Owners and operators of power transformers know that these assets are usually very reliable, but that they can offer significant challenges in:
- Identifying their health and fitness for purpose
- Looking for anomalous performance
- Identifying candidates for replacement in a strategic program
Decisions may be immediate and practical:
- Can I return a transformer to service after a fault?
- Will a unit last 5 years so I can replace it as part of a capacity upgrade project?
The GSU transformer in Figure 1 was the subject of an International Conference of Doble Clients in 2003, “GSU Replacement Following Condition Assessment”; the GSU looks in good condition, but do we know it will perform adequately between off line tests?
In many cases the decision is reasonably easy – failure rates on transformers are low and units are expected to last decades. However, in some cases the data is less conclusive and there is a great degree of uncertainty. This uncertainty leads to tough decisions, which are supported through having the best available data and appropriate tools, including doblePRIME for condition monitoring and dobleARMS for strategic risk management.
Figure 2 shows a transformer after a bushing failure, which also destroyed the adjacent bushing and sent shock waves through the main transformer tank; it is possible this bushing may have been saved with appropriate condition monitoring, as part of an asset management program.
Many organizations have been successfully managing their assets for decades; compliance with the newly published standard on Asset Management, ISO 55000, provides a means to demonstrate that the organisation has a comprehensive and coherent asset management system. The standard also gives a framework for condition monitoring and for strategic risk management, supported by doblePRIME and dobleARMS.
This discussion gives examples of the benefits of application of these tools as part of a complete asset centric approach which yields both technical and financial benefits.
The case of the GSU bushing
This is a practical case taken from the International Conference of Doble Clients. A 345/26 kV 968 MVA GSU, manufactured by GE in 1984, supplied power via a DC link to a large city in the United States. The bushings were being monitored as they were of a type with a known failure mode where adequate warning of failure could be given by changes in leakage current and online power factor. A Doble IDD bushing monitor was used to record the raw current and derive the required parameters, and then to provide notifications of bushing deterioration. After several months of normal operation, the IDD showed a small but significant change in the relative phase angle associated with the H3 bushing over a period of three days; this was followed by a further variation of over 2o in seven hours, along with a rise in leakage current magnitude of over 8%. Top level alerts were received from the IDD and acted upon – the transformer was removed from service and the bushings tested off line, confirming the on-line results. The leakage current data is summarized in Figure 3. The rapidly deteriorating situation is clear.
A subsequent forensic analysis of the bushing showed advanced deterioration and punctures in the insulation. Though impossible to say how long the bushing had before failure, it was certain that this bushing had moved from ‘normal’ and was heading toward the same catastrophic failure which had been seen with similar units.
Appropriate condition monitoring provided data for a tactical asset management decision- the bushing was replaced, and the GSU likely saved from destruction.
doblePRIME : on-line monitoring
Condition monitoring should be based on a simple initial question: what problem are we trying to solve?
If we are looking for the general health of a transformer, then general monitoring, including DGA, and temperatures may be sufficient. This will not detect all potential problems, and there are failure modes which may still occur without warning. More comprehensive monitoring would include bushings, SCADA data, Partial Discharge (PD) and would include a means to correlate data and develop a health score for a transformer. If there are particular issues with a transformer – bushings which are suspect or a tap changer prone to rapid thermal deterioration – targeted condition monitoring would be included in the application of an advanced suite of tools.
The doblePRIME condition monitoring platform is a scalable and flexible system, gathering and analysing condition data at an individual station. doblePRIME can target a single measurement parameter on a single transformer or can integrate several data sources (DGA, bushings, SCADA) on each transformer; doblePRIME will also provide a comprehensive view of all transformers at a station, aggregating data across different transformers to data-mine the larger data set and seek out anomalous behaviour.
doblePRIME includes analytics that combine available data to produce an indication of asset health; it provides communication through all common channels, and will accept data from any common IED, thus allowing inclusion of pre-existing devices. doblePRIME includes:
- doblePRIME Delphi – a single value device, either composite DGA or Hydrogen
- doblePRIME Domino – solid state moisture in oil
- doblePRIME IDD – bushing monitoring, including voltage reference
- doblePRIME PD-Guard – targeted PD monitoring: bushings, neutral, main tank
- doblePRIME Analytics – a communications hub and data historian that integrates, aggregates and analyses data
- doblePRIME enables technicians, engineers and managers to better understand their assets, manage asset condition and prevent unplanned outages.
Condition monitoring gives alerts and alarms – it is important to manage these through agreed action plans which are implemented without the need for further discussion. doblePRIME gives the indications which may be managed through dobleARMS.
