You Can´t Change The Weather

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About one-quar­ter of elec­tric­i­ty gen­er­at­ed around the world now comes from renew­able ener­gy sources. This was the find­ing of the Glob­al Sta­tus Report 2017 from the Renew­able Ener­gy Pol­i­cy Net­work for the 21st Cen­tu­ry, which also cal­cu­lat­ed that the total out­put of renew­able ener­gy sources in 2016 amount­ed to more than 2,017 gigawatts (GW) of elec­tric­i­ty. That’s over 20 per­cent of glob­al total ener­gy con­sump­tion. And this fig­ure is grow­ing.

In inter­na­tion­al com­par­isons, Ger­many is far ahead in the areas of wind and solar ener­gy, although its mar­ket is in a state of flux. Mean­while, many Asian coun­tries and nations bor­der­ing the Per­sian Gulf and the Sahara are start­ing to fol­low its lead. Tai­wan and South Korea, for exam­ple, have decid­ed to phase out nuclear ener­gy and instead want to expand their wind and solar out­put. Coun­tries which have pre­vi­ous­ly relied on oil are start­ing to dis­cov­er their nat­ur­al ener­gy sources as well.

After the Unit­ed Arab Emi­rates recent­ly announced their inten­tion to invest 163 bil­lion dol­lars in pro­duc­ing green pow­er, Sau­di Ara­bia began fol­low­ing suit and is now plan­ning to con­struct new solar ener­gy instal­la­tions with an out­put in excess of 10 GW. The king­dom is look­ing to invest between 30 and 50 bil­lion dol­lars in the project. On the oth­er side of the Sahara, Moroc­co is sys­tem­at­i­cal­ly expand­ing its wind and solar ener­gy land­scape and is cur­rent­ly com­mis­sion­ing phase two of Noor—a 350-megawatt solar project. For engi­neers and oper­a­tors of wind and solar pow­er sta­tions, this means that things are def­i­nite­ly look­ing up.

KEEPING THE GRIDS RUNNING SMOOTHLY

Solar and wind ener­gy sources are gen­er­al­ly con­sid­ered clean, but these cre­den­tials can only be applied to CO2 emis­sions and not to the grids that are being sup­plied by renew­ables. Fun­da­men­tal­ly, it makes no dif­fer­ence whether the elec­tric­i­ty from a sin­gle wind tur­bine is fed into a local grid or if a wind or solar plant feeds pow­er into medi­um, high, or extra-high volt­age grids. Mul­ti­ple instances of feed­back effects occur at all lev­els when renew­able ener­gies are fed into the grid.

Keep­ing things run­ning smooth­ly requires sig­nif­i­cant tech­no­log­i­cal and finan­cial invest­ment, and one of the key fac­tors that has to be considered—even at the design stage—is oper­at­ing the sys­tems with­in the volt­age range of the applic­a­ble grid con­nec­tion point. It is also essen­tial to keep an open mind to all the tech­ni­cal options that are avail­able, as this is the only way to cre­ate an over­all solu­tion that deliv­ers the very best pos­si­ble per­for­mance, and con­sis­tent­ly opti­mizes the lev­elized cost of ener­gy (LCOE). Putting togeth­er an intel­li­gent com­bi­na­tion of equip­ment is the key to devel­op­ing an ide­al solu­tion for all volt­age lev­els.

ACTIVE AND PASSIVE FILTERS OPTIMIZE COSTS

The sup­ply of ener­gy from wind and solar pow­er sta­tions cre­ates a greater amount of dis­tur­bance in pub­lic elec­tric­i­ty grids. This includes volt­age fluc­tu­a­tions, reac­tive pow­er, har­mon­ics, supra­har­mon­ics, and res­o­nance. In some cas­es, reac­tive pow­er com­pen­sa­tion sys­tems or pas­sive fil­ters alone are not enough to solve this issue, even if they may have been in the past. New ways of think­ing are required, par­tic­u­lar­ly in the face of con­stant­ly chang­ing grid con­nec­tion guide­lines. The only way for engi­neers and oper­a­tors to devel­op the ide­al solu­tion is to com­bine dif­fer­ent options.

