Oil filling - A colossal transformer"For a job like this we need a partner capable of a high degree of accuracy and flexibility," says Mark Wilson, Head of Sales & Tendering at Alstom Grid's transformer factory in Stafford, UK.

The delivery of some 390,000 litres of oil in 15 trucks arriving on a predetermined schedule, mostly in pairs and over a two-day period – that's the challenging task entrusted to Nynas as the latest of Alstom Grid's massive quadrature boosters is prepared for operation. The giant power transformer, installed at the Penwortham Substation near Preston in the northwest of England, will be filled with high-quality transformer oil from Nynas. And once the oil filling has started, the flow needs to be continuous so that the vacuum in the transformer is maintained and moisture is kept out of the system.

"There's a lot of work involved behind the scenes to ensure a project like this goes to plan," says Paul Vann, General Manager, Nynas UK. "For instance, because the particular grade of inhibited, long-life oil specified for this project meets the requirements of international standards, it is in great demand. Therefore, simply ensuring that the right oil is available locally in sufficient quantities requires good communication between all parties. Then there is the challenge of delivering this quantity of oil in a suitably dried state for processing. Nynas extended its local oil-drying capability for this type of oil solely to accommodate the throughput requirements for the quad boosters."

A quad booster is not a transformer in the traditional sense, as it doesn’t transform power. A variant of a dualcore phase-shifting transformer, it is used to control and optimise the flow in power lines and to manage the load sharing in three-phase electricity transmission networks. Quad boosters are needed because the focus for power generation is shifting from fossil fuels to renewable energy. Where historically in the UK the main power generating plants were located in the Midlands, massive wind farms are currently being built off the northwest coast. This means that parts of the power network traditionally used for power distribution now have to be upgraded in order to cope with large volumes of generated power. With 40 years of experience in building this type of unit, Alstom Grid is the only quad booster supplier in the UK and one of only a handful of suppliers worldwide. Since 1997, Alstom has supplied nine quad boosters to the UK's National Grid Company, with power ratings of up to 2,750 MVA.

"We build on average 24 transformer units per year to serve both domestic and global demand," says Mark Wilson, Head of Sales & Tendering at Alstom Grid's transformer factory in Stafford, UK. "Of these, one or two are quad boosters." The Stafford factory is Alstom Grid's Global Centre of Excellence for high-voltage transformers. The quad boosters and other transformer units built at the factory are all typically large scale. Varying in weight from 120 to 340 tonnes, the size-limiting factors for these units are the factory doors and the weight limits on roads. The transport of such large units to their final destination in the UK or to a port for delivery overseas can be tricky. "Sometimes traffic lights need to be dismantled, and between our factory and the port a roundabout has been specially designed so that our lorries can drive straight over rather than around it," says Wilson.

Alstom Grid's latest quad booster is one of the largest in the world. Measuring 10.6 x 5 x 5 metres, the quad booster comprises two sections, a shunt unit and a series unit, each weighing around 270 tonnes. Once on site, the two units are connected and engineering tests are carried out. "And then our team is pleased to coordinate and deliver the 350 tonnes of oil that's needed," says Vann. 

Business: Alstom Grid is part of the Alstom Group, a global leader in power generation, power transmission and rail infrastructure. Alstom Grid ranks amongst the top three in the electrical transmission sector. Number of employees: Alstom Grid employs 20,000 of the Group's 96,500 employees. Turnover: In 2009, Alstom Grid had a sales turnover of approximately EUR 3.5 billion. The company's latest contract for the design, supply and commissioning of two 400kV 2750MVA Quadrature Boosters is valued at £18 million.

 

The delivery of oil has started and will take place over a two-day period.

Oil analysis keeps Nacka Energi in powerWith 27,000 subscribers depending on electricity from Nacka Energi, the company keeps a close eye on its nine main transformers. Each transformer is made to measure, so if one fails, it could take a year to replace it.

Southeast of Stockholm lies the largely residential municipality of Nacka Kommun. Its proximity to the capital and the availability of large recreational areas, not least the Stockholm

archipelago, has made it an increasingly popular place to live. "There is a lot of construction going on here all the time," says Örjan Melander, plant manager at local power utility Nacka Energi. "We need to keep an eye on this and invest in new equipment in time to be able to provide the power that is required."

Nacka energy has around 27,000 subscribers among the municipality's almost 90,000 inhabitants. The company runs a pure distribution operation with no energy sales or production of its own. "We have nine substations, each equipped with a power transformer," says Melander. "If one were to fail it would put us in a situation where it would be difficult to provide our subscribers with electricity."

To keep the power transformers in shape Nacka Energi has signed an agreement with Nynas for services within the area of insulating oil management (IOM). This means that oil samples are regularly taken from the transformers and sent to Nynas for laboratory analysis. In return, Nacka Energi gets an analysis report, including diagnostics and recommendations for maintenance measures if this is required. The company also receives an annual summary together with a suggested test and maintenance programme for the coming years, including a consultation with one of Nynas' experts.

"We decided to buy this service and get expert advice because we don't have that kind of specialized know-how in the company," says Melander. Since the two main purposes of transformer oil are electrical insulation and cooling, the oil must always be kept in good condition. If not, the transformer could, in the worst case, short circuit and break down. A new transformer has to be custom-made and costs approximately SEK 2.5 million with a one-year delivery time. "This is something you really want to avoid," says Melander. "That's why it is so important to keep track of this."

Business: Distribution of electricity to 27,000 subscribers in Nacka Kommun, Sweden Network: 20/10/0.4 and 30/10/0.4 kilovolts via nine substations and 268 net stations Number of employees: 35 Turnover: SEK 104 million (2009)

  

Insulating oil management

A specialist in transformer oils, Nynas offers a number of services within the area of Insulating Oil Management (IOM). One of them is insulating oil analysis in Nynas' four laboratories. Through oil samples taken from customers' transformers experts can assess both the oil and transformer condition.

The result is delivered in an analysis report, including diagnostics and maintenance recommendations. There is also a web tool, IOMonitor, which customers can use to access their analysis results, view trend data and get support in their maintenance budget process.

The tests follow relevant international norms and standards, and it is not necessary to disconnect the transformer to take the oil samples, says Lars Fröberg, sales executive IOM Nordic.

Customers can use specially designed sampling kits with equipment that allows them to take oil and dissolved gas samples and return them safely to the laboratory. There are different types of kits, depending on the needs of the customer.

"It's a simple and cost-effective method for customers to map the status of their transformers and plan ahead for future maintenance," says Fröberg.

  

Barth in Austria - Solve transformer problems, and fast"Barth, we've got a problem!" This is a statement heard regularly on answering the phone at Barth GmbH in Austria. Luckily for the caller they've come to the right place.

"We try to solve our customer's problem the same day they contact us," says Georg Barth, Managing Director and owner of Barth GmbH. He explains that rapid response is one of the keys to the company's success. "A transformer is often the very life blood of an industry. Which is why 'tomorrow' may well be too late." Now Barth is concentrating on exporting its recipe to Austria's neighbours.

Barth GmbH is centrally located in the capital of Austria, Vienna. In addition to administrative

facilities, the site includes Europe's biggest depot of transformer and switchgear oils in drums from Nynas, as well as around 200 new and used transformers. The oil depot, normally holding around 30 metric tons of mostly Nytro 4000X and Nyswitcho 3X, and the stock of transformers are the keys to Barth being able to provide whatever services their customers need with such speed in emergency situations.

Most of the company's customers are in Austria, but some are in neighbouring countries such as Germany, Hungary and Switzerland. Clients are primarily utilities and industries such as pulp, paper, steel and chemical producers, but also include the likes of breweries and railway companies. Barth continues: "The common denominator for most of these clients is that they basically aren't focussed on their transformers, but rather on their core business. That's why it can, and does, happen that they suddenly discover that they have a transformer that needs an immediate oil change or repair, or quite simply has to be replaced. "And that's when they call us and say that their entire production process is at risk and can we help them now, at once!" And for the most part Barth can. Oil in drums is normally sent the same day, whilst transformer repairs and servicing are carried out the same day or the following day.

Benedikt Elbling, deputy MD at Barth, says that the company can, in extreme situations, which actually are not that uncommon, pick up, check, repair, return and link a transformer back into the network within six hours. But Barth is not just about emergency measures. There are three components to the business: power distribution, which includes sales of transformer oil and of distribution and power transformers; drive engineering, which includes selling equipment such as electrical motors and generators; and, finally, servicing and repairs of transformers from 630 kV to 2000 kV and of electrical machines and engines up to 1000 kW, as well as oil analyses including thermographic analysis. "This," continues Barth, "means we offer a complete package of products and services."

Elbling chips in that he is keen to emphasize how important transformer servicing is as a preventive measure to avoid standstills or panic rescues. "A simple preventive measure is to choose a high quality oil from Nynas when making an oil change. We always recommend Nynas Nytro 4000X, because we know it will maintain its high quality under severe stress conditions over a long period." On the whole, he points out, the quality level of transformer oils is generally high in Austria. But at Barth they have also noticed a clear link that the transformers where problems occur after perhaps ten, sometimes even after five, years are commonly filled with oils of only standard quality.

"Sometimes we hear the argument that high quality oils only need to be used for power transformers," continues Elbling. "But that is not quite true. Many contemporary distribution transformers have a consistently high load, and that means you have to have a really good oil to withstand the stress over a long period of time." Barth adds, "Of course high quality oil costs more, but at the same time you are buying freedom from problems over a long period of time, up to 40 years. What's more, the added cost is relatively low in this context, especially if you factor in the cost of a break in production." "At the same time of course our customers have their own demands of us," continues Elbling. "Many of them want only oils of the highest quality, therefore we are happy that we are able to offer them Nynas products."

Barth has chosen to market and sell oils from Nynas both on account of its high quality and because the two companies have a long mutual history. "We have always sold transformer oils of the highest quality," says Robert Plenk, who is in charge of innovation and marketing at Barth. "As long ago as the 1960s we were collaborating with the former Austrian company Technol, an important and respected company in its field. When Nynas, which has a reputation at least equally as good, acquired Technol in 2002 it was therefore natural for us to start buying transformer oils in drums from the new owner."

The collaboration is beneficial to all concerned, Barth and its customers, and Nynas, who would find it difficult to supply small volumes equally fast, explains Plenk. "Since there are many customers in this region who buy oil in drums, and since we already had well functioning logistics and routines for that business, Nynas let us become exclusive supplier of their drummed oils in Austria. Larger accounts that need bulk deliveries are supplied directly by Nynas."

Another preventive measure that Barth offers is analysis of transformer oils. "We advocate carrying out analyses at regular intervals since we can then compare tests and perceive trends as to how the oil and the cellulose in the transformer are ageing," explains Elbling. "Carrying out one test only gives you a snapshot of the condition of the oil at one point in time. With a series of tests we can judge when it is time for various preventive measures." He adds that it goes without saying that Barth can carry out the sampling and analyses without the transformer having to be shut down. Barth offers most of the test methods used in the trade to assess transformers' various components, plus the company also offers analysis with infrared photography: thermography. From the resulting pictures the analyst can see exactly where a transformer or a motor is overheated and get information about where the source of the problem can be found, and can then apply the appropriate specific measures.

