Ageing infrastructure is a major problem for almost every power utility in the developed world, and it's a problem that becomes particularly acute when the original manufacturer gives notice that support will soon be withdrawn for an asset that has long been obsolete but nevertheless continues to fulfil a critical role. Hans Picard of power management company Eaton and Dirk Boender of Stedin, a major Dutch utility, explain how their companies have worked together to successfully address just such a problem
The most satisfactory way to minimise the risks associated with obsolete assets that have reached or passed the end of their design lives or that are no longer supported by the manufacturer, is undoubtedly to replace them as quickly as possible with new equipment. Unfortunately, in the real world where budgets and resources are limited, this utopian solution is rarely possible. This was exactly the situation that recently confronted Stedin, the Dutch regional network operator responsible for the transmission and distribution of electricity in Randstad, the region of the western Netherlands that includes the major cities of Rotterdam, Utrecht and The Hague.
Stedin has around 2,200 bays of switchgear based on minimum-oil circuit breakers (MOBs) produced by Eaton's predecessors (Hazemeyer and COQ) in its HV substations, which operate at voltages from 10 kV to 25 kV, and a further 3,200 similarly equipped bays in distribution cabinets. Most of the switchgear was installed when the electricity grid in the Netherlands was reconstructed in the years following the Second World War. Today, the 2,200 bays of MOB's comprise about 35% of all 10 through 25 kV bays in operation in Stedin's HV substations. Clearly, replacement cannot be done overnight.
Recently, Eaton announced it would soon be withdrawing support for Hazemeyer and COQ MOBs. This decision was not taken lightly, but no manufacturer can continue to support obsolete equipment indefinitely, and these products have been out of production for many decades.
Stedin knew that termination of support for the MOBs would lead to risks that include an increase in failure rates, since routine maintenance might no longer be possible, long repair times in the event of a fault, because of the poor availability of spares, and even non-repairable loss of function after a fault if suitable spares could not be sourced. In the worst-case scenario, failure of a circuit breaker could lead to a switchgear cubicle bursting under fault conditions, releasing hot oil or gas and possibly leading to fire. In such a case there is significant potential for injury to employees and for prolonged supply outages.
To devise a strategy for minimising these and other operational risks, engineers from Stedin and Eaton worked closely together. They started by analysing failure modes for MOBs, drawing on the operating experience of both organisations, and on third-party research findings. A major conclusion was, apart from well-documented age-related failures of 25 kV bushings, no salient age-related failure mechanisms could be clearly identified for the MOBs being used by Stedin.
Operating experience, however, suggested the most likely failure modes would be no movement or uncontrolled movement of the circuit breaker main contacts, and faults with the insulation. In addition, there are many minor failure types, including small oil leaks and problems with control and signalling.
Working with the results of this analysis, Stedin assessed the impact of the risks identified in relation to its five core business values: safety, quality of supply, law and regulations, financial performance and reputation. Based on the predicted frequency and severity of the possible failure events Stedin then generated a risk evaluation on the Stedin risk appreciation matrix – this is shown in Figure 1, where the arrow indicates the potential long-term effect of Eaton's termination of support if no remedial action were taken.
It was clear taking no action would lead to unacceptably high risks. The loss of an HV substation that typically feeds 20,000 consumers would, for example, have an enormous impact on Stedin's CML (customer minutes lost) and SAIDI (system average interruption duration index) performance. With this in mind, options were examined for ensuring the future safety and reliability of the HV substations and distribution equipment.
These options included:
1. Failed irreparably
2. Preventative maintenance using old technology
3. Replacing critical components with new technology
4 Complete replacement of the switchgear
5. The above options, plus cannibalisation
Option 1 was ruled out as it involves unacceptably high risk. Option 2 is potentially attractive in financial terms, but its scope is limited. Since there are no commercially available standard solutions, Option 3 is unattractive as customised solutions are inevitably expensive and they also involve technical risks. Option 4 is the preferred solution, but because of budgetary and other constraints, it can only be implemented over a relatively long timescale.
Ultimately, after carefully evaluating these options, Stedin and Eaton devised a two-track approach: the switchgear would all be replaced within 20 years and, throughout this period, Stedin and Eaton would work together to conserve the old equipment prior to replacement. The 20 year time period was chosen because aiming to replace all of the equipment within 10 years would be unrealistic, and extending the timescale to, say, 40 years would make it very difficult to ensure continued reliable operation of the old equipment.
To lay the foundations for implementing this two-track strategy, considerable effort was put into determining the optimum order of replacement for the MOBs. For the HV substations, it was considered reasonable to assume that the probability of failure was proportional to the number of bays of switchgear, n, and that the effect of a failure would be proportional to the peak in power consumption, S, for the substation over the previous year.
This means the relative risk for any given HV substation is n*S. On this basis, the relative risk order for each substation was computed and plotted, as shown in Figure 2. The cumulative risk was also calculated and plotted on the same graph. This reveals that the substations ranked as the top 5 for relative risk account for 20% of the risk, whereas the bottom 5 account for well below 1% of the risk.
For switchgear in distribution cabinets, a different approach was adopted, with the MOBs considered at highest risk being those using oil but no resin-bonded paper for busbar insulation, as these have no barriers between individual phases, and no barriers between the phase conductors and ground. They are, therefore, particularly at risk of failure due to possible moisture ingress and subsequent degrading of the breakdown strength of the oil.
Other factors taken into account in deciding the order in which the old switchgear would be replaced, included amongst others the need to install remote network control, the condition of the protection devices, capacity planning issues, and geographic spread, as it is not practical to replace a large number of devices in a small geographic area at the same time.
For the second track of the strategy – conservation of existing equipment for 20 years – Eaton and Stedin devised a time-based preventative maintenance approach based on experiences relating to failures, on operational requirements and on acceptable levels of non-availability. It was also noted that both organisations have found that long inspection intervals allow minor problems to develop into major failures.
The condition of all the existing equipment was determined by inspection, and the maintenance work was standardised as preventative rather than condition based. Stedin asked Eaton to take charge of the maintenance for many reasons, including Eaton's access to:
• Specific product knowledge and experience
• Design, test and other product-related documentation
• Drawings and production specifications for spare parts
• Technology and tooling for spare part manufacture
• Assembly and adjustment tools, and special test equipment
The result of the joint work carried out by Stedin and Eaton was a conservation agreement between the two parties that focuses on safety, reliability, availability and efficiency. This agreement runs until 2032.
To ensure the agreement can be implemented effectively, Eaton had to address the issue of product knowledge, bearing in mind that for much of the term of the agreement, the workers involved with conserving Stedin's MOBs will never have seen the breakers in their new state. This issue was further complicated because, at the time when the breakers were originally manufactured, standards of documentation were much lower than they are today, and a lot of critical knowhow existed only in the minds of the workforce.
This situation has been corrected by interviewing employees in Eaton's inspection and overhaul workshop who have personal experience of working on the MOBs. Many of these employees are approaching retirement age, but their expertise has been preserved by recording it in detail. A similar process of recording knowledge from the members of the field service group has also been put in place.
In addition, a training scheme has been implemented to give younger team members experience of working with MOBs, and steps have been taken to ensure the future availability of essential materials and components. These include climate-controlled storage for breaker components, careful conservation of critical machines, tools and moulds, and long-term agreements with sub-suppliers.
While the old MOBs remain in use, detailed information will be collected on failures, modifications and maintenance activities, allowing standardised reports to be produced and distributed. This will mean both Stedin and Eaton will have a full picture of the condition and performance of the ageing assets, which can be used to fine-tune the replacement and conservation programmes.
The strategies discussed in this article have only recently been implemented, so it would be premature to provide a final assessment of their overall effectiveness. However, both Stedin and Eaton are confident that the safety and security of the HV substations and distribution cabinets that use the obsolete MOBs can be maintained until the replacement programme is finally completed in 2032.
