Image: Fraunhofer IEE

Grid con­trol 2.0: Pow­er con­vert­ers can keep the inter­con­nect­ed grid sta­ble even with very high shares of renew­able energies

In the future, the pow­er sup­ply sys­tem can also be oper­at­ed with­out the sta­bi­liz­ing effect of the syn­chro­nous gen­er­a­tors of con­ven­tion­al pow­er plants. This was joint­ly stat­ed by the experts of the joint project “Grid Con­trol 2.0” at the final con­fer­ence in Kas­sel today. The research project was able to show that plants with grid-form­ing pow­er con­vert­ers can pro­vide instan­ta­neous reserve and thus sta­bi­lize the sys­tem even in extreme sit­u­a­tions. At the end of the project, pow­er grid oper­a­tors will dis­cuss the detailed results and ques­tions regard­ing the intro­duc­tion of the new tech­nol­o­gy with tech­ni­cal experts and rep­re­sen­ta­tives of the Fed­er­al Min­istry of Eco­nom­ics and Cli­mate Pro­tec­tion and the Fed­er­al Net­work Agency.

Up to now, it has main­ly been the syn­chro­nous gen­er­a­tors of large pow­er plants that have ensured that the fre­quen­cy and volt­age require­ments in the pow­er grid are met. How­ev­er, with the ener­gy tran­si­tion, pow­er plants are increas­ing­ly being replaced by wind ener­gy and pho­to­volta­ic sys­tems that are cou­pled to the elec­tri­cal grid with pow­er con­vert­ers. The joint project “Grid Con­trol 2.0”, coor­di­nat­ed by Fraun­hofer IEE, was able to demon­strate that gen­er­a­tion plants with grid-form­ing pow­er con­vert­ers can pro­vide instan­ta­neous reserve and thus sta­bi­lize the sys­tem even in extreme sit­u­a­tions. At the final con­fer­ence, pow­er grid oper­a­tors will dis­cuss the results of the research project and issues relat­ing to the intro­duc­tion of the tech­nol­o­gy with experts and rep­re­sen­ta­tives of the Ger­man Fed­er­al Min­istry of Eco­nom­ics and Cli­mate Pro­tec­tion and the Ger­man Fed­er­al Net­work Agency. On the pre­vi­ous day, grid-form­ing pow­er con­vert­ers were demon­strat­ed in oper­a­tion in lab­o­ra­to­ries on the new cam­pus of Fraun­hofer IEE in Kas­sel, among others.

The results of the joint project “Grid Con­trol 2.0” on con­trol and sta­bil­i­ty in the con­vert­er-dom­i­nat­ed inter­con­nect­ed grid con­tribute to the fur­ther devel­op­ment of appli­ca­tion rules. Spon­sored by the pro­gram, which start­ed in 2018 and is now com­ing to an end, is the
joint project from the Ger­man Fed­er­al Min­istry for Eco­nom­ic Affairs and Cli­mate Pro­tec­tion with around 10 mil­lion euros.

Alexan­der Folz, Gov­ern­ment Direc­tor in the Sys­tem Secu­ri­ty Divi­sion of the Fed­er­al Min­istry of Eco­nom­ics and Cli­mate Pro­tec­tion states, “The Grid Reg­u­la­tion 2.0 project has addressed key chal­lenges for future sys­tem oper­a­tion. The results come at just the right time for the Roadmap Sys­tem Sta­bil­i­ty of the Fed­er­al Min­istry of Eco­nom­ics and Cli­mate Protection.”

“We are con­vinced that the inter­con­nect­ed grid — and, in the event of a fault, sub­grids as well — can be kept sta­ble even with very high con­vert­er shares. How­ev­er, this requires suit­able con­trol meth­ods. We have deter­mined the require­ments for these pro­ce­dures and devel­oped con­trol pro­ce­dures to ensure that the pow­er con­vert­ers can serve safe and sta­ble sys­tem oper­a­tion,” explains project man­ag­er Dr. Philipp Strauß, deputy direc­tor of Fraun­hofer IEE in Kas­sel. Par­tic­u­lar atten­tion is paid to the devel­op­ment of a suit­able trans­for­ma­tion path. “New tech­nolo­gies must be seam­less­ly inte­grat­ed into exist­ing grid con­trol process­es. The tran­si­tion must be designed so that the emerg­ing sys­tem is at least as sta­ble as the cur­rent one,” says Strauß.

In addi­tion to Fraun­hofer IEE, the project involves the Tech­ni­cal Uni­ver­si­ty of Braun­schweig and the Uni­ver­si­ty of Kas­sel, the dis­tri­b­u­tion grid oper­a­tor EWE NETZ GmbH, E‑ON SE with its sub­sidiaries West­netz GmbH and Mit­teldeutsche Net­zge­sellschaft Strom mbH, the con­vert­er man­u­fac­tur­ers and sys­tem providers SMA Solar Tech­nol­o­gy AG and SiemensEn­er­gy Glob­al GmbH & Co. KG, the Euro­pean Dis­trib­uted Ener­gy Resources Lab­o­ra­to­ries (DER­lab e.V.) for inter­na­tion­al net­work­ing, the Deutsche Energie-Agen­tur GmbH (dena), the four oper­a­tors of the Ger­man trans­mis­sion grid, and the Forum Net­ztech­nik und Net­z­be­trieb (FNN) in VDE as the insti­tu­tion defin­ing the grid con­nec­tion rules. The project thus includes rep­re­sen­ta­tives of all the main stake­hold­er groups affect­ed by this issue. “This was essen­tial to the suc­cess of our work,” Strauß notes.

