Partial Discharge Testing allows manufacturers to be certain that their product will have a long life-span before a failure occurs. This confidence allows the product to be installed into equipment that operates in hard to replace and/or service environments such as off-shore wind farms.
What is Partial Discharge?
Partial discharge is the movement of the tiny electrical corona that is produced by a voltage difference. The corona is miniscule and will find routes through seemingly solid electrical dielectric insulation material that is used to protect equipment from having an electrical breakdown failure.
The dielectric material will have inclusions, possibly due to the make-up of the material and voids which can be between two materials (plastic of the container and the potting placed in the container). The two materials may appear to have contact with each other and therefore have no gap, however, if the contact is not adhesive and the materials fully integrated, there can be an air gap. The continuous impact of the corona will enlarge these gaps to eventually allow a high voltage breakdown failure to occur.
This process of enlarging the gaps between the materials to ultimately result in a high voltage breakdown may take 10’s of years however, this can still be considerably less than the expected operational life-span.
How do you test for partial discharge?
A partial discharge test consists of the following stages:
- Set the Inception Voltage:
Slowly introducing an increasing voltage difference (which is higher than the item will be exposed to) between the two parts of the circuit that should be protected from each other. When the discharge increases, and reaches a defined level (usually 5pC) due to the increase in the voltage, the corona will start to damage the insulation and is classified as reaching its “inception voltage”.
- Hold it:
Once the inception level is known, the test voltage is then held at that level for a period of time.
- Discover the Extinction Voltage:
The test voltage is reduced in stages and the level of discharge monitored to confirm that it is falling. When the level of discharge reaches the defined level, the voltage that this happened at is classified as the “extinction voltage”
How does a unit pass?
To pass a partial discharge test, the extinction voltage must be higher than that to which the unit will be exposed. If this is the case, no harmful discharge will be seen by the unit at its normal operating voltage and therefore it will have a long operational life-span.
When designing a product, remembering the following points will allow you to develop a unit that can have a long operational life-span and pass the Partial Discharge Test.
- Mechanical layout of the component allowing enough space
- Use of materials with bonded insulation layers
- Using insulation material that has a very tight and almostimpervious mechanical structure
Want to know more?
Here at AGW we have invested in the development, manufacture and testing of products with PD requirements. We now have automated test equipment which supplements the existing manually controlled equipment. The equipment is used to confirm if the products have achieved their requirements. Click here for more information on our PD Testing capabilities.
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