Optimal Planning of Virtual Inertia Installations to Improve the Power System Frequency Response

In recent years, the power system has seen a fast transformation from one primarily based on fossil energy to one where renewable energy, especially wind and solar power, takes a more significant proportion in the energy profile. With the shift in energy profile come the changes in the electricity generation units. The solar panels and wind turbines replace the synchronous generators in electricity generation. Most solar and wind generation units are converter-interfaced. In contrast, the synchronous generator is connected to the power grid directly. For this reason, the future power system of a high level of renewable penetration will exhibit dynamic properties different from the traditional power system, which poses many challenges. One of the challenges is related to frequency stability. The frequency stability of a traditional power system is secured with a three-level frequency control scheme. The scheme is composed of three frequency regulation mechanisms at different time scales. The fastest control mechanism, named primary frequency control, needs about 5 s to be fully deployed to arrest the frequency drops or overshoots. After that, the other two frequency secondary and tertiary frequency control mechanisms are then slowly deployed to bring the system frequency back to the nominal value. Under this control scheme, the overall active power generation and consumption in a power system get balanced, and the power frequency variation is limited within a narrow range around a nominal value. However, before the primary frequency control is sufficiently deployed, the system relies on the natural inertia response of the synchronous generators to maintain the active power balance at the sacrifice of changes in the generators' rotational speeds. As the power frequency is decided collectively by the rotational speeds of all synchronous generators in the system, larger system inertia means smaller power frequency variation when subject to the same disturbance. Since there is no lack of system inertia in a synchronous generator-dominant power system, the power frequency variation with the help of the tertiary control scheme is usually contained within a limited range. For a future power system with more and more synchronous generators being displaced by converter-interfaced generation (CIG) units, the system inertia decreases. The tertiary frequency control scheme alone can no longer limit the power frequency variation within an acceptable range. For this reason, techniques were proposed to emulate inertia response on a converter-interfaced generation unit. Apart from the level of total system inertia, studies show that the spatial distribution of system inertia can also influence the frequency response. Under this context, a well-planned virtual inertia installation at selected locations can achieve a satisfactory level of improvement on frequency response at a low investment cost. This thesis work aims at developing a systematic method to search for the most economical plan of virtual inertia installations while ensuring a satisfactory level of frequency response. In order to derive the most economical plan of virtual installation, a mathematical optimization problem is proposed with constraints formulated with the help of a newly proposed metric of inertia response that quantifies the influence of inertia on the system frequency response. The formulation of the optimization problem considers all possible combinations of loading and renewable generation profiles. Two methods are proposed to solve the optimization problem of the mixed-integer type. The first one is based on the classic scheme of dynamic programming. The second method adopts a relaxation technique based on the sparsity promotion or Majorize-Minimization (MM) method. Furthermore, parallel and cloud programming techniques are used to facilitate computation speed. Other minor contributions include a design of a supplementary controller on top of the inertia emulation control to improve the voltage stability of a converter-interfaced generation unit. Finally, case studies were conducted on a modified Southeast Australian power system against different types of faults to validate the performance and investment cost of the virtual inertia installation plan givens by the proposed method in comparison with two other methods. The result shows that the virtual inertia installation plan given by the proposed method produces better performance while at lower investment costs

Three-Phase Power Converter Based Real-Time Synchronous Generator Emulation

To bridge the gap between power system research and their real application in power grids, a Hardware Test-Bed (HTB) with modular three-phase power converters has been developed at the CURENT center, the University of Tennessee, Knoxville, to emulate transmission level power systems with actual power flowing. This dissertation focuses on the development and verification of a real-time synchronous generator (SG) emulator in the HTB. The research involved in this dissertation aims at designing a proper control to achieve emulator performance goal and investigating the sources of error and its influence on interconnected SG-emulator networks. First, different interface algorithms (IAs) are compared and the voltage type ideal transformer model (ITM) is selected considering the accuracy and stability. At the same time, closed-loop voltage control with current feed-forward is proposed to decrease the error caused by the non-ideality of the power amplifier. The emulation is then verified through two different ways. First, the output waveforms of the emulator in experiment are compared with the simulation under the same condition. Second, a transfer function perturbation (TFP) based error model is obtained and redefined as the relative error for the amplitude and phase between the emulated and the target system over the frequency range of interest. The major cause of the error is investigated through a quantitative analysis of the error with varying parameters. Third, the stability issue associated with the interconnection of two SG emulators is studied. The small signal models of the two-generation system with constant current and constant impedance load are developed, and the main reasons that cause instability are researched and verified. The developed SG emulator is also verified in the two-area system by comparing the system dynamics visually. At last, the 6th-order SG model including transformer voltages and saturation effect is applied in the three-phase symmetrical fault scenario. Control parameters are designed based on the TFP error evaluation of the fault condition. The developed SG emulator is then tested and verified in line-to-line fault condition. In addition, the stability of the new SG emulator is studied again and compared with the previous emulation

