ABB's service team delivers an extended lease of life for the UK's ageing transformer fleet
17/07/2014 Hitachi Energy UK Limited
New refurbishment and upgrading service is targeted at the two most common points of failure for power transformers - tap-changers and bushingsABB's UK transformer service team has launched a new refurbishment and upgrading service that targets the two most common points of failure for power transformers at 132 kilovolt (kV) and above - the tap-changer and bushings. Replacing one or both of these components offers a cost-effective way to provide a life-extension of at least 10 years for the UK's vast fleet of transformers that are now mid-life or beyond, many having been installed 30 years ago.
The service is available for transformers from any manufacturer, and is particularly suitable for older legacy designs that are no longer supported by the original manufacturer. As part of the service, ABB is able to provide either a direct replacement for the tap-changer or, depending on suitability, an upgrade to a state-of-the-art vacuum tap-changer. The brittle and easily damaged porcelain oil impregnated bushings can be replaced with modern dry type resin-impregnated (RIP) bushings.
The challenge of an ageing transformer fleet
Power transformers are often the most valuable asset in a substation or plant. They are key components of high-voltage equipment for power generation plants, transmission systems and large industrial plant. Unexpected failures can cause major disturbances to operating systems, resulting in unscheduled outages and power delivery problems. Such failures can be the result of poor maintenance, operating conditions, poor protection, undetected faults, or even severe lightning or short circuits. Outages can affect revenue, incur penalties and can cost a company its reputation and its customers.
The enormous cost of power transformer failure provides ample incentive for operators to ensure reliability and availability throughout the life cycle of these key assets. Transformers typically cost from £1 million upward and on the rare occasions they do fail, the financial impact can be even more significant - in extreme cases, they can leave a company facing financial ruin. In addition, as most countries have strict laws in place that control and regulate power supply, non-delivery penalties can be as high as 100 times the price of the energy itself.
Although transformers are normally highly dependable, the UK's current transformer fleet is quite old. The average age for those in industrial plants is 30 years, and 40 years for those used by utilities. While aging transformers are generally not "ticking time bombs," their failure rates, as well as their replacement and repair costs are steadily - albeit slowly - increasing with age.
The risk development curves are steeper for industrial and power generation plants as the transformers in these installations tend to be used more intensively. While age alone does not increase the risk of unexpected failures, it generally is an indication of this risk. Risk of failure is heightened by other factors, including type of application and the tendency to load transformers to their maximum or even over in some cases, to meet the economic needs of the deregulated environment and competitive markets.
The tap-changer - upgrading the transformer's gearbox
The tap-changer is effectively the transformer's gearbox. It connects or disconnects turns in the tap winding to change the ratio between windings to maintain a constant voltage out of the transformer. This is vital for the stabilization of network voltage under variable load conditions. Regulation is performed in around 9 to 35 steps and in normal grid applications the tap-changer can operate between 10 to 20 times a day.
Over the life of the transformer the tap-changer will suffer from wear and may sometimes fail catastrophically. ABB's service starts with a thorough review of the condition of the transformer to determine the most appropriate response.
If the tap-changer is worn then it is possible to fit a direct replacement kit based on conventional technology in which the switching takes place in oil. However, in the quest for greater efficiency and to meet key performance and financial targets like improved availability and cost reduction, many transformer users worldwide are now embracing ABB's VUCG vacuum interrupter technology as an alternative that offers some distinct advantages.
In these new-generation tap-changers, electrical arc quenching takes place in a vacuum and not in oil. This substantially reduces contact wear and residues from arcing do not pollute the insulating oil. The benefits of using a vacuum tap-changer include:
-€¢ Lower lifecycle costs, less maintenance and increased time in operation because of a radical reduction in contact wear
-€¢ A cleaner environment for routine inspection, which reduces downtime
-€¢ Reduced sensitivity to moisture in the insulating oil due to the absence of polluting by-products such as carbon
-€¢ Independence of switching characteristics from the condition of the insulating oil
-€¢ Reliability due to the redundant contact system
-€¢ Full interchangeability with existing installations
-€¢ Possibility of using alternative insulating fluids
If the design allows, then ABB recommends the use of a vacuum tap-changer specifically because of its extended maintenance periods. ABB's service team can carry out the replacement on site. This will require the transformer to be taken off-line for two to three days. However, the work can also be carried out as part of a more extensive upgrading and refurbishment exercise with the transformer transported to ABB's specialised facility in Drammen, Norway.
Bushing replacement
The transformer bushings provide the insulated connection between the internal windings of the transformer and the substation's overhead power lines or busbars. Historically, these bushings have been a solid porcelain or oil impregnated design. They generally perform well, but by the time they have been in service for 20 years their performance starts to degrade by partial discharge and sometimes fail catastrophically by shattering. Bushing replacement is therefore an essential element of a transformer's mid-life service.
If the transformer bushings need replacement then ABB recommends the installation of a new generation composite resin impregnated (RIP) design.
In the RIP design, a web of crepe paper is wound onto a mandrel or a conductor to form the main insulation. Conduction inserts made of aluminum are placed at calculated and very precise axial and radial positions to create the grading of the electric field. The core is heated, vacuum dried and impregnated-۬by using a curable epoxy resin to form the solid condenser core. After curing, the core is machined and the mounting flange and insulator are fitted. An assortment of terminal connections, shields and conductor configurations are available to suit all applications.
RIP bushings offer the following advantages:
-€¢ High mechanical strength and flexibility combined with reduced weight, increasing the ability to withstand shocks such as earthquakes
-€¢ Increased safety to personnel due to the nature of the material - does not shatter and disperse on failure, as can the porcelain technology
-€¢ Because no oil is in the bushing, there is no need to have an expansion volume at its highest point. This means that even if RIP bushings are stored horizontally, they can be energized immediately after installation
-€¢ Eliminates the risk of bushing oil being sprayed over the equipment and causing a fire in the event of an explosive failure.
-€¢ The transformer is sealed, which means that the risk for moisture ingress to the transformer is reduced in the event of flashovers.
-€¢ Downtime in the event of major transformer failures is also reduced because no porcelain fragments are left inside the transformer.
For more information please contact:
Karen Strong
ABB Power Products & Power Systems Divisions UK
-۬Tel: +44 1785 825050-۬
Email: Karen.strong@gb.abb.com
Web: www.abb.com
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