Partial discharge is often one of the earliest indicators that insulation in high-voltage equipment is under stress. While the detection of partial discharge is a valuable sign, understanding the types of partial discharge is essential for insulation health. Utilities and industrial operators routinely monitor partial discharge activity because it can reveal developing defects long before they progress into costly failures. Yet the presence of partial discharge alone rarely tells the full story. Two similar partial discharge measurements may originate from entirely different sources, carry different levels of risk, and require different maintenance responses
This creates a common diagnostic challenge for engineers: once partial discharge has been detected, what exactly is causing it? Is the discharge developing inside the insulation itself, travelling along an insulating surface, or occurring within the surrounding air? The answer can significantly affect how the condition is interpreted and what actions should follow.
Understanding the type of partial discharge present is therefore an important step in insulation diagnostics. By distinguishing between internal, surface, and corona discharge, engineers can gain deeper insight into defect location, degradation mechanisms, and potential failure risks.
This article explores the characteristics of these three partial discharge types and explains how their identification supports more informed maintenance decisions.
Why the Type of Partial Discharge Matters?
A partial discharge is a localized electrical breakdown that does not completely bridge the insulation between conductors. Although each event involves only part of the insulation system, repeated activity can progressively degrade insulating material.
That definition covers several physically different phenomena.
A discharge occurring inside a microscopic void in solid insulation is not equivalent to activity tracking across a contaminated termination. Neither should automatically be interpreted in the same way as corona around a sharp exposed conductor.
The distinction matters because the discharge location provides clues about the underlying defect. Partial discharge classification therefore gives engineers more useful information than partial discharge magnitude alone.
Consider three identical measurements showing detectable partial discharge activity. One may originate from a void inside epoxy insulation, another from contamination across an insulator surface, and the third from an exposed sharp metallic point. Similar detection levels can represent completely different defects and maintenance requirements.
This is why partial discharge monitoring programs do more than simply detect electrical activity. Their value lies in helping engineers distinguish between different discharge mechanisms, assess defect severity, and understand what the measurements reveal about insulation condition.
Effective partial discharge diagnostics must therefore answer three questions:
- Is the signal genuine partial discharge rather than interference?
- What type of discharge is occurring?
- Where is the likely source?
Only after those questions are addressed can the measurement become useful condition information.
To answer all these diagnostic questions, engineers generally classify partial discharge activity according to the location of discharge occurrence and how the physical mechanism is responsible for it. In high-voltage electrical equipment, most partial discharge activity falls in three broad categories. Internal, surface and corona partial discharge. Although all three involve localized electrical discharges, they develop in different regions of the insulation system and often indicate different underlying defects. Understanding the characteristics of each type is the first step toward interpreting partial discharge measurements correctly and identifying the most appropriate maintenance response.
Internal Partial Discharge Develops Inside the Insulation
Internal partial discharge occurs within defects enclosed by an insulation system. Typical sources include gas-filled voids, cavities, cracks, and delamination in solid dielectric materials. IEEE technical references describe internal discharge as occurring within voids or cavities in solid insulation.
The underlying mechanism is electrical field concentration.
A void has different dielectric properties from the surrounding solid insulation. When voltage is applied, the electric field across the void can become sufficiently high to ionize the gas inside it, even though the surrounding insulation continues to withstand the overall applied voltage.
Repeated discharges then stress the surfaces surrounding the cavity. Over time, this activity can contribute to chemical, thermal, and physical degradation of the insulation.
Internal partial discharge can occur in assets including power cables, cast-resin equipment, rotating-machine windings, and other solid insulation systems.
For example, a manufacturing defect may leave a small void inside an epoxy insulation component. The component can initially pass normal operation without complete breakdown, while repetitive discharge occurs within the cavity whenever local electrical stress exceeds the discharge inception condition.
From a diagnostic perspective, internal partial discharge often points to a defect concealed within the insulation system, making visual confirmation difficult and increasing the importance of electrical condition assessment.
Surface Partial Discharge Travels Along an Insulating Interface
Surface partial discharge develops along the surface of an insulating material rather than inside its bulk structure. It commonly occurs where electric-field distribution along an insulation surface becomes unfavourable.
Contamination, moisture, inadequate creepage distance, damaged surfaces, poor field grading, and installation defects can increase local surface stress. Once discharge activity begins, continued exposure can alter the surface and make further discharge easier.
This mechanism makes environmental conditions particularly relevant.
A dry, clean insulating surface may behave normally while the same surface develops partial discharge under elevated humidity or contamination. Consequently, surface activity may vary substantially as operating conditions change.
Typical locations include cable terminations, bushings, spacers, support insulators, and interfaces where insulating materials meet energized or grounded components.
The resulting maintenance response is fundamentally different from that for an internal void. Engineers may need to investigate contamination, moisture, termination installation, surface damage, or field-control components rather than assuming if the bulk insulation has deteriorated internally.
Corona Partial Discharge Occurs in a Gaseous Medium
Corona partial discharge occurs when the electric field around a conductor becomes strong enough to ionize the surrounding gas without producing complete breakdown across the insulation gap.
The formation of corona is largely influenced by conductor geometry and local electric-field concentration. Sharp points, protrusions, rough surfaces, small-radius conductors, and poorly finished connections can create localized regions of high electrical stress. Once the electric field exceeds the ionization threshold of the surrounding gas, corona activity can begin.
