Filters for the high-frequency range

The air is clear, the sky is blue, and a soft breeze is blow­ing across the fields of the Pala­tine munic­i­pal­i­ty of Offen­bach an der Que­ich. This breeze is enough to set in motion the huge rotor blades of the six wind tur­bines oper­at­ed by region­al ener­gy sup­pli­er EnergieSüdp­falz GmbH & Co. KG. They rotate cease­less­ly – and every rota­tion pro­duces envi­ron­men­tal­ly friend­ly pow­er. Each year, this amounts to a total of 40 mil­lion kilo­watt-hours for 13,800 house­holds, and the wind farm has a total out­put of 15.8 megawatts. There are thou­sands of pow­er plants just like this one across the globe. But a small con­crete struc­ture at the foot of the over one-hun­dred-meter-high wind tur­bines turns the Offen­bach wind farm into some­thing spe­cial: The lit­tle build­ing con­tains the first high-fre­quen­cy fil­ter sys­tem from MR, which ensures dis­rup­tion-free grids at fre­quen­cies up to 9 kHz.

The backstory

This is nec­es­sary because, although pow­er from renew­able ener­gy sources is envi­ron­men­tal­ly friend­ly, in con­trast to con­ven­tion­al pow­er sta­tions, it is dif­fi­cult to keep the grid fre­quen­cy in wind pow­er plants at a sta­ble lev­el. The rotors turn faster or slow­er depend­ing on the strength of the wind, caus­ing the amount of pow­er gen­er­at­ed to vary. In order to bal­ance out these fluc­tu­a­tions, rec­ti­fi­er trans­form­ers with what are known as IGBTs (insu­lat­ed gate bipo­lar tran­sis­tors) are brought into play in mod­ern wind pow­er plants. The prob­lem here, how­ev­er, is that this tech­nol­o­gy caus­es har­mon­ics that can be dam­ag­ing to elec­tric­i­ty grids.

A wind farm oper­a­tor is there­fore only allowed to sup­ply its pow­er to the grid if cer­tain lim­it val­ues are not exceed­ed. This was exact­ly the prob­lem that EnergieSüdp­falz and project devel­op­er juwi Wind GmbH were faced with: The elec­tric­i­ty gen­er­at­ed by the wind tur­bines exceed­ed the lim­it val­ues for har­mon­ics at fre­quen­cies up to 9 kHz. That being the case, the two part­ners turned to the Pow­er Qual­i­ty experts at MR for advice. “Har­mon­ics per se are not unusu­al and can be dealt with using pas­sive fil­ter sys­tems. We already have many years of expe­ri­ence in this field. But nor­mal­ly we would be deal­ing with fre­quen­cy ranges up to 2.5 kHz,” explains Dr. Thomas Schlegel, Head of Engi­neer­ing in the Pow­er Qual­i­ty depart­ment.

The filter concept

Schlegel and his team accept­ed the chal­lenge and start­ed on the devel­op­ment of a suit­able fil­ter con­cept. The aim was to drop the har­mon­ic lev­el in the fre­quen­cy range up to 9 kHz over a broad spec­trum, while also sig­nif­i­cant­ly reduc­ing indi­vid­ual har­mon­ic fre­quen­cies. The experts start­ed by tak­ing com­pre­hen­sive mea­sure­ments at the wind farm before com­bin­ing them with a math­e­mat­i­cal sim­u­la­tion. “We can only come to an opti­mum solu­tion if we link our mod­els to real mea­sured val­ues,” states Schlegel. A pas­sive har­mon­ic fil­ter sys­tem is always made up of the same com­po­nents. These include reac­tors, capac­i­tors, cir­cuit break­ers, and volt­age trans­form­ers.

“We aim to pro­vide our cus­tomers with com­pre­hen­sive solu­tion con­cepts. That is why we over­saw the project from the ini­tial con­cept all the way through to com­mis­sion­ing.” Dr. Thomas Schlegel
Head of Engi­neer­ing in the Pow­er Qual­i­ty depart­ment 

“The key was to match these com­po­nents with each oth­er so that they would actu­al­ly fil­ter out the har­mon­ics to a suf­fi­cient degree in the high-fre­quen­cy range. In the end, every fil­ter cir­cuit sys­tem is dif­fer­ent, as it is always pre­cise­ly tai­lored to the con­di­tions on site. The grid struc­ture plays a role here – but so too do many oth­er para­me­ters.” The solu­tion turned out to be a con­cept involv­ing three fil­ter cir­cuits with tun­ing fre­quen­cies of 189/3800/5600 Hz and a total out­put of 2800 kvar. The Pow­er Qual­i­ty experts also com­plet­ed the instal­la­tion on site. “We aim to pro­vide our cus­tomers with com­pre­hen­sive solu­tion con­cepts. That is why we over­saw the project from the ini­tial con­cept all the way through to com­mis­sion­ing,” says Schlegel. Con­for­mi­ty tests con­firmed the per­for­mance of the sys­tem in oper­a­tion. As a result, 13,800 house­holds have been receiv­ing clean elec­tric­i­ty from clean wind pow­er since April 2017.

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Your contact

Do you have any ques­tions about high-fre­quen­cy fil­ters?
Dr. Thomas Schlegel is hap­py to assist you:
t.schlegel@reinhausen.com