HVDC inertia provision

The frequency control mechanisms of electric power systems are based on the inertia of traditional generation units with synchronous machines. However, the equivalent inertia in a power system will be reduced when traditional generators are replaced by renewable energy sources with power electronic grid interfaces. This will cause increasing challenges with frequency control which can lead to stability problems and increased risk of power system failures when the equivalent inertia becomes very low. In the Nordic countries, one of the most challenging scenarios will be operation with high import from HVDC connections during low load in the system, since this will allow for stopping most of the reservoir-based hydropower generators. However, if the power electronic converters of the HVDC interconnections between the Nordic countries and Central Europe or UK can be controlled to provide virtual inertia they could also help to alleviate the problems resulting from operating the system with few synchronous generators. This implies that the HVDC converter terminals have to be controlled to emulate the inertia and the general control characteristics of a synchronous generator. This project has addressed the challenges of utilizing HVDC interconnections to support the power system operation in conditions with low inertia from synchronous generators by pursuing three main lines of activities. i) From the perspective of power system operation, the project has developed methods for identifying the most suitable location and the most useful control response from power electronic converters that can provide transient power injection or inertial support. As a part of this activity, the project is funding a PhD study at KU Leuven, Belgium, working on the development of methods for identifying the value of providing virtual inertia and other ancillary services from HVDC converters. ii) The second main activity in the project has been oriented towards development, analysis and comparison of control system implementations for providing virtual inertia support from HVDC transmission schemes. Conducted comparative evaluations have identified differences in the stability characteristics among the studied control strategies, but have also demonstrated that they can all provide equivalent inertial response when operated in an isolated power system where the power electronic converter has significant impact on the frequency transients. The project activities have provided significant contributions to the analysis and design of methods for handling handle unbalanced conditions and other severe power system transients in control strategies including inertia emulation functionality. Related to the activity on control system design, the project has funded a PhD study at CentraleLille, France. The topic of the PhD work has been the control design and protection of HVDC converters operated as general "grid-forming" units with capability for islanded operation with or without explicit inertia emulation. iii) The third main activity of the project is related to the analysis of how inertial support and grid forming control from HVDC converters can influence the dynamics and stability of a power system. Within this activity, the project has funded a postdoctoral researcher at NTNU who has developed a new numerical method for analysis of power system stability. The project has also developed methods for eigenvalue-based analysis of power system small-signal stability when including detailed models of power electronic converters with their control systems as well as models representing the electromagnetic transients in ac- or dc cables. Experimental testing in laboratory environments has been conducted both at SINTEF/NTNU and at CentraleLille to support the research activities on control system design for individual HVDC converters and the analysis of power system dynamics. The most relevant control system implementations have been demonstrated by laboratory experiments with small-scale models of HVDC converters. Several cases have been tested for verifying the performance of the studied control strategies under regular operating conditions as well as during unbalanced grid faults or other severe transient events. For evaluating the impact on a larger power system configuration, Power-Hardware-in-the-Loop tests including a simplified real-time simulation model of the Nordic power system have been conducted.

Forskningsaktivitetene i prosjektet har vært rettet mot utvikling av metoder for å designe og analysere reguleringsstrategier for kraftelektronikkomformere som kan bidra med virtuell svingmasse i kraftsystemet. Dette omfatter også matematiske modeller for å analysere mulig økonomisk gevinst ved å utnytte kapasitet i høyspente likestrøm (HVDC)-forbindelser for å tilby virtuell svingmasse og andre støttefunksjoner for kraftsystemet. Slike modeller kan utnyttes til å prioritere bruken av HVDC-forbindelser, og bidra til å redusere kostnader med å sikre stabilitet i kraftsystemet. Videre har prosjektet bidratt til utvikling av metoder for stabilitetsanalyse i kraftsystemer med stor andel kraftelektronikk-basert produksjon. For å utvide bruksområdet for reguleringsstrategier som gir virtuell svingmasse, har prosjektet også utviklet nye metoder for å håndtere feilsituasjoner og ubalanserte trefasespenninger i kraftnettet. Selv om prosjektet har vært rettet mot HVDC systemer, kan flere av metodene som er utviklet også være relevante for andre bruksområder. De mulige effektene av prosjektet på lang sikt er derfor først og fremst knyttet til at forskningsaktivitetene har vært en del av en omfattende internasjonal innsats for å håndtere utviklingen mot stor andel kraftelektronikk-basert produksjon i kraftsystemet. Behovet for slik forskning og kunnskapsutvikling har økt gradvis gjennom storskala utvikling av fornybar energiproduksjon basert på vindturbiner og solceller, og har i Europeisk sammenheng blitt ytterligere kritisk i dagens energisituasjon. I Norsk sammenheng kan det forventes at den samme kunnskapen også vil bli nødvendig for å muliggjøre oppfyllelse av politiske mål om utnyttelse av fornybar energi, eksempelvis knyttet til utvikling av 30 GW havvindkapasitet. Virkningene av prosjektet er ellers i stor grad knyttet til kunnskapsutviklingen som har blitt mulig ved gjennomføring av forskningsaktivitetene. Siden metoder og teknologi for å håndtere en stor andel kraftelektronikk-basert produksjon har blitt viktig for å støtte utviklingen av kraftsystemet, har også den internasjonale forskningsaktiviteten innen relaterte temaområder økt sterkt i løpet av prosjektperioden. Dette prosjektet har samtidig vært det største forskningsinitiativet i Norge relatert til bruk av kraftelektronikkomformere for å stabilisere kraftsystemer med synkende fysisk svingmasse. Prosjektet har dermed gjort det mulig for de involverte forskningsgruppene å holde seg oppdatert på en forskningsfront under rask utvikling og samtidig bidra med resultater av internasjonal interesse innenfor prioriterte temaområder. Prosjektet har også støttet et betydelig internasjonalt samarbeid, og aktiviteten i prosjektet har vekket interesse fra flere internasjonale bedrifter. Kunnskapen opparbeidet gjennom prosjektet har også blitt en viktig basis for forskningsaktiviteter i andre pågående prosjekter av nasjonal interesse, eksempelvis Grønn plattform-prosjektet "OceanGrid Research."

Future power systems are facing new challenges when traditional thermal generation units are replaced by renewable energy sources with power electronic grid interfaces. In central Europe, an increasing share of wind and PV generation is leading to periods with few synchronous generators in operation, and the resulting low equivalent rotating inertia in the grid can introduce stability problems. Scenarios with low inertia, leading to challenges with frequency control and grid stability, are already requiring attention in the transmission networks in UK and Ireland. In the Nordic countries of Europe, low equivalent inertia is expected to produce potential issues within 2025. Different control schemes have been proposed for providing virtual inertia from power electronic converters distributed generation and other low voltage applications. Especially HVDC converters may represent an effective solution for alleviating issues caused by decreasing rotating inertia due to the significant installed power rating. However, identification of the most suitable implementation methods for HVDC converters with different topologies is still an open issue. This project will develop methods for assessing the value of and the requirements for inertia emulation from HVDC transmission schemes. Furthermore, control strategies suitable for inertia emulation by HVDC converter stations with different power converter topologies will be developed, and their performance and stability characteristics will be analysed. Detailed models of HVDC transmission schemes with inertia emulation capability will also be developed for analysing the influence on the stability of large-scale power systems. The methods and techniques resulting from the research activities within the project will support the development of "smarter transmission systems" in a future context with limited physical inertia from traditional generation plants.