Addressing Instability Issues in Microgrids Caused By Constant Power Loads Using Energy Storage Systems

Renewable energy sources, the most reasonable fuel-shift taken over the naturally limited conventional fuels, necessarily deal with the self-functional microgrid system rather than the traditional grid distribution system. The study shows that the microgrid system, a comparatively low-powered system, experiences the challenge of instability due to the constant power load (CPL) from many electronic devices such as inverter-based systems. In this dissertation, as a methodical approach to mitigate the instability complication, AC microgrid stability is thoroughly investigated for each and every considerable parameter of the system. Furthermore, a specific loading limit is depicted by evaluating the stability margin from the small signal analysis of the microgrid scheme. After demonstrating all cases regarding the instability problem, the storage-based virtual impedance power compensation method is introduced to restore the system stability and literally extend the loading limit of the microgrid system. Here, a PID controller is implemented to maintain the constant terminal voltage of CPL via current injection method from storage. Since the system is highly nonlinear by nature, advanced nonlinear control techniques, such as Sliding Mode Control and Lyapunov Redesign Control technique, are implemented to control the entire nonlinear system. Robustness, noise rejection, and frequency variation are scrutinized rigorously in a virtual platform such as Matlab/Simulink with appreciable aftermaths. After that, a comparative analysis is presented between SMC and LRC controller robustness by varying CPL power. From this analysis, it is evident that Lyapunov redesign controller performs better than the previous one in retaining microgrid stability for dense CPL-loaded conditions. Finally, to ensure a robust storage system, Hybrid Energy Storage System is introduced and its advantages are discussed as extended research work

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

Grid-Forming Converter Control Method to Improve DC-Link Stability in Inverter-Based AC Grids

As renewable energy sources with power-electronic interfaces become functionally and economically viable alternatives to bulk synchronous generators, it becomes vital to understand the behavior of these inverter-interfaced sources in ac grids devoid of any synchronous generation, i.e. inverter-based grids. In these types of grids, the inverters need to operate in parallel in grid-forming mode to regulate and synchronize their output voltage while also delivering the power required by the loads. It is common practice, therefore, to mimic the parallel operation control of the very synchronous generators that these inverter-based sources are meant to replace. This practice, however, is based on impractical assumptions and completely disregards the key differences between synchronous machines and power electronic inverters, as well as the dynamics of the dc source connected to the inverter. This dissertation aims to highlight the shortcomings of conventional controllers and derive an improved grid-forming inverter controller that is effective in parallel ac operation without sacrificing dc-link stability. This dissertation begins with a basis for understanding the control concepts used by grid-forming inverters in ac grids and exploring where existing ideas and methods are lacking in terms of efficient and stable inverter control. The knowledge gained from the literature survey is used to derive the requirements for a grid-forming control method that is appropriate for inverter-based ac grids. This is followed by a review and comparative analysis of the performance of five commonly used control techniques for grid-forming inverters, which reveal that nested loop controllers can have a destabilizing effect under changing grid conditions. This observation is further explored through an impedance-based stability analysis of single-loop and nested-loop controllers in grid-forming inverters, followed by a review of impedance-based analysis methods that can be used to assess the control design for grid-forming inverters. An improved grid-forming inverter controller is proposed with a demonstrated ability to achieve both dc-link and ac output stability with proportional power-sharing. This dissertation ends with a summary of the efforts and contributions as well as ideas for future applications of the proposed controller

Modulation and Control Techniques for Performance Improvement of Micro Grid Tie Inverters

The concept of microgrids is a new building block of smart grid that acts as a single controllable entity which allows reliable interconnection of distributed energy resources and loads and provides alternative way of their integration into power system. Due to its specifics, microgrids require different control strategies and dynamics of regulation as compared to ones used in conventional utility grids. All types of power converters used in microgrid share commonalities which potentially affect high frequency modes of microgrid in same manner. There are numerous unique design requirements imposed on microgrid tie inverters, which are dictated by the nature of the microgrid system and bring major challenges that are reviewed and further analyzed in this work. This work introduces, performs a detailed study on, and implements nonconventional control and modulation techniques leading to performance improvement of microgrid tie inverters in respect to aforementioned challenges

Contributions to impedance shaping control techniques for power electronic converters

