Condition measurement pays dividends

Recently developed techniques to measure the condition of live power assets are transforming the way assets are managed around the world. Converts report dramatically improved network reliability and better customer service at lower cost. So where is the evidence to support their claims? And what can Australia learn from it?

By Charles Williamson, EA Technology Australia

Just like people, assets age and deteriorate. And as with human medicine, there have been tremendous advances in recent years in the ability to diagnose the ailments suffered by assets in their early stages, enabling us to keep them healthy and productive for longer. The revolution in asset management is being driven by two factors which are inextricably linked: new techniques for accurately measuring the condition of live assets, plus new methodologies for managing assets more effectively, based on their actual condition. There are many techniques available for assessing the condition of live assets, including oil sampling and analysis, plus thermal imaging. But the most useful is the detection, location and measurement of Partial Discharge (PD) activity in HV and MV assets which are typically found in substations.


Value of Partial Discharge (PD) Data
The latest PD instruments are extremely valuable for identifying faults as they develop at an early stage, and preventing them from developing into failures – the ‘quick health check’ approach.


But the information they gather is also playing a major part in transforming the industry’s approach to asset management itself: from one based on time-scheduled maintenance and replacement, to one based on a detailed understanding of the condition of the asset base. Maximising the value of PD data is essentially carried out at two levels:


1. Asset condition registers
Expert analysis and interpretation of PD activity readings gives a clear indication of the condition of assets, including accurate predictions of when they are likely to fail. In EA Technology’s case, this is based on a unique database, built up over more than 30 years, which shows how tens of thousands of asset types have deteriorated over time.


This approach enables operators to develop registers of assets, in which each asset is accorded a ‘health index’ showing its present condition, its predicted date of failure and/or its remaining service life. There is strong evidence (see below in this article) that basing maintenance and replacement on actual condition is far more cost effective than time-scheduled interventions or a ‘wait until it stops working’ approach. This is because condition based regimes:

• Prevent expensive unexpected failures
• Prevent needless invasive maintenance, which is costly, disruptive and often creates more faults
• Enables operators to prioritise expenditure on a strictly ‘need to’ basis

2. Condition Based Risk Management (CBRM)
CBRM is a comprehensive new methodology, which takes condition based asset management to a higher level, enabling managers to take more intelligent decisions on revenue and capital spending. It also reduces the cost of network operation, while improving their efficiency and reliability. The effectiveness of CBRM derives from factoring together PROBABILITY (derived from the asset condition) and CONSEQUENCES of asset failure, to determine risk in terms of financial cost. In addition to managing the health of assets, CBRM provides the answers to the key questions:

• If an asset costing $XX fails, what will be the consequential loss to the business?
• If an asset is refurbished or replaced at a cost of $YY, what will be the benefit to the business
• Therefore, where should we prioritise our spending?
  
  

Understanding Partial Discharge
EA Technology’s experience of working with network assets for more than 30 years shows that Partial Discharge (PD) activity is a factor in around 85% of disruptive substation failures. It has thus become increasingly clear that the ability to detect and measure PD is key toassessing the health of assets and implementing CBRM. A partial discharge is an electrical discharge or spark that bridges a small portion of the high voltage insulation between two conducting electrodes or a conducting electrode and earth. Partial discharge can occur at any point in the insulation system, where the electric field strength exceeds the breakdown strength of the insulating material. Partial discharge can occur in voids within solid insulation, across the surface of insulating material due to contaminants or irregularities,within gas bubbles in liquid insulation or around an electrode in gas(corona activity). Once present, partial discharge ALWAYS increases.


PD failure Process
PD activity provides clear evidence that an asset is deteriorating in a way that is likely to lead to failure. The process of deterioration can propagate and develop, until the insulation is unable to withstand the electrical stress, leading to flashover. The ultimate failure of HV/MV assets is often sudden and catastrophic. The best case scenario is that protection systems trip out unexpectedly, causing outages. The worst case scenario is major damage, injury and death.


PD Detection & measurement
Partial discharges emit energy, in the form of effects which can be detected, located, measured and monitored:

• Electromagnetic emissions, in the form of radio waves, light and heat
• Acoustic emissions, in the audible and ultrasonic ranges
• Ozone and nitrous oxide gases
• The most effective techniques for detecting and measuring PD activity in live assets are based on quantifying emissions from discharges
  
  

Transient Earth Voltages (TEVs)
The importance of TEV effects (discharges of radio energy associated with PD activity) was first identified by EA Technology in the 1970s. Measuring TEV emissions is the most effective way to assess internal PD activity in metalclad MV switchgear.


Ultrasonic emissions
PD activity creates emissions in both the audible and ultrasonic ranges. The latter is by far the most valuable for early detection and measurement. Measuring ultrasonic emissions is the most effective way to assess PD activity where there is an air passage e.g. vents or door in the casing of an asset.


UHF emissions
PD activity can also be measured in the UHF range, and is particularly useful in monitoring EHV assets.


PD instruments
The latest generation of PD instruments typically use a combination of ultrasonic and TEV sensor technologies, characterised by the EA Technology UltraTEV range. These include:

• Handheld dual sensor instruments which provide an instant indication of critical levels of PD activity, ideal for ‘first pass’ PD surveys and safety checks. Traffic light warning levels are precisely calibrated using a database of known patterns of asset deterioration
• More sophisticated handhelds, which provide audible and numerical readings of ultrasonic and TEV activity
• PD location instruments which pinpoint and quantify the source of PD activity
• PD monitoring instruments, which measure, record and analyse PD activity over time
• PD alarm systems, which give immediate warning of critical PD activity in groups of assets or whole networks
• Specialist PD monitoring systems for strategically important assets, including Gas Insulated Switchgear (GIS)
  
  

Other asset classes
Condition based management is by no means confined to assets which present faults in the form of PD activity. The same principle is equally effective, using a range of condition measurement techniques, with all types of electricity network assets including substations and cables. It can apply to the complete asset, such as an overhead line, as well as to the component parts, such as the overhead conductors, poles, towers and footings.


Condition Measurement: the Bottom line
The UK Business case taken as a whole, the UK electricity network is relatively efficient. But an in-depth analysis by EA Technology Consulting of preventable, condition-related failures, shows there is considerable scope for improvement.


SP Powergrid’s network includes nearly 10,000 substations, 40,000 switchgear sets, 14,000 transformers and 30,000km of cable. Since incorporating condition monitoring into its systems, it has dramatically improved an already excellent performance. The System Average Interruption Duration Index (SAIDI) has averaged less than 1 minute pa over the last three years.

SP Powergrid estimates that over the last eight financial years, condition monitoring has enabled it to avert 450 network failure incidents, with a net financial saving of US$29 million. In addition to improving customer service, it has been able to pass cost savings on to them.


CLP Hong Kong
The China Light and Power network in Hong Kong includes nearly 13,000 substations and 22,000km of overhead lines and underground cables, serving 2.26 million customers. As a result of focusing over the last 10 years on condition based maintenance, to predict faults and improve reliability , it has reduced its SAIDI figures from more than 40 to 2.68 minutes lost per year. Demand from customers has continue to grow, but in the last two years, greater operating efficiencies have enabled CLP to reduce tariffs.


Conclusion
There is a world-wide ground-swell of interest in moving to a more efficient and cost-effective methodology for electricity network asset management. Modern PD instrumentation gives network operators the tools to collect good asset condition information. This is the fundamental input to a rigorous methodology to quantify risk. Applied to the whole asset population, a utility company is thereby able to develop and prioritise its investment strategies based on a comprehensive understanding of its risk profile.


www.powertrans.com.au Transmission & Distribution