The company's geographical location in the centre of Europe is naturally favourable for Barth when it comes to fast service. "Previously," continues Barth, "when the iron curtain divided Europe, Austria and Vienna formed something of a bridge between east and west. Many companies from Western Europe set up a 'bridgehead' here when they wanted to establish a business or do business in the east. It is a practice that to some extent still applies even today." Now Barth's goal is increase its presence to include more of Central Europe. "In the first instance we are aiming at sales of transformer oil in drums, transformers and electric motors," explains Barth. "We'll then look at our other services but that would require further expansion as when it comes to service and maintenance the short lead time some of our customers are used to means that we have to be relatively close to their plants."

Asked what it is that Barth's customers primarily demand, Plenk is quick to reply: "Competence and quality! Well, in addition to the short lead times we have already mentioned. And since Nynas is a company that works with the same high standards, our collaboration works extremely well. But there is also a human aspect to this collaboration. We know those who work at Nynas personally, they are highly professional, easy to communicate with and get answers from. The fact that Nynas is a well known and strong brand also strengthens us further." At Barth they are looking to the future with confidence. "The strength of our business concept is that we supply a complete package of products and services, thus making it easy for our customers when they run into problems or need service," explains Barth. "That we have a successful concept is obvious,

not least shown by the fact that we have demonstrated steady growth for many years, and grown by 15–20 percent every year in the last few years. "And," he adds, "as long as there are companies and industries that need energy, there will be work for us to do!"

Dr Valery Davydov is leading the research conducted at the Centre for Power Transformer Monitoring, Diagnostics and Life Management, in Melbourne, Australia. "Here we have the possibility to achieve things that have not previously been done." IMAGE: Adrian Richards 

The "impossible" transformer"The aim of our research is not just to find out what the remaining life of a transformer is. We also want to know exactly WHEN it has to be replaced. The ideal situation would be to replace it the day before it fails!"

Admittedly somewhat exaggerated and simplified, but that is basically how Dr Valery Davydov describes the goals of the research he is in charge of at Monash University, Melbourne, Australia. An important tool in this work is a unique, specially constructed transformer that is filled with Nynas oil.

It is with a great deal of pride and enthusiasm that Research Director Valery Davydov tells us about the research he and his nine colleagues are conducting at the Centre for Power Transformer Monitoring, Diagnostics and Life Management. "I started working at Monash University in 1994," explains Davydov. "It was on a project funded by the US-based Electrical Power Research Institute (EPRI) that was expected to run for 18 months. We are now working on the fifth and biggest EPRI project, which will run until 2009."

Backing the research into power transformers, the Centre was established in 2005, following a successful bid for a competitive grant under the Science, Technology and Innovation Initiative of the Victoria State Government, Australia. The Centre is also more than 50% funded by a number of industry sponsors, a condition for entitlement to public funding. One of the principal sponsors is a transformer manufacturer, Wilson Transformer Company (WTC), Australia, which also constructed the special research transformer that was installed on the university premises in late 2006. 

"In my opinion we are conducting unique research here," continues Davydov. "Bits and pieces of what we are working on in our Centre are also being done at perhaps a few other research institutions in the world. But nobody has our breadth or can represent the entire range of the transformer industry! Our Centre is the first of its kind. Among our sponsors are transformer suppliers, component manufacturers, utilities, insurance companies and companies that provide analytical services. They all have, in different ways, a great interest in obtaining more in-depth knowledge about how to measure a transformer's remnant life, and how to manage it most efficiently, i.e.: to retire the transformer or repair it? To dry, reclaim or retrofill? To extend life or do nothing?"

"The problem issues in respect to power transformers are well known," continues Davydov. "Today there are many transformers that have been in operation for 50-60, some even for 80 years, although they were originally designed to last 30-40 years. At the same time the loads they have to cope with have increased, and they will continue to increase. It's hardly surprising then that problems arise."

These problems come about because the insulating materials in an old transformer, the cellulose and the oil, degrade. One effect is a growing percentage of water in the transformer, which causes deterioration in the cellulose's insulating ability and can also turn to steam evolving in the form of bubbles in the case of heavy loads. "And there we have an excellent scenario for a transformer breakdown, which in the worst case scenario can take the form of an explosion," explains Davydov. He searches out some statistics on power transformers that have exploded in the USA during the last few decades and shows that there has been a significant increase starting in the late 1990's. 

"It is this fear of accidents, and the costs that hit both companies and society in connection with a transformer breakdown, costs that can amount to billions of USD, that is the background to our research."

One of the key issues for utilities is thus "How long can we safely use our transformers?" "To be able to answer that question we have to study both the oil and the cellulose," answers Davydov, "since they affect each other. We have to find out what happens in a transformer in the case of various loads and stresses, and we have to develop systems for both monitoring and diagnostics."

The Centre conducts research in three steps. In the first step physical models of the cellulose and oil systems are studied. "We take the information we get from these physical models, transfer them into algorithms and mathematical models. We then develop and work further with them in our special test transformer, to check that the theoretical results are correct."

In the last stage, studies are conducted on real transformers, some at the WTC's factory, others on operating transformers in the field. The Centre has a special field programme, using transformers that are owned by the project's sponsors. These transformers are monitored continuously and the Centre has access to all the online and offline data for its research work. "The results and solutions we obtain are to be implemented in running transformers. Some of our findings belong to our collaboration partners, while other findings can be published."

An important part of the Centre's research work is that, under controlled conditions, with very comprehensive monitoring, it is possible to find answers to numerous practical questions using its specially constructed test transformer. It took Davydov a number of years to convert the initial concept of this ideal "test" transformer into a reality. "We had a long list of things we wanted to be able to do with the transformer, which we presented to Wilson’s engineers as a two-page 'wish list'. 'That is impossible to make!', was their spontaneous reaction", remembers Davydov with a smile, "but then they rose to the challenge." He goes on to say, "The project turned into something of a nightmare. The transformer was given numerous specially designed attributes, and we could have up to a dozen experts sitting in on some of the meetings. It also turned out more expensive than budgeted, but the project was saved by the University and a few new sponsors coming up with more funding. It became clear that there are many parties who are interested in this research. And the end result has been well worth all the work. Since the Wilson Transformer Co. uses Nynas oils, the Centre's transformer was of course filled with Nynas oil."

One of the many unique features of the transformer is online real time monitoring of 60+ variables. A unique feature allows paper samples to be obtained in a non-destructive way in order to conduct degree of polymerization (DP) measurments. The transformer can also be opened. An external heater for establishing temperature and moisture equilibriums, and a special moisturising system for increasing the moisture content of the transformer insulation, are other important features, as is the array of five round windows on the side walls and lid, where you can quite simply look into the transformer and see what happens when it is loaded, for example the development of bubbles.

The Centre has already produced a number of concrete results. These include both software applications and tests, for example for the solubility of water in oil, that are available commercially. In the future the Centre, through the cross-disciplinary involvement of electrical engineering and chemistry, will lead the development of new test methods, algorithms and diagnostic techniques, which will have the potential to be marketed worldwide. "This is a unique project," says Davydov. "Here we have the possibility to achieve things that have not previously been done. It is extremely challenging, at the same time as it is, for me as an engineer and a scientist, extremely rewarding."

CARLO LASZLO

Facts

The projectNynas oil user: The Centre for Power Transformer Monitoring, Diagnostics and Life Management.

Location: Monash University, Melbourne, Australia.Consortium supporting the Centre

Victoria State Government (via the STI Grant); and an unincorporated joint venture consisting of Monash University, Wilson Transformer Company, Dynamic Ratings (Australia) and TJ|H2b Analytical Services (Australia and USA).

Sponsors: Country Energy, Energex, Energy Australia, Ergon Energy, Powercor/CitiPower, Powerlink and SP AusNet (Australia); Factory Mutual Insurance Company - FM Global (USA), Transpower (New Zealand) and Weidmann (Switzerland)

Equipment Suppliers: Luxtron (USA) and Nynas (Sweden).

Research Director: Dr Valery Davydov.Research team: Dr Andrew Reykherdt, Dr Dejan Susa, Dr Daniel Martin, Dr David Fernandez, Dr Wenyu Guo, Mr Nick Lelekakis and Mr Jaury Wijaya; consultants: Dr Oleg Roizman and Mr Arne Petersen.Primary objectives: To develop new technologies, computer algorithms and equipment for monitoring, diagnostics and life management of power transformers.Website: www.ecse.monash.edu.au/centres/cptm/index.html

The test transformerManufacturer: Wilson Transformer Company, 2006Nameplate data: 468/936 kVA, 22/4.5/0.4 kV, 3-phase, 50 Hz, ONAN, ONAF, OFAN, OFAF; total mass 6850 kg, mass of oil 2650 kg; conservator with rubber bag.

Online monitoring systems and devices:

Data acquisition systems: a special in-house built multi-channel system and a DRMCC unit. Temperature probes: 16 fibre-optic, 20 thermocouples, 8 RTDs, 1 Winding Temperature 

Indicator. Other transmitters: 5 moisture in oil, 1 oil flow, 2 oil pressure, 3 voltage and 4 current.

Other features: specially arranged winding clamping pressures, 5 glass observation windows, 2 paper sample holders, external heater for establishing temperature and moisture equilibriums, special moisturising system for increasing moisture content, valves for connecting on-line dryout systems and additional coolers for testing, validation and improvement, valves, ports and flanges for inserting additional devices, monitors and samples into transformer, valves and ports for sampling oil from various locations and injecting oil samples containing known concentrations of dissolved gases of interest, overhead crane.

  

Jaury Wijaya (left) is part of the research team along with supporting engineer Ian Reynolds. IMAGE: Carlo Laszlo 

Extend transformer lifetime through maintenance and oil changes

The lifetime of a transformer can often be extended by 20–30 years by conducting regular check-ups and making timely oil changes. This is what Jürgen Scholz, owner of EES Jürgen Scholz GmbH in Hamburg, Germany, firmly believes.

He and his team have more than 30 years experience in servicing local plant and power transformers. "As today's transformers have no 'overcapacity' and are pushed harder right from the start they need even more checks and maintenance than in the past if they are to last a long time without causing problems," he says. With a broad smile, Jürgen Scholz describes himself as a "true Hamburger lad". Born and bred in the city, he qualified as an electrical engineer in the early 1960s. As time went by he found himself working increasingly with transformers and soon realised that there was a market for the mobile maintenance of transformers at customers’ premises. In 1976 he started his own company, EES Jürgen Scholz GmbH.

"In the early days I used our dining room as an office, installed the oil lab in the attic and stored my stock of spare parts in the cellar of the house where I lived with my wife and our little daughter," he reminisces. 30 years on, EES Jürgen Scholz has 28 employees working at two premises in Hamburg and 5 special service trucks equipped with oil tanks and mobile units for vacuum oil regeneration and transformer drying. The company also has a transformer workshop, a huge stock of new and overhauled transformers and an officially approved waste treatment plant for authorized disposal of both PCB-free and PCB-contaminated transformers.

Each day the mobile service units are despatched to places all over Germany. Most of the company's customers are within a radius of 500 kilometres from Hamburg, the second largest German city. But sometimes they get called further afield to neighbouring countries like Belgium and the Netherlands and even as far as Ukraine. EES has established itself as a reliable service partner for transformer owners in different industries. Thanks to globalization EES is also becoming an extended workbench for transformer and plant manufacturers in Germany and abroad by offering services and support for the installation and filling of new transformers and the dismantling and disposal of old ones. Customers like ABB, Alstom, Cegelec, e-on, EWE, Pauwels, Siemens, SGB, Vattenfall, as well as overseas manufacturers like Mitsubishi, appreciate using the EES service package.

EES Jürgen Scholz concentrates on three core areas of business:

1. Testing, maintenance and repair of transformers; 2. Purchasing old transformers and selling them again after completely overhauling them 

(including drying and either oil regeneration or, if necessary, changing the oil); and 3. Dismantling transformers that have come to the end of their service life.

"In my experience 93% of the components from old transformers can be reused," explains Scholz. "The removed spare parts can often be reused in repairs, steel and copper are recycled and we sell the electromagnetic steel to customers in China, India and Pakistan for reuse." The oil from old transformers is also recycled. In Germany environmental protection and waste management laws regulate the disposal of used oil, which is classified as environmentally hazardous PCB waste if its PCB-content exceeds 20 mg/kg. For handling this waste a special permit is needed. "With our high-class equipment and transport vehicles for old transformers and

our dedicated storage facilities we can take care of PCB-contaminated oil waste too. Way back in 1984 we were the first German company to get a permit to take care of environmentally hazardous waste," adds Scholz proudly.

The company also buys in used transformers for overhauling and renovation followed by certified resale. Scholz estimates that these transformers then have a remaining lifetime of about 20–30 years. They have a rated capacity between 50 kVA and 35,000 kVA and are all listed on the company's website. Occasionally EES receives customers' transformers for overhauling and repair in the transformer workshop. But usually one of the mobile service units travels to the transformer in question. Thanks to the extensive know-how within the company, all types and sizes of transformers from all manufacturers can be maintained and repaired.

"Apart from having a huge information base about transformer types and manufacturers and the necessary tools and instruments for performing the services we offer, we have skilled and experienced employees most of whom have worked for us for 10 to 20 years." One thing the company doesn't offer is replacement of windings, as Scholz is convinced that in general it is more economical to replace the whole transformer.

Scholz explains that most of the problems that their customers call with can be remedied directly, and if necessary they send an emergency service unit to the customer's premises. "In most cases it is cheaper, faster and more efficient for us to drive to the customer with one of our mobile units than to disconnect and transport the transformer elsewhere. That was my service concept right from the start and it has been successful. With our high-level range of services which we optimize continuously we are now the country’s leading company in our field." "Once on site the first thing we do is an oil analysis to get some idea of the transformer's condition. After that the necessary maintenance and service work is carried out. Apart from measures performed to ensure industrial and environmental safety, this work includes cleaning, resolving leaks, degassing and drying and, if necessary, completely changing the insulating oil."

Scholz continues: "From an early stage we chose to focus on changing, rather than regenerating, oil. There are several reasons for this but the main ones are that it is cheaper and faster to carry out a change than regeneration and most of our customers prefer to have new oil."

"We came into contact with Nynas via what used to be Technol in Austria and at several events organized by VDEW (the German Electricity Association)," explains Scholz. "What we appreciate about Nynas is that their prices are competitive, the staff are pleasant and competent and we can rely on promises they make about product quality and delivery dates."

So, what are the most common problems EES Jürgen Scholz comes across? "Well," says Scholz with a smile, "many customers don’t seem to realize they have responsibility for a transformer until it becomes a problem…" As Scholz sees it, many transformers become damaged through keeping them too hot for too long at a time. This is often caused by insufficient ventilation and poor air conditioning systems, and the fact that many transformers are run at maximum levels, even during the summer when outside temperatures are higher.

"We come across many transformers that constantly work at 100% of their capacity," explains Scholz, "often with reduced air circulation and cooling, which further increases operating temperatures. One detrimental effect of running a transformer like this is accelerated aging of the oil and solid insulating materials, which leads to formation of acids, water and sludge in the transformer." Scholz says that transformers that were installed in the 1960s and 70s had more "overcapacity" than those installed nowadays. "In the past transformers used to run at about 60–70% of their maximum capacity whereas today they are constantly run at full power. This will obviously result in a shortening of their lifetime."

Acute problems could of course be averted by upgrading to a transformer with a higher capacity in pace with increased demand. "But," continues Scholz, "most transformer owners choose to push their existing systems as far as possible, usually for economic reasons. They favour repairing an old transformer instead of buying a new one for two reasons:

1. A properly maintained transformer can be expected to have a service life of about 50–60 years, maybe even more, and 

2. Replacing a transformer usually interferes with other technical equipment and may result in further adjustment demands."

"While new transformers are undeniably very efficient and functional, they are pushed harder right from the start," explains Scholz. "Therefore they need even more regular check-ups and service." Scholz believes that his company can continue to grow, slowly but surely. He sees two contradictory trends within his field which he bases this belief on: "On the down side for us, an increasing number of manufacturing plants in Germany are moving their production abroad. However, on the up side, and so far this trend seems to be the stronger one and shows signs of continuing for a long time to come, is that an increasing number of old transformers need attention and manufacturers from abroad show an increasing interest in using EES as their extended workbench and service base."

CARLO LASZLO

Facts

Company: EES Jürgen Scholz GmbH. Founded: 1976. Owner/ Chairman of the Board: Jürgen Scholz.Number of employees: 28. Head office: Hamburg, Germany. Main business: Service, maintenance and repair of transformers. Market: Primarily Germany, but also the Netherlands and Belgium. Website: www.ees-hamburg.de

  

Jürgen Scholz's 5 top tips for transformer owners

1. Install a contact dial thermometer with a maximum indicator, and a dehumidifier in your transformer. 

2. At least twice a year (Spring/ Autumn) check the transformer's temperature in relation to its load, as well as the maximum temperature achieved in the relevant period as shown by the maximum indicator and the oil volume in relation to the transformer's temperature. If the temperature is too high over longer periods of time then this indicates that there is a fault in the transformer. 

3. Carry out regular quality tests on your transformer oil. From the results you can get a great deal of information about the condition of your transformer. 

4. Check the composition of the gas in the oil. Gas can give an indication as to whether there are "hot-spots" or cellulose degradation in your transformer. 

5. Check regularly whether your transformer is leaking.

  

Lars Frode Askheim, head of Hafslund Entreprenör AS’s division for special services, believes there is a growing market for transformer service. IMAGE: Claes Löfgren 

Maintenance and investment lead to tip-top transformers Power networks in many parts of the world are suffering because of the high cost of replacing aging infrastructure.

And the low level of maintenance performed on older transformers is not helping the situation. Yet things could be substantially improved through the use of well-planned monitoring and service activities. A company with considerable experience and extensive expertise in this field is Hafslund Entreprenör of Norway.

To claim that many owners of power plants, electricity transmission and distribution systems are sitting on ticking bombs is perhaps going a bit too far. But the risk of aged transformers in bad condition failing whilst in operation, and thereby causing major power outages, is growing day by day. The reason, quite simply, is that in most countries the average age of the electrical infrastructure, including transformers, is increasing and too many transformers are getting close to the end of their useful life. In Norway, as in many other parts of the world, many of the transformers now in use were produced and commissioned during the period 1945-55, as reinstating the infrastructure for power transmission was an important part of the reconstruction process after the Second World War.

When consumption of electricity rapidly increased at the end of the 1960s, it marked the start of a second extensive wave of infrastructure investments, which also included major investments in hydroelectric power. This wave of expansion also has parallels in many other countries worldwide. Most of these transformers are still in operation and maintenance has fallen behind for many of them. This, added to the increased demand for electricity over the past few decades is putting serious strain on the system; and the rate of replacement of old transformers is slower than needed to substantially improve the situation. "Transformers age over time. But low fast and to what extent depends on a number of factors," explains Lars Frode Askheim, divisional manager for Hafslund Entreprenör's specialist services.

The temperature at which a transformer operates is very important for its expected life time; and the temperature is influenced by a number of factors, for example the load, the cooling system, oil type etc. The increased load and changes in operating mode caused by modern day demands are leading to various degrees of degradation and a faster deterioration of the overall condition of many transformers. "The oil molecules themselves also age over time, mainly impacted by temperature and oxygen availability", continues Askheim. "This leads to a deterioration of the insulation capabilities and other properties of the oil. And the more the oil ages, the more negatively the cellulose is affected." If the cellulose insulation deteriorates too much the result may be an electrical flashover between the windings inside the transformer.

"The ultimate consequence of this is a breakdown of the transformer," explains Askheim. "And there is a risk that, due to local overheating and pressure build up the transformer tank could burst. In the worst-case scenario this could result in a fire or even an explosion that could risk human lives and have serious environmental consequences." The simple solution it would seem would be to replace all old transformers with new ones. "But of course this is not a realistic solution for the majority of transformer owners," continues Askheim. "It would be an economic impossibility for most companies to replace all of their old transformers that are in the risk zone in one go."

But there is light at the end of the tunnel. Askheim proposes a different and financially more viable solution: analysis followed by conditioning (degassing/dehydration) and, if necessary,

regeneration of the transformer oil or an oil change depending on the level of degradation. Through oil regeneration and replacement of consumed inhibitors, natural or synthetic, the transformer oil can be restored to a good condition. If this procedure is carried out before the transformer's insulation has deteriorated too far, then the degradation process continues at a much lower rate, though not quite as low as for a new transformer. To put it simply, its useful lifetime is extended. Naturally, by how much it is extended varies from case to case depending on the initial quality of the oil, design of the transformer, operating conditions, etc.

However if the oil is badly oxidized, it will not respond well to regeneration, and then an oil change is probably the most favourable alternative. Obviously this is more expensive. It is therefore important that all larger transformers are tested at regular intervals, and proper maintenance actions are introduced as soon as it is deemed necessary based on results of the analysis. "Through a thorough condition assessment of each transformer you obtain detailed knowledge about the overall condition of the transformer population," Askheim explains. "If you combine this with a carefully thought through maintenance plan with appropriate service actions – for example conditioning, regeneration or changing the oil – the useful life of a transformer can in many cases be extended and so the total lifetime of your transformer population can be improved. Furthermore you will have a safer and a more reliable operation. Then long-term investments can be made in a better planned and controlled manner."

Askheim makes a theoretical estimation of what a transformer owner can save through regular testing and maintenance with regeneration when needed. "In the ideal scenario, where the transformer is regenerated twice and at the right points during its lifetime, we would dare, speaking theoretically and generalising, to claim that you can extend a transformer's lifetime by up to 50 percent. Or, if you look at it from a financial point of view: you reduce the yearly cost for the transformer by a third." "I think many of our customers see the value of and need for regeneration," says Askheim. "As by doing this they can defer some of their investments into the future."

