Online PD Monitoring vs Online PD Testing

What is Online PD Monitoring

Online Partial Discharge (PD) Monitoring, also known as permanent or fixed PD monitoring, is a condition-based monitoring solution in which PD sensors are permanently installed inside or on the surface of switchgear or electrical equipment. A dedicated PD monitoring unit（HMI） continuously acquires and analyzes PD signals 24/7, and presents the insulation condition of the equipment via a local display or a remote monitoring system. This solution is primarily used for continuous asset condition monitoring and early warning of insulation faults.

What is Portable Online PD Testing

Portable Online PD Testing, also referred to as portable insulation diagnostic testing, is a short-term diagnostic approach using mobile test instruments. Sensors are temporarily deployed on energized electrical equipment to capture and analyze PD signals. This method is suitable for routine inspection, fault investigation, and insulation condition assessment.

Portable Online PD Testing vs. Online PD Monitoring – What’s the Difference？

Both Online PD Monitoring and portable Online PD Testing are performed while the equipment remains energized, eliminating the need for a power shutdown. This distinguishes them from traditional offline PD testing, which is gradually being phased out.

However, the two approaches differ significantly in terms of deployment method, monitoring duration, and applications.

Comparison	Portable Online PD Testing	Online PD Monitoring
Mode	Handheld portable device; temporary on-site visits or short-duration, targeted inspections.	PD sensors are permanently fixed in place; operates continuously, 24 hours a day.
Usage	Suitable for both de-energized and energized equipment; used for periodic spot checks and ad-hoc troubleshooting.	Real-time data acquisition while equipment remains energized; continuous, unattended monitoring.
Feature	Measure-and-go operation; highly flexible and mobile; performs intermittent measurements.	Continuous real-time data streaming; trend analysis; automatic alerts for parameter excursions.
Application	Routine inspections, re-verification of suspected defects, and pinpointing faults in the field.	Long-term online early warning and condition-based maintenance for high-voltage switchgear, busbars, and transformers.

When to Choose Online PD Monitoring or Online PD Testing

Online PD Monitoring is recommended for critical distribution circuits, such as main incoming panels, busbar sections, and feeders supplying important loads. It is particularly suitable for high-risk or mission-critical environments—such as mining, chemical plants, energy storage systems, and data centers—where unplanned outages are not acceptable.

In such cases, continuous 24/7 monitoring is required. Permanently installed sensors enable ongoing PD signal acquisition, early detection of insulation defects, trend analysis, and automatic alarming. Combined with local displays or remote monitoring platforms, this approach transforms maintenance from reactive repair to proactive prevention, ensuring compliance with safety and operational requirements.

Portable Online PD Testing is more suitable for substations or switchgear rooms with a large number of panels of moderate criticality, or where budget constraints and structural limitations prevent permanent sensor installation.

It is typically used for routine inspections, commissioning of new equipment, condition surveys of aging assets, and fault localization. A single portable device can be deployed across multiple panels, significantly reducing investment while meeting diagnostic needs. It is also commonly used to verify alarms and perform precise fault localization following indications from online monitoring systems.

Conclusion

Online PD Monitoring and Online PD Testing are complementary approaches for insulation assessment under energized conditions. Monitoring enables continuous, real-time detection and early warning for critical assets, while testing provides flexible, cost-effective diagnostics for inspections and fault verification. A combined strategy helps balance reliability, risk, and cost.