Understanding Partial Discharge (PD) Testing

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Partial Discharge Testing

Predictive maintenance helps ensure reliability and longevity of equipment and electrical systems, especially within industrial settings. Given the multitude of equipment and components, each with unique specifications and operating conditions, it is imperative that suitable mix of technologies and testing techniques be adopted for the maintenance effort to be comprehensive. Partial discharge (PD) testing is yet another way to monitor the health of electrical systems in a facility. In this article we take a look at the what, when, and where of partial discharge and how best to use it to avoid insulation failures in electrical systems.

Before attempting to measure or test PD, let’s first understand what we are looking for!

Partial Discharge – What, Where and When?

Partial discharge is a tiny spark of electricity that occurs within the insulation of electrical equipment. This discharge travels through the dielectric material and connects the energised conductors inside the casing. It is important to note that PD activity can occur anywhere within the dielectric, where the breakdown strength of the material is no longer adequate to counter strength of the electric field generated in the system.

Breakdown strength is indicative of the health of the insulation. It tends to weaken as a result of cracks, cavities, contamination and other problems with the dielectric material, which are ideally signs of aging, wear and tear, or exposure to elements of weather.

When not detected and repaired on time, these discharges which generally occur at voltages of 2,000V or above are capable of totally eroding insulation and causing un-expected outages.

Most disruptive failures of MV/HV equipment are as a result of PD activity.

Partial Discharge – Testing

PDs, typically lasting a few nanoseconds, are measured in picoCoulombs (pC). Test instruments use a combination of inductive and capacitive analogue sensors to capture these discharges, filter noise, amplify signals and convert them to digital data, which is then used for further analysis and decision-making.

There are several PD testing solutions available to suit specific testing requirements, including but not limited to:

  • Offline PD Testing of MV Equipment
  • Online PD Testing of MV Equipment
  • Continuous PD Monitoring of MV Equipment
  • Very Low Frequency (VLF) Cable PD Mapping – Megger / SEBA TDS NT range
  • On-line cable PD monitoring

Offline PD Testing

Typically, PD testing conducted on new equipment as a part of acceptance testing is done offline. The solution proves valuable when testing for specific test conditions, such as stress levels under different voltages without triggering a fault. It is also known to be more accurate in detecting fault location, especially in aged equipment. Offline testing often proves expensive in live units as it requires equipment to be de-energised, causing losses in production and productivity as well, but can be a part of scheduled predictive/preventive maintenance programs.

Non-Intrusive Online PD Testing

This solution is ideal for MV equipment as it is carried out in real-time when the equipment is functioning under normal voltages, operating conditions, and stress levels. Online testing is relatively cost-effective as the process is non-intrusive (no downtime, no changes in operating voltages), non-destructive (does not attempt to trigger any failures), and does not expose the system to any undue voltage stresses. Both surface and internal partial discharge activity can be detected using ultrasonic and transient earth voltage (TEV) measurement techniques. These tests can be carried out using instruments such as the UltraTEV Detector, UltraTEV Plus or UltraTEV Locator.

Continuous PD Monitoring

It is possible to remotely record PD activity in critical medium-voltage equipment continuously using on-line PD monitors. Recording and analysing PD data real-time proves valuable, especially in case of unmanned centres or aging systems. Continuous monitoring reveals an accurate picture on capability and state of the equipment, warning of any imminent failures that are likely to be costly to set right. The UltraTEV Monitor is ideal for this application.

Very Low Frequency (VLF) Cable PD Mapping

For new or existing installations, where the customer can obtain an outage, Very Low Frequency (VLF) Cable PD Mapping test equipment can be used to determine cable condition. This test allows the user to accurately determine PD magnitude and location along the length of the cable. The Megger / SEBA TDS NT range is ideal for this application.

On-line Cable PD Monitoring

It is also possible to install on-line PD monitors on MV/HV cables. Instruments such as the CableData Collector™ (CDC) allow you to detect and report on PD activity in LIVE MV/HV cable networks. This early identification helps to reduce the need for disruptive maintenance outages, by highlighting high risk areas in ample time for repairs and replacements to be made.

Partial Discharge Testing — Applicability

PD, though characteristic of aging systems, has however no limits as a result. Testing equipment right before and after installation can help detect breakdown of the insulation caused by improper installation, operating conditions, or even design faults. It is also useful in securing baseline data for future comparisons.

PD testing is suitable for all types of electrical equipment powered by medium or high voltages such as:

  • Transformers and bushings
  • Switchgear
  • Motors & Generators
  • And most importantly cables, terminations, and splices

We often fail to note that cables do not figure as a part of scheduled preventive/predictive maintenance programs, despite being crucial components in electrical systems. This is partly due to the lack of tests that can offer meaningful insights on health of cables, and partly due to the concern of likely damages cables are bound to suffer during the testing process.

PD testing now provides an effective way to ascertain the condition and capability of cables, determine the effectiveness of insulation, and prevent any imminent insulation failures in electrical systems. It helps enhance the reliability of electrical installations when included in the scheduled maintenance programs, along with other testing technologies such as IR imaging.

As with other tests, it is important to record baseline values whenever a new equipment or cable is installed and PD values measured and tracked as needed to ensure successful execution of PD testing. Baseline data can be obtained either from the manufacturer’s test data if available, or captured as a part of acceptance testing before the new component is integrated with the existing system.

For further information, contact us by email or call us on 057 866 2162. We’d be happy to answer any questions you might have.

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