Chase Fell
Technical Education Committee Chair
Jay Industrial Repair
For service professionals working with large electric motors and generators, machines with voltage ratings 5 kV and higher can be vulnerable to invisible threats of partial discharge (PD). Corona and PD activity have long been a challenge to detect before failure occurs. Traditionally, detecting these issues required indirect methods like radio interference voltage (RIV) testing, ultrasonic detection or offline insulation tests. But recent advances in partial discharge imaging—sometimes called corona cameras—are giving engineers and technicians a powerful new tool: the ability to see electrical discharge activity directly and in real time.
In this article, we’ll explore how partial discharge cameras work, their practical applications for motors and generators and how they can support proactive maintenance strategies in high-voltage equipment.
What is Partial Discharge?
Partial discharge (PD) refers to small electrical discharges that occur within the insulation system of high-voltage equipment without fully bridging the insulation. PD activity is both a symptom and a cause of insulation degradation. Over time, partial discharge can erode insulation, leading to complete failure, flashover or catastrophic breakdown.
PD can occur in:
- Stator winding end turns
- Slot discharge between conductor and core
- Surface tracking along insulating supports
- Terminal and bushing connections
The challenge is that these discharges often emit only subtle electrical and acoustic signals, which are not easily detected without specialized equipment.
How Does a Corona Camera Work?
PD cameras, also known as corona cameras, combine two imaging technologies in a single handheld or mounted device:
- Energize the winding with AC voltage at power frequency. The voltage level needs to be line-to-ground voltage = (VLL)(.58). An AC hipot is ordinarily used for the excitation of the winding. Test each phase separately and ground circuits that are not under test. Due to the capacitance of the winding, it is likely that a resonator will be needed to keep test current within the limits of the power supply.
- Ultraviolet (UV) detection: The camera detects the UV light emitted by corona and PD events, which are normally invisible to the human eye (Figure 1). 3. Visible light imaging: The camera overlays the UV signal onto a normal optical image, allowing the user to pinpoint the discharge source visually. When viewed through the camera, the discharge appears as a bright spot or halo superimposed on the actual equipment image (Figure 2). The result is a powerful diagnostic: the technician can see exactly where the discharge is happening, even in complex winding assemblies or in hard-to-access areas.
Many corona cameras include built-in software to quantify discharge intensity and log images or videos for documentation.
| Applications for Motor and Generators |
| Stator End Winding Inspection |
Corona cameras can reveal PD activity in the end winding regions—especially around stress grading coatings, support rings or where contamination or loose bindings create surface discharge paths. |
| Terminal and Bus Connection Assessment |
External discharge from improperly shielded or contaminated terminal connections can be detected before it results in tracking, pitting or failure. |
| Insulation Surface Condition Monitoring |
Surface PD on epoxy-glass supports, brackets or insulating structures can be identified visually, allowing cleaning o |
Benefits for Service Providers
For EASA service centers and field technicians, PD imaging offers several key advantages:
- Immediate visual results: Unlike indirect measurement methods, corona cameras provide an intuitive, visual indication of discharge locations.
- Documentation for trending: Captured images allow baseline creation and comparison over time, supporting predictive maintenance.
- Improved troubleshooting: Helps differentiate between external discharge sources (e.g., contamination, loose connections) versus internal winding issues.
In the service of high-voltage motors and generators, being able to “see” discharge bridges the gap between traditional testing and hands-on maintenance.
Limitations and Considerations
It’s important to note that corona cameras have some limitations:
- Sensitivity is affected by lighting: UV detection works best in low-light or shaded environments; bright sunlight may mask weaker signals.
- Cannot detect internal discharge: Discharges occurring deep inside insulation (e.g., internal voids) may not be visible externally.
- Requires line-of-sight: Discharge must be accessible to the camera’s field of view.
Therefore, corona cameras are best used as complementary tools alongside electrical testing methods like surge comparison, PD sensors or insulation resistance tests.
Looking Ahead
As the fleet of aging high-voltage motors and generators continues to grow, and as uptime expectations rise, service providers are seeking more effective ways to monitor insulation health. Partial discharge imaging represents an important addition to the condition-monitoring toolbox.
By giving technicians and engineers the ability to see where insulation stress is manifesting as corona or PD, this technology helps prioritize repairs, reduce unplanned outages and extend the life of critical assets.
In today’s high-voltage world, being able to see the invisible is more than a technical advantage—it’s a business necessity.
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Safety First: Precautions for Using Partial Discharge Cameras
While partial discharge cameras allow non-contact inspections, working near high-voltage equipment—especially energized machines—requires strict adherence to safety protocols:
- Maintain safe approach distances per applicable electrical safety standards (e.g., NFPA 70E, IEEE 516).
- Wear appropriate PPE for arc-flash and electrical hazards, even when using the camera from a distance.
- Use insulated platforms or barriers if working near exposed conductors.
- Do not exceed equipment voltage ratings for the camera or operator’s safety zone.
- Coordinate with plant operations to ensure equipment status (energized/de-energized) is verified and tagged.
Remember: While the camera itself is safe to operate at a distance, the inspection environment may pose hazards that require proper training, protective equipment and procedures.
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