In order to decide upon the best maintenance and service needs for oil-filled equipment, Hafslund Entreprenör purchases oil analysis and diagnostic services from Nynas IOM Norway, an analysis laboratory that specializes in insulating oil analysis. The relationship between Hafslund Entreprenör and the manager of the Nynas IOM laboratory, John Bongard, goes back a long way before Nynas took over the former E-CO Diagnose laboratory in 2005. "Working with John Bongard," continues Askheim "gives us a great feeling of security, since he is a recognised authority within this field. And since Nynas IOM Norway came into the picture we have started to use the wider spectrum of services available within the company. We know that they are backed by their IOM and research lab in Sweden which is of a very high international standard, and we have started to order other special analyses like furan and PCB analysis which are carried out by the Nynas IOM UK laboratory. We also buy cable oil analyses from Nynas IOM Norway."

"Based on the test results from Nynas, such as acidity, interfacial tension, breakdown voltage, water and inhibitor content, we can determine if it is time for any maintenance action," continues Askheim. "These decisions are normally based on national and international guidelines." The action resulting from the test results could be conditioning, regeneration or if there is serious

degradation, an oil change. Conditioning and regeneration of transformer oils in service are the two most common oil improvement processes. Conditioning removes dissolved gases, water and particles from the oil, whilst regeneration in addition removes oil oxidation products harmful to the cellulose. When an oil in a transformer is regenerated it is passed through an active filter until oxidation products and other impurities are down to acceptable levels.

Hafslund Entreprenör undertakes the regeneration of transformers of all sizes. The company's experience with the process dates back to 1960, when they bought their first regenerating equipment. In August this year a second, specially designed regenerator, with the latest and most environmentally friendly technology was purchased. "With both pieces of equipment the oil is regenerated by being passed through active clay. However, in the old machine the active clay can only be used once, and then has to be treated as environmentally hazardous waste. In our new equipment the active clay is in the form of pellets which can be reactivated and used between 200 and 300 times."

"Since regeneration costs both time and money we want to know as fast as possible when the oil is sufficiently treated. In the final stages we therefore take samples at very frequent intervals and send them to Nynas IOM Norway, which provides us with an analysis report within the hour if needed. It goes without saying that we appreciate getting the results so fast."

Fast results are not the only advantage of working with Nynas. "There is also considerable added value gained from using Nynas as a reference company and business partner in our customer contacts," Askheim concludes. "For us the close cooperation with Nynas means more than merely buying analysis, we also value their competence and the opportunity to share experiences."

CARLO LASZLO

Facts

Company name: Hafslund Entreprenör AS Founded: 2002 (1900) Owner: Hafslund AS Chairman of the Board: Björn Frogner Managing Director: Lars Bangen Head Office: Oslo, Norway Core business: Services related to planning, construction and maintenance of electrical infrastructure. Service and oil regeneration of transformers in operation.Employees: 365 Turnover: €65 million (2005) Website: www.hafslund.no

  

Regeneration a growing market

Hafslund Entreprenör AS has its roots in the Norwegian electric power industry that developed in the early 20th century. Today the company is an independent subsidiary of the Hafslund group, with two main areas of operation: the construction, operation and maintenance of distribution networks, primarily in southeast Norway; and various forms of service and maintenance of transformers.

Hafslund Entreprenör is among Norway's 3-4 largest in the field of distribution network services, and has about 70 percent of the Norwegian market for transformer maintenance. Some smaller, but growing, parts of the operation are made up of the regeneration of cable oil, and the analysis and drying of transformer paper insulation.

The regeneration of transformer oil is also seen as a growing market thanks to the large number of old transformers, both in Norway and other countries.

Hafslund Entreprenör carries out regeneration all over Norway. In recent years the company has started to enter new markets, primarily in Scandinavia, but the rest of Europe is also seen as a potential market. Their strategy is to grow in Scandinavia through acquisition, since these markets are so mature that it is difficult to grow organically there.

Number of two-winding transformers that have been put into service in Norway between 1901 and 1998.(Source: Statkraft/E-CO Partner.)

places great demands on suppliersSalto Grande is both Argentina's and Uruguay's most important power plant. Around 10% of Argentina's and 60% of Uruguay's electricity comes from here.

"That's why our demands on the equipment supplied are more far-reaching than those stated in international standard norms," explains Eduardo Briosso.

Salto Grande is an impressive sight. The plant – situated about 450 kilometres north of Buenos Aires – is situated on the Uruguay River, which also forms the boundary between Uruguay and Argentina. The dam is 39 metres high and has created an enormous lake upstream that has developed into a popular tourist area.

The power station is a joint project in which both countries collaborate on all levels, from the government to the technical staff working there. Eduardo Briosso, Substations Department Supervisor, from Uruguay, and his colleague Fernando Marull, Transmissions Manager, from Argentina, are among those who share the day-to-day responsibility for the running and maintenance of the power supplies.

It is with an obvious sense of pride that they talk about the plant and its importance for both

countries. "Salto Grande's annual capacity is 6,700,000MWh," explains Fernando Marull. "There are 748 kilometres of lines in Argentina and 473 kilometres in Uruguay." "The power station houses transformers with a total capacity of 74,500kV, as well as 36 induction transformers."

"Since the plant is so important to power supplies we must have a high level of operational dependability and so we do a great deal of preventive maintenance," he continues. "Also, an increasing number of our transformers are approaching 25 years of age, which accentuates the need for preventive measures." An important link in this maintenance work is Salto Grande’s own modern laboratory, which has been amassing experience over the last 25 years.

"Here we can test everything from the oil in the transformers to the cellulose in the insulation. We can run all the samples and tests that we need, even Dissolved Gas Analysis." Based on numerous tests that have been performed over many years, Marull and Briosso, together with their colleagues, have built up a comprehensive bank of experience of transformer oils. They can therefore specify exactly what kind of oils they want in any new transformers purchased for the plant.

When Salto Grande needed a new 500kV transformer, the management of the plant, for the first time, approached a South American supplier, the Argentinean company Faraday in Buenos Aires. When asked "Why?" Briosso responds: "There are several answers to that question. Though probably the main one is that we wanted to stimulate development of a local supplier. Faraday had not manufactured many transformers of that size previously, and it is more usual to turn to producers with long experience. But obviously Faraday has met all the specifications and demands that we placed on the transformer."

Fernando Marull adds that the advantages for Salto Grande in collaborating with a local supplier are that they get a faster response in the case of problems and when obtaining spare parts and the like. With the experience Salto Grande has of their ageing transformers, those responsible for technical aspects stressed that the oil that the new transformer was to be filled with must be of the highest international quality.

"Faraday contacted us and asked whether it would be appropriate to use oil from Nynas," continues Marull. "And it was, since Nynas transformer oils meet, and even surpass, the demands of IEC 60296 and IEC 60422. Moreover, Nynas meets the special specifications and demands that we place over and above the IEC’s."

"We expect Nynas oils to have a long life expectancy and a high oxidation stability. We are confident of this, as we have already had many years of good experience of Nynas' transformer oils since they are in many of the transformers already in our plant."

CARLO LASZLO

This is what it looks like – the large 500 kV power transformer that Faraday delivered to Salto Grande. IMAGE: Carlo Laszlo

Faraday invests in a growing market – with the help of oils from NynasWhen transformer manufacturer Faraday decided to compete with big international rivals in the large power transformer market, they did it in style, winning a contract with Argentina's and Uruguay's biggest power supplier, Salto Grande.

Engineer Jorge Claudio Buonarcorso, factory manager at Faraday, explains: "Building a large power transformer for Salto Grande was a huge challenge with a highly successful outcome."

Faraday S.A.I.C. y F. was founded in 1957 and has its production centre in the capital of Argentina, Buenos Aires. Faraday is one of the leading South American transformer manufacturers, doing its own design, computation and construction of large power transformers with capacities up to 450 MVA and a rated voltage of up to 500 kV for three-phase constructions and up to 1200 MVA in single-phase constructions. One factor that has helped them achieve and maintain this position is that they have Argentina's biggest test laboratory. "We have experienced a large number of technologies," continues Buonarcorso. "We used to have a joint venture agreement with a large European company, and so we are thoroughly familiar with their technology. These days we produce transformers on our own behalf, without any joint venture agreement."

Most of Faraday's sales are to the major power companies – both domestic and foreign – that are active in Argentina. Up until recently the production focus was on transmission transformers,

and Jorge Buonarcorso maintains that the company has about 75% of the Argentine market in that sector. But recently they took a step up to manufacturing 500 kV power transformers as well. And with the sale of one such transformer to Salto Grande it is hoped that the company has managed to successfully break into a new market. "Salto Grande bought our second 500 kV transformer," says engineer Ricardo Sirabonian, who joined the company in 1974 at the same time as the manufacture of power transformers started its development. Today he works as a technical adviser and is an information source on anything to do with transformers.

"They realised that we could offer the best value. Their decision was probably also based on a desire to support and endorse a local supplier. And, at the same time, they saw the advantages of being in close contact with our production processes, after sales service and so on." The fact that the demands Salto Grande places on transformers are way above international norms has not brought any problems at all, according to Buonarcorso. He explains why: "We have a reliable, tried-and-tested design from a company of high international repute. We also have considerable know-how and many years' experience of computation and construction. Another significant asset of our company is our team of constructors, which comprises both older experienced constructors and talented young engineers."

"Altogether, this gives our products a level of excellence that is highly comparable with international transformer manufacturers," sums up Buonarcorso. In order to hold its own in a global market, Faraday naturally complies with a series of international standards such as ISO, DIN, IEC, ANSI and IEEE. Materials and equipment are primarily purchased from European manufacturers.

Nynas oils came to Faraday's attention about five years ago. "We now use Nynas oils in several of our customers' transformers, since we want to take advantage of the special characteristics these oils possess," explains Sirabonian. "In our opinion, Nynas naphthenic oils have excellent electrical properties which cater to the special electrical and physical requirements of Extra High Voltage transformers."

In addition to the transformer sold to Salto Grande, Faraday has supplied transformers with Nynas oils to the South American nuclear power plant Nuclear Eléctrica Argentina S.A. "Of course there are strong competitors to Nynas," he continues. "But there were several influential factors that made us choose Nynas as our supplier of transformer oils, apart from the fact that the company fully answers all our technical demands."

The most important reason, says Sirabonian, is Nynas' local presence. "Through General Manager Ana Cura we have always rapidly received the technical support we want. A further plus is that Nynas has a depot in Buenos Aires – which guarantees fast deliveries. We find Nynas is a flexible company, with an ability to listen to our requests, with fast lines of communication, and a willingness to share its knowledge and research, which suits us perfectly."

Both Ricardo Sirabonian and Jorge Buonarcorso feel that there is good reason for Faraday to view the future brightly. Buonarcorso explains: "The sale to Salto Grande is proof of the fact that we have managed to meet the toughest demands that are placed on an international market. In addition, energy needs are growing in Argentina and large parts of Latin America. This means

new investments in power transmission, and I am convinced that we will be able to hold our own in this market, both now and in the future."

CARLO LASZLO

The bright yellow cone is the top of a generator, the main part of which is on the floor below; this is in turn driven by a turbine, yet another floor below.

