Voltage control strategies for loss minimzation in autonomous microgrids

This dissertation investigates the novel idea of flexible-voltage autonomous microgrids (MG), employing several interconnectable dc buses operating in a minimum-voltage mode. In comparison with the traditional fixed-voltage MGs, the proposed MGs reduce losses to gain significant enhancement in efficiency. It is widely believed that energy systems of the future will heavily depend on MGs rich in power electronics converters (PECs). This dissertation is focused on MGs with a high degree of self-sufficiency, without precluding sporadic links with the power grid. Potential applications of those MGs include: (a) distributed generation power systems, (b) ships, land vehicles, aircraft, and spacecraft, (c) users in need of power supply impervious to vulnerabilities of the grid, and (d) localities lacking an access to a grid.Modern pulse-width modulated PECs allow rapid and wide-range changes of voltages and currents. High switching frequencies result in high power quality and fast dynamic response, but each switching event causes energy loss related to the magnitudes of input voltage and output current. In the existing MGs, the bus voltages are maintained at a fixed level. However, many heavy loads, such as electric drives, operate most of the time with a reduced voltage, which is adjusted by decreasing the voltage gain of the feeding converter. This makes the voltage pulses high and narrow. If instead the pulses were made wide and low, then with the current unchanged the conduction losses would remain unchanged, but the switching losses would greatly decrease. This observation leads to the main idea of the dissertation, namely MGs whose dc-bus voltages are allowed to fluctuate and which are maintained at the lowest possible level. Loss minimization, apart from energy savings, may be critical for autonomous MGs with a tight balance of power.In this dissertation, two methods are proposed for calculating the minimum (optimum) required dc voltage level. In the first method, a central control unit allocates the minimum required dc voltages to individual buses by employing the information obtained from control systems of the adjustable voltage loads. For example, most of the variable-speed ac motors employ the so-called constant volts per hertz strategy, in which the relation between frequency and voltage is clearly specified. In the more sophisticated high-performance drives, the instantaneous values of the desired speed, torque, and current are available, allowing the required voltage estimation from the equation of power balance.In the second method, the problem of determining the optimal dc voltage and power settings is formulated as an optimization problem with the objective function of minimizing the converter losses. Genetic algorithm is utilized in solving the optimization problem. Due to limited available power from renewables, reducing the converter losses will enhance the survivability of the microgrid and ease the cooling requirements, resulting in a more compact system. A model of a 20-bus microgrid with the dc distribution network is employed to verify the effectiveness of the proposed methods

Nonlinear Modeling of Power Electronics-based Power Systems for Control Design and Harmonic Studies

The massive integration of power electronics devices in the modern electric grid marked a turning point in the concept of stability, power quality and control in power systems. The evolution of the grid toward a converter-dominated network motivates a deep renovation of the classical power system theory developed for machine-dominated networks. The high degree of controllability of power electronics converters, furthermore, paves the way to the investigation of advanced control strategies to enhance the grid stability, resiliency and sustainability. This doctoral dissertation explores four cardinal topics in the field of power electronics-based power systems: dynamic modeling, stability analysis, converters control, and power quality with particular focus on harmonic distortion. In all four research areas, a particular attention is given to the implications of the nonlinearity of the converter models on the power system

Stability Boundary Analysis of Islanded Droop-Based Microgrids Using an Autonomous Shooting Method

This paper presents a stability analysis for droop-based islanded AC microgrids via an autonomous shooting method based on bifurcation theory. Shooting methods have been used for the periodic steady-state analysis of electrical systems with harmonic or unbalanced components with a fixed fundamental frequency; however, these methods cannot be directly used for the analysis of microgrids because, due to the their nature, the microgrids frequency has small variations depending on their operative point. In this way, a new system transformation is introduced in this work to change the droop-controlled microgrid mathematical model from an non-autonomous system into an autonomous system. By removing the explicit time dependency, the steady-state solution can be obtained with a shooting methods and the stability of the system calculated. Three case studies are presented, where unbalances and nonlinearities are included, for stability analysis based on bifurcation analysis; the bifurcations indicate qualitative changes in the dynamics of the system, thus delimiting the operating zones of nonlinear systems, which is important for practical designs. The model transformation is validated through time-domain simulation comparisons, and it is demonstrated through the bifurcation analysis that the instability of the microgrid is caused by supercritical Neimark–Sacker bifurcations, and the dynamical system phase portraits are presented