Hybrid sys­tems, cre­at­ed by com­bin­ing active and pas­sive fil­ters, deliv­er fea­tures such as a com­pre­hen­sive fil­ter effect and reac­tive pow­er com­pen­sa­tion at the grid con­nec­tion point of wind and solar pow­er instal­la­tions. The pas­sive fil­ters oper­ate over an exten­sive range, reach­ing supra­har­mon­ic fre­quen­cies. Active fil­ters selec­tive­ly reduce har­mon­ics in the low-fre­quen­cy range.

Pas­sive fil­ters com­bine a capac­i­tor with induc­tance. They can have a pos­i­tive impact on line imped­ance and reduce har­mon­ic com­po­nents in the grid. In addi­tion, pas­sive fil­ters pro­vide a fixed lev­el of capac­i­tive reac­tive pow­er and can have a range of dimen­sions to suit the require­ments of each appli­ca­tion.

Active fil­ters are based on pow­er elec­tron­ic IGBT tech­nol­o­gy and are increas­ing­ly being used to reduce grid feed­back effects and improve volt­age qual­i­ty, par­tic­u­lar­ly in indus­tri­al grids. The sup­plied com­pen­sa­tion cur­rent in phase oppo­si­tion to the dis­tur­bance smooth­ly reduces har­mon­ics and reac­tive pow­er with high dynam­ics. In addi­tion, the right sys­tems can pro­vide com­pen­sa­tion for asym­met­ri­cal loads and flick­er. Anoth­er impor­tant con­sid­er­a­tion is the abil­i­ty to extend sys­tems on a mod­u­lar basis. If new wind tur­bines are built or the pro­duc­tion process and dis­tur­bance con­di­tions change, for exam­ple, oper­a­tors can respond quick­ly to the new sit­u­a­tion by installing addi­tion­al fil­ter mod­ules.

The ide­al dimen­sions of the com­bined fil­ters and com­pen­sa­tion sys­tems are deter­mined by spe­cial­ists who con­duct exten­sive sim­u­la­tion stud­ies as ear­ly as the design stage or mea­sure the volt­age qual­i­ty dur­ing oper­a­tion. This makes it pos­si­ble to ful­fill the applic­a­ble stan­dards and suc­cess­ful­ly achieve the nec­es­sary cer­ti­fi­ca­tions. Anoth­er area where the over­all design of wind and solar sys­tems can be opti­mized by com­bin­ing equip­ment is the dimen­sion­ing of rec­ti­fi­er trans­form­ers.

Since the semi­con­duc­tor mod­ules have a fixed cur­rent-car­ry­ing capac­i­ty, there are lim­its on the pow­er that can be trans­mit­ted at the net­work node in ques­tion. In such cas­es, dimen­sion­ing is pri­mar­i­ly depen­dent on the width of the volt­age band in the input range of the AC volt­age. This is where the trans­former with tap chang­er comes into play.

KEEPING COSTS DOWN

The trans­former with tap chang­er offers a sig­nif­i­cant­ly small­er input volt­age range at the AC side of the rec­ti­fi­er trans­former. Equipped with an on-load tap-chang­er, it helps the com­pa­ny that is using it to get the best out of its rec­ti­fi­er trans­form­ers. Ohmic loss­es in the grid are min­i­mized. The nar­row­er volt­age band increas­es the amount of rec­ti­fi­er trans­former pow­er that can be trans mit­ted. Con­verse­ly, this means that if con­trol tech­nol­o­gy is used con­sis­tent­ly across all volt­age lev­els, then engi­neers can reduce the size of all rec­ti­fi­er trans­form­ers accord­ing­ly. Reduc­ing the LCOE is the name of the game. Com­bin­ing reg­u­lat­ed trans­form­ers and rec­ti­fi­er trans­form­ers, as well as active and wide-band pas­sive fil­ters, brings engi­neers of solar and wind pow­er instal­la­tions one step clos­er to their main goal: to cre­ate sys­tems with max­i­mum finan­cial yield by min­i­miz­ing invest­ment costs and, at the same time, max­i­miz­ing sys­tem performance—at all volt­age lev­els.

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YOUR PERSONAL CONTACT

Do you have any ques­tions about active and pas­sive fil­ters?
Tobias Funk is here to help:
t.funk@reinhausen.com

Do you have any ques­tions about tap chang­ers?
Fran­co Piz­zut­to is here to help:
f.pizzutto@reinhausen.com