Corona may generate electrical pulses as well as acoustic noise, light, ultraviolet radiation, ozone, and other chemical by-products. These characteristics allow different sensing technologies to be used depending on the equipment and application.
From a diagnostic perspective, corona is often associated with conductor geometry, exposed high-voltage components, or localized field enhancement rather than defects embedded within solid insulation. Corona detected around an exposed conductor should therefore not automatically be interpreted as evidence of an internal insulation void.
All three are forms of partial discharge, but they do not indicate the same type of insulation problem. The value of partial discharge classification lies in understanding what the discharge reveals about the asset condition. Internal, surface, and corona PD originate from different locations and mechanisms, which means each provides a different clue about where engineers should investigate and what may require attention.
Difference between Internal, Surface and Corona Partial Discharge.
The easiest way to understand the difference between internal, surface and corona discharge is to compare where the activity occurs and what creates the local electrical stress.
| Characteristic | Internal PD | Surface PD | Corona PD |
| Primary location | Within insulation | Along an insulation surface or interface | In gas around a stressed conductor |
| Typical source | Void, cavity, crack or delamination | Contamination, moisture, surface damage or field-grading issue | Sharp point, protrusion or high field curvature |
| Environment sensitivity | Depends on insulation and defect conditions | Often strongly influenced by surface and environmental conditions | Influenced by gas conditions and electrode geometry |
| Typical diagnostic concern | Embedded insulation deterioration | Surface/interface deterioration | Localized gaseous ionization around conductor geometry |
| Visual inspection | Source may remain completely hidden | Surface condition may sometimes be inspectable | Physical source may sometimes be accessible |
| Maintenance implication | Investigate internal insulation condition | Inspect interfaces, contamination and field control | Locate and correct high-field source where applicable |
The key takeaway is that each partial discharge type points engineers toward a different investigation pathway. Internal partial discharge suggests a defect within the insulation system itself, surface partial discharge often directs attention toward interfaces and environmental influences, while corona partial discharge is commonly associated with conductor geometry and localized electric-field concentration. Identifying the discharge type therefore helps narrow the search for the underlying defect and supports more effective maintenance planning.
How Engineers Identify the Partial Discharge Type?
Although internal, surface, and corona partial discharge differ in their physical mechanisms, identifying the discharge type in practice is not always straightforward. Different defects can produce overlapping signal magnitudes, while signal propagation through the asset and measurement system can influence what reaches the sensor.
Engineers typically use a structured diagnostic process:
Step 1: Confirm that the detected signal is genuine partial discharge
Measurements may contain switching noise, communication signals, power-electronic interference, and other electromagnetic disturbances. Engineers first verify that the activity originates from the asset rather than from external sources.
Step 2: Analyse discharge behaviour
One of the most useful tools under AC excitation is phase-resolved partial discharge (PRPD) analysis. Instead of displaying only partial discharge magnitude, a PRPD plot maps discharge activity against the phase position of the applied voltage over many cycles. Different defect mechanisms can produce characteristic discharge distributions.
Step 3: Correlate multiple measurement techniques
PRPD patterns alone should not be treated as definitive fingerprints. Defect geometry, insulation material, voltage level, sensor response, signal propagation, and operating conditions can all influence the measured pattern. Engineers therefore combine electrical measurements with techniques such as UHF, acoustic, ultrasonic, or optical detection where appropriate.
Step 4: Interpret the results in context
The final diagnosis depends on correlating measurement data with asset design, operating conditions, environmental influences, and historical trends. The objective is not simply to detect discharge activity, but to identify the underlying defect mechanism and assess its significance to asset condition.
Understanding the defect mechanism is important because it directly influences how engineers respond to the condition. Once the discharge type has been identified, the focus shifts from diagnosis to deciding the most appropriate maintenance strategy.
From Partial Discharge Classification to a Maintenance Decision
Partial discharge classification provides valuable context for maintenance planning. Identifying whether the activity is internal, surface, or corona helps engineers focus their investigation on the most likely source of the problem.
Internal partial discharge may justify deeper investigation of the insulation system. Surface partial discharge may direct inspection toward contamination, moisture, terminations, or field grading. Corona may lead engineers toward exposed conductors, protrusions, connection geometry, or other regions of concentrated electric field. Classification therefore helps narrow the diagnostic search and identify the most relevant maintenance actions.
However, maintenance decisions should consider more than discharge type alone. Engineers also need to assess whether activity is stable or increasing, where the source is located, how critical the asset is, and what the consequences of insulation failure would be. For instance, intermittent surface partial discharge on a redundant feeder and increasing internal partial discharge on a critical cable circuit should not receive identical maintenance priorities simply because both exhibit measurable discharge activity.
This is where continuous partial discharge monitoring becomes particularly valuable. Trending can reveal whether a known source remains stable or changes in magnitude, repetition rate, pattern, or operating correlation over time. While partial discharge classification helps determine what type of defect may be present, continuous monitoring helps establish how that condition is changing.
Rugged Monitoring’s partial discharge monitoring solutions support this approach by continuously monitoring and trending partial discharge activity across critical electrical assets. This provides maintenance and reliability teams with visibility between periodic inspections, helping them identify meaningful changes in discharge behaviour and prioritize actions based on asset condition and risk rather than a single partial discharge measurement.
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