Energy Shaping Control for Stabilization of Interconnected Voltage Source Converters in Weakly-Connected AC Microgrid Systems

With the ubiquitous installations of renewable energy resources such as solar and wind, for decentralized power applications across the United States, microgrids are being viewed as an avenue for achieving this goal. Various independent system operators and regional transmission operators such as Southwest Power Pool (SPP), Midcontinent System Operator (MISO), PJM Interconnection and Electric Reliability Council of Texas (ERCOT) manage the transmission and generation systems that host the distributed energy resources (DERs). Voltage source converters typically interconnect the DERs to the utility system and used in High voltage dc (HVDC) systems for transmitting power throughout the United States. A microgrid configuration is built at the 13.8kV 4.75MVA National Center for Reliable Energy Transmission (NCREPT) testing facility for performing grid-connected and islanded operation of interconnected voltage source converters. The interconnected voltage source converters consist of a variable voltage variable frequency (VVVF) drive, which powers a regenerative (REGEN) load bench acting as a distributed energy resource emulator. Due to the weak-grid interface in islanded mode testing, a voltage instability occurs on the VVVF dc link voltage causing the system to collapse. This dissertation presents a new stability theorem for stabilizing interconnected voltage source converters in microgrid systems with weak-grid interfaces. The new stability theorem is derived using the concepts of Dirac composition in Port-Hamiltonian systems, passivity in physical systems, eigenvalue analysis and robust analysis based on the edge theorem for parametric uncertainty. The novel stability theorem aims to prove that all members of the classes of voltage source converter-based microgrid systems can be stabilized using an energy-shaping control methodology. The proposed theorems and stability analysis justifies the development of the Modified Interconnection and Damping Assignment Passivity-Based Control (Modified IDA-PBC) method to be utilized in stabilizing the microgrid configuration at NCREPT for mitigating system instabilities. The system is simulated in MATLAB/SimulinkTM using the Simpower toolbox to observe the system’s performance of the designed controller in comparison to the decoupled proportional intergral controller. The simulation results verify that the Modified-IDA-PBC is a viable option for dc bus voltage control of interconnected voltage source converters in microgrid systems

Energy Storage Control and Requirements For Inverter-Based Microgrids

The intermittent nature of distributed renewable sources such as wind or solar requires integration of energy storage systems. In this dissertation a distributed form of the Hamiltonian Surface Shaping and Power Flow Control (HSSPFC) method is used to determine the energy storage requirements of three-phase inverter-based microgrids. The overall control is appropriate to be integrated into a hierarchical control system. As the primary control, a novel dq droop control sets the local references and is supported by a level-zero Hamiltonian controller which includes energy storage feed-forward and feedback, and an inverter feed-forward controls. Here, the energy storage element performs as the sole actuator of the system and enforces the references that are set by the droop control while the inverter feed-forward matches the voltage levels of the inverter to the local bus. The control method as well as the power flow and energy transfer model of the microgrid system enables the capacity and bandwidth of the storage system to be determined. The Hamiltonian control is further derived for parallel operation of hybrid, band-limited and reduced-order battery and flywheel storage systems. Moreover, a control scheme is proposed to enable sharing of power between parallel battery and flywheel storage systems according to their bandwidth support capabilities. Here, battery storage systems are considered as the primary storage elements while flywheel systems are controlled to complement the deficit for higher power fluctuations. Power and energy sizing guidelines are presented and relevant trade-offs are addressed in illustrative examples. Energy storage baseline requirements for pulsed power loads are also presented in this work. Here, the energy storage system combination with the pulsed load is controlled to mimic a constant power load that can further be integrated into power buffer systems. Examples of control and requirements for ideal, band-limited and reduced-order battery and flywheel storage systems are given. By considering these requirements, a system designer can derive the specifications for source-side or load-side energy storage elements and their control systems

Passivity voltage based control of the boost power converter used in photovoltaic system

