It's a bright and sunny Monday in April in Regensburg. Service technicians Christoph Friedl and Andreas Singerer meet at MR to load their vehicle, because they have a job to do. This time they are traveling to the Czech Republic to maintain the three tap changers of a phase-shifting transformer. They were contracted by ČEPS, the Czech Republic's only transmission system operator and thus responsible for keeping the lights on in the country.
Top service for a long transformer life
Around 300 MR service technicians are on call worldwide. An order from the transmission grid operator ČEPS in the Czech Republic shows how regular tap-changer maintenance can support the secure energy supply of an entire country.
Czech Republic's central network node
The Czech energy grid is highly dependent on transit. A large proportion of the energy imported and later exported comes from Germany and enters the country via the Hradec substation near Kadaň, which is operated by ČEPS. It is one of the largest and most important substations in the Czech Republic. The four phase-shifting transformers located there, with a capacity of 850 MVA, are of particular importance. When there is a high level of wind energy production in the neighboring country, there are sometimes major voltage fluctuations in the cross-border electricity transmission. The phase shifters compensate for these fluctuations by shifting the overload in such a way that the grid as a whole is better utilized, which leads to a higher transmission capacity in the extra-high voltage grid. This also makes expensive redispatch measures unnecessary. The phase shifters in Hradec thus act as a buffer against fluctuating volumes of German wind power, which has to pass through these transformers – and thus also through the on-load tap-changers from MR.

„Working directly with MR ensures that we get the best possible service quality for our systems and the availability of original spare parts.“
Jan Lála, Head of Maintenance Region West at CEPS
When Christoph and Andreas arrive at ČEPS in Hradec, the transformer to be serviced is no longer energized. After draining the insulating oil, the ČEPS employees use a crane to lift the three tap changers, blackened by the oil, out of the transformer. Now the two MR service technicians can take the tap changers to the substation workshop and get to work.
Andreas and Christoph are two of 300 service technicians working for MR worldwide. As head of Global Execution & Product Services, Uwe Seltsam is responsible, among other things, for ensuring that their training is a success and that service work is carried out to the highest quality standards. “Our service calls can basically be divided into two areas. On the one hand, there are those that involve unplanned and very time-critical cases that require quick action. In such cases, our specialists set out immediately and usually arrive at the customer's site within 24 hours.
Often, the problem is so serious that the transformer is already damaged,” says Uwe Seltsam. If production comes to a halt in such a case, it costs a lot of money with every hour that passes. On the other hand, our more frequent deployments involve “routine maintenance” as in Hradec. Seltsam emphasizes: “The on-load tap-changer is very robust. If it is maintained as we recommend, it can even outlive its transformer. But for maintenance, a high level of system knowledge and experience is required.”

„Our technicians are extensively trained, and deliver MR quality around the world.“
Uwe Seltsam, Department Manager
Measure, clean, check
In the workshop, the service technicians now go through all the necessary steps with practiced ease. Christoph knows the tap changers like the back of his hand. “Our products are designed for maximum efficiency. There isn't a single screw too many, and every small component has a very specific function. If you don't know what that is, you can't take care of the part.”

The tap changers get a good soaking in the oil pan. Christoph starts with a visual check here and looks for any signs of defects.

On the open tap changer, they meticulously put all components through their paces.

Cleaned, tested and serviced, the on-load tap-changers are returned to the transformer. The insulating oil is then reintroduced and test switching operations are carried out.
A workshop like a transformer plant
The workshop in Hradec is as well equipped as a transformer plant and even has a large oil pan in which the 100-kilogram diverter switch inserts can drip dry. Once everything has been measured, tested, cleaned and serviced, the diverter switch inserts are reassembled. A tap-changer service normally takes eight to ten hours, so there is not much leeway for delays. After three days, the service at ČEPS is complete: the tap changers are reinstalled by crane, the oil is refilled, and test switching operations are carried out. The service technicians then officially hand the transformer back to the operator together with their service report.
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