Oil tests and maintenance extend transformers' life expectancySafety and maintenance are two of the cornerstones of operations at the Norwegian power company Sira-Kvina.

"We want to have a safe plant, as well as wanting to maximise the life expectancy of our transformers and avoid costly losses in production," explains Geir Vårdal, Area Plant Manager, and one of the people in charge of the maintenance and safety work.

Sira-Kvina is one of Norway's biggest power companies, producing around 6 TWh a year. Its seven hydro power stations provide about 5% of Norway's total power production. The company's production centres are scattered across southern Norway, and the biggest power station is in Tonstad. In total, Sira-Kvina has 13 large generator transformers of around 100 MVA, and several smaller station transformers of about 250 to 2500 kVA. The company’s biggest transformer is a 360 MVA unit.

Most of the transformers are now around 30 years old which, generally speaking, means that they should be approaching the end of their useful life. But Geir Vårdal and his colleagues hope that a very thorough maintenance program will enable them to postpone the retirement of the transformers for many years to come. He explains: "There are many things you can do to extend a transformer's life. Without going into all of them, I can mention the importance of keeping the temperature down, changing drying filters frequently and ensuring the levels of acids, oxygen and moisture in the oil are kept low. All of these are relatively simple and small measures that we have taken on a regular basis. A more far-reaching task is oil regeneration or treatment with an ion exchange filter. That is something we have done to some transformers and are planning to pursue on others."

Vårdal emphasises the need for testing transformer oils regularly for safety reasons. That's also why Sira-Kvina has had a protective system installed that disconnects the transformer if there is an electrical fault. Gas relays have been installed that issue a warning if gas forms in a transformer's expansion tank. In addition to these precautions, there are also plans to install sensors that can measure Hydrogen gas content online. H2can be formed if there is an electrical fault in the transformer and can be used for diagnostic purposes. In the future Sira-Kvina wants to keep a constant check on this, so it does not exceed acceptable levels.

"To assess the condition of the oil and the transformer we carry out both the General Oil Test (GOT), and Dissolved Gas Analysis (DGA)," continues Vårdal. Among other things GOT reveals whether the oil contains acid and water and provides information on its condition in general. DGA shows whether the oil contains certain gases. "But it is very difficult to establish exactly the internal condition of a transformer," explains Vårdal. "This applies particularly to the state of the cellulose insulation, which at the end of the day determines the length of the transformer's working life."

Regeneration is a standard maintenance action, and is recommended, for example, by ABB in Norway. The main purpose of this treatment is to reduce the level of acid in the oil. But the method is relatively expensive and requires considerable resources. "We therefore decided a while back to try a simpler and cheaper method using an ion exchange filter," explains Vårdal. "We naturally wanted to know if the method was reliable, so we contacted Nynas Naphthenics' IOM unit in late 2004 to investigate the long-term effects of the method with the aid of an accelerated oxidation test." It turned out that the short-term effects were good, but that the method doesn't work as well as the regeneration method does in the long term.

"Nynas carried out a very thorough in-depth test for us," says Vårdal. "We are very satisfied with Nynas Naphthenics' efforts, but unfortunately not fully satisfied with the new method. It was clearly evident from Nynas' tests that we were not managing to remove everything we hoped from the oil."

"Clearly there is a high level of competence at Nynas Naphthenics," says Vårdal. "I believe that an oil company that refines oil, with a solid R&D department, and great in-depth knowledge about transformer oil, is well suited to conduct high level tests and evaluations of used oils."

Sira-Kvina's strategy will now be to concentrate maintenance resources on the best transformers

and in that way get the longest possible operational time from their total transformer population. Despite the partly negative results, Sira-Kvina is still considering further work with the ion exchange filtering method. Possibilities include doing it more often and in combination with vacuum filtering to remove water from the oil more efficiently. Regardless of whether they use the regeneration method or the simpler ion exchange filter method, the levels of antioxidants in the transformers will be reduced as a result of the treatment.

"Previously it was something of a problem to add antioxidants after a treatment like this," continues Vårdal. "They were sold in a form that was difficult to mix in with the oil, and you had to use expensive special equipment. But in Nynas Naphthenics' product NYHIB 10, the antioxidants are already dissolved in. We buy drums with a 10% concentration of antioxidants, which we can simply add at the end of the process. "And that," he concludes with a smile, "is just great!"

CARLO LASZLO

Extreme demands on the sea bed

Working on oil fields at depths up to 2000m below sea level leaves no room for mistakes. In some respects the demands placed on oil pumps, transformers and electrical circuits used on the sea bed exceed those placed on space vessels.

Operational disruptions that halt oil production can be extremely costly which is why every link in the production chain has to be extremely reliable.

Most underwater oil fields that are below relatively shallow seas are already being exploited, so oil companies are increasingly turning their attention towards finds beneath deeper water. But these oil fields are ever further from land and at increasingly greater depths. Construction of rigs to extract oil from these finds involves colossal levels of investment.

"As far back as the mid 1980s we started to investigate the theoretical possibilities of locating electrical submersible pumps and transformers on the sea bed," says Esa Virtanen, who is head of development at the distribution transformer factory of ABB's Power Technology Division in Finland. ABB's 180 employees, based at Vasa on Finland's west coast, work on a range of products, including transformers which have to withstand extreme demands. The transformers ABB produces are used on oil rigs, ships, power stations and plants of the process and mining industries. "We used to be part of the Strömberg group which was very strong in the field of power electronics," continues Esa Virtanen. "Through this group we gained experience of transformers with frequency converters which, for example, can be used to regulate the speed of an oil pump.

"The oil industry tends to be conservative and for a long time no one was willing to take the first steps with this new and untried technology." "But in 1997, in collaboration with Norwegian pump supplier Framo Engineering and end user Exxon Mobile, we started work on a project to develop the new pump technology for use on the Zafiro oil field off the coast of Equatorial Guinea. Exxon wanted to use a booster pump at a depth of 600m that could pump oil to a ship located 8km from the well."

"When the pressure in an oil well is less than the pressure on the sea bed, you need to use a special solution to help bring the oil up. One method is to pump sea water or gas down into the oil well but this has the disadvantage of diluting the extracted oil with water or gas. Alternatively, the oil can be pumped to the surface." ABB worked with oil pump specialist Framo Engineering to develop a solution which was put into action at the end of 1998.

The 'package' to be lowered into the sea was formed by a specially designed sub sea structure containing a pump, transformer, pipes and valves and weighed in at 70t. The actual transformer is about 2.5m high, 2m long and 1.2m wide, weighs around 7t, contains about 2m3 of insulating oil and has a power output of 1,600kW. Heavy structures such as this cannot just be lowered onto the sea bed because they could sink into the mud and sand. They need to be supported by a special piled foundation. The underside of the pump structure is fitted with tubes that slot onto the piles, and hold the structure a few metres above the sea bed.

Installing and using equipment at these depths creates its own particular difficulties. "The high water pressure means that we have been forced to develop new, specially designed components for the structure," explains Esa Virtanen. "Once installed on the sea bed, the scope for doing anything to the equipment is very limited." Deep sea divers can work at depths down to about 100 metres but any deeper and Remotely Operated Vehicles (ROVs) are needed. ROVs are manoeuvred by operators on the surface who can 'see' via the ROV's cameras. The ROV can operate valves and can also start and shut down equipment, but in situ repairs and service are out of the question.

Corrosion is another problem and the transformer casing has to be constructed from special high-grade steel to offer more resistance. Johan Backa is Sales Manager at ABB Bergen in Norway and his unit within ABB has global responsibility for sub sea electrical systems and frequency converter systems. The unit played a key role in putting together components for the project and was also involved in getting the electrical system to function as a complete unit.

"The equipment's quality and reliability are very important in a project of this kind because it is extremely expensive to raise equipment up from the sea bed for repair." But how do you define 'extremely expensive'? According to Johan Backa, ships that can lower and lift equipment, along with divers and other personnel, cost about 240,000 US dollars a day. Cost of repair and losses due to drops in production have to be added on too.

The extreme water pressure on the sea bed presents a big challenge, but this was not the biggest difficulty facing the team. "Transformers are completely filled with oil which, in an ideal situation, should not be compressed. But at the depths at which these installations operate, the oil is affected and becomes compressed. Therefore it is extremely important that the oil in the transformer has been treated carefully to remove all traces of gas from it."

Other, greater problems include finding materials that are suitable for use on the sea bed, and developing electrical circuits that can withstand both sea water and great depths. "Heat presents one of the biggest challenges because the transformers become hot while in operation causing the oil to expand," continues Johan Backa. "The transformer is in a solid tank that cannot expand, so a rubber membrane which acts as a pressure compensator is included to overcome the problem." Heat also leads to chemical reactions in the casing that can allow organisms to start growing on it, reducing the insulating effect. To prevent organism growth it is important that the transformer maintains as low a temperature as possible.

Esa Virtanen worked closely with Nynas Naphthenics' R&D department throughout the project to gain a better understanding of the changes which the transformer oil would undergo at depth. "Together we discussed a range of potential oil problems, including how the oil's insulating properties would be affected by the extreme pressure." ABB chose to work with Nynas Naphthenics for this project because the company has many years of experience of transformer oil products, explains Esa Virtanen.

"We have known for a long time that the company can supply high quality insulating oils, and we know their properties. When it came to choosing oil for this highly demanding application, it was obvious that we needed an oil that we were familiar with. Moreover we already had compatibility test data for Nynas Naphthenics' oils, so we knew that they would function well with the other materials and components. This prior knowledge helped to reduce the total development time, which was also important." The first oil pumping project has been followed by others, and Esa Virtanen says proudly that up to now there have been no stops in production due to transformer faults.

Another deep sea project, known as Pipe Heating Systems, which ABB is involved in is under way in the Gulf of Mexico, at a depth of about 2,000m below sea level. The project involves warming pipelines on the sea bed where the oil has 'frozen' through the formation of hydrates

because of intense pressure and cold. Johan Backa explains: "To get the lump to dissolve you have to heat the oil pipeline but there are several ways of doing this. Our technique uses mobile plant that is transported to the frozen pipeline site by boat and consists of an electrical system, a sub sea cable and a sub sea skid containing the equipment."

The skid, which includes a sub sea transformer and electrical connectors for connecting the pipeline, is lowered down to the seabed and with the help of an ROV, the electrical connectors are attached to the pipeline. Once connected, the electricity supply from the boat is turned on. The ship's diesel generator generates 480 volts and a step-up transformer is used to raise and regulate the voltage between 1kV and 11kV. The power is used to heat the pipeline and normally the blockage clears after a few days. "These transformers, like all our sub sea transformers, also contain oil from Nynas Naphthenics," adds Johan Backa.

These techniques for getting the equipment necessary for extraction of oil at great depths down to the sea bed have an enormous potential and represent a growing market. Even though the investments are huge – around 20 million US dollars – it can increase production by about 10,000 barrels of oil a day. At today's oil prices (October 2003) this level of investment will pay for itself in about 100 days. This means that an increasing number of oil companies are willing to take the financial risk.