Adaptive virtual inertia controller based on machine learning for superconducting magnetic energy storage for dynamic response enhanced

The goal of this paper was to create an adaptive virtual inertia controller (VIC) for superconducting magnetic energy storage (SMES). An adaptive virtual inertia controller is designed using an extreme learning machine (ELM). The test system is a 25-bus interconnected Java Indonesian power grid. Time domain simulation is used to evaluate the effectiveness of the proposed controller method. To simulate the case study, the MATLAB/Simulink environment is used. According to the simulation results, an extreme learning machine can be used to make the virtual inertia controller adaptable to system variation. It has also been discovered that designing virtual inertia based on an extreme learning machine not only makes the VIC adaptive to any change in the system but also provides better dynamics performance when compared to other scenarios (the overshoot value of adaptive VIC is less than -5×10-5)

Control of AC/DC microgrids with renewables in the context of smart grids including ancillary services and electric mobility

Microgrids are a very good solution for current problems raised by the constant growth of load demand and high penetration of renewable energy sources, that results in grid modernization through “Smart-Grids” concept. The impact of distributed energy sources based on power electronics is an important concern for power systems, where natural frequency regulation for the system is hindered because of inertia reduction. In this context, Direct Current (DC) grids are considered a relevant solution, since the DC nature of power electronic devices bring technological and economical advantages compared to Alternative Current (AC). The thesis proposes the design and control of a hybrid AC/DC Microgrid to integrate different renewable sources, including solar power and braking energy recovery from trains, to energy storage systems as batteries and supercapacitors and to loads like electric vehicles or another grids (either AC or DC), for reliable operation and stability. The stabilization of the Microgrid buses’ voltages and the provision of ancillary services is assured by the proposed control strategy, where a rigorous stability study is made. A low-level distributed nonlinear controller, based on “System-of-Systems” approach is developed for proper operation of the whole Microgrid. A supercapacitor is applied to deal with transients, balancing the DC bus of the Microgrid and absorbing the energy injected by intermittent and possibly strong energy sources as energy recovery from the braking of trains and subways, while the battery realizes the power flow in long term. Dynamical feedback control based on singular perturbation analysis is developed for supercapacitor and train. A Lyapunov function is built considering the interconnected devices of the Microgrid to ensure the stability of the whole system. Simulations highlight the performance of the proposed control with parametric robustness tests and a comparison with traditional linear controller. The Virtual Synchronous Machine (VSM) approach is implemented in the Microgrid for power sharing and frequency stability improvement. An adaptive virtual inertia is proposed, then the inertia constant becomes a system’s state variable that can be designed to improve frequency stability and inertial support, where stability analysis is carried out. Therefore, the VSM is the link between DC and AC side of the Microgrid, regarding the available power in DC grid, applied for ancillary services in the AC Microgrid. Simulation results show the effectiveness of the proposed adaptive inertia, where a comparison with droop and standard control techniques is conducted.As Microrredes são uma ótima solução para os problemas atuais gerados pelo constante crescimento da demanda de carga e alta penetração de fontes de energia renováveis, que resulta na modernização da rede através do conceito “Smart-Grids”. O impacto das fontes de energia distribuídas baseados em eletrônica de potência é uma preocupação importante para o sistemas de potência, onde a regulação natural da frequência do sistema é prejudicada devido à redução da inércia. Nesse contexto, as redes de corrente contínua (CC) são consideradas um progresso, já que a natureza CC dos dispositivos eletrônicos traz vantagens tecnológicas e econômicas em comparação com a corrente alternada (CA). A tese propõe o controle de uma Microrrede híbrida CA/CC para integrar diferentes fontes renováveis, incluindo geração solar e frenagem regenerativa de trens, sistemas de armazenamento de energia como baterias e supercapacitores e cargas como veículos elétricos ou outras (CA ou CC) para confiabilidade da operação e estabilidade. A regulação das tensões dos barramentos da Microrrede e a prestação de serviços anciliares são garantidas pela estratégia de controle proposta, onde é realizado um rigoroso estudo de estabilidade. Um controlador não linear distribuído de baixo nível, baseado na abordagem “System-of-Systems”, é desenvolvido para a operação adequada de toda a rede elétrica. Um supercapacitor é aplicado para lidar com os transitórios, equilibrando o barramento CC da Microrrede, absorvendo a energia injetada por fontes de energia intermitentes e possivelmente fortes como recuperação de energia da frenagem de trens e metrôs, enquanto a bateria realiza o fluxo de potência a longo prazo. O controle por dynamical feedback baseado numa análise de singular perturbation é desenvolvido para o supercapacitor e o trem. Funções de Lyapunov são construídas considerando os dispositivos interconectados da Microrrede para garantir a estabilidade de todo o sistema. As simulações destacam o desempenho do controle proposto com testes de robustez paramétricos e uma comparação com o controlador linear tradicional. O esquema de máquina síncrona virtual (VSM) é implementado na Microrrede para compartilhamento de potência e melhoria da estabilidade de frequência. Então é proposto o uso de inércia virtual adaptativa, no qual a constante de inércia se torna variável de estado do sistema, projetada para melhorar a estabilidade da frequência e prover suporte inercial. Portanto, o VSM realiza a conexão entre lado CC e CA da Microrrede, onde a energia disponível na rede CC é usada para prestar serviços anciliares no lado CA da Microrrede. Os resultados da simulação mostram a eficácia da inércia adaptativa proposta, sendo realizada uma comparação entre o controle droop e outras técnicas de controle convencionais

