© Fraunhofer ISE

Sta­ble pow­er sup­ply due to grid-form­ing inverters

Large pow­er plants with their syn­chro­nous gen­er­a­tors ensure sta­bil­i­ty in the pow­er grid, but are grad­u­al­ly being shut down in the course of the ener­gy tran­si­tion. To address this prob­lem, researchers at the Fraun­hofer Insti­tute for Solar Ener­gy Sys­tems ISE are inves­ti­gat­ing how grid-form­ing invert­ers can ensure a reli­able sup­ply of sinu­soidal alter­nat­ing cur­rent and sta­ble grid fre­quen­cy in the future.

Switch­ing on the lamp, charg­ing the mobile phone, putting the milk in the fridge and quick­ly vac­u­um­ing the flat — in our every­day lives we rely on elec­tric­i­ty com­ing reli­ably from the sock­et. But our pow­er grid is very com­plex in its struc­ture and the equi­lib­ri­um in which it nor­mal­ly finds itself is frag­ile. Ide­al­ly, the cur­rent flow­ing through Europe’s elec­tri­cal lines has a sinu­soidal alter­nat­ing volt­age with an approx­i­mate­ly con­stant fre­quen­cy of 50 Hertz. This sta­bil­i­ty is made pos­si­ble by the phys­i­cal prop­er­ties of syn­chro­nous gen­er­a­tors in large pow­er plants. These bring iner­tia and thus the so-called momen­tum reserve into the sys­tem via their rotat­ing mass. They can com­pen­sate for any gen­er­a­tion deficits in the short term using the stored kinet­ic ener­gy and thus bridge the time until fur­ther pro­tec­tive mea­sures such as the pro­vi­sion of con­trol reserves are acti­vat­ed. This means that even in crit­i­cal sit­u­a­tions, such as the unplanned fail­ure of large gen­er­a­tion capac­i­ties or the dis­in­te­gra­tion of the grid into grid sec­tions, a so-called sys­tem split, nation­wide pow­er fail­ures do not occur immediately.

Now, how­ev­er, large nuclear and coal-fired pow­er plants are increas­ing­ly being tak­en off the grid and replaced by renew­able forms of ener­gy gen­er­a­tion. “In this way, the syn­chro­nous gen­er­a­tors are lost, which are a very essen­tial basis for grid con­trol,” explains Dr. Sönke Rogal­la, head of the Pow­er Elec­tron­ics and Grid Inte­gra­tion depart­ment at Fraun­hofer ISE. He and his research team see grid-form­ing invert­ers as a promis­ing alter­na­tive for main­tain­ing grid stability.

The right pro­gram­ming makes the difference

Invert­ers are pow­er elec­tron­ic devices whose pri­ma­ry task is to con­vert direct cur­rent into alter­nat­ing cur­rent. Depend­ing on the pow­er class, they vary in appear­ance from small bat­tery stor­age sys­tems to large megawatt sys­tems. Their elec­tri­cal behav­iour is not phys­i­cal­ly defined, but must first be deter­mined accord­ing­ly via cer­tain con­trol algo­rithms. Nowa­days, invert­ers are usu­al­ly pro­grammed to feed a desired pow­er into a rigid­ly assumed pow­er grid pro­vid­ed by pow­er­ful large-scale pow­er plants. Grid-form­ing invert­ers, on the oth­er hand, are pro­grammed to behave like a volt­age source. Com­pa­ra­ble to the behav­ior of con­ven­tion­al pow­er plants, grid-form­ing invert­ers thus react at short notice to the demand of the grid and pro­vide instan­ta­neous reserves.

“It is impor­tant, for exam­ple, that the devices react cor­rect­ly and reflex­ive­ly in spe­cial cas­es such as over­load sit­u­a­tions, defec­tive lines or sys­tem splits and keep the grid sta­ble,” says Roland Singer, group leader of con­vert­er-based grids. “To this end, we are research­ing the devel­op­ment of devices and algo­rithms. We can test var­i­ous appli­ca­tion sce­nar­ios with the help of sim­u­la­tions as well as using the test infra­struc­ture in our insti­tute’s own mul­ti-megawatt lab in Freiburg.”

Holis­tic view in the “Ver­bund­net­zSta­bil” project

Rogal­la explains that there is now a con­sen­sus among trans­mis­sion sys­tem oper­a­tors that grid-form­ing invert­ers will be nec­es­sary for a large pro­por­tion of the plants that are new­ly con­nect­ed to the grid. For exam­ple, Fraun­hofer ISE pro­vides con­sult­ing ser­vices to var­i­ous elec­tric­i­ty grid oper­a­tors and has been work­ing with var­i­ous coop­er­a­tion part­ners from sci­ence and indus­try in the “Ver­bund­net­zSta­bil” project since 2017 (see info box below). “In a unique con­stel­la­tion, we have suc­ceed­ed in bring­ing togeth­er com­pe­tences from the field of pow­er elec­tron­ics and con­trol tech­nol­o­gy with com­pe­tences in grid dynam­ics and grid con­trol. This allowed us to take a holis­tic look at the use of and the exact require­ments for grid-form­ing invert­ers on a larg­er scale,” says Rogalla.

In the first project step, the require­ments for future pow­er grids were clar­i­fied and crit­i­cal sit­u­a­tions defined. This formed the basis for con­crete device devel­op­ment and pro­gram­ming togeth­er with the invert­er man­u­fac­tur­er KACO new ener­gy. In the Mul­ti-Megawatt Lab, the researchers were then able to recre­ate a small-scale pow­er grid and inves­ti­gate how the pro­por­tion of syn­chro­nous machines and grid-form­ing invert­ers, as well as the imple­ment­ed con­trols, affect­ed volt­age sta­bil­i­ty in var­i­ous fault scenarios.

Rogal­la and Singer are very sat­is­fied with the results. “Our inves­ti­ga­tions once again clear­ly show that a switch from syn­chro­nous gen­er­a­tors to grid-form­ing invert­ers works and is also becom­ing increas­ing­ly urgent,” empha­sizes Singer. “At the same time, we were able to clear­ly define what the grid of the future will real­ly need and, with the help of a test guide­line that we devel­oped, pro­vide sug­ges­tions for impor­tant tech­ni­cal details where there is still no clear stan­dard,” adds Rogal­la. “In this way, we want to pro­vide the indus­try with assis­tance in the tech­ni­cal eval­u­a­tion of suit­able devices for the upcom­ing mar­ket launch of grid-form­ing inverters.”

Cur­rent­ly, the final report of the project is being writ­ten. At the same time, the researchers want to test their devices and find­ings on the real pow­er grid in one of the insti­tute’s office wings. In a fur­ther research project, which is cur­rent­ly being planned, the tech­nol­o­gy devel­oped is to be imple­ment­ed in a large pho­to­volta­ic stor­age pow­er plant and grid inter­ac­tions are to be inves­ti­gat­ed under real conditions.