ABB has made it possible to reach previously untapped oil reserves at depths of up to 3000m but Esa Virtanen believes that access to even deeper supplies will soon be possible. "Depths up to 3,000m are no problem these days. In theory the absolute limit is 10,000m where the pressure reaches 1,000 bars and the transformer oil will become solid. Such extremes are rarely encountered so depth will not be a limiting factor and our studies also show that it is possible to have transformers up to 70km from the mainland." Johan Backa continues: "The potential of future technology makes this a very interesting and challenging area to work in, with lots of developmental work and a large portion of pioneer spirit. Right now sub sea activities are the most exciting area of the oil industry."

"What is more, having supplied the major part of commercial installations, ABB is at the leading edge of this field. This is a good position that we aim to keep. But to achieve that, all the components must be very high quality, and that also applies to the oil, since it plays a key role in the entire installation."

CARLO LASZLO

Extend transformer lifetime through maintenance and oil changesThe lifetime of a transformer can often be extended by 20–30 years by conducting regular check-ups and making timely oil changes. This is what Jürgen Scholz, owner of EES Jürgen Scholz GmbH in Hamburg, Germany, firmly believes.

He and his team have more than 30 years experience in servicing local plant and power transformers. "As today's transformers have no 'overcapacity' and are pushed harder right from the start they need even more checks and maintenance than in the past if they are to last a long time without causing problems," he says. With a broad smile, Jürgen Scholz describes himself as a "true Hamburger lad". Born and bred in the city, he qualified as an electrical engineer in the early 1960s. As time went by he found himself working increasingly with transformers and soon realised that there was a market for the mobile maintenance of transformers at customers’ premises. In 1976 he started his own company, EES Jürgen Scholz GmbH.

"In the early days I used our dining room as an office, installed the oil lab in the attic and stored my stock of spare parts in the cellar of the house where I lived with my wife and our little daughter," he reminisces. 30 years on, EES Jürgen Scholz has 28 employees working at two premises in Hamburg and 5 special service trucks equipped with oil tanks and mobile units for vacuum oil regeneration and transformer drying. The company also has a transformer workshop, a huge stock of new and overhauled transformers and an officially approved waste treatment plant for authorized disposal of both PCB-free and PCB-contaminated transformers.

Each day the mobile service units are despatched to places all over Germany. Most of the company's customers are within a radius of 500 kilometres from Hamburg, the second largest German city. But sometimes they get called further afield to neighbouring countries like Belgium and the Netherlands and even as far as Ukraine. EES has established itself as a reliable service partner for transformer owners in different industries. Thanks to globalization EES is also becoming an extended workbench for transformer and plant manufacturers in Germany and abroad by offering services and support for the installation and filling of new transformers and the dismantling and disposal of old ones. Customers like ABB, Alstom, Cegelec, e-on, EWE, Pauwels, Siemens, SGB, Vattenfall, as well as overseas manufacturers like Mitsubishi, appreciate using the EES service package.

EES Jürgen Scholz concentrates on three core areas of business:

1. Testing, maintenance and repair of transformers; 2. Purchasing old transformers and selling them again after completely overhauling them 

(including drying and either oil regeneration or, if necessary, changing the oil); and 3. Dismantling transformers that have come to the end of their service life.

"In my experience 93% of the components from old transformers can be reused," explains Scholz. "The removed spare parts can often be reused in repairs, steel and copper are recycled and we sell the electromagnetic steel to customers in China, India and Pakistan for reuse." The oil from old transformers is also recycled. In Germany environmental protection and waste management laws regulate the disposal of used oil, which is classified as environmentally hazardous PCB waste if its PCB-content exceeds 20 mg/kg. For handling this waste a special permit is needed. "With our high-class equipment and transport vehicles for old transformers and our dedicated storage facilities we can take care of PCB-contaminated oil waste too. Way back in 1984 we were the first German company to get a permit to take care of environmentally hazardous waste," adds Scholz proudly.

The company also buys in used transformers for overhauling and renovation followed by certified resale. Scholz estimates that these transformers then have a remaining lifetime of about 20–30 years. They have a rated capacity between 50 kVA and 35,000 kVA and are all listed on the company's website. Occasionally EES receives customers' transformers for overhauling and repair in the transformer workshop. But usually one of the mobile service units travels to the transformer in question. Thanks to the extensive know-how within the company, all types and sizes of transformers from all manufacturers can be maintained and repaired.

"Apart from having a huge information base about transformer types and manufacturers and the necessary tools and instruments for performing the services we offer, we have skilled and experienced employees most of whom have worked for us for 10 to 20 years." One thing the company doesn't offer is replacement of windings, as Scholz is convinced that in general it is more economical to replace the whole transformer.

Scholz explains that most of the problems that their customers call with can be remedied directly, and if necessary they send an emergency service unit to the customer's premises. "In most cases it is cheaper, faster and more efficient for us to drive to the customer with one of our mobile

units than to disconnect and transport the transformer elsewhere. That was my service concept right from the start and it has been successful. With our high-level range of services which we optimize continuously we are now the country’s leading company in our field." "Once on site the first thing we do is an oil analysis to get some idea of the transformer's condition. After that the necessary maintenance and service work is carried out. Apart from measures performed to ensure industrial and environmental safety, this work includes cleaning, resolving leaks, degassing and drying and, if necessary, completely changing the insulating oil."

Scholz continues: "From an early stage we chose to focus on changing, rather than regenerating, oil. There are several reasons for this but the main ones are that it is cheaper and faster to carry out a change than regeneration and most of our customers prefer to have new oil."

"We came into contact with Nynas via what used to be Technol in Austria and at several events organized by VDEW (the German Electricity Association)," explains Scholz. "What we appreciate about Nynas is that their prices are competitive, the staff are pleasant and competent and we can rely on promises they make about product quality and delivery dates."

So, what are the most common problems EES Jürgen Scholz comes across? "Well," says Scholz with a smile, "many customers don’t seem to realize they have responsibility for a transformer until it becomes a problem…" As Scholz sees it, many transformers become damaged through keeping them too hot for too long at a time. This is often caused by insufficient ventilation and poor air conditioning systems, and the fact that many transformers are run at maximum levels, even during the summer when outside temperatures are higher.

"We come across many transformers that constantly work at 100% of their capacity," explains Scholz, "often with reduced air circulation and cooling, which further increases operating temperatures. One detrimental effect of running a transformer like this is accelerated aging of the oil and solid insulating materials, which leads to formation of acids, water and sludge in the transformer." Scholz says that transformers that were installed in the 1960s and 70s had more "overcapacity" than those installed nowadays. "In the past transformers used to run at about 60–70% of their maximum capacity whereas today they are constantly run at full power. This will obviously result in a shortening of their lifetime."

Acute problems could of course be averted by upgrading to a transformer with a higher capacity in pace with increased demand. "But," continues Scholz, "most transformer owners choose to push their existing systems as far as possible, usually for economic reasons. They favour repairing an old transformer instead of buying a new one for two reasons:

1. A properly maintained transformer can be expected to have a service life of about 50–60 years, maybe even more, and 

2. Replacing a transformer usually interferes with other technical equipment and may result in further adjustment demands."

"While new transformers are undeniably very efficient and functional, they are pushed harder right from the start," explains Scholz. "Therefore they need even more regular check-ups and service." Scholz believes that his company can continue to grow, slowly but surely. He sees two

contradictory trends within his field which he bases this belief on: "On the down side for us, an increasing number of manufacturing plants in Germany are moving their production abroad. However, on the up side, and so far this trend seems to be the stronger one and shows signs of continuing for a long time to come, is that an increasing number of old transformers need attention and manufacturers from abroad show an increasing interest in using EES as their extended workbench and service base."

CARLO LASZLO

Facts

Company: EES Jürgen Scholz GmbH. Founded: 1976. Owner/ Chairman of the Board: Jürgen Scholz.Number of employees: 28. Head office: Hamburg, Germany. Main business: Service, maintenance and repair of transformers. Market: Primarily Germany, but also the Netherlands and Belgium. Website: www.ees-hamburg.de

  

Jürgen Scholz's 5 top tips for transformer owners

1. Install a contact dial thermometer with a maximum indicator, and a dehumidifier in your transformer. 

2. At least twice a year (Spring/ Autumn) check the transformer's temperature in relation to its load, as well as the maximum temperature achieved in the relevant period as shown by the maximum indicator and the oil volume in relation to the transformer's temperature. If the temperature is too high over longer periods of time then this indicates that there is a fault in the transformer. 

3. Carry out regular quality tests on your transformer oil. From the results you can get a great deal of information about the condition of your transformer. 

4. Check the composition of the gas in the oil. Gas can give an indication as to whether there are "hot-spots" or cellulose degradation in your transformer. 

5. Check regularly whether your transformer is leaking.

South American power collaboration places great demands on suppliersSalto Grande is both Argentina's and Uruguay's most important power plant. Around 10% of Argentina's and 60% of Uruguay's electricity comes from here.

"That's why our demands on the equipment supplied are more far-reaching than those stated in international standard norms," explains Eduardo Briosso.

Salto Grande is an impressive sight. The plant – situated about 450 kilometres north of Buenos Aires – is situated on the Uruguay River, which also forms the boundary between Uruguay and Argentina. The dam is 39 metres high and has created an enormous lake upstream that has developed into a popular tourist area.

The power station is a joint project in which both countries collaborate on all levels, from the government to the technical staff working there. Eduardo Briosso, Substations Department Supervisor, from Uruguay, and his colleague Fernando Marull, Transmissions Manager, from Argentina, are among those who share the day-to-day responsibility for the running and maintenance of the power supplies.

It is with an obvious sense of pride that they talk about the plant and its importance for both countries. "Salto Grande's annual capacity is 6,700,000MWh," explains Fernando Marull. "There are 748 kilometres of lines in Argentina and 473 kilometres in Uruguay." "The power station houses transformers with a total capacity of 74,500kV, as well as 36 induction transformers."

"Since the plant is so important to power supplies we must have a high level of operational dependability and so we do a great deal of preventive maintenance," he continues. "Also, an increasing number of our transformers are approaching 25 years of age, which accentuates the need for preventive measures." An important link in this maintenance work is Salto Grande’s own modern laboratory, which has been amassing experience over the last 25 years.

"Here we can test everything from the oil in the transformers to the cellulose in the insulation. We can run all the samples and tests that we need, even Dissolved Gas Analysis." Based on numerous tests that have been performed over many years, Marull and Briosso, together with their colleagues, have built up a comprehensive bank of experience of transformer oils. They can therefore specify exactly what kind of oils they want in any new transformers purchased for the plant.

When Salto Grande needed a new 500kV transformer, the management of the plant, for the first time, approached a South American supplier, the Argentinean company Faraday in Buenos Aires. When asked "Why?" Briosso responds: "There are several answers to that question. Though probably the main one is that we wanted to stimulate development of a local supplier. Faraday had not manufactured many transformers of that size previously, and it is more usual to turn to producers with long experience. But obviously Faraday has met all the specifications and demands that we placed on the transformer."

Fernando Marull adds that the advantages for Salto Grande in collaborating with a local supplier are that they get a faster response in the case of problems and when obtaining spare parts and the like. With the experience Salto Grande has of their ageing transformers, those responsible for technical aspects stressed that the oil that the new transformer was to be filled with must be of the highest international quality.

"Faraday contacted us and asked whether it would be appropriate to use oil from Nynas," continues Marull. "And it was, since Nynas transformer oils meet, and even surpass, the

demands of IEC 60296 and IEC 60422. Moreover, Nynas meets the special specifications and demands that we place over and above the IEC’s."

"We expect Nynas oils to have a long life expectancy and a high oxidation stability. We are confident of this, as we have already had many years of good experience of Nynas' transformer oils since they are in many of the transformers already in our plant."

CARLO LASZLO

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News & Events

10/5/2012 New Marketing Director at Nynas – focuses on growth Dr. Valentina Serra Holm has been appointed Marketing Director for Nynas Naphthenics business.

8/31/2012 Efacec awarded Nynas as best supplier 2011 The largest Portuguese group in the field of electricity recognices Nynas

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This is what it looks like – the large 500 kV power transformer that Faraday delivered to Salto Grande. IMAGE: Carlo Laszlo

Faraday invests in a growing market – with the help of oils from NynasWhen transformer manufacturer Faraday decided to compete with big international rivals in the large power transformer market, they did it in style, winning a contract with Argentina's and Uruguay's biggest power supplier, Salto Grande.

Engineer Jorge Claudio Buonarcorso, factory manager at Faraday, explains: "Building a large power transformer for Salto Grande was a huge challenge with a highly successful outcome."

Faraday S.A.I.C. y F. was founded in 1957 and has its production centre in the capital of Argentina, Buenos Aires. Faraday is one of the leading South American transformer manufacturers, doing its own design, computation and construction of large power transformers with capacities up to 450 MVA and a rated voltage of up to 500 kV for three-phase constructions and up to 1200 MVA in single-phase constructions. One factor that has helped them achieve and maintain this position is that they have Argentina's biggest test laboratory. "We have experienced a large number of technologies," continues Buonarcorso. "We used to have a joint venture agreement with a large European company, and so we are thoroughly familiar with their technology. These days we produce transformers on our own behalf, without any joint venture agreement."

Most of Faraday's sales are to the major power companies – both domestic and foreign – that are active in Argentina. Up until recently the production focus was on transmission transformers, and Jorge Buonarcorso maintains that the company has about 75% of the Argentine market in that sector. But recently they took a step up to manufacturing 500 kV power transformers as well. And with the sale of one such transformer to Salto Grande it is hoped that the company has managed to successfully break into a new market. "Salto Grande bought our second 500 kV transformer," says engineer Ricardo Sirabonian, who joined the company in 1974 at the same time as the manufacture of power transformers started its development. Today he works as a technical adviser and is an information source on anything to do with transformers.

"They realised that we could offer the best value. Their decision was probably also based on a desire to support and endorse a local supplier. And, at the same time, they saw the advantages of being in close contact with our production processes, after sales service and so on." The fact that the demands Salto Grande places on transformers are way above international norms has not brought any problems at all, according to Buonarcorso. He explains why: "We have a reliable, tried-and-tested design from a company of high international repute. We also have considerable know-how and many years' experience of computation and construction. Another significant asset of our company is our team of constructors, which comprises both older experienced constructors and talented young engineers."

"Altogether, this gives our products a level of excellence that is highly comparable with international transformer manufacturers," sums up Buonarcorso. In order to hold its own in a global market, Faraday naturally complies with a series of international standards such as ISO,

DIN, IEC, ANSI and IEEE. Materials and equipment are primarily purchased from European manufacturers.

Nynas oils came to Faraday's attention about five years ago. "We now use Nynas oils in several of our customers' transformers, since we want to take advantage of the special characteristics these oils possess," explains Sirabonian. "In our opinion, Nynas naphthenic oils have excellent electrical properties which cater to the special electrical and physical requirements of Extra High Voltage transformers."

In addition to the transformer sold to Salto Grande, Faraday has supplied transformers with Nynas oils to the South American nuclear power plant Nuclear Eléctrica Argentina S.A. "Of course there are strong competitors to Nynas," he continues. "But there were several influential factors that made us choose Nynas as our supplier of transformer oils, apart from the fact that the company fully answers all our technical demands."

The most important reason, says Sirabonian, is Nynas' local presence. "Through General Manager Ana Cura we have always rapidly received the technical support we want. A further plus is that Nynas has a depot in Buenos Aires – which guarantees fast deliveries. We find Nynas is a flexible company, with an ability to listen to our requests, with fast lines of communication, and a willingness to share its knowledge and research, which suits us perfectly."

Both Ricardo Sirabonian and Jorge Buonarcorso feel that there is good reason for Faraday to view the future brightly. Buonarcorso explains: "The sale to Salto Grande is proof of the fact that we have managed to meet the toughest demands that are placed on an international market. In addition, energy needs are growing in Argentina and large parts of Latin America. This means new investments in power transmission, and I am convinced that we will be able to hold our own in this market, both now and in the future."

CARLO LASZLO

Flow characteristics of transformer oil depend on the viscosity. 

Viscosity of oil important in transformer coolingBack to Knowledge Tank 

Transformer oil has two roles to play in a transformer: act as an electrical insulator and cool the transformer. This article focuses on its role as cooling medium.

The cooling ability of a transformer oil affects the basic premises for the design of the transformer. With efficient cooling it can be much more compact since it can be produced with considerably fewer or smaller radiators. Re-filling a transformer with an oil that is an effective coolant can also have a positive effect on a transformer where sludge has resulted in deposits on the windings. Also, the ageing of the cellulose in the transformer is affected directly by temperature, which means that efficient cooling results in increased life expectancy.

Flow speed is difficult to calculate

When making a comparison of the cooling ability of various oils, they can be roughly divided into categories. The basic principle is a division into oils with normal cooling abilities and oils with extremely good cooling abilities. It is only really interesting to compare oils within the same category. We shall here limit ourselves to oils with extremely good cooling abilities – arctic grade oils. 

When oils in the same category are compared, the results show that a number of factors that influence cooling are constant. These are for example the heat capacity and thermal conductivity. If you compare properties in a given type of transformer design – for example one with natural oil flow (no pump) and natural air flow (no fan) – results show that it is the oil’s viscosity properties that determine its cooling ability. This fact enables us to derive the formula that describe flow speed and heat exchange factors.

The true flow speed in a transformer can actually be rather difficult to calculate, but with a given design one can assume that it is steered by viscosity at a given temperature. This has been demonstrated in many types of equipment and is considered to be a given fact in transformer design. Flow speed follows Newton’s and Bernoulli’s laws and can be calculated using the formula:

where:p = pressurev = kinematic viscosityl, d = dimensions of the channely = specific weightw = oil speed in the channelg = gravity constant

Another way of expressing the formula is:

If we merge all factors that are constant at a particular temperature to become a constant, f, then we get the following equation:

In other words, the speed is inversely proportional to viscosity, i.e. the lower the viscosity the higher the flow speed (1).

Heat exchange factor is important

The heat exchange factor plays an important role in the cooling of a transformer. It is actually a matter of heat exchange between the oil and the windings. This factor is highly affected by viscosity. The Reynolds number (Re) is the parameter that is used as a measure of the heat exchange factor in the transformer industry.

The Re value also shows whether the oil flow is laminar or turbulent. Laminar flow means that there is an even layer of oil along the boundary between windings and oil. In a turbulent flow this layer is disturbed and the oil is constantly mixed, so that new parts of the oil phase continuously come into contact with the windings. This means that cooling is much more effective with a turbulent flow.

The connection between Re, flow speed and viscosity can be represented thus:

The lower the viscosity, the higher the Re value. When Re is higher than 2300 there is a turbulent flow.

Transformer oils of high quality always have as low a viscosity as possible, with the proviso that flash point must still be maintained. The existing standards for transformer oils state that the viscosity should be at most 12 mm2/s at 40?C (2, 3). Transformer oils with a particularly good cooling ability have viscosities in the region of 7–8 mm2/s at 40?C.

Comparison in practice

A transformer manufacturer wanted to reduce the number of radiator banks from three to two in a 1500 kVA ONAN (Oil Natural Air Natural) transformer. The reason was that the company wanted to design a transformer that was smaller, needed fewer cooling radiators, needed less oil,

and which could function at low temperatures. They wanted to achieve this at the same time as making the transformer cheaper to manufacture.

Since there was a great deal to be gained by using such a design it was possible to consider using a more expensive oil with better cooling properties. In the study conducted, a catalytically dewaxed paraffinic oil was compared with a naphthenic arctic type oil.

A specific unit was selected for making a comparative study. The transformer was filled with the paraffinic oil, run through the test, emptied of the oil, filled with naphthenic oil and run through the same test again. The test was conducted in accordance with recommendations in standards for what are known as heat run tests (4, 5). The test was designed to determine the average increase in winding temperature. It should not increase more than 65°C above the temperature of the surroundings.

The transformer was first filled and put under vacuum. After vacuum treatment the oils were tested for water content and breakdown voltage (ASTM D 877, IEC 60156). After being prepared for the heat run, the transformer was loaded by simulating loading using the short circuit method for more than 24 hours with total losses (sum of non- load and load losses) to raise the temperature of the windings and the oil from the starting (ambient) temperature to the maximum operating temperature.

During that procedure, all relevant temperatures were measured and plotted. These are:

1. Top oil temperature (measured by a sensor in the transformer tank near to the oil surface)2. Oil temperature in the radiator – top and bottom3. Ambient cooling air temperature (as an average of 3 sensors each being a distance of approx. 

1.2 m from the transformer at about half the height of the transformer (approx. 1 m high))

The transformer was protected against air currents to avoid any disturbance of the measurements.

Before starting to load the transformer, the cold resistance of the windings was determined. After that, following the standard procedure, the transformer was loaded (heated) with total losses until the unit’s temperature did not rise (vary) by more that 1°C during 3 consecutive hours.

This was reached after 27 hours. Then, the load was reduced to rated current for 60 minutes and after that period, the load was switched off to determine the average winding temperature by the resistance method (hot resistance).

The average winding temperature is determined by the equation below from (4):T = R/R0 (Tk + T0 ) – Tk where:T = temperature (°C) corresponding to heat resistance R,T0 = temperature (°C) at which cold resistance R0 was measuredR0 = cold resistance (Ohm)R = heat resistance (Ohm)

Tk = 234.5 °C for copper (and 225.0 °C for aluminium)

Results and conclusions

The values obtained from the test corresponded well with calculated values. The study showed that both the oils gave similar results, which was expected since the oils had approximately the same low viscosity.

Since the temperature rise in both cases was at a good level, i.e. under 65?C, it was possible to draw the conclusion that the desired design with fewer radiators would function satisfactorily. 

It also showed that it was not necessary to use a very expensive catalytically dewaxed paraffinic oil in order to get sufficiently effective cooling. The naphthenic oil accomplished the task equally well. Since an important driving force behind the entire development work had been to develop a design that would be cheaper to manufacture, all that remained was for the buyer to compare the price of the oils.

But in many cases paraffinic transformer oils have a higher viscosity and moreover for reasons of their higher viscosity index, an increased viscosity at operating temperature of the transformer.

