Optimising power system frequency stability using virtual inertia from inverter-based renewable energy generation

Thesis (MEng)--Stellenbosch University, 2019.ENGLISH ABSTRACT: Inverter-based renewable energy generation are integrated into power systems at an increasing rate. Governments continuously set higher goals for renewable energy generation considering only the impact on the environment, development time, financial and economical side, while ignoring the impact of high penetration of inverter-based renewable energy generation on the current power system's stability. This thesis investigates the impact of increased variable renewable energy generation integration into the power system, with a specific focus on inertial response for system frequency stability. The focus is on wind and solar power generation, which uses inverters to interface with the power system network. These generation sources have a detrimental effect on the generation/load power balance, which reduces the system frequency stability. The power system becomes more sensitive, with increased RoCoF, lower frequency nadir and increased difficulty to control system frequency with generation/load balancing. Current mitigation measures and regulations for the decreased frequency stability are reviewed, indicating that for high share of renewable generation not just long-term energy storage is required, but also short-term energy storage with fast power response capabilities. To evaluate the impact, power system components are examined and modeled to implement in a power system simulation. Power system operation and stability (rotor, voltage and frequency) are reviewed and discussed. The research then focuses on transient frequency dynamics and stability. For the mitigation of reduced frequency stability the concept of virtual inertia is introduced. Virtual inertia is then explained and simulated for wind and solar PV plants. Lastly the H2 -norm metric is used to evaluate power system frequency stability, rather than using the amount of inertia present in a power system. The metric follows from Lyapunov theory for analyzing non-linear system stability through energy functions. The distribution of virtual inertia in a network is then optimised using the Genetic Algorithm with the H2 -norm, which is used to analyze the system robustness against disturbances, as the cost-function. The results show significant performance improvement in transient stability for the Western Transmission network of the Eskom power system in South Africa.AFRIKAANSE OPSOMMING: Omsetter-gebaseerde hernubare energie opwekking word in toenemende mate in kragstelsels geïntegreer. Regerings stel voortdurend hoër doelwitte vir hernubare energie opwekking waar slegs die impak op die omgewing, ontwikkelingstyd, finansiële en ekonomiese faktore in ag geneem word, terwyl die impak van hoë penetrasie omsetter-gebaseerde hernubare energie opwekking op die huidige kragstelsel se stabiliteit geïgnoreer word. Hierdie tesis ondersoek die impak van toenemende hernubare energie opwekking integrasie op die huidige kragstelsel. Die fokus is spesifiek op die traagheidsreaksie vir stelselfrekwensiestabiliteit, as ook op wind- en son-kragopwekking wat deur middel van omsetters aan die kragstelselnetwerk koppel. Hierdie hernubare bronne het 'n negatiewe effek op die generasie/las kragbalansering, wat die stelselfrekwensie se stabiliteit bedreig. Die kragstelsel word meer sensitief, met verhoogde RoCoF, laer frekwensie nadir en dit word moeiliker om die stelselfrekwensie te beheer met generasie/las balansering. Huidige regulasies en oplossings vir die agteruitgang van die frekwensie-stabiliteit word hersien. Om hoër aandele van hernubare energie opwekking te bewerkstellig, vereis dit langtermyn energiestoor sowel as korttermyn energiestoorkapasiteit met vinnige drywing uittree reaksie vermoëns. Om die impak op die krag stelsel te evalueer, word die stelsel komponente ondersoek en gemodelleer vir die doel om dit in 'n kragstelsel simulasie te implementeer. Kragstelseloperasie en stabiliteit (rotor, spanning en frekwensie) word hersien en bespreek. Die navorsing fokus dan op oorgangs-frekwensie-dinamika en stabiliteit. Die konsep van virtuele traagheid word voorgestel as 'n oplossing vir die agteruitgang van frekwensiestabiliteit. Virtuele traagheid word dan verduidelik en gesimuleer vir winden son-kragopwekking. Laastens word die H2 -norm gebruik om die stelsel se frekwensie-stabiliteit te evalueer, eerder as om die hoeveelheid traagheid in 'n kragstelsel te gebruik. Die metode volg uit die Lyapunov-teorie waar nie-line^ere stelsels se stabiliteit ge-analiseer word deur die opstel en gebruik van funksies wat die energie in die stelsel beskryf. Die verspreiding van virtuele traagheid in 'n netwerk word dan ge-optimeer met die gebruik van die Genetiese Algoritme. Die koste-funkise wat ge-minimeer word is die, H2 -norm. Die H2 -norm verteenwoordig die stelsel robuustheid teen versteurings en dus, word ge-mimimeer. Die resultate toon beduidende prestasie verbetering in oorgang-stabiliteit vir die Westelike transmissie netwerk van die Eskom krag-stelsel in Suid-Afrika