PD APPLICATIONS - POWER, DISTRIBUTION & INSTRUMENT TRANSFORMERS

On- and off-line testing

Performing partial discharge (PD) testing on power, distribution, or instrument transformers in the field presents significant challenges for service engineers or commissioning teams. These on-site assessments can be performed on-line, meaning the transformer is connected to the grid and energised, or performed while the transformer is off-line, meaning it is powered by a mobile power system. Both methods have advantages and disadvantages, as listed below:

On-line PD testing advantages:

  • Transformer is under load condition (full current): Testing occurs while the transformer is operating at its usual load, providing real-world insights.
  • Rated voltage and frequency: Measurements reflect actual operating conditions.
  • Up-to-date insights into insulation system condition: Continuous monitoring ensures real-time data on insulation health.
  • Early detection of insulation degradation or potential faults: Identifies issues before they lead to failures, reducing downtime and maintenance costs.
  • Continuous operation of the asset: Allows the system to remain operational during testing.
  • Integration with condition monitoring systems: Enables comprehensive analysis of equipment health by combining PD data with other monitoring information.

On-line PD testing limitations:

  • No variable voltage and frequency: Testing is limited to the operating conditions of the transformer, eliminating access to additional insights.
  • Almost no access (or limited access) to the transformer tank: Restricted ability for further inspection.
  • Difficult for fault investigations
  • Cannot distinguish PD inception voltage (PDIV) and extinction voltage (PDEV): These critical parameters cannot be separately identified.
  • Requires special equipment and expertise 
  • Highly affected by electrical noise: External electrical interference can impact measurement accuracy.

Off-line PD testing advantages:

  • Full variable voltage and switchable frequencies
  • More detailed analysis of partial discharge activity
  • Reduced electrical noise: Asset disconnection from the grid minimizes interference. 
  • Manipulation of the power supply: Independent from grid conditions, allowing tailored testing scenarios.
  • Concurrent testing capabilities: Ability to perform other diagnostic tests, such as insulation resistance, tan delta (power factor), and winding resistance. 
  • Cost savings and the portability of power supplies for on-site settings

Off-line PD testing limitations:

  • May not capture transient or intermittent partial discharge events: These events occur during normal operation and might be missed during offline testing.
  • Requires downtime: The asset must be taken out of service, impacting operational schedules. 
  • Lacks real-time insights: Does not provide ongoing information about the insulation system’s condition.
  • Potentially undetected faults: Issues might go unnoticed between maintenance periods, leading to possible risks.

Typical Packages

1) Set for power transformer onsite testing: