Refurbishing ESB’s transformer units – why refurbish instead of replace?
15 November 2016
Transformers are key items of equipment used in electricity infrastructures worldwide to step power up to transmission voltages and then systematically step power down to distribution voltages to supply both domestic and industrial customers. However, they do have a limited lifetime, which ultimately depends on their operating regime both in terms of load and environmental factors.
Many transformers in the developed world are approaching or have passed the end of their typical life period – twenty to forty years, depending on their application.Regulators have become less willing to allow their transformers to be replaced simply on the basis of age, preferring to consider other factors such as asset health and criticality.
For this reason, most regulated and private network operators are now considering refurbishment as an alternative to replacement.
In this article, I will consider reasons why operators may opt for refurbishing rather than replacing transformers, before going on to reflect upon which transformers should or may be refurbished, and those that should not. Following my recent presentation at the Doble ‘Life of a Transformer’ Seminar in Dublin, I will detail what type of refurbishment can be undertaken either at site in the substation or in a transformer factory, and the results and limitations of refurbishment.
Why refurbish instead of replace?
1) Saving money and time
The cost of refurbishing a transformer is normally considerably less than the cost of purchasing a new transformer. While cost is a factor, depending on the extent of the refurbishment carried out on the transformer, time saving may be more important, as the time required to refurbish a transformer, may be considerably less than the time required to specify and purchase a new transformer.
In some cases, refurbishment can be carried out under revenue expenditure, rather than capital expenditure, which allows the work to be carried out without having to go through a lengthier and more complicated approvals procedure.
Extending the life of assets is good for customers; as long as it can be achieved without adding significant risk to the continuity of their electricity supply. This is good because it results in lower absolute levels of capital investment and hence lower bills.
2) Extended asset life
In addition to the regulatory pressures mentioned above, the electricity industry generally is being encouraged to make responsible use of resources and to demonstrate environmental sustainability as part of its corporate responsibility.
One way of achieving this is to prolong the life of transformers. Many transformers have hitherto been replaced as peak demand at specific substations exceeds the firm capacity of the transformers.
In such circumstances, it may well be possible to increase the capacity of the asset by adding more cooling, whilst ensuring the transformer remains within the temperature rise limits. This might include increasing the capacity of ancillary items such as tap changers and bushings.
3) Remaining plant life
This is especially relevant for generator transformers and related auxiliary transformers, because of the business model used for some power plants. The investment incurred to build the plant is recouped over the expected lifetime of the plant, after which time it may be closed down.
Many new power plants were built in the 1990s with an expected lifetime of around 20 years, and have now reached the end of their expected life. However, due to power shortages and for commercial reasons, some power plants are being operated beyond their end of life. Older power plants are also having their useful life extended for the same reasons – to ‘keep the lights on’ until new power plants and other forms of generation can be installed to meet the demand.
As the expected remaining life of these plants is now measured in years rather than decades, it is not cost-effective to replace large pieces of capital plant such as transformers, but refurbishment of transformers gives the operator more confidence that the plant will continue to operate for the required period, reducing the risk of unexpected failure.
If a transformer has been replaced for reasons other than failure, it is possible to refurbish the old transformer and keep it as a spare. Transformers that are being moved from one site to another can also be refurbished at a suitable workshop before re-installation.
Initial assessment and results of refurbishment
It is always necessary to assess a transformer, or fleet of transformers, to determine if refurbishment is possible, and if so, to what extent.
Many tools are available to carry out assessments of transformers both on and off-line, and continuous monitoring can be used to determine if the transformer condition is deteriorating to the extent that is has to be removed from service in advance of failure.
If the paper insulation is shown to have a high degree of degradation, or if a fault is indicated in the transformer, the decision may have to be made to take the transformer out of service. If this is not immediately possible, due to the time required to obtain a replacement transformer, measures may be taken to keep the transformer in service temporarily.
If only external works are carried out, the operator has to make a judgment on whether the active part of the transformer will continue to be fit for service over the anticipated lifetime of the transformer.
If limited funding is available and transformer refurbishment has to be prioritised, then these tools can be used, along with a measure of the critical nature of the transformer, to rank transformers. Those that are in most need of refurbishment can then have the work undertaken first.
Refurbishment – when and why
Refurbishment can be carried out on site, particularly for external works, or in a dedicated facility with drying and test capabilities. The latter is more suitable for complex but assured procedures.
Where the time, cost or practicality of removing a transformer from site is prohibitive, it is possible to erect a protective structure around the transformer and to carry out the work within this structure. Typically, this would be a temporary building or tent-like structure. This temporary arrangement can include air-conditioning to keep the temperature steady and to control moisture in the atmosphere around the transformer.
The advantages of using a dedicated facility is that more complicated procedures can be carried out, and the transformer can be tested at the end of the refurbishment process. If the transformer has been fitted with new windings or the insulation has been replaced, the tests can be carried out at the full test levels according to the appropriate standard.