Con­ti­nen­tal Euro­pean inter­con­nect­ed net­work in view

The new con­trol pro­ce­dures and their sta­bi­liz­ing influ­ence were sim­u­la­tive­ly eval­u­at­ed under extreme sce­nar­ios, such as a sys­tem split — i.e. a grid sep­a­ra­tion across Europe. Strong volt­age dips were not only cal­cu­lat­ed, but also car­ried out in the accred­it­ed lab­o­ra­to­ry accord­ing to exist­ing stan­dards. A par­tic­u­lar chal­lenge for the grid-form­ing pow­er con­vert­ers here are suit­able and extreme­ly fast cur­rent lim­it­ing process­es. The research teams were able to demon­strate that even under such harsh con­di­tions, a con­tri­bu­tion to net­work for­ma­tion can be made.

Grid-form­ing prop­er­ties are now also required of sys­tems with very large con­vert­ers, which are used, for exam­ple, for pow­er fac­tor cor­rec­tion in the trans­mis­sion grid or in head-end sta­tions of high-volt­age direct cur­rent trans­mis­sion (HVDC). In addi­tion, the project will con­tribute to how these require­ments can be reflect­ed in the tech­ni­cal appli­ca­tion rules for gen­er­a­tion plants in the future. In all of this, the project part­ners also take into account the inter­na­tion­al per­spec­tive — after all, the Ger­man pow­er grid is embed­ded in the con­ti­nen­tal Euro­pean inter­con­nect­ed grid.

Research ques­tions of the project

At the final con­fer­ence, Philipp Strauß sum­ma­rized the results on the research ques­tions: “In the project, grid-form­ing con­trol meth­ods were fur­ther devel­oped with opti­mized cur­rent lim­it­ing meth­ods. Spa­tial dis­tri­b­u­tion of grid-form­ing equip­ment is nec­es­sary and instan­ta­neous reserve can be pro­vid­ed in the trans­mis­sion and dis­tri­b­u­tion grids. New island­ing detec­tion meth­ods have been devel­oped. Bat­tery sys­tems, wind tur­bines, pho­to­volta­ic sys­tems, rotat­ing phase shifters, stat­coms, and elec­tric loads, among oth­ers, can be grid-form­ing with the new con­trol meth­ods. A pure con­vert­er grid by grid-form­ing con­trol is pos­si­ble and the seam­less tran­si­tion with dif­fer­ent shares of syn­chro­nous machines is real­iz­able. A spec­i­fi­ca­tion and new test meth­ods have been devel­oped for elec­tri­cal iner­tia, net for­ma­tion and damp­ing, among others.”

A cen­tral top­ic here is the inter­ac­tion between pow­er con­vert­ers and syn­chro­nous gen­er­a­tors. For exam­ple, the part­ners ana­lyzed in which com­bi­na­tion and at which volt­age lev­el syn­chro­nous gen­er­a­tors as well as cur­rent-influ­enced or volt­age-influ­enced con­vert­ers are nec­es­sary or per­mis­si­ble in order to main­tain sys­tem sta­bil­i­ty. In addi­tion, the experts eval­u­at­ed the volt­age qual­i­ty at the var­i­ous oper­at­ing points of the future sys­tem against the back­ground that the share of syn­chro­nous gen­er­a­tors will vary more and more with the fur­ther expan­sion of renew­able ener­gies. In this con­text, the experts also inves­ti­gat­ed which robust­ness require­ments result from this for the stress imprint­ing controls.

Last but not least, they also addressed the ques­tion of the extent to which sta­ble sys­tem behav­ior is pos­si­ble even with com­plete­ly con­vert­er-based gen­er­a­tion. The project thus makes an impor­tant con­tri­bu­tion to achiev­ing Ger­many’s cli­mate pro­tec­tion targets.

Fur­ther need for research

“The instan­ta­neous reserve, which has so far come from the rotat­ing mass­es of con­ven­tion­al, fos­sil-fuel pow­er plants and sta­bi­lizes the pow­er sys­tem’s grid fre­quen­cy, can also be effec­tive­ly pro­vid­ed in a decen­tral­ized man­ner by grid-form­ing invert­ers at all volt­age lev­els of the dis­tri­b­u­tion grid. “, spec­i­fies Prof. Dr. Bernd Engel of TU Braun­schweig, “In a fur­ther research project, we want to address remain­ing tech­ni­cal chal­lenges, such as pos­si­ble pow­er oscil­la­tions between the invert­ers, com­pat­i­bil­i­ty with exist­ing pro­tec­tion devices and the risk of unwant­ed islanding.”