Modeling & Small Signal Analysis of Grid Forming Inverter

There is a rising number of inverter-based resources (IBRs) being integrated with distribution systems are becoming a more common occurrence. With integration of IBRs inverters, power utilities are experiencing an increase of number of operations with regards to voltage and frequency support. To maintain grid stability and reliability, IBRs need to provide some of the services currently (or formerly) provided by synchronous generators. Interconnection standards, like the IEEE 1547. 2018 has include requirements for IBRs to have the capability to provide some of these services—such as frequency and voltage support—and the procurement and deployment of the services can be implemented either as mandatory interconnection requirements or as market products. All the IBRs deployed today are grid-following (GFL), and read the voltage and frequency of the grid and inject current to provide the appropriate amount of active and reactive power. The fundamental GFL IBR design assumption is that there are still enough synchronous generators on the grid to provide a relatively strong and stable voltage and frequency signal, which GFL IBRs can “follow.” But since levels of GFL are increasing, there will be a limit to how far GFL controls can be pushed, and, at some point, new advanced inverter controls (termed grid forming (GFM)) will be needed to maintain system stability. GFM IBRs will also be needed to establish voltage and frequency during operating conditions when there are zero synchronous machines (100 percent IBR penetration). Power systems around the world are at the point of now needing to make this technological leap; however, system operators and planners, equipment owners, and manufacturers today are facing a circular problem regarding the deployment of advanced IBR controls. Which comes first, the requirement for a capability or the capability itself? How do grid operators know what performance or capability is possible from new equipment (and therefore what they could require)? How can they evaluate costs and benefits of having such equipment on the grid? What drives manufacturers to invest in modern technology without its being mandated for interconnection to the grid or otherwise incentivized by the market? The objective of this thesis is to provide a better understanding of ride through fault capabilities of Grid Forming Inverter (GFM) tied into the generation side of the power grid when using control functions. Furthermore, to investigate the robustness of implementing time delay with a PLL system within the control settings for grid forming inverters. To this end, to identify the contributing factors that affects the stability of the time delay to better design and future models of GFMs. As discussed, the microgrid is a potential solution for future distributed generation systems. However, controlling a microgrid is still a complex issue and many proposed solutions, are only based on locally measured signals without any communications; thus, it is difficult to achieve global optimization. Future works on this topic will analyse the role of restoration practices, communication control techniques to better approximate the delay. The specific areas below will be discussed in this thesis

Modeling & Small Signal Analysis of Grid Forming Inverter

There is a rising number of inverter-based resources (IBRs) being integrated with distribution systems are becoming a more common occurrence. With integration of IBRs inverters, power utilities are experiencing an increase of number of operations with regards to voltage and frequency support. To maintain grid stability and reliability, IBRs need to provide some of the services currently (or formerly) provided by synchronous generators. Interconnection standards, like the IEEE 1547. 2018 has include requirements for IBRs to have the capability to provide some of these services—such as frequency and voltage support—and the procurement and deployment of the services can be implemented either as mandatory interconnection requirements or as market products. All the IBRs deployed today are grid-following (GFL), and read the voltage and frequency of the grid and inject current to provide the appropriate amount of active and reactive power. The fundamental GFL IBR design assumption is that there are still enough synchronous generators on the grid to provide a relatively strong and stable voltage and frequency signal, which GFL IBRs can “follow.” But since levels of GFL are increasing, there will be a limit to how far GFL controls can be pushed, and, at some point, new advanced inverter controls (termed grid forming (GFM)) will be needed to maintain system stability. GFM IBRs will also be needed to establish voltage and frequency during operating conditions when there are zero synchronous machines (100 percent IBR penetration). Power systems around the world are at the point of now needing to make this technological leap; however, system operators and planners, equipment owners, and manufacturers today are facing a circular problem regarding the deployment of advanced IBR controls. Which comes first, the requirement for a capability or the capability itself? How do grid operators know what performance or capability is possible from new equipment (and therefore what they could require)? How can they evaluate costs and benefits of having such equipment on the grid? What drives manufacturers to invest in modern technology without its being mandated for interconnection to the grid or otherwise incentivized by the market? The objective of this thesis is to provide a better understanding of ride through fault capabilities of Grid Forming Inverter (GFM) tied into the generation side of the power grid when using control functions. Furthermore, to investigate the robustness of implementing time delay with a PLL system within the control settings for grid forming inverters. To this end, to identify the contributing factors that affects the stability of the time delay to better design and future models of GFMs. As discussed, the microgrid is a potential solution for future distributed generation systems. However, controlling a microgrid is still a complex issue and many proposed solutions, are only based on locally measured signals without any communications; thus, it is difficult to achieve global optimization. Future works on this topic will analyse the role of restoration practices, communication control techniques to better approximate the delay. The specific areas below will be discussed in this thesis