The case of the ageing fleet – where to concentrate?
Managing a fleet of transformers is a challenge – it means managing a lot of transformers individually but also using the statistics of the population to help drive priorities and decisions. A population may have a historic failure rate, but that applies to the population and is only an indication for a particular transformer.
Ageing of the population is a key analysis – all assets are ageing, nominally at the rate of one year per year, but some may be ageing more quickly due to higher impact of through faults, or more tap changer operations, or have more thermal deterioration of paper insulation. This information can be factored into the health analysis for individual transformers and also used to prioritize and identify candidates for replacement. Age alone is an indicator – and it is often the case that older units are built bigger and stronger and are, in fact, in better condition than some newer ones. Figure 4 shows a predicted failure rate rising with transformer age, based on statistics shared by Hartford Steam Boiler Insurance Company at the International Conference of Doble Clients in 2012.
A fleet allows for analysis of relevant statistics; if we have 1,000 transformers and a historic failure rate of 0.5%, we can expect, over the coming year, 5 failures. But which transformers? It is important to review the manufacturer, design and data from industry sources, something which Doble does on a regular basis through participation in international conferences, standards bodies and through its own unparalleled database of over 44 million separate test results. If we look at off line bushing power factor tests, we can see that in North America alone, in 2012, over 45,000 separate C1 measurements were made and reported, of which, over 1.5% were in a condition which required replacement – where the power factor was above 1% or had doubled since installation. This is a lot of bushings that have been replaced before failure. It is more difficult to do the same with a whole power transformer as the capital costs are so high and the lead time so long.
Figure 5 gives summary information for an organization with power transformers in several locations. The colour coding helps focus attention – both in terms of map ‘pins’ for individual divisions of the organization, and for transformers, through asset health scores. Ranked asset health scores allow for identification of priority work; criticality analysis allows for timing of that work to suit the risk.
In figure 5 the highest asset health index (AHI) score is for a station transformer, followed by a power transformer and a circuit breaker.
So – the tough decisions:
- Which transformers should be targeted for replacement, in what timescale, and with what justification?
- How do we manage the risk associated with those units before they are replaced?
Managing a population is best achieved through a consistent and standards based approach, as exemplified by dobleARMS.
dobleARMS: strategic planning and intervention
dobleARMS is a comprehensive and action-oriented Asset Risk Management System (ARMS). It takes all available condition, asset, operational, test and maintenance data and generates transformer health indices for a single unit or an entire fleet; dobleARMS integrates transformer criticality (consequence/impact) to give a view of risk, based on user specified dimensions, and identifies both short and long term priorities and variations to the operational and capital plans.
dobleARMS accepts criticality metrics for safety, environmental impact, business interruption and financial loss, calibrated through a common denominator to ensure cohesion of analysis and results. By ensuring that we keep track of the original risk quantities, transformer owners, managers and operators can address risks as they develop and manage plans for intervention: what, when and where. Figure 6 shows dobleARMS in some detail, used to manage transformers in a North American location – the data is real, but has been relabelled for security reasons.
There are some key features in Figure 6 to note:
- The top concern transformer is shown in detail – drill down to the available monitoring, SCADA and asset data
- The smart WatchList tracks those units which are in the plan for action
- The fleet summary is a filterable and sortable list which identifies vulnerable transformers at key locations
- The individual transformer health scores and criticality are listed in an interrogable manner
Within an ISO 55000 framework, dobleARMS provides both short term tactical support for a rapidly changing operational environment, and long term strategic support for financial considerations and replacement and contingency planning.
Conclusions
There are tough decisions to be made with regards to short term individual transformer viability, and longer term transformer population management. Doble has almost a century of practical field experience and associated asset management of transformers, which have been the basis of the development of key tools to support these tough decisions: doblePRIME and dobleARMS.
doblePRIME is a condition monitoring platform which is applied at the station level to generate decision support data with respect to transformer health and viability. doblePRIME is a vital tactical tool in the face of operational contingencies and the need for targeted response in a timely manner.
dobleARMS is an enterprise system for managing risk across a fleet of assets – combining all available and relevant data, building on Doble experience across the globe, and prioritising assets on condition, on criticality and on risk – justifying capital decisions and the long term strategic asset plan.
As individual tools, doblePRIME and dobleARMS are leading-edge and world-class products. Together, doblePRIME and dobleARMS underpin smart decisions for an intelligent grid and can provide value from day one of their deployment.
For further information, to see a demonstration or to arrange a local trial implementation, please contact Doble at [email protected] or +44 1483 514120.
