Partial Discharge Monitoring has emerged as a crucial method for detecting failures in electrical insulation. Unlike traditional offline methods, PD condition monitoring now allows asset experts to predict faults before they escalate into catastrophic failures, such as grid blackouts and safety risks. As a result, a majority of businesses are investing in advanced PD monitoring solutions to ensure the integrity and reliability of their electrical systems.
But what’s driving this surge in investment?
In this blog, we’ll explore what partial discharge monitoring is, the common detection techniques used, and the growing necessity for proactive monitoring solutions.
What is Partial Discharge?
Partial discharge is a tiny electrical discharge caused when local electric stress becomes too high. Occurring on or inside the surface of an insulating material, these discharges do not bridge the full gap between conductors. Thus, tokening the word “partial”.
They may seem insignificant on their own, as each individual discharge is microscopic. However, when these discharges occur repeatedly, they can cause degradation of insulation over time. To effectively monitor and address these faults, it is crucial to first understand the various types of partial discharges.
Understanding the different types also helps in selecting the right partial discharge monitoring solutions.
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Internal Discharge
Such PD activity occurs inside the solid dielectric voids or cavities, or as bubbles in liquids, or as delamination between layers. These are mainly caused by either manufacturing defects or insulation shrinkage from asset ageing. A key signature for identifying internal discharge during partial discharge monitoring is checking for pulse patterns that are symmetrical in both half cycles of AC.
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Surface Discharge
As the name suggests, such PD occurs on the surface of the insulation, such as bushings, insulator surfaces, contaminated surfaces, or a moisture layer. Much of this activity is due to environmental factors, including pollution, poor maintenance, and sharp edges. Since the PD activity occurs on the surface, it is easily identified during PD monitoring. If not, it can also be detected through acoustic emissions or mixed-phase/half-cycle patterns.
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Corona Discharge
These partial discharges form in air or gar near sharp points, edges, or under high fields. Corona discharges can also occur externally to the insulation or in air gaps, due to non-uniform fields, elevated voltages, or pollution. This type of PD emits visible glow, audible hissing, electromagnetic noise, or UV, making partial discharge monitoring easy. Often considered a lesser, imminent threat than internal PD, it still causes some environmental damage.
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Treeing
Usually found in cable insulation, treeing is caused within solid dielectrics that branch out as “trees” to cause damage under repeated PD. As the name suggests, this type of PD occurs over long-term activity due to local defects, moisture penetration, or poor insulation design. It is also an indicator for both internal and surface PD, making it more dangerous when compared to all. But with real-time partial discharge monitoring, this threat can be identified early on and prevented before it results in catastrophic failure.
Besides these common types of PD, the phase-resolved PD patterns (PRPD) also help in differentiating other types of partial discharge activity. This is done by relating PD pulses to the phase of the applied AC voltage.
How to identify PD Activity in each asset?
Different types of electrical assets show partial discharge activity differently. Therefore, each of them would require partial discharge monitoring that best suits their needs.
| Asset Type | Common PD Sources | Detection Challenges |
| Power Transformers (Oil‐filled, Dry type) | – Voids in winding insulation
– Paper degradation; oil bubbles – Discharges at bushings or leads – Internal contamination. |
– Noise from transformer load
– Switching internal echoes – Difficulty physically accessing internal sensors – Oil filtering, temperature effects – Dielectric windows attenuate UHF |
| Switchgear (AIS, GIS, MV Panel) | – Voids, sharp edges, damaged contacts, loose connections
– SF₆ defects (if applicable) – surface contaminated insulators. |
– Metal enclosures attenuate some signals
– High electromagnetic interference – Compartmentalization – Difficulty distinguishing external vs internal PD – Humidity and weather influences. |
| Cables, Joints, Terminations | – Voids in insulation
– Moisture ingress – Poor sealing in joints/terminations – Partial discharges at connectors and accessories. |
– Attenuation over cable length (high frequency components lose strength), reflections, and dispersion in the signal
– Distinguishing PD from other pulses (switching transients, load changes); ambient noise – Access to both ends for localization. |
| Rotating Machines (Generators, Motors) | – Turn-to-turn insulation, slot discharge, end winding PD, aging insulation, and moisture. | – Motion, vibration, electromagnetic interference
– Variation with speed/load – Sensor mounting challenges – Cooling system noise, brush gear noise. |
As these events only affect a particular section of the insulation system, the PD becomes invisible to traditional partial discharge monitoring techniques or manual inspections. Over time, a cumulation of such faults going unnoticed leads to complete asset failure.
The need for partial discharge monitoring
Besides the technical and business aspects, partial discharge monitoring is crucial for several other reasons:
Minimize unplanned downtime
Complete insulation failure can lead to fires, system-wide outages, and safety hazards. With traditional methods, repair or replacement after the failure is expensive and disruptive. But with proactive PD monitoring, issues can be preempted to minimize asset downtime and improve reliability.
Asset life extension & maintenance optimization
Using condition-based maintenance (CBM) rather than time-based methods, experts can act when the condition warrants, not on schedule alone. Partial discharge monitoring delivers quantitative metrics supported by data-driven insights that strengthen O&M operations.
Risk management & safety
For high-voltage assets, a sudden insulation failure can be dangerous. It can also produce harmful by-products (gases, ozone, heat), which may accelerate other failure mechanisms. By implementing intelligent partial discharge monitoring techniques, such metrics can be recorded and monitored in real-time.
Regulatory and financial incentives
Regulatory bodies often require reliability or safety standards. Also, insurance premiums, warranties, and compliance may depend on proving the health of assets. Advanced partial discharge monitoring solutions also support such documentation of care and risk mitigation.
Operational efficiency and cost savings
Avoiding emergency replacements, reducing collateral damage, optimizing spare parts inventory, and improving planning all come from knowing asset health in advance. Condition monitoring systems provide AI-driven insights by analyzing real-time and historical data. This can be used to improve overall operational efficiency and cost savings.
Partial discharge monitoring offers asset experts’ valuable insights into hidden faults. However, it’s important to note that it isn’t a one-size-fits-all solution. Each asset presents unique challenges regarding access, noise environments, and detection capabilities, as previously discussed.
In our next blog post, we will further explore partial discharge monitoring, including techniques, the differences between offline and online PD testing, and the latest advancements in PD condition monitoring.