Introduction. This paper presents a robust nonlinear control of the DC-DC boost converter feeding by a photovoltaic system based on the passivity control. The control law design uses the passivity approach. Novelty. The novelty consists in designing a control law for a photovoltaic system using a passivity approach based on energy shaping and associated with damping injection. Purpose. The purpose consists to develop a tool for design and optimize a control law of the photovoltaic system in order to improve its efficiency under some conditions such as the variations of the temperature, the irradiation and the parameters. Also, the control law design should be simple with a lower overshoot and a shorter settling time. Methods. This work uses the port Hamiltonian mathematical approach with minimization of the energy dissipation in boost converter of the photovoltaic system to illustrate the modification of energy and generate a specify duty cycle applied to the converter. Results. The results with MATLAB/SimPowerToolbox® have proven the robustness against parameter variations and effectiveness of the proposed control. Practical value. The experimental results, carried out using a dSPACE DS1104 system, are presented to show the feasibility and the robustness of the proposed control strategy against parameter variations.Вступ. У статті представлено надійне нелінійне керування живленням перетворювача постійного струму, що підвищує, фотоелектричною системою на основі керування пасивністю. У створенні закону управління використовується пасивний підхід. Новизна. Новизна полягає у розробці закону управління фотоелектричною системою з використанням пасивного підходу, заснованого на формуванні енергії та пов'язаного з упорскуванням демпфування. Мета. Мета полягає в тому, щоб розробити інструмент для проектування та оптимізації закону керування фотогальванічною системою для підвищення її ефективності за деяких умов, таких як зміни температури, опромінення та параметрів. Крім того, будова закону управління має бути простою, з меншим перерегулюванням і коротшим часом встановлення. Методи. У роботі використовується математичний підхід Гамільтона до порту з мінімізацією розсіювання енергії у перетворювачі фотоелектричної системи, що підвищує, щоб проілюструвати зміну енергії і створити заданий робочий цикл, що застосовується до перетворювача. Результати. Результати з використанням MATLAB/SimPowerToolbox® довели стійкість до змін параметрів та ефективність запропонованого керування. Практична цінність. Представлені експериментальні результати, отримані з використанням системи dSPACE DS1104, щоб показати здійсненність та стійкість запропонованої стратегії управління при  зміні параметрів

Modeling, Simulation and Control of Doubly-Fed Induction Machine Controlled by Back-to-Back converter

