Because renewable energy sources are environment-friendly and inexhaustible, more and more renewable energy power plants have been integrated into power grids worldwide. To compensate for their inherent variability, STATCOMs are typically installed at the point of common coupling (PCC) to help their operation by regulating the PCC voltage. However under different contingencies, PCC voltage fluctuations in magnitude and frequency may impede the STATCOM from tracking the grid frequency correctly, hence worsening its overall compensation performance, and putting at risk the operation of the power plant. Further, the virtual synchronous machine (VSM) concept has recently been introduced to control grid-connected inverters emulating the behavior of rotating synchronous machines, in an effort to eliminate the shortcomings of conventional d-q frame phase-locked loops (PLL). In this dissertation, the VSM concept is extended by developing a STATCOM controller with it, which then behaves like a fully-adjustable synchronous condenser, including the adjustment of its "virtual" inertia and impedance. An average model in two D-Q frames is proposed to analyze the inherent dynamics of the VSM-based STATCOM controller with insight into impacts from the virtual parameters and a design guideline is then formulated. The proposed controller is compared against existent d-q frame STATCOM control strategies, evincing how the VSM-based approach guarantees an improved voltage regulation performance at the PCC by adjusting the phase of its compensating current during frequency fluctuations, in both simulation and experiment. Secondly, the dynamics of the VSM-based STATCOM controller in large signal sense is studied, especially its capability to ride through faults. Analysis is carried out with phasors to obtain a fundamental understanding at first and followed by state space equations to predict the transients analytically, which is validated by matching both simulation and experiment. The effects of two outer loops are also reviewed and some possible solutions are suggested and evaluated. Moreover, the relationship between the virtual inertia and the actual inertia is established and the dc capacitor sizing is discussed in a possibly more economical way. The start-up process of a VSM-based STATCOM is presented to implement a practical prototype as well. / Master of Science

This thesis applies the concept of a virtual-synchronous-machine- (VSM-) based control to a conventional 250-kW utility-scale photovoltaic (PV) inverter. VSM is a recently-developed control scheme which offers an alternative grid-synchronization method to the conventional grid-tracking control scheme, which is based on the dq phase-locked-loop- (PLL-) oriented vector control. Synchronous machines inherently synchronize to the grid and largely partake in the stabilization of the grid frequency during power system dynamics. The purpose of this thesis is primarily to present the design of a grid-forming control scheme based on the VSM and the derivation of the terminal dq-frame ac impedance of the small-signal model of the inverter and control scheme. This design is also compared to the design of the conventional grid-tracking control structure, both from a loop design and terminal dq-frame ac impedance standpoint. Due to the inherent lax power-balance synchronization, the grid-forming control scheme results in 1 to 2 decades' lower frequency range of negative incremental input impedance in the diagonal elements, which is a favorable condition for stability. Additionally, the stability of the grid-forming control scheme is compared to the conventional grid-tracking control using the generalized Nyquist criterion (GNC) for stability under three modes of operation of active and reactive power injection. It is found that the connection is stable for both control schemes under unity power factor and fixed reactive power modes; however, the grid-forming control is able to inject twice the amount of active power under the voltage regulation mode when compared to the grid-tracking control. / Master of Science / Concerns about the current and future state of the environment has prompted government and non-profit agencies to enact regulatory legislation on fossil fuel emissions. In 2017, electricity generation comprised 28% of total U.S. greenhouse gas emissions with 68% of this generation being due to coal combustion sources. As a result, utilities have retired a number of coal power plants and have employed alternative means of power generation, specifically renewable energy sources (RES). Most RES operate as variable-frequency ac sources (wind) or dc sources (solar) and are interfaced with the power grid through ac-dc-ac or dc-ac converters, respectively, which are power-electronic devices used to control the injection of power to the grid. Conventional converters synchronize with the grid by tracking the phase of the voltage at the point of common coupling (PCC) through a phase-locked loop (PLL). While power system dynamics significantly affect the performance of a PLL, and, subsequently, inverters' operation, the initial frequency regulation during grid events is attributed to the system's inherent inertia due to the multitude of synchronous machines (SM). However, with the steady increase of RES penetration, even while retaining the number of SM units, the net inertia in the system will decrease, thus resulting in prolonged responses in frequency regulation to the aforementioned dynamics. This thesis investigates the control of variable-frequency sources as conventional synchronous machines and provides a detailed design procedure of this control structure for photovoltaic (PV) inverter applications. Additionally, the stability of the connection of the inverter to the grid is analyzed using innovative stability analysis techniques which treat the inverter and control as a black box. In this manner, the inner-workings of the inverter need not be known, especially since it is proprietary information of the manufacturer, and the operator can measure the output response of the device to some input signal. In this work, it is found that the connection between the inverter and grid is stable with this new control scheme and comparable to conventional control structures. Additionally, the control based on synchronous machine characteristics shows improved stability for voltage and frequency regulation, which is key to maintaining a stable grid.