Control and Stability of Residential Microgrid with Grid-Forming Prosumers

The rise of the prosumers (producers-consumers), residential customers equipped with behind-the-meter distributed energy resources (DER), such as battery storage and rooftop solar PV, offers an opportunity to use prosumer-owned DER innovatively. The thesis rests on the premise that prosumers equipped with grid-forming inverters can not only provide inertia to improve the frequency performance of the bulk grid but also support islanded operation of residential microgrids (low-voltage distribution feeder operated in an islanded mode), which can improve distribution grids’ resilience and reliability without purposely designing low-voltage (LV) distribution feeders as microgrids. Today, grid-following control is predominantly used to control prosumer DER, by which the prosumers behave as controlled current sources. These grid-following prosumers deliver active and reactive power by staying synchronized with the existing grid. However, they cannot operate if disconnected from the main grid due to the lack of voltage reference. This gives rise to the increasing interest in the use of grid-forming power converters, by which the prosumers behave as voltage sources. Grid-forming converters regulate their output voltage according to the reference of their own and exhibit load sharing with other prosumers even in islanded operation. Making use of grid-forming prosumers opens up opportunities to improve distribution grids’ resilience and enhance the genuine inertia of highly renewable-penetrated power systems. Firstly, electricity networks in many regional communities are prone to frequent power outages. Instead of purposely designing the community as a microgrid with dedicated grid-forming equipment, the LV feeder can be turned into a residential microgrid with multiple paralleled grid-forming prosumers. In this case, the LV feeder can operate in both grid-connected and islanded modes. Secondly, gridforming prosumers in the residential microgrid behave as voltage sources that respond naturally to the varying loads in the system. This is much like synchronous machines extracting kinetic energy from rotating masses. “Genuine” system inertia is thus enhanced, which is fundamentally different from the “emulated” inertia by fast frequency response (FFR) from grid-following converters. Against this backdrop, this thesis mainly focuses on two aspects. The first is the small-signal stability of such residential microgrids. In particular, the impact of the increasing number of grid-forming prosumers is studied based on the linearised model. The impact of the various dynamic response of primary sources is also investigated. The second is the control of the grid-forming prosumers aiming to provide sufficient inertia for the system. The control is focused on both the inverters and the DC-stage converters. Specifically, the thesis proposes an advanced controller for the DC-stage converters based on active disturbance rejection control (ADRC), which observes and rejects the “total disturbance” of the system, thereby enhancing the inertial response provided by prosumer DER. In addition, to make better use of the energy from prosumer-owned DER, an adaptive droop controller based on a piecewise power function is proposed, which ensures that residential ESS provide little power in the steady state while supplying sufficient power to cater for the demand variation during the transient state. Proposed strategies are verified by time-domain simulations

The Impact of Renewable Power Generation and Extreme Weather Events on the Stability and Resilience of AC Power Grids