Aquesta Tesi estudia el control d'un sistema complex, un sistema d'emmagatzement d'energia cinètica, incloent les seves especificacions de control, modelat, disseny de controladors, simulacions, muntatge i validació experimental.Primerament, s'estudia l'interconnexió i control dels sistemes electromecànics. Es presenta el formalisme Hamiltonià (PCHS) en general, i després particularitzant en els sistemes electromecànics, inclòs els sistemes d'estructura variable (VSS).L'IDA-PBC (Interconnection and damping assignment-passivity based control) és una tècnica de control basat en els PCHS. En aquesta Tesi s'estudien el problemes que apareixen en controlar, per IDA-PBC, sortides de grau relatiu u quan el paràmetres nominals del controlador són incerts. Per evitar-los es proposa introduir una acció integral que pot ésser interpretada dins l'estructura Hamiltoniana.En aquesta Tesi també es presenten dues modificacions que permeten millorar el rang d'aplicacions de la tècnica IDA-PBC. Primer, es demostra que el fet de descomposar la tècnica de l'IDA-PBC en deformar la funció d'energia i una injecció de fregament, redueix el conjunt de sistemes que es poden estabilitzar mitjançant aquest mètode. Per evitar aquest problema, es proposa fer simultàniament els dos passos donant lloc a l'anomenat SIDA-PBC. Per altre costat, el mètode IDA-PBC requereix el coneixement de la funció energia (o Hamiltonià). Això representa un problema perquè, en general, el punt d'equilibri depèn de paràmetres incerts. En aquest treball es desenvolupa una metodologia per seleccionar l'estructura Hamiltoniana que redueix aquesta dependència dels paràmetres. Aquesta tècnica permet millorar la robustesa dels les sortides d'ordre relatiu superior a u.El sistema d'emmagatzement d'energia cinètica consisteix en una màquina d'inducció doblament alimentada (DFIM) amb un volant d'inèrcia, controlada pel rotor per un convertidor de potència back-to-back (B2B). L'objectiu és gestionar el flux d'energia entre la DFIM i una càrrega local connectada a la xarxa, commutant entre diferents punts de funcionament. Per això es planteja una gestió de l'energia, basada en la velocitat òptima de la DFIM.Pel què fa al control de la DFIM, es proposa un nou esquema de control que ofereix importants avantatges, i que és considerablement més senzill que el mètode clàssic, el vector control. Aquest nou controlador permet una fàcil descomposició de les potències activa i reactiva de l'estator, i el seu control a través de les tensions de rotor. Aquest disseny s'obté aplicant el procediment que millora la robustesa de l'IDA-PBC.S'han estudiat d'altres controladors, com el vector control clàssic. També a partir de la tècnica IDA-PBC, on l'equació en derivades parcials que apareix en aplicar el mètode es pot resoldre fixant l'energia en llaç tancat, i afegint nous termes a la matriu d'interconnexió. Per obtenir un controlador definit globalment s'afegeix un terme de fregament depenent dels estats, que desacobla la part elèctrica i mecànica del sistema. Finalment, també es prova que mitjançant el SIDA-PBC es pot modelar l'energia total (elèctrica i mecànica) de la DFIM. Tots aquest controladors han estat simulats i comparats. El controlador robust IDA-PBC s'ha validat experimentalment amb uns resultats satisfactoris. A la Tesi també es presenta un controlador que permet el flux bidireccional de potència pel B2B. L'estudi de la dinàmica zero adverteix que les tècniques de control estàndard no garanties en l'estabilitat en ambdós direccions, i per això s'utilitza un controlador IDA-PBC. Pel disseny s'utilitza un model basat en GSSA (generalized state space averaging), on es descomposa i es trunca el sistema per determinades freqüències, i que permet expresar els objectius de control (tensió constant al bus de contínua i factor de potència unitari) com un problema de regulació. Les simulacions i els resultats experimentals validen, tant la llei de control, com les simplificacions efectuades.Els controladors proposats i validats experimentalment són usats, finalment, per implementar la gestió de potència del sistema d'emmegatzement d'energia cinètica. Els resultats confirmen el bon comportament del sistema i dels controladors IDA-PBC proposats.This Thesis studies a complex multidomain system, the Flywheel Energy Storage System, including the control objectives specification, modeling, control design, simulation, experimental setup assembling and experimental validation stages.The port interconnection and control of electromechanical systems is studied. The port Hamiltonian formalism is presented in general, and particularized for generalized electromechanical systems, including variable structure systems (VSS).Interconnection and damping assignment-passivity based control (IDA-PBC) is a well known technique for port Hamiltonian systems (PCHS). In this Thesis we point out the kind of problems that can appear in the closed-loop structure obtained by IDA-PBC methodsfor relative degree one outputs, when nominal values are used in a system with uncertain parameters. To correct this, we introduce an integral control, which can be cast into the Hamiltonian framework.This Thesis also presents two new approaches which improve the range of applicability of the IDA-PBC technique. First, we show that the standard two-stage procedure used in IDA-PBC consisting of splitting the control action into the sum of energy-shaping and damping injection terms is not without loss of generality, and effectively reduces the set of systems that can be stabilized with IDA-PBC. To overcome this problem we suggest to carry out simultaneously both stages and refer to this variation of the method as SIDA-PBC.Secondly, we present an improvement of the IDA-PBC technique. The IDA-PBC method requires the knowledge of the full energy (or Hamiltonian) function. This is a problem because, in general, the equilibrium point which is to be regulated depends on uncertain parameters. We show how select the target port-Hamiltonian structure so that this dependence is reduced. This new approach allows to improve the robustness for higher relative degree outputs.The Flywheel Energy Storage System consists of a doubly-fed induction machine (DFIM), controlled through the rotor voltage by a power electronics subsystem (a back-to-back AC/AC converter (B2B)), and coupled to flywheel. The control objective is to optimally regulate the power flow between the DFIM and a local load connected to the grid, and this is achieved by commuting between different steady-state regimes. A police management based on the optimal speed for the DFIM is proposed.In this Thesis we propose a new control scheme for the DFIM that offers significant advantages, and is considerably simpler, than the classical vector control method. This controller allows an easy decomposition of the active and reactive powers on the stator side and their regulation, acting on the rotor voltage, via stator current control. This design was obtained applying the new robust IDA-PBC procedure.Other controllers are also designed along the dissertation. The classical vector control is studied. We also apply the classic IDA-PBC technique. It is shown that the partial differential equation that appears in this method can be circumvented by fixing the desired closed-loop total energy and adding new terms to the interconnection structure. Furthermore, to obtain a globally defined control law we introduce a state--dependent damping term that has the nice interpretation of effectively decoupling the electrical and mechanical parts of the system. This results in a globally convergent controller parameterized by two degrees of freedom. Finally, we also prove that with SIDA-PBC we can shape the total energy of the full (electrical and mechanical) dynamics of the DFIM. These different controllers (vector control, IDA-PBC, SIDA-PBC and robust IDA-PBC) are simulated and compared. The IDA-PBC robust controller is also experimentally tested and shown to work satisfactorily.A controller able to achieve bidirectional power flow for the B2B converter is presented. Standard techniques cannot be used since it is shown that no single output yields a stable zero dynamics for power flowing both ways. The controller is computed using standard IDA-PBC techniques for a suitable generalized state space averaging truncation of the system, which transforms the control objectives, namely constant output voltage dc-bus and unity input power factor, into a regulation problem. Simulation and experimental results for the full system confirm the correctness of the simplifications introduced to obtain the controller.The proposed and tested controllers for the DFIM and the B2B are used to implement the power management policy. These results show a good performance of the flywheel energy storage system and also validate the IDA-PBC technique, with the proposed improvements

