A partial discharge (PD) is an electrical breakdown phenomenon that is localised in an insulating medium, between two conductors that are at different potentials. Partial discharges occur due to the existence of a high, non-uniform electric field and their effect is the deterioration of the insulation system over time, that is to say, they reduce its service life (breakdown of the dielectric strength).
Partial discharge is produced by the dielectric stress caused by the presence of a high and non-homogeneous voltage in the insulating materials. The main causes for PD to appear are:
- Non-homogeneous distribution of the electric field.
- Presence of bubbles in solid and liquid insulation.
- Point effects that localise the dielectric stress in the insulation.
- Presence of moisture, cracks or water treeing.
- Presence of contaminants on the insulation surface.
- Voltage exceeding the dielectric strength of the insulating materials.
The great advantage offered by PD testing is that it allows the equipment to be analysed and diagnosed during its normal operation, since a drop in PD intensity occurs just before failure. Once the problem has been detected, the shutdown would be scheduled in order to carry out the corrective action.
Partial discharges fall into 4 main groups:
Internal
Internal discharges are normally due to cavities inside the electrical insulation. Cavities are weak points within the insulator and are normally produced by poor manufacturing or by ageing of the material.
The electric field existing inside a cavity is equal to or greater than the electric field surrounding the insulation. This is because the gas has a lower dielectric strength than the insulation surrounding it and, as a result, cavities are weak points within the insulation, where PD activity starts.
The following figure shows how PD emits pulses in relation to the applied voltage. In this figure Vc is the voltage in the cavity, which is a fraction of the applied voltage Va. When the voltage in the cavity reaches U +, which is the breakdown voltage of the gas inside the cavity, the discharge occurs and the voltage in the cavity drops to V +, where the discharge stops.
We must bear in mind that moisture and environmental contaminants can form layers on the insulator. This unwanted layer is a path for leakage current over the surface of the insulator. This current will produce heat on the surface and will generate evaporation that will cause physical disruption in the contaminant layer.
Large potential differences are generated across the gaps in the contaminant layer and small sparks can bridge those gaps. The heat from the sparks causes carbonisation of the insulation and leads to the formation of permanent carbon tracks on the surface.
Under such conditions, this process will develop over time and will eventually lead to a disruptive discharge and a complete failure of the insulation.
Surface
We must bear in mind that moisture and environmental contaminants can form layers on the insulator. This unwanted layer is a path for leakage current over the surface of the insulator. This current will produce heat on the surface and will generate evaporation that will cause physical disruption in the contaminant layer.
Large potential differences are generated across the gaps in the contaminant layer and small sparks can bridge those gaps. The heat from the sparks causes carbonisation of the insulation and leads to the formation of permanent carbon tracks on the surface.
Under such conditions, this process will develop over time and will eventually lead to a disruptive discharge and a complete failure of the insulation.
Corona
Corona is a type of discharge that occurs around sharp conducting points at high voltage when the voltage gradient exceeds a critical value. The corona effect is produced by the ionisation of the medium surrounding a conductor; it can also occur at a sharp point at earth potential, and it may be visible in the form of light, typically a purple glow, since corona generally consists of micro-arcs.
The corona effect is a low-energy process but, over a prolonged period of time, it can substantially degrade the insulation, causing failures due to dielectric breakdown. That is to say, the presence of corona effect can reduce the reliability of an insulation system. We must also point out that corona effects are cumulative and permanent, and failure can occur without prior warning.
The corona effect can generate:
- Light
- Ultraviolet radiation
- Sound (hissing, crackling caused by the expansion of visible gases)
- Ozone
- Nitric and other acids
- Salts (sometimes seen as white powder deposits)
- Mechanical erosion of surfaces by ion bombardment
- Heating (generally slight and especially in insulating material)
- Carbon deposits, creating a potential arcing path
In the case of electrical switchgear, if the enclosed equipment is not well ventilated, ionisation can reduce the dielectric strength of the air and the shortest insulation distance will eventually cause a high-energy electric arc from phase to phase or from phase to earth. The build-up of nitric acid inside an enclosed space such as an air-insulated switchboard will create carbon marks on the insulating materials. Ion bombardment also destroys some solid insulating materials, eventually turning them into dust
Electrical treeing
Electrical treeing originates from a defective point such as a small gas void, a sharp electrode edge or a metallic particle, where the electric field is high. This partial discharge can generate ozone and ultraviolet light and, over time, react with the surrounding dielectric material and cause decomposition, thus generating a new void. This weak point can grow and grow over time, forming a tree inside the insulation. This tree can grow to the point that it causes a complete breakdown.
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