Next Generation Inverters Equipped with Virtual Synchronous Compensators for Grid Services and Grid Support

L'abstract è presente nell'allegato / the abstract is in the attachmen

Grid-forming wind power plants

With growing concerns over climate change, the power system is witnessing an unprecedented growth in electricity generation from intermittent renewable energy sources (RES) such as wind and solar, which are commonly interfaced to the grid by power-electronic converters. However, increasing the penetration level of converter-interfaced generation units reduces the number of synchronous generators (SGs) in the grid that provide system services to support voltage and frequency, either inherently or through mandatory requirements and market products. This brings several challenges for the grid operators, which include increasing risk of harmonic interactions, decreasing system inertia and reduction in the short-circuit power of the grid, which all together might jeopardize the security and availability of the power systems. As a countermeasure, it is necessary that the power-electronic-based generation units not only provide grid support services that are originally provided by the SGs, but also operate in harmony with other generation units in all kinds of grid conditions. As a result, the concept of grid-forming (GFM) control, which mimics the beneficial properties of the SGs in converter systems, has emerged as a viable solution to allow effective and secured operation of power systems with increased penetration of converter-based resources.\ua0\ua0 This thesis investigates the application of GFM control strategies in wind power plants (WPPs). In particular, the focus of the work will be on developing an effective GFM control strategy for the energy storage systems (ESS) in WPPs that not only supports the operation of the WPP in various grid conditions, but also offers a certain degree of GFM properties to the overall WPP. To start with, the selection of the most suitable GFM control strategy for wind power applications is made by evaluating and comparing various control strategies available in the literature. The comparison is based on their influence on the frequency characteristics of the converter and robustness of the controller in varying grid strength. To address the transient stability problem of GFM converters during current limitation, a novel strategy based on the limitation of converter\u27s internal voltage vector is developed, which effectively limits the converter current to a desired value and retains the GFM properties of the converter at all times. An experimental setup is used to validate the effectiveness of the proposed limitation strategy in case of various grid disturbances. By implementing the proposed GFM control strategy for the ESS in a test WPP model, it is shown using detailed time-domain simulation results that the GFM behaviour can be offered to the overall WPP. The Network Frequency Perturbation (NFP) plots are used to verify the GFM behaviour of the considered WPP. Furthermore, an overview of various energy storage technologies (ESTs) suitable for providing ancillary services from WPPs is presented. With a focus on the two most suitable ESTs, i.e., batteries and supercapacitors, recommendations are given for design and sizing of the ESS for a given application. Finally, a coordinated control strategy between the WPP and SGs is developed, which facilitates the provision of frequency support from the WPP and at the same time reduces the energy storage requirements for the converter system