MODELING AND CONTROL OF MICROGRID COMPONENTS

Due to the increase in the integration of renewable energy resources into electrical power systems, there are various challenges that modern power systems are facing. A lot of issues in this subject are discussed under the concept of microgrid and their operational and control concerns. Power electronic interfaces (converters, inverters) are necessary for connecting generation units based on renewable energy resources to the power grid. Consequently, inverter control is a primary issue in operating microgrids. Fast dynamics of power electronic interfaces results in different operating concerns and strategies for inverter-based generation units as compared to large conventional synchronous generators. To provide simplicity in operating inverter-based generation units, there are various control strategies based on emulating the critical properties of a conventional synchronous generator such as inertia and damping. This dissertation designs a novel operational and control model for controlled power electronic loads and inverter-based generators inspired by synchronous generators' equations and stated in port-Hamiltonian systems' formulation. This inverter generator controller is added to the inverter switching controller to enable the generator to behave in a manner similar to a synchronous generator. We develop a control methodology based on Interconnection and Damping Assignment Passivity Based Control (IDA-PBC) strategy for the proposed inverter-based generator dynamics. We prove the stability of the designed closed loop system and develop a simulation model for the projected control strategy that includes an example system consisting of a constant impedance load, a $\pi-$modeled line and an inverter-based generator. We also develop a generic port-Hamiltonian model for loads that allows through the appropriate selection of structure and controls the mimicking of the behavior of complex loads that are connected to the grid through controlled power electronic interfaces

Development of a Port-Hamiltonian Model for use in oscillating water column control scheme investigations.