The unconventional improvements in the power electronics field have been the primary reason for massive deployment of renewable energy sources in the electrical power grid over the past several decades. This needed trend, together with the increasing penetration of micro-, and nano- grids, is bringing significant improvements in system controllability, performance, and energy availability, but is fundamentally changing the nature of electronically-interfaced sources and loads, altering their conventionally mild aggregate dynamics, and inflicting low- and high- frequency dynamic interactions that never before existed at this magnitude. This problem is not restricted only to the grid; modern electronic power distribution systems built for airplanes, ships, electric vehicles, data-centers, and homes, comprise dozens, even hundreds of power electronics converters, produced by different manufacturers, who provide very limited details on converters' dynamic behavior - distinctiveness that has the highest impact on how two converters, or converter and a system interact. Consequently, substantial dispersion of power electronics into the future grid will significantly depend on engineers' capability to understand how to model and dynamically control power flow and subsystem interactions. It is therefore essential to continue developing innovative methods that allow easier system-level modeling, continuous monitoring of dynamic interactions, and advanced control concepts of power electronics converters and systems. The dissertation will start with a "black box" approach to modeling of three-phase power electronics converters, introducing a method to remove source and load dynamics from in-situ measured terminated frequency responses. It will be then shown how converter, itself, can perform an online stability assessment knowing its own unterminated dynamics, and being able to measure all terminal immittances. The dissertation will further advance into an approach to control power electronics converters based on the electro-mechanical duality with synchronous machines, and end with selected examples of system-level operation, where small-signal instability in multi-source power systems can be mitigated using this concept. / Ph. D. / The modern technological advancements and ever-increasing needs for a sustainable future silently demand a serious revision of the conventional practice in electricity production, distribution, and utilization. These technologies are already challenging the limits of the biggest and most complex system ever built by humankind - the electrical grid. One practical solution to this problem is much higher dispersion of electronic power conversion systems capable of decoupling dynamics between system sources, distribution, and loads, while improving system controllability, reliability, and efficiency. Such a trend is already happening, and there has been an increased immersion of power electronics converters in electric cars, ships, airplanes, and the grid, in an effort to replace their traditional thermal, mechanical, hydraulic, and pneumatic systems. The goals have been to reduce the size, weight, and operational costs while increasing efficiency and reliability. In all these applications, a majority of energy sources and loads are interfaced to the power system through power electronics converters ranging in power from few watts to hundreds of megawatts. However, massive dispersion of power electronics into the future grid will significantly depend on engineers’ capability to understand how to model and dynamically control power flow and subsystem interactions. It is important to continue researching innovative methods that allow easier system-level modeling, continuous monitoring of interactions, and advanced control concepts of power electronics converters and systems. This dissertation hence addresses modeling of power electronics converters using their behavioral models, and shows how these models can assist the stability assessment of the system converters operate in. Additionally, dissertation presents an alternative way to control power electronics converters to behave as synchronous machines, and how this concept can be used to mitigate some stability problems.

