Partial discharges are low-intensity electrical phenomena that occur in regions with constructive imperfections in a dielectric medium subjected to an electric field.
What are partial discharges?
Partial discharge (PD) is the term used to describe a low-intensity electrical discharge that occurs in a region with imperfections in a dielectric medium subjected to an electric field.
In this context, the IEC 60270 standard defines partial discharge as "localized electrical discharges that partially bridge the gap between two conductors through the insulation. They usually result from a localized concentration of electrical stress in the insulation or on an insulating surface. These discharges often manifest as pulses lasting less than 1 microsecond."
Partial discharges can be classified according to the nature of their origin. Among the identified types are: surface, corona, internal void, contamination in resins, gas bubbles in liquid dielectrics, among others.
Understanding the nature and origin of these discharges is crucial for implementing effective monitoring and maintenance measures, thus contributing to the integrity and durability of electrical systems.
Surface discharges
These are phenomena that occur in gases or liquids on the surface of a dielectric material, usually originating from the electrode towards the surface.
The onset of this process, called tracking, occurs when the component of the electric field tangent to the surface exceeds a pre-established critical value.
This condition can trigger the formation of unwanted conductive paths, representing a significant threat to insulation integrity.
The persistence of this phenomenon over time can result in complete insulation breakdown, highlighting the crucial importance of monitoring and controlling electric field levels to prevent damage and ensure the safe operation of electrical systems.
External discharges
Discharges known as "external" or "corona" refer to discharges resulting from the ionization of ambient air when subjected to an electric field intense enough to partially break the established dielectric.
When voltage is initially induced, glow and discharge currents can occur. These discharges originate in gases, arising from sharp tips located on metal electrodes, especially in areas with small radius of curvature.
The process results in the formation of regions near the pointed objects with an intense electric field, exceeding the gas breakdown value.
As a consequence, the chemical process triggered by these discharges in the gas produces by-products that integrate into the gaseous medium. In pure environments, such processes are generally considered reversible and harmless.
However, corona discharges in the air can generate ozone, which can cause cracks in polymeric insulation. Additionally, the presence of nitrogen oxides with water vapor can corrode metals and deposit conductive materials on insulators, leading to tracking of the insulating material.
Therefore, understanding and controlling these phenomena is essential to ensure the integrity and durability of electrical systems.
Internal discharges
Manifest in the spaces, often voids and filled with gas, present in the solid and liquid materials used in insulation systems.
In solid insulation situations, partial discharges can arise in gas capillary cavities, constructive imperfections, gaps, or cracks, often resulting from defects in the molecular structure.
In liquid insulators, these partial discharges can occur in gas bubbles, originating from thermal and electrical phenomena, and in water vapors formed in regions of intensified electric field.
A specific type of internal discharge is represented by electric trees. This pre-breakdown phenomenon occurs inside the insulation of electrical equipment, such as insulated power cables.
Its origin is associated with the continuity of internal partial discharges, which occur in voids or result from electrode failures.
Understanding these processes is fundamental to preventing damage and ensuring the efficiency and safety of electrical systems.
How do partial discharges affect the electrical system:
Electrical discharges can have significant impacts on equipment or the entire electrical system. Below, see how they can affect the system:
Insulation degradation:
PDs can contribute to the degradation of insulation in assets present in the network, such as transformers, circuit breakers, cables, and connectors. Over time, these damages can lead to failures, increasing the risks of short circuits.Premature aging:
Equipment subject to frequent PDs may age prematurely due to additional stress on the insulation, resulting in a reduced service life.Equipment failure:
Repeated partial discharges can lead to failures in critical equipment. Equipment damaged by partial discharges may require frequent maintenance or replacement, resulting in additional costs and interruptions in power supply.Loss of efficiency:
Equipment subject to partial discharges may experience efficiency losses due to additional electrical stress. This can result in less efficient operation and higher energy consumption.Fire risk:
In extreme situations, partial discharges can contribute to heat buildup in electrical equipment, increasing the risk of electrical fires. This is especially concerning in areas where fire safety is critical.