El conformado de la impedancia o admitancia mediante control para convertidores electrónicos de potencia permite alcanzar entre otros objetivos: mejora de la robustez de los controles diseñados, amortiguación de la dinámica de la tensión en caso de cambios de carga, y optimización del filtro de red y del controlador en un solo paso (co-diseño). La conformación de la impedancia debe ir siempre acompañada de un buen seguimiento de referencias. Por tanto, la idea principal es diseñar controladores con una estructura sencilla que equilibren la consecución de los objetivos marcados en cada caso. Este diseño se realiza mediante técnicas modernas, cuya resolución (síntesis del controlador) requiere de herramientas de optimización. La principal ventaja de estas técnicas sobre las clásicas, es decir, las basadas en soluciones algebraicas, es su capacidad para tratar problemas de control complejos (plantas de alto orden y/o varios objetivos) de una forma considerablemente sistemática. El primer problema de control por conformación de la impedancia consiste en reducir el sobreimpulso de tensión ante cambios de carga y minimizar el tamaño de los componentes del filtro pasivo en los convertidores DC-DC. Posteriormente, se diseñan controladores de corriente y tensión para un inversor DC-AC trifásico que logren una estabilidad robusta del sistema para una amplia variedad de filtros. La condición de estabilidad robusta menos conservadora, siendo la impedancia de la red la principal fuente de incertidumbre, es el índice de pasividad. En el caso de los controladores de corriente, el impacto de los lazos superiores en la estabilidad basada en la impedancia también se analiza mediante un índice adicional: máximo valor singular. Cada uno de los índices se aplica a un rango de frecuencias determinado. Finalmente, estas condiciones se incluyen en el diseño en un solo paso del controlador de un convertidor back-to-back utilizado para operar generadores de inducción doblemente alimentados (aerogeneradores tipo 3) presentes en algunos parques eólicos. Esta solución evita los problemas de oscilación subsíncrona, derivados de las líneas de transmisión con condensadores de compensación en serie, a los que se enfrentan estos parques eólicos. Los resultados de simulación y experimentales demuestran la eficacia y versatilidad de la propuesta.Impedance or admittance shaping by control for power electronic converters allows to achieve among other objectives: robustness enhancement of the designed controls, damped voltage dynamics in case of load changes, and grid filter and controller optimization in a single step (co-design). Impedance shaping must always be accompanied by a correct reference tracking performance. Therefore, the main idea is to design controllers with a simple structure that balance the achievement of the objectives set in each case. This design is carried out using modern techniques, whose resolution (controller synthesis) requires optimization tools. The main advantage of these techniques over the classical ones, i.e. those based on algebraic solutions, is their ability to deal with complex control problems (high order plants and/or several objectives) in a considerably systematic way. The first impedance shaping control problem is to reduce voltage overshoot under load changes and minimize the size of passive filter components in DC-DC converters. Subsequently, current and voltage controllers for a three-phase DC-AC inverter are designed to achieve robust system stability for a wide variety of filters. The least conservative robust stability condition, with grid impedance being the main source of uncertainty, is the passivity index. In the case of current controllers, the impact of higher loops on impedance-based stability is also analyzed by an additional index: maximum singular value. Each of the indices is applied to a given frequency range. Finally, these conditions are included in the one-step design of the controller of a back-to-back converter used to operate doubly fed induction generators (type-3 wind turbines) present in some wind farms. This solution avoids the sub-synchronous oscillation problems, derived from transmission lines with series compensation capacitors, faced by these wind farms. Simulation and experimental results demonstrate the effectiveness and versatility of the proposa

Virtual synchronous-machine control of voltage-source converters in a low-voltage microgrid

In order to facilitate the further integration of distributed renewable generation into existing power systems, enhanced control schemes for grid-tied power electronic converters are necessary to ensure non-synchronous power sources can provide power and support to the grid. The virtual-synchronous-machine concept proposes the use of control schemes to enable static generators to operate with the dynamics of rotating synchronous generators. In this paper, a control scheme is presented based on the principle of active-power synchronization to regulate the active power of a grid-tied voltage-source converter based on an emulation of the synchronous-machine swing equation. Design of a cascaded inner-loop voltage and resonant current control is presented to regulate the output voltage as specified via the outer-loop virtual-machine control scheme responsible for power regulation. The performance of this control scheme is investigated within the context of microgrid operation for the provision of active and reactive power to the system, and microgrid frequency support. Experimental validation is provided via the use of a 15 kVA three-phase VSC in a 90 kVA 400V microgrid

Small signal modeling and analysis of microgrid systems

This dissertation focuses on small-signal modeling and analysis of inverter based microgrid systems. The proposed microgrid consists of two microsources placed on two different buses. The buses are connected using a distribution feeder with some impedance. The proposed microgrid can operate with the grid support, or without the grid support. When operated without the grid support, the standalone system’s microsources participate in controlling the system voltage and frequency. For a non-inertia source, such as the inverter, the load perturbations play an important role in system dynamics. In paper-I, such complex system was studied. In the grid-tied mode, the microsources share the load demand with other sources that are present in the main grid. The control algorithm for such system is much simpler than that of the islanded system. However, when aggregated in multi-bus system, prohibitively higher order state-space models are formed. In paper-II, a reduced order modeling of such systems was considered. Singular perturbation method was applied to identify the two time-scale property of the system. In paper-III, a similar approach was taken to develop a reduced order model of the islanded system that was developed in paper-I. Application of such reduced order models were illustrated by using them to simulate a modified IEEE-37 bus microgrid system. The islanded microgrids system’s stability is characterized in paper-IV by the Markov Jump Linear System Analysis. Conservative bounds on the expected value of the state were determined from a combination of the Markov process parameters, the dynamics of each linear system, and the magnitude of the impulses. The conclusions were verified with the simulation results. --Abstract, page iii