Partial discharge refers to a localised electrical discharge that affects only a limited region of an insulation system and does not represent a full breakdown between two electrodes. It occurs when the electric field strength in small regions exceeds the dielectric strength of the medium, while the overall insulation remains intact.
Put simply: the insulation still holds, but is already being damaged at individual locations. Partial discharges are particularly critical because they act gradually and can lead to complete insulation failure over time.
In the electrical industry, in motor and generator manufacturing, and in high-voltage applications, partial discharge is considered one of the most important ageing mechanisms of insulation systems.
Physical fundamentals of partial discharge
Partial discharges arise from inhomogeneous electric fields. These typically occur at locations where material transitions, air inclusions or geometric changes are present.
Typical discharge locations
- Gas-filled cavities within solid insulating materials
- Interfaces between different materials
- Sharp edges or points with field enhancement
- Surface regions with contamination or moisture
The discharge locally ionises the medium, generates short-term current pulses and releases energy in the form of heat, UV radiation and chemically reactive particles.
Types of partial discharges
Several forms are distinguished depending on the point of origin:
Internal partial discharge
Occurs in cavities or pores within a solid insulating material. Particularly critical, as it is difficult to detect and acts directly within the material.
Surface partial discharge
Arises along insulation surfaces, usually in the presence of contamination, moisture or insufficient creepage paths.
Corona discharge
A specific form of partial discharge in gases, typically in air. It occurs at sharp edges or wires and is often accompanied by audible crackling.
Edge and corner discharge
Local discharges at transitions between conductor, insulation and air, often caused by poor geometry or assembly defects.
Causes of partial discharge
Partial discharges are rarely random. Common causes include:
- Air inclusions from insufficient impregnation
- Unsuitable material combinations with differing permittivity
- Insufficient creepage and clearance distances
- Sharp edges, burrs or tight bend radii
- Thermal ageing and crack formation
- Moisture ingress or contamination
- Elevated operating voltage or rapid voltage transients
In high-voltage and medium-voltage machines these effects tend to reinforce one another.
Effects on insulation systems
Partial discharge does not cause immediate failure, but it does cause progressive damage:
- Chemical decomposition of the insulation
- Erosion and material removal
- Microcracks and delamination
- Reduction in dielectric strength and insulation resistance
- Accelerated thermal ageing
In the long run this process almost always ends in full breakdown.
Partial discharge in motors, generators and transformers
In electrical machines, partial discharge occurs particularly frequently in:
- Form coils and field coils with incomplete impregnation
- Mica or laminate insulation systems with voids
- High-voltage windings above around 3 kV
- Hairpin windings with tight radii and material transitions
- Transitions between conductor, layer insulation and groundwall
Partial discharge resistance is therefore a central criterion in the design of modern insulation systems.
Measurement and evaluation of partial discharge
Partial discharges are typically measured according to IEC 60270.
Common parameters
- Partial discharge charge in picocoulombs (pC)
- PD inception voltage (PDIV): voltage at which partial discharge starts
- PD extinction voltage (PDEV): voltage at which partial discharge stops
Measurement methods
- Electrical PD measurement according to IEC 60270
- High-frequency measurement (HF, UHF)
- Acoustic emission measurement
- Online monitoring of rotating machines
The lower the measured partial discharge at rated voltage, the higher the quality of the insulation system.
Measures to avoid partial discharge
Design and process measures are decisive:
- Use of partial-discharge-resistant materials such as mica, Nomex or polyimide
- Multilayer groundwall insulation systems
- Clean edge handling and sufficient bend radii
- Void-free impregnation, ideally VPI
- Optimised creepage and clearance distances
- Avoidance of harsh material transitions without equalising layers
In practice, partial discharge avoidance is always a system topic, not just a material question.
GOBA Takeaway
Partial discharge is one of the most important early indicators of insulation damage in electrical machines. It acts locally, gradually and destructively. Anyone who fails to control partial discharge risks premature failures, high maintenance costs and safety issues. Reliable insulation systems require clean design, suitable material combinations and controlled impregnation processes. In the electrical insulation industry, partial discharge resistance is therefore not a supplementary feature but a central quality criterion.