Partial discharge is a localized electrical breakdown that occurs within or along the surface of insulation material, but does not fully bridge the gap between two conductors. Think of it as a tiny spark that jumps partway across an insulating barrier rather than completing the full path. These small discharges happen repeatedly under high-voltage stress, and over time they erode insulation from the inside out, eventually leading to complete equipment failure if left undetected.
How Partial Discharge Works
Electrical insulation in cables, transformers, and switchgear is never perfectly uniform. It contains microscopic imperfections: gas-filled voids, cracks, contaminants, or areas where different materials meet. These tiny flaws have a lower breakdown strength than the surrounding insulation. When the electric field in one of these weak spots exceeds a critical threshold, the air or gas inside ionizes and a small discharge fires across that localized area.
The key distinction is “partial.” The discharge burns across the defect but stops there. It doesn’t create a full conductive path between the two energized conductors. Each individual discharge is tiny, often measured in picocoulombs of charge. But these pulses repeat with every cycle of the AC voltage, sometimes thousands of times per second, steadily degrading the insulation around them.
Types of Partial Discharge
Partial discharge takes several forms depending on where it occurs and how the electric field concentrates.
- Internal discharge happens inside gas-filled voids or cavities trapped within solid insulation. These voids are common manufacturing defects in materials like epoxy or cross-linked polyethylene. The gas inside the void breaks down at a lower voltage than the surrounding solid, so discharge activity starts there first. This is one of the most damaging types because it’s hidden inside the material and directly erodes the insulation walls.
- Surface discharge occurs along the boundary between insulation and another material, such as where a cable’s insulation meets air. Electric field distortion at these interfaces can trigger discharges that creep along the surface.
- Corona discharge results from ionization of air surrounding a conductor when the local electric field exceeds a critical threshold, typically near sharp edges or irregular surfaces. It often appears as a faint bluish glow and produces a hissing sound. Corona is generally less immediately destructive than internal discharge, but it still causes energy losses and gradual surface degradation. Its electrical signature also differs: corona signals tend to concentrate energy below 200 kHz, while internal partial discharge pulses produce broadband signals extending beyond 800 kHz.
- Electrical treeing is the long-term consequence of sustained partial discharge inside solid insulation. Repeated discharges carve microscopic hollow channels that branch outward from the original defect, forming a pattern that looks like a tree under magnification. The process has three stages: initiation at a single point, propagation as branching channels spread through the material, and final breakdown when the channels connect the two conductors and the insulation fails completely.
What Causes It
The root cause is almost always an imperfection in the insulation. In solid insulation like epoxy or polymer cable sheathing, the most common culprit is a void, a small air pocket trapped during manufacturing. X-ray inspection of failed equipment routinely confirms that partial discharge originates from these internal defects. Moisture intrusion, foreign particles embedded in the insulation, mechanical damage, and poor joints or terminations all create similar weak points.
Age matters too. Insulation that has been thermally stressed, exposed to moisture over years, or subjected to repeated voltage surges develops new micro-defects. Each new defect becomes a potential site for discharge activity. This is why partial discharge tends to accelerate over time: the discharges themselves create new damage, which hosts more discharges, in a self-reinforcing cycle.
Why It Matters for Electrical Equipment
Left unchecked, partial discharge destroys insulation from within. Each pulse deposits energy into the defect site, breaking chemical bonds in the insulation material. In polymer-insulated cables, this creates the branching tree-like channels described above. In gas-insulated switchgear, the process is different but equally concerning. Discharge activity inside equipment filled with insulating gas breaks the gas molecules apart, producing toxic and corrosive byproducts including hydrogen fluoride, sulfur dioxide, and various fluorine compounds. These byproducts attack metal surfaces and further degrade the insulation, compounding the original problem.
The progression from first discharge to total failure can take months or years, depending on the voltage level, the size of the defect, and the insulation material. But the endpoint is always the same: complete dielectric breakdown, which in high-voltage equipment means a catastrophic fault. This is why utilities and industrial facilities invest heavily in detecting partial discharge early.
How Partial Discharge Is Measured
The international standard for partial discharge measurement is IEC 60270, most recently updated in 2025. It defines the primary quantity of interest as “apparent charge,” measured in picocoulombs. This isn’t the actual charge moving inside the defect (which is impossible to measure directly) but rather the charge that flows in the external circuit to compensate for the discharge event. Higher apparent charge readings generally indicate more severe discharge activity.
Several sensor technologies are used to detect partial discharge in the field:
- High-frequency current transformers (HFCTs) clamp around cable earth connections and pick up the high-frequency current pulses that partial discharge produces. They’re commonly used for monitoring underground power cables.
- Ultrasonic sensors detect the acoustic waves that discharge activity generates inside equipment like transformers and switchgear. They can help pinpoint the physical location of the discharge source.
- Transient earth voltage (TEV) sensors detect electromagnetic signals that escape through small gaps in metal-clad switchgear enclosures. These are popular for routine surveys of indoor switchgear because they require no direct electrical connection.
Handheld instruments now combine multiple sensor types in a single device, allowing technicians to screen equipment quickly during routine maintenance walks.
Reading Discharge Patterns
One of the most useful diagnostic tools is the phase-resolved partial discharge (PRPD) plot, which maps discharge pulses against the phase angle of the AC power cycle. Different types of insulation defects produce visually distinct patterns. Void discharges create an “elevated arc” shape. Surface discharges form crescent-shaped clusters. Corona discharges produce triangular patterns. Gap-type discharges appear as block-shaped distributions.
Experienced analysts can look at a PRPD plot and identify what kind of defect is causing the discharge, which helps maintenance teams decide how urgently the equipment needs attention and what kind of repair is likely needed. Automated pattern recognition using image-matching algorithms is increasingly used to make this analysis faster and more consistent, with matching confidence scores routinely above 80%.
Where Partial Discharge Is Most Common
Any equipment that relies on electrical insulation under high voltage is susceptible. In practice, the assets that get the most attention for partial discharge monitoring are medium- and high-voltage power cables, cable joints and terminations, power transformers, gas-insulated switchgear, and rotating machines like large motors and generators. Cable joints are particularly vulnerable because they introduce interfaces between different insulation materials, and installation quality varies. Rotating machines develop slot discharges where windings sit in the stator core, producing their own characteristic “baby stroller” PRPD pattern.
For facilities that depend on uninterrupted power, such as hospitals, data centers, and manufacturing plants, routine partial discharge surveys are a core part of predictive maintenance. Catching a developing insulation defect early enough to schedule a planned repair is far less costly than dealing with an unplanned outage from a catastrophic insulation failure.