Der erste Teil dieser Arbeit beschäftigt sich mit der Frage, welchen Einfluss kurzzeitige Schwankungen der erneuerbaren Energiequellen auf die synchrone Netzfrequenz haben. Zu diesem Zweck wird eine lineare Antworttheorie für stochastische Störungen von dynamischen Systemen auf Netzwerken hergeleitet. Anschließend wird diese Theorie verwendet, um den Einfluss von kurzfristigen Wind- und Sonnenschwankungen auf die Netzdynamik zu analysieren. Hierbei wird gezeigt, dass die Frequenzantwort des Netzes weitestgehend homogen ist, aber die Anfälligkeit für Leistungsschwankungen aufgrund von Leitungsverlusten entlang des Leistungsflusses zunimmt. Der zweite Teil der Arbeit befasst sich mit der Modellierung von netzbildenden Wechselrichterregelungen. Bislang existiert kein universelles Modell zur Beschreibung der kollektiven Dynamik solcher Systeme. Um dies zu erreichen, wird unter Ausnutzung der inhärenten Symmetrie des synchronen Betriebszustandes eine Normalform für netzbildende Akteure abgeleitet. Anschließend wird gezeigt, dass dieses Modell eine gute Annäherung an typische Wechselrichter-Dynamiken bietet, aber auch für eine datengesteuerte Modellierung gut geeignet ist. Der letzte Teil der Arbeit befasst sich mit der Analyse des Risikos von Stromausfällen, welche durch Hurrikans verursacht werden. Hohe Windgeschwindigkeiten verursachen häufig Schäden an der Übertragungsinfrastruktur, welche wiederum zu Überlastungen anderer Komponenten führen und damit eine Kaskade von Ausfällen im gesamten Netz auslösen können. Simulationen solcher Szenarien werden durch die Kombination eines meteorologischen Windmodells sowie eines Modells für kaskadierende Leitungsausfälle durchgeführt. Durch Monte-Carlo-Simulationen in einer synthetischen Nachbildung des texanischen Übertragungsnetzes können einzelne kritische Leitungen identifiziert werden, welche zu großflächigen Stromausfällen führen.The first part of this thesis addresses the question which impact short-term renewable fluctuations have on the synchronous grid frequency. For this purpose, a linear response theory for stochastic perturbations of networked dynamical systems is derived. This theory is then used to analyze the impact of short-term wind and solar fluctuations on the grid frequency. It is shown that while the network frequency response is mainly homogenous, the susceptibility to power fluctuations is increasing along the power flow due to transmission line losses. The second part of the thesis is concerned with modeling grid-forming inverter controls. So far there exists no universal model for studying the collective dynamics of such systems. By utilizing the inherent symmetry of the synchronous operating state, a normal form for grid-forming actors is derived. It is shown that this model provides a useful approximation of certain inverter control dynamics but is also well-suited for a data-driven modeling approach. The last part of the thesis deals with analyzing the risk of hurricane-induced power outages. High wind speeds often cause damage to transmission infrastructure which can lead to overloads of other components and thereby induce a cascade of failures spreading through the entire grid. Simulations of such scenarios are implemented by combining a meteorological wind field model with a model for cascading line failures. Using Monte Carlo simulations in a synthetic test case resembling the Texas transmission system, it is possible to identify critical lines that trigger large-scale power outages

A Universal Grid-forming Inverter Model and Simulation-based Characterization Across Timescales

The evolution of the power grid has given rise to a variety of innovations in inverter control architectures. Among these advances, a class of controllers has emerged with the aim of enabling 100\% inverter-based grids and these are known as grid-forming methods. Since these strategies are still under active development, well validated models are needed by equipment manufacturers as well as system planners and operators. In particular, a system operator may be unable to determine specifications and services that are required from grid forming devices without having the ability to represent them in a simulation environment with trusted models. A universal grid forming model that is portable across multiple simulation domains will be valuable in addressing this issue. In this paper, we develop a practical implementation of such a model that has the ability to represent four different grid-forming methods in a variety of simulation software packages while accurately capturing dynamics across from microsecond to millisecond timescales

Solutions to Improve Transient Stability of Universal Grid-forming Inverter-based Resources

In the future power grids, the share of grid-forming inverter-based resources must be increased in order to guarantee stability of the power system during rapid changes in generation, consumption and network topology. Therefore, it is important that the behavior and response of grid-forming inverters is also stable during different types of operation modes, events and faults. Previously, frequency stability improvement of grid-forming battery energy storages with universal frequency-locked-loop after load change in small HV network has been studied. However, in this paper the focus is on more severe disturbances i.e. unbalanced 2-phase and balanced 3-phase faults that are typically most challenging for the different synchronization methods. In this paper, based on multiple PSCAD simulations, new solutions are proposed to improve the transient frequency and voltage stability and fault-ride-through capability of grid-forming inverter-based resources with universal frequency-locked-loop during and after unbalanced and balanced faults.© 2023 The Authors. Published by Praise Worthy Prize S.r.l. This article is open access published under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) Available online by June 30th, 2023.fi=vertaisarvioitu|en=peerReviewed

Tradeoffs between AC power quality and DC bus ripple for 3-phase 3-wire inverter-connected devices within microgrids

Visions of future power systems contain high penetrations of inverters which are used to convert power from dc (direct current) to ac (alternating current) or vice versa. The behavior of these devices is dependent upon the choice and implementation of the control algorithms. In particular, there is a tradeoff between dc bus ripple and ac power quality. This study examines the tradeoffs. Four control modes are examined. Mathematical derivations are used to predict the key implications of each control mode. Then, an inverter is studied both in simulation and in hardware at the 10 kVA scale, in different microgrid environments of grid impedance and power quality. It is found that voltage-drive mode provides the best ac power quality, but at the expense of high dc bus ripple. Sinusoidal current generation and dual-sequence controllers provide relatively low dc bus ripple and relatively small effects on power quality. High-bandwidth dc bus ripple minimization mode works well in environments of low grid impedance, but is highly unsuitable within higher impedance microgrid environments and/or at low switching frequencies. The findings also suggest that the certification procedures given by G5/4, P29 and IEEE 1547 are potentially not adequate to cover all applications and scenarios