Giants in the grid

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Fields, mead­ows and pas­tures – the region around the Low­er Sax­ony town of Lin­gen in north­west Ger­many, not far from the Dutch bor­der, is rur­al and com­par­a­tive­ly sparse­ly pop­u­lat­ed. And yet the amount of elec­tric­i­ty mov­ing through here is already enough to sup­ply mil­lions of peo­ple with ener­gy. Even more elec­tric­i­ty will soon be flow­ing through the area: The trans­mis­sion sys­tem oper­a­tor Ampri­on will expand the Haneken­fähr sub­sta­tion and switch­ing sta­tion a few kilo­me­ters south of Lin­gen into Germany’s largest grid node by 2028 and is invest­ing 200 mil­lion euros in this mega project. 

The dimen­sions of the plant are cor­re­spond­ing­ly huge: it is 24 hectares in size, an area in which St. Peter’s Basil­i­ca in Rome would fit approx­i­mate­ly 15 times. The sub­sta­tion is locat­ed on an already very busy north-south cor­ri­dor in Amprion’s elec­tric­i­ty trans­mis­sion grid. Ener­gy from nine off­shore wind tur­bines ends up in the sub­sta­tion, which sends it on to indus­tri­al cen­ters in the south. In a few years, the new BorWin4 and DolWin4 con­nec­tions will add anoth­er 1.8 gigawatts of off­shore wind ener­gy from the North Sea. The site is there­fore mak­ing a sig­nif­i­cant con­tri­bu­tion to the suc­cess of the ener­gy tran­si­tion in Ger­many. 

Ampri­on installed two phase-shift­ing trans­form­ers in Haneken­fähr in order to con­trol the ever-increas­ing ener­gy flows and increase the uti­liza­tion of the grid. The equip­ment enables the con­trol of pow­er flows and helps to avoid over­loads in the grid, which are becom­ing increas­ing­ly com­mon due to the feed-in of volatile ener­gy gen­er­a­tors. 

Their appear­ance alone shows just how big their task is: each one mea­sures 30 × 26 × 11 meters and weighs an incred­i­ble 905 tons. The tech­ni­cal data is no less impres­sive: 400 kV rat­ed volt­age, 2,500 MVA max­i­mum through­put capac­i­ty and 3,600 A max­i­mum con­tin­u­ous cur­rent. The world has nev­er seen more pow­er­ful phase-shift­ing trans­form­ers. 

Dr. Daniel Eich­hoff, Head of Sta­tion Tech­nol­o­gy and HVDC (High Volt­age Direct Cur­rent Trans­mis­sion) at Ampri­on, is respon­si­ble for the pro­cure­ment of spe­cial equip­ment such as phase-shift­ing trans­form­ers. While he is used to man­ag­ing large infra­struc­ture projects, he is also impressed by the dimen­sions: “What par­tic­u­lar­ly fas­ci­nates me is the uti­liza­tion of the tech­ni­cal­ly pos­si­ble lim­it range: these pow­er sizes in a two-tank design are an absolute nov­el­ty.” The fact that the phase-shift­ing trans­form­ers could be built in this way at all is not a mat­ter of course. “With­out on-load tap-chang­ers from MR, the trans­former could not have been real­ized the way we want­ed it to be.” 

Maximum availability   

Since Ampri­on want­ed high avail­abil­i­ty and low main­te­nance for the on-load tap-chang­ers, it was clear from the out­set that low-main­te­nance vac­u­um tech­nol­o­gy should be used. Dr. Eich­hoff explains: “The phase-shift­ing trans­form­ers are housed in sound­proof enclo­sures. In order to remove the on-load tap-chang­er insert for main­te­nance, the roof would first have to be removed, which is time-con­sum­ing. How­ev­er, low main­te­nance costs are very impor­tant to us.” But not only because main­te­nance alone is very expen­sive, but also because any down­time can cost a lot of mon­ey. This is because a cen­tral func­tion of the phase shifter is to avoid so-called redis­patch costs, which are incurred when­ev­er less elec­tric­i­ty can be trans­mit­ted in the grid than demand­ed by the mar­ket between gen­er­a­tors or con­sumers. 

“We can trans­port much more elec­tric­i­ty with the phase shifter than in a pure­ly pas­sive grid and there­fore only reach the area where we have to inter­vene with redis­patch mea­sures much lat­er.” Since this saves Ampri­on around €36 mil­lion per year, every dis­rup­tion of the phase-shift­ing trans­form­ers is asso­ci­at­ed with enor­mous loss­es. The only prob­lem was that at the time of the inquiry to MR, an on-load tap-chang­er in this per­for­mance class did not yet exist. Pre­vi­ous­ly, vac­u­um tech­nol­o­gy was lim­it­ed to a rat­ed cur­rent of 2,600 amperes. How­ev­er, this was nowhere near enough to achieve the large phase angle that Ampri­on want­ed: “Our grid sim­u­la­tions showed that we need­ed an adjust­ment angle of 20 degrees in order to achieve a cor­re­spond­ing effect on the load flow dis­tri­b­u­tion in the grid.” 

The engi­neers at MR there­fore had to push the lim­its and devel­op a new on-load tap-chang­er. This was no easy task, as the instal­la­tion space could not be sig­nif­i­cant­ly larg­er despite the high out­put. But the engi­neers at MR achieved a mas­ter­piece: The VACUTAP® VRL® can even top the required rat­ed cur­rent of almost 3,000 A by 200 A and, thanks to the high switch­ing capac­i­ty of up to 10,000 kVA, it is pos­si­ble to real­ize a high phase angle. 

Without forced current splitting 

But these are not the only high­lights: Despite their enor­mous dimen­sions, the phase-shift­ing trans­form­ers are still com­par­a­tive­ly com­pact, which is pos­si­ble because the VACUTAP® VRL® does not require forced cur­rent split­ting. As a result, the wind­ings are less com­plex and addi­tion­al con­nec­tions between the series trans­former and the exci­ta­tion trans­former, which make up such a phase-shift­ing trans­former, can be omit­ted.  

An impor­tant side effect: few­er con­nec­tions also mean few­er poten­tial weak points, which also has a pos­i­tive effect on the avail­abil­i­ty of the phase-shift­ing trans­form­ers. Breuer sum­ma­rizes the advan­tages as fol­lows: “The VACUTAP® VRL® enables a low­er oil vol­ume, low­er trans­port weight, low­er main­te­nance require­ments and there­fore opti­mized oper­at­ing costs for both oper­a­tors and trans­former man­u­fac­tur­ers.” For Dr. Eich­hoff, one thing is cer­tain: “The VACUTAP® VRL® is an enabler for the entire tech­ni­cal solu­tion.” 

High-performance transformers are the future 

The giants at the sub­sta­tion in Haneken­fähr will not be the only ones for Ampri­on: “We are cur­rent­ly plan­ning a total of five sta­tions, each with two phase-shift­ing trans­form­ers, to be con­nect­ed to the grid by 2030,” says Eich­hoff. And the VACUTAP® VRL® will switch in all of them. The trend towards ever larg­er trans­form­ers is not just a phe­nom­e­non in Amprion’s trans­mis­sion grid. All over the world, more and more ener­gy has to be moved as elec­tri­fi­ca­tion con­tin­ues to advance. In order to trans­port high­er cur­rents over long dis­tances, con­trol ener­gy flows, com­pen­sate for reac­tive pow­er or sup­ply ener­gy-inten­sive indus­tries, ever more pow­er­ful equip­ment with cor­re­spond­ing­ly strong on-load tap-chang­ers is required.