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High-volt­age direct cur­rent (HVDC) trans­mis­sion makes it pos­si­ble to trans­port large amounts of ener­gy – espe­cial­ly from renew­able ener­gy sources – over long dis­tances to con­sumers with vir­tu­al­ly no loss­es. To do this, con­vert­er sta­tions con­vert the incom­ing AC volt­age into DC volt­age at the begin­ning of the HVDC lines and back into AC volt­age at the end. For many years, grid oper­a­tors relied on line-com­mu­tat­ed con­vert­ers (LCC) which work with thyris­tors that are depen­dent on the sup­ply volt­age of the AC grid for fault-free oper­a­tion. This is where a reg­u­lat­ed trans­former comes into play, which con­nects the con­vert­er sta­tion to the AC grid. Here, a tap-chang­er reg­u­lates the input volt­age pre­cise­ly in dis­crete steps to ensure that the thyris­tors switch in the opti­mum range and that the oper­a­tion of the con­vert­er remains sta­ble.

New converter technology

How­ev­er, with the intro­duc­tion of con­vert­ers using volt­age source con­vert­er (VSC) tech­nol­o­gy, the sig­nif­i­cance of on-load tap-chang­ers in HVDC sys­tems is chang­ing: this con­vert­er tech­nol­o­gy uses IGBTs, i.e. semi­con­duc­tor ele­ments that can switch inde­pen­dent­ly and flex­i­bly. This means that VSC con­vert­ers can con­trol volt­age and cur­rent inde­pen­dent­ly of the sup­ply volt­age of the AC grid and react to fluc­tu­a­tions in the pow­er grid even with­out a reg­u­lat­ed trans­former. But does this mean the end of on-load tap-chang­ers in mod­ern HVDC sys­tems?

Better switching

The answer is no. This is the con­clu­sion of an inde­pen­dent study by TU Ilme­nau, which looked at the char­ac­ter­is­tics of mod­ern HVDC sys­tems. The study shows that HVDC sys­tems with VSC sys­tems have high­er loss­es when they are oper­at­ed with­out a tap-chang­er in the trans­former. The rea­son for this is the cur­rent flow through the semi­con­duc­tor ele­ments, which is often high­er than nec­es­sary with­out tar­get­ed volt­age reg­u­la­tion. The result: more heat loss and less usable ener­gy. This is pre­cise­ly where the on-load tap-chang­er comes in. It increas­es the volt­age applied to the invert­er based on the respec­tive oper­at­ing sit­u­a­tion and in com­pli­ance with the per­mis­si­ble sys­tem lim­its, and does so in such a way that the cur­rent flow decreas­es while the pow­er remains the same.

This sig­nif­i­cant­ly reduces loss­es and increas­es the ener­gy yield. This pays off, espe­cial­ly for long trans­mis­sion dis­tances and large amounts of ener­gy: More green ener­gy reach­es con­sumers, and oper­a­tion becomes more effi­cient and eco­nom­i­cal. In addi­tion, the low­er ther­mal load reduces wear and extends the ser­vice life of the invert­er com­po­nents. And while these have to be replaced after around 15 years, on-load tap-chang­ers last up to 60 years – an invest­ment that pays off both eco­log­i­cal­ly and eco­nom­i­cal­ly for grid oper­a­tors in the long term.

Con­clu­sion: Tap-chang­ers will remain a key tech­nol­o­gy for HVDC sys­tems in the future because there is cur­rent­ly no equiv­a­lent tech­ni­cal solu­tion for pre­cise volt­age reg­u­la­tion dur­ing oper­a­tion with max­i­mum reli­a­bil­i­ty and max­i­mum ser­vice life. This is why Maschi­nen­fab­rik Rein­hausen will con­tin­ue to focus on the fur­ther devel­op­ment of on-load tap-chang­ers for HVDC sys­tems in the future – with mod­ern con­trol con­cepts, dig­i­tal net­work­ing and the tar­get­ed use of arti­fi­cial intel­li­gence.

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“On-load tap-changers simplify operational management”

Three questions for Prof. Dirk Westermann, who led the study at TU Ilmenau.

What general trends do you see for HVDC applications?

Sev­er­al trends can cur­rent­ly be observed. More HVDC sys­tems are need­ed because the expan­sion of renew­able ener­gies is increas­ing, and glob­al elec­tric­i­ty trad­ing is also grow­ing. In addi­tion, there are con­tin­u­ous tech­no­log­i­cal advances that are lead­ing to more effi­cient and cost-effec­tive HVDC sys­tems, such as self-com­mu­tat­ed mod­u­lar mul­ti­level con­vert­ers in half- or full-bridge tech­nol­o­gy.

Will we even need on-load tap-changers in HVDC systems in the future?

Def­i­nite­ly. Although self-con­trolled invert­ers can take over a cer­tain volt­age reg­u­la­tion func­tion, tap-chang­ers give you the chance to imple­ment improved con­trol strate­gies and min­i­mize loss­es.

Don’t HVDC systems become more vulnerable with additional components such as on-load tap-changers?

No, because on-load tap-chang­ers have been tried and test­ed for decades and have a long ser­vice life and very low sus­cep­ti­bil­i­ty to faults, espe­cial­ly with estab­lished man­u­fac­tur­ers. In addi­tion, on-load tap-chang­ers do not increase the con­trol effort in the sys­tem, but rather sim­pli­fy oper­a­tional man­age­ment.