With global energy demand estimated to rise considerably and global warming accepted by the majority of scientists, the pressure to reduce fossil fuel usage is increasing. To this end, the UK government has set a target of generating 50% of electricity from renewable energy sources by 2050. It can therefore be deduced that decreasing the cost of renewable energy by increasing the energy capture is critical. Oscillating Water Columns (OWCs) employing bidirectional turbines coupled with generators can be used to capture energy from oceanic waves and convert it to electrical energy. This thesis includes a study to quantify the potential power smoothing that can be achieved from a wave farm of ideal OWC devices and from auxiliary hardware such as flywheel energy storage systems. Also detailed are the upgrades to the OWC test facility at Cranfield University, including the world-first capability to simulate polychromatic waves. This test facility has been employed to validate turbine characteristics derived from Computational Fluid Dynamic (CFD) numerical results. This thesis contains a literature review of the existing control strategies for OWCs that concludes that the optimization of power capture from individual components in the energy chain forces system-level compromises. This conclusion drove the development of an unique energy-based model of the complete wave-to-wire system utilizing port-Hamiltonian mechanics which mandated two modifications to the port-Hamiltonian framework. The first modification to the port-Hamiltonian framework resulted in a new generalized means of modeling systems where the potential energy is dependent on the momentum variables. The second modification expands the port-Hamiltonian framework to allow the modeling of ow source systems in addition to effort source systems. The port-Hamiltonian wave-to-wire OWC model enables the future development of a control approach that optimizes power capture at a system level. As a first step to achieving this goal an Injection Damping Assignment (IDA) Passivity Based Control (PBC) strategy was successfully applied to an OWC system and an energy storage flywheel system. These strategies pave the way for future developments utilizing optimization techniques, such as the use of cost functions to identify the peak efficiency operating condition.Engineering and Physical Sciences (EPSRC)PhD in Energy and Powe

Passivity-based analysis and control of AC microgrids: Integration, operation and control of energy storage systems

Microgrids are essential subsystems of modern electric power systems. They allow providing electrical energy service for millions of people around the world by integrating multiple distributed energy resources and energy storage technologies at a small scale. This thesis studies these systems from the dynamical analysis and control point of view, to ful ll three main objectives: rst, to model pulse-width-modulated voltage and current source converters for integrating distributed energy resources in ac microgrids (Grids) with single-phase and three-phase topologies; second, to develop Hamiltonian models for representing the whole dynamics of ac Grids via classical circuit theory, since this model exhibits interconnection and dissipation structures typical in Lagrangian and Hamiltonian modeling; third, to design passivity-based controllers for guaranteeing stable operation of the entire Grids when these are operated under grid-connected or isolated modes. Hamiltonian modeling of power electronic converters based on voltage and current source technologies as well as Hamiltonian models of electrical Grids facilitate the dynamical analysis under the passivity paradigm with stability and scalability criteria. The main contributions of this thesis are: integrating supercapacitors and superconducting coils in ac power grids through a uni ed control model; uni ed ac grid modeling via circuit theory and active and reactive power decoupling in power converters under grid-connected mode as well as voltage and frequency control for isolated Grid con gurations. Finally, simulation results corroborate the theoretical developments presented in this thesis

Interconnection and damping assignment passivity-based controller for multilevel inverter

This thesis proposes an Interconnection and Damping Assignment Passivity- Based Controller (IDA-PBC) to control a 5-level Cascaded H-Bridge Multilevel Inverter (CHMI). The proposed IDA-PBC uses the Port-Controlled Hamiltonian (PCH) theory to modify the CHMI system energy by adding damping, thereby modifying dissipation structures related to dynamics and stability. The objective is to maintain output voltage regulation, resulting in fast response and low Total Harmonic Distortion (THD) values. Although the proposed IDA-PBC control algorithm showed outstanding performance during transient and nonlinear load condition, further improvements are required during no-load condition. To address this, improvements in the form of modification to the proposed IDA-PBC algorithm was made by adding a single loop Proportional-Integral (PI) controller at the voltage side, which was aimed at regulating the voltage before it was fed back into the IDAPBC. In order to verify the viability of the proposed IDA-PBC-PI controller for the CHMI, a simulation study was conducted using MATLAB/Simulink at a 20 kHz switching frequency and 1 µs sample time. The controller was tested at five load conditions, namely, steady state, no-load to full-load, load uncertainty, structural uncertainty and nonlinear load condition. The performance of the proposed controller showed regulated output voltage while maintaining THD values below 5% in all load conditions and a maximum of 220 µs response time during load uncertainty. The simulation results revealed the superiority of the proposed controller compared to the conventional double loop PI controller and the conventional IDA-PBC in terms of transient response, THD value, as well as regulation of the output voltage. The feasibility of the proposed IDA-PBC-PI controller was validated by developing its proof-of-concept hardware prototype. The simulation and experimental results obtained based on a 3 kHz switching frequency and 38 µs sample time were found to be consistent, which confirmed the capability of the proposed controller in controlling the 5-level CHMI output voltage