For this reason naphthenic oils have a real technical advantage.

In this example, good cooling properties were of particular importance, but one should remember that regardless of the design of the transformer, improved cooling always gives positive effects. A lower temperature always means a reduction in the thermal ageing of the cellulose and thereby a lengthening of the transformer’s life expectancy.

References

(1) Dubbel Taschenbuch für Maschinenbau Ed. 17, K.-H. Grote, J. Feldhusen (Eds.), Springer, Berlin (1990)

(2) IEC 60296: Specification for mineral insulating oil for transformers and switchgear, ed. 3. (2003)

(3) ASTM D3487: Standard specification for mineral insulating oil used in electrical apparatus

(4) IEEE C57.12.90-1999: IEEE standard test code for liquid-immersed distribution, power, and regulating transformers and IEEE guide for short-circuit testing of distribution and power transformers (1999)

(5) IEC 60076 Part 2: Power Transformers (1993)

  

Through regular sampling, DGA and Furan analysis can give a good indication of how fast the degradation of insulation in a transformer is progressing. By doing this one can avoid extreme situations like this.

Reducing the risk of insulation failureBack to Knowledge Tank

By monitoring the content of gases and furanic compounds in the insulation it is possible to reduce the risk of failures in transformers containing traditional paper and oil insulation systems. The gas content of the oil can provide an indication of the condition of the transformer. Whilst measuring furfural gives an indication of the condition of the cellulose insulation and can provide a basis for estimating the rate of degradation.

The combination of oil and cellulose as insulating material has been used for almost a century now. When combined they have a much greater electrical insulating capacity than either of the materials has individually. Together they have a dielectric strength, or insulating ability, of as much as 64 kV per mm, considerably more than the sum of the individual value for each material. In spite of the apparent lack of mechanical strength of the oil-cellulose combination, together they form a very effective barrier against electrical breakdown.

The problem with using these materials as insulation in electrical equipment is that both age and decompose over time. Ageing accelerates when they are exposed to high temperatures and moisture. Electrical equipment often also contains certain chemical substances that can accelerate the degradation of oil, among them copper, oxygen, paint and varnish.

To ensure safe and reliable transformer operation, ageing of the oil and cellulose needs to be carefully and regularly monitored using accurate and established methods. The results should provide a good basis for estimating the chemical status of the oil and the cellulose, and preferably also how this status is likely to develop over time and what maintenance measures might become necessary. A simplified flowchart of the methodology is given in Figure 1: A condition-based approach to maintenance methodology for transformers and other equipment of critical importance.

Figure 1

There are several methods that can be used in combination to paint a picture of the condition of the oil and cellulose. But let us take a closer look at the link between the chemical status of the oil and the degradation of the cellulose.

When oil is exposed to high temperatures and moisture, or some other catalyst, the oil oxidises. Subsequently substances such as acids and other polar substances are formed, which in turn accelerate the degradation of the cellulose. The degree of paper degradation can be estimated through analysis and monitoring of the gases dissolved in the oil over time. This is a long-established method, known as Dissolved Gas Analysis or DGA, that has proved to be effective in preventing failures.

The DGA method remains the best method for detecting various abnormalities in transformers to date. The procedures for taking samples of the oil and extracting the gas for this analysis are well laid out in the standard IEC60567. Unfortunately the interpretation of the data obtained is not equally clear cut, and different ways of doing this may have the same objective but could lead to different conclusions. One of the most common methods for interpreting DGA measurements is to study the gas concentration ratio. This is illustrated in Figure2: When a transformer has been diagnosed with an abnormality through DGA analysis, the Duval Triangle is one of the most

efficient tools to identify what type of fault condition you may experience. This is very important in order to decide upon further maintenance or repair measures.

Figure 2

DGA is relatively easy and cheap to perform, and has proven to be an efficient way of reducing the number of transformer accidents but it does not give a complete picture of the condition of a transformer. However, it can be combined with other methods that measure the degradation of the cellulose.

Cellulose is made up of long chains of glucose units. The strength of the cellulose is dependent on how long these chains are. The length of the chain is usually measured in number of glucose units, and is expressed as degree of polymerization, DP. When many products of degradation from the oil are present, the a-1.4 glucosidic bonds between the glucose units are gradually broken down resulting in the cellulose chains becoming shorter and shorter which leads to a reduced DP value.

In new cellulose insulation made of kraft paper the DP value is about 1400. In combination with oil the DP value for new electrical insulating cellulose is usually about 1000, which gradually drops with age down to about 200. At the same time the tensile strength of the cellulose also deteriorates, which gradually increases the risk of faults in the insulation and thereby the risk of breakdown.

The degree cellulose degradation can be determined by measuring the content of furfural in the oil. Furfural is an aromatic aldehyde with five carbon atoms that is a product of degradation. The correlation between furfural content and the DP value is not perfect, but it is sufficiently good to give an indication of how the strength of the cellulose is deteriorating. It should be noted that depending on the refining technique some oils might contain traces of 2furfural from the refining process. Although 2furfural has no known negative effect on the performance of oil in-service at

these concentrations, it is important to measure furfural content of the oil from the outset and before energisation of the equipment. This enables a baseline to be established against which increases in 2furfural concentrations can be monitored in the future.

Combining analysis of the gas content and ratios (DGA) with analysis of furanic compounds in the insulating oil provides a very good basis from which to take measures to reduce the risk of breakdowns. The analysis can give information about whether there is an immediate risk of problems, and if this is the case, further electrical investigations can indicate where the damage is located. Of decisive importance for the effectiveness of such analyses is how often measurements are made and the efficiency of routines that are in place to remedy any problems that might occur.

Measuring peroxides in oil, the clean, clear and easy wayBack to Knowledge Tank

A new method for the determination of peroxides in oil developed by researchers at Nynas promises to become a useful tool for both product development and analysis. Able to detect the very first signs of oxidative ageing, the INPOX method is versatile, accurate, clean, and easy to use.

At Nynas we continually perform research into how our products behave in application and investigate new ways to analyse product performance. Resistance to oxidation is important in many applications of specialty oils and so we have invested a large amount of effort in

developing tools to study and analyse oxidation phenomena.

One such tool is the newly developed INPOX method for the determination of peroxide levels in oil. This method has many advantages over other methods aimed at judging the level of oxidation. INPOX is especially attractive as it is designed to detect the very first oxidation products: peroxides.

Peroxides play a central role in the degradation of oil. Like all organic matter, oil is constantly under threat of degradation by peroxidation, i.e. autooxidation by atmospheric oxygen brought about by radical chain reactions of organic peroxides. It is important to control and prevent this destructive effect in both the technical applications of hydrocarbon liquids and in food preparations containing lipids of biological origin.

The process of hydrocarbon autooxidation (see Figure 1) starts with thermal generation of a hydrocarbon radical, which quickly reacts with available oxygen to form a peroxy radical. This radical in turn abstracts a hydrogen from another hydrocarbon molecule (generating another radical) to form a hydroperoxide. This is the core of the autooxidation chain reaction and it is only limited by the availability of oxygen once it has started.

Hydroperoxides are the initial semi-stable products of hydrocarbon autooxidation. The radical chain reactions of hydroperoxides and peroxides that inevitably follow ultimately lead to familiar oxygenated hydrocarbon derivatives such as alcohols, keto-compounds and carboxylic acid derivatives (and carbon dioxide, CO2) via radical terminations and subsequent radical and/or non-radical pathways.

To date oil oxidation stability and the effects of antioxidants have generally only been judged against one of the ultimate products of hydrocarbon oxidation, i.e. carboxylic acids. The reason for this is the ease with which one can determine acidity by acid/base titration. However, for many applications it is of potential interest to determine the very earliest signs of oxidation, i.e. peroxides or, more specifically, hydroperoxides.

There are a variety of redox (titration or voltametry) methodologies (see Figure 2) for the determination of hydroperoxides that have been applied for vegetable oils, biofuels and jet fuel as well as transformer oil. In the most established method (ASTM D3703-99) hydroperoxides are allowed to oxidize iodide ions to iodine, which is then determined by colorimetric titration with thiosulfate using starch as the indicator. However, from an environmental point of view, this method is problematic as it requires the use of large volumes of Freon (originally) or other halogenated solvents. It is also very labour intensive and requires prior knowledge of the expected amount of peroxides in the sample.

The technique we have developed involves adding a reagent selective for hydroperoxides to the oil sample. The peroxide level can then be determined by a modern gas chromatography-mass spectrometry (GC-MS)-based determination method. This method is applicable for all light mineral oils and vegetable oils used for electrical insulation (transformer oil), but can equally well be applied to liquid fuels or lubricating oils.

The dynamic range of the method is so wide that only one concentration of the reagent solution is necessary to cover all the ranges of peroxide content that are of interest - from new to very severely aged oils. The latter is also the main reason for utilization of GC-MS (nowadays a standard instrument) as Single Ion Monitoring (SIM) enables good detection and quantification, even in the very complex matrixes of aged mineral oils.

Furthermore, the method is based on the internal standard (IS) technique, as this provides much better reproducibility over time. Our method can also simultaneously determine the oxidation inhibitor content of the oil, which is why we have named it INPOX.

Compared to other analysis methods routinely used to judge oxidation in transformer oil, the INPOX method has clear advantages. The normal methods of analysis of transformer oil result in values for Total Acid Number (TAN), Interfacial Tension (IFT) and Dielectric Dissipation Factor (DDF). Of these only TAN values are unambiguously linked to oil oxidation. Low IFT and high DDF values can be caused by the effects not only of oxidation products but also of contaminants from other sources. Oil samples with intermediate to high TAN values can have very different IFT and DDF values.

The problem with TAN is that it only provides a measurement of oxidation that has already happened. On the other hand, peroxides indicate ongoing oxidation, as they are the very first oxidation products.

Early indication of oxidation is important, not least because it is likely that peroxides, like acids, can damage the solid insulation as well as contribute to copper corrosion/ dissolution.

From our studies on used transformer oil, peroxide levels appear to be reasonably stable in oil samples stored at room temperature (in the dark). If oil samples (even inhibited grades) are stored in glass bottles openly in the lab, the peroxide content can become very high. Sampling and storage routines are therefore important.

In general, oils that showed significant TAN values had low peroxide content, showing that oxidation had already happened. In full accordance with how inhibitors function, there were no significant peroxide levels in inhibited oil as long as there was some inhibitor left. For uninhibited grades, the peroxide level clearly shows when the oils have exhausted the natural inhibitors, something which is not possible to measure any other way.

The INPOX method also measures levels of dibutyl para-cresol (DBPC) through direct detection of the molecules themselves. In contrast, the generally used infrared (IR) method measures only an absorption, which in principle can arise from other molecules. In general the IR method appears to work just as well as the more reliable INPOX method for oil samples with low TAN values. However, at higher TAN values there is evidence of disturbance from oil oxidation products in the IR method.

The INPOX method will be a useful tool both in the development of new products in cooperation with our customers, and in analysis of field samples of oils already in service. We also believe

that this versatile method can be utilized to further the scientific and technical knowledge of how hydrocarbon fluids age under different conditions.