This effectively provides the operator with the same level of assurance as with a new transformer and the manufacturer may provide a limited warranty over the work carried out. Care needs to be taken that all components, e.g. bushings, can withstand the full test voltage if they have not been replaced.
If the windings or other parts or components are not replaced, then the tests are carried out at a lower voltage (80% to 100%, according to IEC 60076), but this still provides a level of confidence that the transformer is fit for service and that the work has been carried out competently.
Depending on the extent or refurbishment and testing, a transformer can have another 10-20 years more life than if no work or limited work was carried out. Unless there has been extensive refurbishment of the windings and major components, there is no guarantee of lifetime extension, so it is up to the operator, in conjunction with a prudent service provider, to assess the risk for each individual transformer.
Although much work can be done to extend the life of transformers, it is not possible to give life back to cellulose insulation once it has started to degrade. For transformers with low degree of polymerisation or where the paper is known to be in a poor state, the windings and insulation need to be replaced as part of any refurbishment process.
Case study – ESB transformers
ESB engaged Winder Power Ltd to complete the refurbishment of 3 x 5MVA transformers, supporting its efforts to make responsible use of resources and demonstrate environmental sustainability by refurbishing the units instead of replacing them. The three transformers were manufactured in 1975, 1977 and 1978. They were refurbished in 2015.
As a leading Irish utility with an asset base of approximately €9 billion, and 43% of electricity generation capacity in the all-island Irish market, ESB requires significant amounts of reliable power to provide its 1.5 million customers throughout Ireland every day. This is where Winder Power stepped in.
ESB carried out an assessment of the transformers before opting for refurbishment, deciding that the core and windings were in good enough condition to warrant the refurbishment. Most of the work specified was to the outside of the tank, including the instrumentation.
Using the services of a local heavy haulage company to dismantle and transport the transformers to Leeds, Winder Power undertook significant refurbishment works on the ESB transformers, which involved disassembling them, inspecting all elements, supplying and fitting new components, and re-assembling them, before shipping them back to Ireland.
The work commenced with removing the core and windings from the tank for inspection, including the tapping connections. Nothing untoward was found, so the core and windings were dried in a hot-air oven for seven days. After drying out, the core clamps were tightened up to ensure the core and windings were adequately clamped.
Apart for tightening the clamping, the core and windings was unchanged. All HV, LV and neutral bushings were examined for signs of damage. One bushing had a broken porcelain insulator; this was replaced.
The tapchanger, used to maintain the output voltage of the transformer at a constant value, was inspected and maintained by a specialist contractor. Because the tapchanger contains moving parts and also performs switching operations to change the transformer ratio, it is subject to wear in service, and is therefore the component of a transformer most likely to require significant maintenance.
All gaskets on the tank were replaced. The external surface of the tank, tapchanger and coolers was cleaned and prepared for painting, before applying a full coat of paint to Winder’s paint specification for coastal areas. Transformers are also supplied with protection instruments, which operate alarm or trip relays if the transformer operation goes outside certain pre-determined limits. If there is a fault in the transformer, the insulating oil breaks down and gases are generated. In extreme cases, an explosion occurs within the transformer and there is an oil surge from the transformer tank towards the conservator, which is a header tank used to ensure the oil level is always above the main tank.
The Buchholz relay is a device that collects any free gas generated in the oil and detects oil surges. It generates the appropriate alarm or trip signal, depending on the severity of the fault. This was replaced with a new relay, including a tap at ground level to allow for sampling of any gas collected without having to climb onto the top of the tank. The transformer was also fitted with a new oil temperature indicator, which detects overheating in the oil and provides alarm and trip signals if the temperature exceeds pre-determined limits.
A new stainless steel marshalling box was provided, to allow the customer to connect the instrument alarms and trips into his system.
All valves, including the valves between the main tank and the cooling radiators, were replaced. The core and windings were tanked and connected, and the transformer was filled again with the oil that had been removed. In some instances, refurbished transformers are filled with new oil, but if the oil is in good enough condition it can be re-used.
The transformer was subjected to routine tests, including loss measurements and dielectric tests. Because the transformer had previously been in service, dielectric tests were carried out oat 75% of the original voltage levels.
One additional test requested by the customer was to measure the capacitance between windings and to earth, and the power factor of the insulation. Insulation power factor is a measure of the quality of the insulation. As insulation ages the power factor will increase, and this can be used to gauge the level of deterioration of insulation. Finally, all instruments were checked for correct operation.
The work on the first transformer was completed in 15 working days from arrival at the factory until test was completed. The second transformer followed three weeks later, while the third, which was not required urgently, was completed a few months later.
Eur Ing Elizabeth MacKenzie BSc, MSc, CEng, FIET, technical director at Winder Power, a leading manufacturer of power and distribution transformers