Tap-changer and bushing upgrade service

Craig Carrington, ABB’s Transformer Service Sales Manager, outlines a new service targeted at the two most common points of failure for power distribution transformers.
Our new refurbishment and upgrading service 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 we 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 resin impregnated (RIP) bushings or the next generation of dry-type resin-impregnated synthetic (RIS) bushing.
The challenge of an ageing transformer fleet
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. Carrying out diagnostic testing on a continuous basis can support fleet and asset management by providing assurance of asset condition.
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. One option is to fit a direct replacement 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 vacuum interrupter technology such as our VUBB and VUCG models.
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
- Interchangeable with existing installations
- Possibility of using alternative insulating fluids.
If the design allows, then we recommend the use of a vacuum tap-changer specifically because of its extended maintenance periods. This is because vacuum tap changers can deliver 300,000 operations between maintenance, versus 100,000 for conventional technology. Our 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
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 are a solid porcelain or oil impregnated design. They generally perform well but by the time they have been in service for 20 years it is possible that their performance will be affected by partial discharge and they 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) or resin impregnated synthetic (RIS) design. 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 – unlike porcelain, the material does not shatter and disperse on failure
- 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.
A new generation composite resin impregnated (RIP) bushing

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