Finding Partial Discharge: How often to test?

What is Partial Discharge?

Partial Discharge (PD) is a localised dielectric breakdown of a small portion of electrical insulation. This happens when electrical stress in a high voltage (HV) environment is too high, and when insulation cannot cope, it breaks down.

The 3 main types of Partial Discharge

Internal PD - The silent killer

Internal PD occurs silently, giving no external warning signs such as sound, smell or visual indication. It is often the main cause of unanticipated equipment failure.

Surface PD - A more visible concern

Surface PD can be seen with the naked eye and is often accelerated by environmental conditions such as, humidity. Although it can be easily identified by visual inspection, the real challenge here is early detection.

Corona PD - Onset for surface PD

Corona PD is what we often hear in outdoor switchyards. Although it is not particularly harmful in outdoor switchyards, it can initiate surface PD if left unchecked in enclosed chambers.

What is Partial Discharge Testing?

Partial discharge testing is an established condition monitoring test that plays a pivotal role in determining HV asset health. When embarking on a PD testing program, the greatest challenge lies in tailoring a testing schedule that is cost-effective and identifies faults promptly, allowing time for intervention before failure.

The P-to-F Curve: How often should I test?

Commonly used in predictive maintenance, the P-to-F curve represents the progression of equipment failure over time.

• P (Potential Failure): The point at which a failure first becomes detectable. Equipment is still functioning properly.
• Condition Monitoring Phase: Between P and F, condition monitoring tools can detect worsening conditions, allowing for interventions before a functional failure.

When dealing with HV switchgear and the multiple variables at play, the P-to-F curve becomes more complex. Each type of failure or variable may have its own distinct P-to-F curve, which can vary in length and shape. When creating your optimal test regime, it is important to consider this P-to-F interval.

What are the factors influencing the P-to-F interval?

• Type of partial discharge
• Type of HV asset
• Discharge amplitude
• Insulation type
• Environmental conditions
• Physical position of discharge

Determining the perfect PD test schedule for your HV assets

Periodic testing VS Permanent monitoring

Periodic annual testing is useful for identifying emerging PD defects but may not be prompt at tracking deterioration if the P-to-F interval is relatively short - you do not want a failure happening right before your next annual test is due!

This is where permanent, around the clock monitoring comes in. Data insights from permanent monitoring provides the operator with invaluable information and ample time to take preventive measures. This is not only great for early detection of PD issues but also, monitoring the condition of end-of-life assets.

Considering the P-to-F curve is key

For optimal test design, it is important to account for the shortest possible P-to-F interval within the network. By narrowing the testing interval, we can enhance our chances of identifying defects closer to their onset and provide time to act before a full-scale failure occurs.

A risk evaluation of your assets and their life stages will help determine the most optimal test regime. Contact us for a free consultation today!

For further information on PD testing regimes, read our published article "PD Testing of HV Assets: How often should we test?". Real-life case studies featured!