buying or financing a wind farm, technical due diligence asks about the condition of the plant. Further measurements during ongoing operation are the power curve measurement and the sound emission measurement. You will find further tests under electrical inspections.
