Controlled by four giants

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Sun and wind don’t send a bill? Sure they do: If more and more renew­able ener­gy flows into our pow­er grids, there will also be more pow­er fluc­tu­a­tions due to the weath­er. To pro­tect line sec­tions from over­loads and bot­tle­necks, expen­sive redis­patch mea­sures — inter­ven­tions in the pre-planned gen­er­a­tion out­put of pow­er plants — are then nec­es­sary. These have to be request­ed by the respec­tive trans­mis­sion sys­tem oper­a­tor. Accord­ing to the BDEW (Ger­man Asso­ci­a­tion of Ener­gy and Water Indus­tries), this cre­at­ed costs of around 590 mil­lion euros in Ger­many in 2021. In 2020, the fig­ure was 240 mil­lion euros — anoth­er exam­ple show­ing that the ener­gy tran­si­tion also means more expense for the con­ver­sion and expan­sion of the elec­tric­i­ty net­works.

Ten­neT TSO, one of the four largest trans­mis­sion sys­tem oper­a­tors in Ger­many, could cer­tain­ly sing a song about this. That’s why the Bayreuth-based com­pa­ny is set­ting the course for the future and invest­ing in all areas. Among oth­er things, they are rely­ing on pow­er­ful phase-shift­ing trans­form­ers (PSTs) to make partic­ularly stressed lines fit for the increas­ing feed-in of green elec­tric­i­ty. The four cur­rent­ly largest weigh 940 tons each, are each the size of a bun­ga­low and are locat­ed at the Wür­gau sub­sta­tion in the Bam­berg dis­trict. From the end of 2022, they will reg­u­late two impor­tant north-south pow­er cir­cuits in the Bavar­i­an regions of Upper and Mid­dle Fran­co­nia and help to bet­ter dis­trib­ute wind pow­er from the Ger­man north to the south.

PSTs shift the over­load from a pow­er line in a way that makes bet­ter use of the over­all grid. How is this done? If you think of the trans­mis­sion grid as a net­work of water pipes with many inflows and out­flows, elec­tric­i­ty, like water, always seeks the path of least resis­tance. If ener­gy fed into a cir­cuit becomes con­gest­ed or there is a bot­tle­neck, phase shifters con­nect­ed in par­al­lel can open or reg­u­late the cor­re­spond­ing inflows or out­flows upstream and down­stream like valves, so that the cur­rent takes the desired path.

HELPS THE ENERGY TRANSITION

In this way, load flows in the grid can be opti­mal­ly con­trolled. This leads to an over­all high­er trans­mis­sion capac­i­ty in the extra-high volt­age grid, empha­sizes Dr. Flo­ri­an Mar­tin, Head of Asset Tech­nol­o­gy at Ten­neT, and explains fur­ther: “Cost­ly redis­patch mea­sures are no longer nec­es­sary. In view of the cur­rent ener­gy cri­sis, this can even lead to the sit­u­a­tion that the grid can con­tinue to be oper­at­ed in a sta­ble man­ner, even if redis­patch pow­er plants required for this should no longer be avail­able.” There­fore, in addi­tion to oth­er mea­sures, more and larg­er phase-shift­ing trans­form­ers are need­ed to counter the more volatile feed­ers.

The Wür­gau phase shifters demon­strate the cut­ting-edge tech­ni­cal require­ments involved. One of these colos­si con­sists of two units, so-called series, and exci­ta­tion trans­form­ers. Each dou­ble unit has a through­put of 1,420 mega­volt amperes (MVA), mak­ing a total of 2,840 MVA.

Powerful bouncer

There are good rea­sons for this enor­mous pow­er: While the entire load flow of a line pass­es through the trans­former, it is the on-load tap-chang­er inside that allows the load flow to be braked and accel­er­at­ed in the first place. As a key func­tion­al PST ele­ment, it con­trols the high pow­ers. Four on-load tap-chang­ers are installed in the Wür­gauer phase shifters for each trans­former bank. The demands on per­for­mance and reli­a­bil­i­ty are extreme here.

That´s why Ten­neT turned to the com­pa­ny that invent­ed the on-load tap-chang­er and con­tin­ues to opti­mize it: “Togeth­er with MR, we devel­oped a cus­tomer-spe­cif­ic solu­tion from their port­fo­lio based on our tech­ni­cal spec­i­fi­ca­tions, test­ed it and put it into oper­a­tion,” says Flo­ri­an Mar­tin. Com­plete­ly planned and built, the trans­port of the PST colos­si was the last major hur­dle before their instal­la­tion and com­mis­sion­ing. In the sum­mer of 2021, eight heavy-duty trans­ports brought the indi­vid­ual PST boil­ers, each weigh­ing up to 326 tons, to the Wür­gauer Berg in two days via rail, ship and road (see info box).

Ten­neT spent two years prepar­ing the sub­sta­tion, which was mod­ern­ized in 2015, and invest­ed 70 mil­lion euros in the PST sys­tem alone. When the trans­form­ers go into oper­a­tion, the project will have tak­en four years to set up. A huge effort, but well worth it, because phase shifters help to bring the ener­gy tran­si­tion into the grid in a future-proof way. “And by sav­ing on redis­patch costs, phase-shift­ing trans­form­ers pay for them­selves in a short time, usu­al­ly after just one year or less,” assures MR expert Jür­gen Koll­manns­berg­er (see inter­view in next arti­cle).

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