Understanding the different types of partial discharge is essential for the diagnosis, monitoring, and prevention of insulation degradation in power systems. This article provides a technical overview of the major types of partial discharge and their characteristic features.
What is partial discharge?
Partial Discharge (PD) is a localized dielectric breakdown in a small region of an insulating material, which does not completely bridge the space between two conducting electrodes. It is a critical phenomenon that can deteriorate insulation systems in high-voltage equipment, ultimately leading to electrical failure. To protect your equipment from partial discharge, you need to install partial discharge monitoring system.
6 types of partial discharge
- Surface Discharge
Surface discharge occurs along the surface of an insulating material, typically at the interface between a solid insulation and a surrounding gas (like air or SF₆).
Unlike internal discharges (which happen inside insulation voids) or corona discharges (which occur around conductors in a gas), surface discharges travel along the surface of an insulator under the influence of a high electric field.
How Does Surface Discharge Happen?
It usually occurs when:
- The insulation surface is contaminated with moisture, dirt, or conductive deposits
- There are sharp edges, voids, or irregularities along the insulation surface
- The electric field along the surface exceeds the local dielectric strength of the surrounding medium (gas or air)
- When these conditions are met, the localized field triggers a series of tiny, rapid dielectric breakdowns along the insulation’s surface, known as surface discharges.
2. Internal Discharge
Internal discharge occurs inside the insulation itself, typically within voids, gas bubbles, delaminations, or other defects embedded within solid or liquid dielectrics.
Unlike surface discharge (which happens along an insulator’s surface), internal discharges are hidden within the bulk of the insulation and are often undetectable by direct visual inspection.
How Does Internal Discharge Happen?
It usually occurs when:
- Air voids, gas inclusions, or delamination exist inside the insulating material
- The electric field inside the void exceeds the dielectric breakdown strength of the gas (which is always lower than that of the solid insulation around it)
- The stress causes local dielectric breakdown inside the void, generating tiny, fast electrical pulses
- Each discharge progressively degrades the surrounding insulation via thermal, chemical, and mechanical erosion, eventually leading to larger defects like treeing or insulation breakdown.
3. Corona Discharge
Corona discharge occurs in a gas surrounding a high-voltage conductor, especially at sharp points, edges, or irregular surfaces. It involves partial ionization of the surrounding gas, forming a visible or invisible electrical glow accompanied by ozone and noise.
How Does Corona Discharge Happen?
It usually occurs when:
- High voltage is applied to a conductor with sharp points, rough surfaces, or small curvature radii
- The local electric field exceeds the dielectric strength of the surrounding gas (about 3 kV/mm in air)
- Ionization of gas molecules starts, creating a faint, continuous, or pulsed electrical discharge into the gas
This localized discharge forms corona, often visible as a bluish glow with associated ozone generation and radio interference.
4. Microdischarge
Microdischarge is a small, localized discharge that occurs in the presence of minor insulation defects or small voids. It is characterized by brief, low-intensity discharges that are difficult to detect but can lead to long-term degradation of the insulation material.
How Does Microdischarge Happen?
It usually occurs when:
- There are tiny voids, micro-cracks, or surface defects in the insulation material
- High voltage causes localized electric fields that exceed the dielectric strength of these small defects
- A short-duration, low-energy discharge takes place, often not visible but contributing to gradual insulation deterioration
Over time, repeated microdischarges can cause cumulative damage, leading to the eventual breakdown of the insulation material.
5. Pulse Discharge
Pulse discharge is a type of partial discharge characterized by brief, high-energy electrical pulses. These discharges occur in rapid, repeated bursts, and they can cause significant stress on insulation materials, leading to potential long-term damage.
How Does Pulse Discharge Happen?
It usually occurs when:
- High voltage causes rapid, short-duration electric pulses in insulation defects or gaps
- The electric field intensity during each pulse exceeds the local dielectric strength
- The insulation is stressed by these high-energy pulses, which can lead to localized breakdowns of the material
The repeated nature of pulse discharges contributes to the degradation and potential failure of the insulation system over time.
6. Treeing
Treeing is a type of partial discharge that occurs within insulation materials, forming tree-like branches of carbonized paths. It is caused by repeated electrical stress, gradually breaking down the insulation and forming conductive channels.
How Does Treeing Happen?
It usually occurs when:
- Repeated electrical stress is applied to the insulation, especially in the presence of moisture or contamination
- Localized partial discharges initiate and progressively form conductive carbonized paths inside the insulation
- Over time, these discharges branch out, creating a tree-like structure of conductive channels that can cause complete insulation failure
Treeing leads to severe insulation degradation and is often a precursor to electrical breakdown.
Partial discharge (PD) is a critical early-warning phenomenon indicating insulation degradation in high-voltage equipment. Different PD types — including surface discharge, internal discharge, corona discharge, microdischarge, pulse discharge, and treeing — each have distinct causes, characteristics, and damage mechanisms. Accurate identification and monitoring of these discharge types help prevent insulation failure, extend equipment service life, and improve system reliability.
