Insulation breakdown is the leading cause of electrical failures in High Voltage assets, with partial discharges being the first and clearest indicator of deterioration of the insulation medium.
In any medium or high-voltage electrical assets, there will be some level of Partial Discharges (PD) which are the result of the electrical breakdown of air pockets or voids within, or on the surface of the insulation. PD generally increases over time as they erode the insulation via ‘hotspots’ caused by the discharge current. Ultimately, if left unchecked, this can lead to insulation failure, in which case the electrical asset is compromised.
Whilst there is a small cost attached to Condition Monitoring of assets and tests such as Partial Discharge, the cost of unplanned outages stemming from catastrophic failures can result in enormous financial losses. Adherence to a long-term Asset Management plan where deterioration of an asset can be monitored will almost always be more cost effective than short-term gains from the reduction of testing & maintenance budgets.
Partial Discharge testing is also one of the most important forms of electrical testing there is, as PD can be monitored and measured long before insulation failure occurs. By monitoring activity over time, managers can make informed, strategic decisions on when to repair or replace their electrical assets.
What can Partial Discharge Testing Monitor?
Partial Discharge testing is particularly effective at measuring and thereby predicting the future performance of key assets. These include:
- Switchgear (Switchboards, Circuit Breakers, CT’s, VT’s, GIS)
- Motors and generators
- Power transformers
- Cables, splices, and terminations
Partial Discharge Testing is not only a key diagnostic tool for aged or aging assets, but is advantageous to perform on newly installed assets for two important reasons.
First, it is useful to check results against manufacturer data and ensure that your asset was not faulty, to begin with, or was not damaged during installation.
Second, baseline data will provide trend information for future testing and establish an accurate rate of deterioration to help predict the life cycle of your assets.
Online Partial Discharge Testing
Recent advances in technology such as digital signal conversion and active filtering significantly improve the accuracy of Online Partial Discharge Testing, which can be performed during real operating conditions. This means no downtime or interruption of services, which is particularly advantageous for systems that run 24/7.
Online PD testing benefits include:
- More economical and quicker option than Offline PD testing
- Testing is nonintrusive and non-destructive
- Strong Trending ability through comparison of baseline and future measurements
Continous Online / Permanent Monitoring
Continuous Online Partial Discharge Testing is an important option for assets that are approaching the end of their life as it allows real time condition assessment in order take precise action, maximizing the lifespan of the equipment, while also avoiding any unplanned shutdowns due to complete failure.
In some cases, equipment or materials that are difficult to access (or involve a significant safety issue) may need to be monitored. For these, permanently mounted sensors can provide a practical, cost-effective solution.
Sensors are installed during an outage or scheduled downtime and once they’ve been correctly installed, they can test and measure continuously without needing the system to go offline.
Offline Partial Discharge Testing
Whilst online testing does provide some advantages, offline testing will provide much more detail, and also offers he following advantages:
- Measurements are calibrated and are in direct conformance with Australian/ International standards (AS/IEC 60270) which allows for the highest level of accuracy.
- The applied test voltage can be increased above the normal operational level, and used to simulate transients/ surges providing a valuable insight into the insulation integrity and assist forecast future PD levels.
- Background noise such as network harmonics/ switching transients (that can dramatically affect the accuracy of online measurements) are significantly reduced or eliminated as the asset is physically isolated from the system.
- Through advanced techniques such as Time Domain Reflectometry (TDR), the exact location of PD can be determined.
What kind of testing do you need?
Whether you choose online or offline testing (or a myriad of other options not covered here), the key consideration is balancing the usefulness of the data you receive with the cost or inconvenience of downtime that results from the testing process.
Once you’ve determined which option most aptly fits the circumstances, Partial Discharge Testing can provide your asset managers with all the information they need to make informed decisions about how to maintain or replace their equipment.