What is partial discharge and why test for it?
Partial discharge (PD) is an electrical discharge that only affects a part of the dielectric existing between two conductors at different voltage.
Partial discharges classification
PD can occur in various points through the insulation system. Partial discharge can be divided in three main categories:
• Corona discharges in gases surrounding metallic parts
• Surface discharge on the surface of the insulation
• Internal discharges in the voids or cavities
Corona discharges: I most cases this is a harmless activity since the material (gas) is regenerated. But if this occurs in closed environments, Ozone will be produced and surrounding materials will breakdown. The corona will also be able to hide other dangerous PD activity.
Surface discharge: Here, a distinction must be made between low- and high-frequency surface discharges. Low frequency surface discharges occur externally due to moisture or dirt on the surface. This is a harmless activity.
High-frequency surface discharges occur internally in components and in the interface between different materials, such as the insulation of a cable and the joint body. This is a critical activity that needs improvement.
Internal discharges: This is the most critical activity in PD. These discharges occur within the cavity of the insulation. The gas inside these cavities has a lower permittivity (ɛ) than in the surrounding material, eg pex.
This results in that the field is not distributed homogeneously in the insulation and a higher field strength is obtained within this cavity. If the field exceeds the electrical strength of the gas, you will get discharges.
Exsample for internal discharge in 33 kV cable
U0=19 kV, d1=4mm, d2=1mm, ɛ1=4,0, ɛ2=1,0
From the example above, we can see that the field strength above the void is 6.3 kV rms which is 8.95 kV peak. In other words, the stress over the void is 8.95 kV, which corresponds to almost three times the strength of air that have a dielectric strength of about 3 kV / mm.This means that the insulation in the voide will breakdown and discharges will occur. This will result in an electric tree starting to grow and the size of the voide increases. If d2 increases from 1 mm to 2 mm, the field strength increases to 10.1 kV.
Why should you test for Partial discharges
PD is regarded as the best early warning indicator of insulation deterioration in high voltage assets. PD measurements is a good way to checking the health status on new and aged electrical components.
PD measurement detects faults that otherwise whould lead to a dielectric breakdown. PD testing is especially important when the asset is critical to the operation
What causes Partial discharges
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Installation faults (Cable, joints and terminations)
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Manufacturing defects
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Transportation damage
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Mechanical damage
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Aging
PD meauserments can be performed on
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Cables and cable accessories
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Power transformers
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Switchgear
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Rotating machines
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VT/CT
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Capacitators
