Partial Discharge Testing: How Often Should You Perform It?

What is Partial Discharge?

Partial Discharge (PD) refers to a localised dielectric breakdown of a small portion of electrical insulation within a high voltage (HV) environment. This breakdown occurs when electrical stress becomes too intense, causing the insulation to fail. Understanding partial discharge is crucial for maintaining the health of HV assets, as it can lead to significant equipment failure if left unaddressed.

The 3 main types of Partial Discharge

There are three main types of partial discharge, each with its own characteristics and implications:

1. Internal Partial Discharge - The silent killer

Internal PD occurs within the insulation material itself, often without any external warning signs like sound, smell, or visual cues. This makes it particularly dangerous, as it can be the root cause of unexpected equipment failures. Regular asset condition assessment is essential to detect internal PD before it leads to severe consequences.

2. Surface Partial Discharge - A more visible concern

Surface PD occurs on the surface of insulation and can often be seen with the naked eye, especially under certain environmental conditions like high humidity. While it is easier to detect through visual inspection, early detection is still crucial to prevent further deterioration. Regular testing helps in identifying surface PD before it causes significant damage.

3. Corona Partial Discharge - A Precursor to Surface PD

Corona PD is typically heard as a buzzing sound in outdoor switchyards. While it is generally less harmful in outdoor environments, if left unchecked, it can initiate surface PD in enclosed spaces. Continuous testing for partial discharge is key to monitoring and mitigating corona PD risks.

What is Partial Discharge Testing?

Partial discharge testing is a vital condition monitoring technique used to assess the health of HV assets. This testing helps identify the early stages of insulation failure, allowing for timely interventions that can prevent complete breakdowns. The challenge lies in developing a testing schedule that is both cost-effective and capable of detecting faults early enough to allow for preventive action.

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

The P-to-F curve is a predictive maintenance tool that illustrates the progression of equipment failure over time. Understanding this curve is essential for determining the optimal frequency of PD testing.

• 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.

Given the complexity of HV switchgear and the multiple variables at play, each type of failure or variable may have its own distinct P-to-F curve. It is important to tailor your partial discharge testing schedule to account for these variations and ensure that testing intervals are optimised for early detection.

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

Several factors that influence the P-to-F interval and, consequently, the frequency of PD testing:

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

Determining the Optimal Partial Discharge Testing Schedule

When planning your PD testing regime, it is important to weigh the benefits of periodic testing against permanent monitoring:

Periodic Testing: Annual testing can help identify emerging PD defects but may not be sufficient if the P-to-F interval is short. You don’t want a failure occurring just before your next scheduled test.

Permanent Monitoring: Continuous monitoring provides real-time data, offering invaluable insights and allowing for preventive measures to be taken well in advance of a failure. This is particularly beneficial for detecting early PD issues and monitoring the condition of ageing assets.

The Importance of Considering the P-to-F Curve

For the most effective partial discharge testing strategy, it is crucial to design your testing intervals with the shortest possible P-to-F interval in mind. By narrowing the testing intervals, you increase the likelihood of detecting defects early, giving you ample time to address issues before they lead to major failures.

A thorough risk evaluation of your HV assets and their life stages will guide you in determining the most effective testing regime. For personalised advice, 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!