Plant ageing

The big switch-around

1 June 2010

Transformers often sit outside the nuclear power plant security fence. But because they control the flow of electricity from the station to the grid, they are as crucial as other main NPP components. Will Dalrymple speaks to two US utilities about their programmes to improve transformer and switchyard reliability.

After the next outage of the Millstone nuclear power station in a few months’ time, Dominion will have replaced all of the step-up transformers that connect the generator to the transmission grid. This will end the major part of a six-year programme by the utility to improve switchyard reliability.

David Roop, Dominion electric transmission operations director explains that Dominion first realised that there was a problem in 2003. “We noticed that there was an increasing number of nuclear power plant unit outages due to switchyard transformers and other equipment failures.” He adds: “Initial failures in 2003 caused the nuclear generators to trip. For example, there were transformer problems, a coupling capacitor failure, a hot connector, and a bushing problem. Although different events were starting to occur, there was a steady upward trend in the number of events, and we wanted to get a handle on that.

“We performed a review and found that there was a cumulative service stress reducing the life expectancy of critical components,” Roop continues.

Further analysis revealed several causes of the problem. First, the transformers had to deal with a changing environment, Roop says. Operational improvements on the nuclear side have made the stations more efficient and improved run times, which added to the stress on the equipment. Secondly, declining refuelling outage durations are causing a shorter window to diagnose and fix problems. Moreover, deregulated electricity markets have changed the business of power supply. In the past, power generation used to be a local business. In today’s deregulated markets, power could come from over 100 miles away. Because of the accumulated impedance of the transmission lines, power coming from long distances tends to be at higher voltage levels. “We saw the high-voltage issue impacting discrete components; transformers, bushings, coupling capacitor voltage transformers, insulators, cables and bus components.”

The analysis has led Roop and his team to appreciate how a switchyard works as a system, with many possible failure points. “We started seeing how specific components and sub-components contribute to end-of-life. Arrestors, coupling capacitors, pump motors, fans, contactors inside control units, all operate more frequently and longer.” He adds: “Old paper-type coupling capacitors failed in service. The manufacturers state that the paper-type coupling capacitors have a 20-25 years of service life. Most of the switchyard coupling capacitors were of the paper type and were approaching their end of service life. New coupling capacitors are a paper film with foil, and they are expected to last 30-35 years.”

At the large US utility Exelon, Amir Shahkarami personally experienced the difficulty of assigning a proper place to transformers. Four years ago, when he was senior vice president of engineering and technical services, he helped set up a steering committee within the utility focused on switchyards, he explains. “We had a lot of switchyard problems; breakers, disconnect switches, and junction boxes. We made training important in the switchyards. We have 17. We colour-coded every line, every component, and established a design review process at every site, and started looking at the health of switchyards. We established preventative maintenance templates for every switchyard component and undertook replacement programmes for circuit breakers, disconnect switches and CCVTs [charged-couple voltage transformers]. What we missed was to include transformers in that population; I blame myself. It didn’t fall on either side; it wasn’t part of the switchyard, it wasn’t part of the plant.” To rectify that oversight, in 2009, Shahkarami set up a steering committee to focus on transformers.

The committee oversees six working groups covering engineering (‘monitoring their health’), installation (‘a lot of people think that it is easy to replace a valve. Replacement tasks are very difficult’), life cycle management, maintenance (‘how often you do the testing’), procurement (‘buying new is not necessarily the right answer’) and logistics (‘how you install spare parts’).

Shahkarami argues that critical transformers should have continuous gas monitoring systems, redundant logic systems and sudden pressure relays so that one hit will not trip a circuit breaker. They should have acoustic monitoring systems that allow workers to hear a loose component, and even antenna array systems that continuously monitor an entire substation, pinpointing partial discharge signals from failing equipment without the need to add sensors to individual devices.

Exelon has developed a standard preventative maintenance template that revises the frequency of electrical testing from every six years to every four. Shahkarami argues that nuclear installations’ superior technical expertise in tasks such as life cycle management and preventative maintenance justifies extending their influence to the switchyard, not normally considered part of a nuclear power plant. “You need to be intrusive and have predictive and preventative tools in place to monitor what is going on for the portion you are not responsible for. You need definitely to build a good relationship with the switchyard owners, and if a transformer is taken offline, you will get a notification and not be surprised. It is difficult, but you can manage it.”

A key document for linking the switchyards and NPPs is an agreement that defines the individual and shared roles and responsibilities of each party. He says that all of Exelon’s 17 nuclear power plants have signed one for their switchyards.

“The fundamental thing is the role of responsibility for ownership of health. If you focus on the health of the components and systems, everything is going to work. If you start with who owns what, you are not going to the heart of the issue. You need to sit down with the owner of the yard and discuss this. If you focus on the robust performance of equipment, and then develop the roles and responsibilities so they do the right preventative maintenance and testing, and at the right time, you will have better performance.”

Who owns the transformer?

This is a controversial point of view, and not one that David Roop at Dominion agrees with. “It comes back to understanding the operational issues; the power plants do not have a broad perspective of enough problems across a fleet of different kinds of power stations to see that there is a challenge with specific equipment.”

Roop was involved in a transmission reorganisation at Dominion that linked nuclear plant switchyards to other power plant switchyards, rather than to nuclear power plants. In 2003, Dominion set up a technical subsidiary to work across the entire fleet of fossil, hydro, and nuclear generators in the utility (although each nuclear plant also has its own service engineer to monitor electrical equipment). Dominion Technical Services now has over 200 people and supports 500 substations, 6000 miles of transmission lines, 3500 circuit breakers, 1870 power transformers, 45,000 relays, and 1600 coupling capacitor voltage devices.

“I had a concern that people who were handling transformers and power circuit breakers might not have a broader view of the challenges that the whole group has, so we modified the organisation in 2003 so it could support everybody with more technical expertise,” he says. “If we see operational issues with a transformer that may be applicable to a nuclear or fossil station, we will send an alert that describes what needs to be done with similar units. A lot of utilities are not set up to do this. Many are no longer vertically integrated.”

Although Roop does not agree that nuclear power plants should take over the responsibility for managing switchyards, he admits that nuclear plants have a different mindset that the transmission owner needs to be sensitive to as it relates to plant reliability and impact. “You have to be very mindful of nuclear power because it is base load. They have done an excellent job in increasing the time intervals between refuelling outages, hence we need to have a very structured maintenance programme that ensures the work that needs to be done gets done in the shorter outage, and we don’t do maintenance while the unit is online.” His organisation has a small subgroup that deals exclusively with Dominion’s nuclear power plants, Kewaunee 1, Millstone 2&3, North Anna 1&2 and Surry 1&2.

He offers a specific example of a new approach taken from the nuclear industry: “We weren’t looking for a finite end-of-life for coupling capacitors and we ran them to failure. We didn’t even consider that there is a defined end-of-life for them. However, this is what nuclear programmes and systems do. We became more aggressive based on what we learned from them.”

Roop also acknowledges that the input of nuclear power plants has benefited his own work. “The transmission business has become more regulated, like nuclear power, and there is a strong emphasis on reliability. The nuclear organisation has improved the reliability of the business with its structure. We have leveraged best practices from the nuclear business and applied it to electric transmission in the areas of human performance, maintenance activities planning and tracking, and operating experience sharing and alerts dissemination.”

Buying a transformer

Keeping a power grid healthy does require replacing old transformers. But Exelon’s Amir Shahkarami argues that the power generation industry does not plan ahead adequately for their transformer needs. “There is a long-term asset management shortfall in really looking five to ten years ahead, and being able to manage the replacement and critical spare parts programme for an important set of transformers,” he says.

The utility’s decision to purchase a new transformer is only the beginning of a long and complex quality assurance process, Shahkarami says. “Only a handful of companies build a transformer of the size needed for nuclear power. If you think buying one is simple, and that you are going to get high quality, you are totally wrong. We bought three brand-new three-phase transformers for two [nuclear power] sites, and as soon as we installed them, they started gassing off, eight to ten months later,” he says. (The problem turned out to be an unidentified internal design flaw). “Customers need to review the whole specification, have a presence in the fabrication shop, and understand every element of the transformer.” Exelon has recently bought 20 new transformers, Shahkarami says.

Making transformers is a labour-intensive process, says Dominion’s David Roop, and human error in the manufacturing process can lead to problems. “The manufacturer could install some oil barrier sheets in the coil assembly to move oil through the winding. If they are not careful about the way they cut the barrier sheets, they could close off the cooling ducts so that oil doesn’t flow, and hot spots result during service. This would fail the unit if it were in service for a long time, especially in a nuclear application where it runs constantly for 18 months.”

To prevent problems, particularly during initial start-up (which are called, rather ghoulishly, ‘infant mortality’) Dominion has started stress-testing at the factory. “We don’t see much infant mortality because of our overstress testing. When such testing is not performed, failures may occur, such as turn-to-turn shorts due to manufacturing flaws, or transformer overheating due to inadequate piping diameter in coolers. These issues cannot be seen in the models the suppliers use, but it is clearly evident when they are tested.”

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Kruemmel's transformer failure

A July 2009 transformer failure at German nuclear plant Kruemmel (KKK) will have cost the 1346MWe BWR almost a full year’s operation. After the failure, joint owners Vattenfall and E.ON decided to shut the plant until two new transformers are installed (in April or May 2010). Ironically, the incident occurred as the plant was coming out of a two-year repair outage caused by a fire started by a step-up transformer in 2007. We recount the sequence of events.

On 16 June 2009, KKK management informed the nuclear regulator that the plant was ready for startup. After the ministry gave its consent on 19 June, the station started up the same day and reached full power on the night of 23/24 June. Then, on 1 July, the turbine went into an automatic shut down due to the failure of a plant supply transformer. That day, the plant had to be disconnected from the grid and the reactor power was, therefore, reduced to a low level. At 19:15 the plant was re-connected to the grid and operated at a reduced power of 700MW. On 2 July, the cause of the first problem was found: a valve on the pressure compensation tank of the transformer was set incorrectly. On Friday 3 July the transformer was operational again, so that power could be increased.

On 4 July, when the plant was operating at a power of about 1200MW, a short circuit in the transformer forced the plant to shut down automatically. The shutdown led to a severe power cut in the Hamburg area, though only for a few milliseconds. Still, the interruption affected about 1500 of the city's 1750 traffic lights. All 14 water pumping stations had to be shut off, so that about 100,000 people had no water. Resulting oscillations in the water supply system cracked water supply pipes, leading to 16 pipe breaks.

Although this second transformer was tested during the maintenance outage, the reason for the new incident has not yet been disclosed. In July, the utilities said that first results of the investigation of the incident showed that a planned supervision device for the transformer had not been installed before start-up of the plant. As a consequence of this omission, plant manager Hans-Dieter Lucht resigned.

In March 2010 Vattenfall declined to provide more information on the transformer failure, citing "open issues in the root cause analyses in our dialogue with the authorities".

transformer transformer
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Millstone transformer Millstone transformer

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