Partial discharge (PD) monitoring is a critical diagnostic tool for assessing the insulation condition of electrical equipment. Both offline and online PD monitoring methods are widely used in the power industry to detect insulation defects before they lead to catastrophic failures. While they share the common goal of insulation assessment, these two approaches differ in methodology, application conditions, and technical implementation. This article will provide a detailed overview of offline and online PD monitoring, highlighting their principles, measurement approaches, and the differences.
What is Offline Partial Discharge Monitoring?
Offline partial discharge monitoring involves measuring PD activity on electrical equipment while it is de-energized. Typically, the device under test is disconnected from the power system and connected to specialized high-voltage test equipment. Various voltage stresses, including AC, DC, or impulse voltages, can be applied to the insulation to provoke partial discharge events.
PD signals are captured using capacitive, inductive, or acoustic sensors, and analyzed to identify insulation defects, their location, and severity. This method is commonly used during commissioning, routine diagnostic testing, or after maintenance activities to verify insulation integrity.
What is Online Partial Discharge Monitoring?
Online partial discharge monitoring measures PD activity while the electrical equipment is energized and in operation. Sensors such as capacitive couplers, high-frequency current transformers, or acoustic emission sensors are installed on live equipment to detect PD pulses without interrupting service.
Online monitoring provides real-time insight into insulation condition under actual operating voltage and environmental stresses. The collected PD signals are processed and analyzed to track insulation degradation trends, enabling condition-based or predictive maintenance strategies.
Offline vs Online Partial Discharge Monitoring
| Feature | Offline PD Monitoring | Online PD Monitoring |
| Equipment State | De-energized | Energized |
| Sensitivity | High | Moderate |
| Operational Impact | Requires outage | Non-intrusive |
| Environmental Representation | Limited | Real operational conditions |
| Data Trend | Single test | Continuous over time |
| Application | Commissioning, diagnostic testing | Condition monitoring, predictive maintenance |
Conclusion
Both offline and online partial discharge (PD) monitoring are critical for comprehensive insulation diagnostics of electrical equipment. Offline monitoring delivers high-resolution measurements under controlled, de-energized conditions, whereas online monitoring provides continuous evaluation under actual operational stresses. Integrating both approaches enables precise identification, localization, and trending of insulation defects, facilitating condition-based maintenance, mitigating insulation-related failures, and optimizing asset lifecycle management.