The modern power system is aiming to progress away from conventional synchronous machine  based power generation towards converter dominated system that leads to extensively high penetration of renewable energy such as wind and PV. This transition of modern power system toward converter based renewable energy comes with new challenges as the conventional synchronous generation is being replaced by converter based power system (CBPS). The converter is commonly interfaced to the power system with Phase Locked Loop (PLL) technique to synchronize the converter with the grid voltage angle and inject the current at the right angle. Therefore, this approach is called grid- ­following converter; this type of configuration of converters may lead to some power system instabilities (e.g., voltage instability, frequency instability, synchronous and sub­synchronous instabilities). In order to overcome the limitation of the grid-­following converters, another converter control concept become present in the literature as a grid-­forming converter where the synchronizing method to the grid eliminates the need for PLL .In this thesis, a grid- ­forming controlled power converter is implemented with an energy storage system to emulate the inertia of the synchronous generator through the VSM control concept. An electromagnetic transient (EMT) simulation has been modeled in the PSCAD simulation environment. The model is the well­known four-­machine two-­area power system. The model has been tested by incrementally replacing the synchronous machines with wind farms connected through power converters; this weakens the grid and may lead to frequency instability during a disturbing event. An Energy Storage System (ESS) has been implemented and added to the system to mitigate the loss of the kinetic energy of the rotating masses of the synchronous generators. The ESS is integrated with a grid-­forming converter that is controlled to mimic the dynamic behavior of a synchronous generator. Thus, the ESS is synchronized to the system based on the swing equation of the synchronous generator. The results show significant improvements in the frequency stability of the system under study. / Det moderna energisystemet har som mål att bortgå från den konventionella synkronmaskinbaserade energiförsörjning mot ett konverteringsdominerat system som leder till  en mycket hög penetration av förnybar energi, som tillexempel vind och solenergi. Den här övergången av modernt energisystem mot konverteringsbaserad förnybar energi medför nya utmaningar i och med att konventionell synkrongenerering byts ut mot konverterarbaserat energisystem (Converter Based Power System, CBPS) . Konverteraren är ofta integrerad i energisystemet via Phase Locked Loop ­teknik (PLL) för att synkronisera konverteraren med kraftnätets spänningsvinkel och injicera strömmen i rätt vinkel. Det här tillvägagångssättet kallas därför för nätföljande konvertering; denna typ av konfiguration av konverterare kan leda till instabilitet i energisystemet (t.ex. instabil elektrisk spänning, frekvensinstabilitet, synkron och sub- ­synkron instabilitet). För att hantera begränsningarna som nätföljande konverterare träder ett koncept om ytterligare en konverteringskontroll fram i litteratur, i form av en nätformande konverterare där synkroniseringsmetoden i nätet eliminerar behovet av PLL.I denna avhandling implementeras en nätformande konverterare med ett energiförvaringssystem för att emulera trögheten i synkrongeneratorn genom VSM-­styrkonceptet. En elektromagnetisk transientsimulering (EMT) har modellerats i simuleringsmiljön PSCAD. Modellen är det välkända energisystemet med fyra maskiner och två områden. Modellen har testats genom att stegvis byta ut synkronmaskinerna med vindkraftverk anslutna genom energikonverterare; detta gör nätet svagare och kan leda till frekvensinstabilitet vid en störande händelse. Ett energiförvaringssystem (Energy Storage System, ESS) har implementerats och kopplats till systemet för att mildra förlusten av kinetisk energi i de roterande massorna hos synkrongeneratorerna. Energiförvaringssystemet ESS är integrerat med en nätformande konverterare som styrs för att härma det dynamiska uppförandet av en synkrongenerator. Således är ESS synkroniserat med systemet baserat påsynkrongeneratorns svängekvation. Resultaten visar betydelsefulla förbättringar av frekvensstabiliteten i systemet under studien.