Steady-State Analysis and Optimal Power Routing of Standalone Unbalanced Hybrid AC/DC Microgrids

The concept of ac microgrids was introduced to integrate distributed generators (DGs) and loads within one entity that can operate autonomously or connected to a utility grid. Furthermore, dc microgrids have received increasing attention as a potential solution to deliver power from DGs to modern dc loads with reduced conversion stages. Moreover, hybrid ac/dc microgrids have been introduced as a paradigm combining the benefits of the two types of microgrids by interconnecting them through interlinking converters (ICs). Steady-state analysis is essential for planning and operation studies of electrical power systems. However, conventional analysis approaches cannot be applied to hybrid ac/dc microgrids due to their distinctive features, such as droop characteristics, lack of a slack bus, and coupling between the ac and dc variables. Additionally, the unbalanced nature of ac microgrids adds to the complexity of modeling and analysis in such networks. Therefore, this thesis is focused on developing steady-state modeling and analysis framework for standalone unbalanced hybrid ac/dc microgrids. First, a steady-state analysis tool for unbalanced hybrid ac/dc microgrids is developed. The ac subgrid's components are modeled in phase coordinates. Furthermore, the dc subgrid's components are modeled and the coupling between the ac and dc variables is formulated. The models of the various system elements are incorporated into a unified power flow formulation, which is solved using a Newton-Trust Region (NTR) method. The developed power flow algorithm is verified through comparisons with time-domain simulations of test microgrids. The analysis tool is used to analyze a larger hybrid ac/dc microgrid through case studies. The case studies shed light on some challenges of these microgrids, namely, imposed limitations on microgrid loadability due to unbalanced ac subgrid's loading, effect of IC settings on microgrid operation, and trade-off between proportional loading of the ac and dc subgrids and proportional power-transfer sharing among ICs. Second, based on the identified microgrid loadability limitation of unbalanced microgrids, a novel adaptive power routing (APR) scheme is proposed to maximize the microgrid loadability. The proposed scheme allows independent control of active and reactive powers flowing through IC phases, so that power can be routed among the ac subgrid's phases. The DPR scheme is integrated into an optimal power flow (OPF) formulation with the objective of minimizing load shedding. A supervisory controller is proposed to solve the OPF problem by adjusting the DG and IC settings. Several case studies are conducted to show the ineffectiveness of conventional supervisory controllers in resolving the loadability issue, and to verify the success of the proposed controller in solving the problem. Third, a power flow approach based on sequence component analysis of the ac microgrid's elements is adopted for faster convergence and improved modeling accuracy as compared to conventional approaches in phase coordinates. This approach breaks down the system model into positive-, negative-, and zero-sequence subsystems that can be solved in parallel for enhanced performance. The positive-sequence power flow is solved using a Newton-Raphson (NR) method, while the negative- and zero-sequence voltages are obtained by solving linear complex equations. The approach is verified through comparisons with time-domain simulations. In addition, the algorithm is utilized to investigate the operation of droop-controlled DGs in larger-scale isochronous unbalanced ac microgrids, and to examine its limit-enforcement abilities at the same time. The algorithm demonstrates significant improvements in terms of accuracy and convergence time when compared against the conventional NTR-based approach in phase coordinates. Finally, the power flow approach developed in the third part is extended to include the IC's and dc subgrid's models so that it can be applied to hybrid ac/dc microgrids. A power flow algorithm is proposed to solve the ac and dc power flows independently in a sequential manner, while maintaining the correlation between the two. The algorithm is verified through comparisons with time-domain models of test hybrid microgrids. Case studies are introduced to test the algorithm's effectiveness in enforcing the DG and IC limits in the power flow solution under various conditions. The algorithm also shows enhanced accuracy and solution speed with respect to the tool developed in the first stage

Control of voltage source converters for distributed generation in microgrids

Microgrids are the near future candidate to reduce the dependence on the carbon-based generation, towards a more environmentally friendly and sustainable energy paradigm. The popularization of the use of renewable energy sources has fostered the development of better technologies for microgrids, particularly power electronics and storage systems. Following the improvements in microgrid technologies achieved in the last decade, a new challenge is being faced: the control and management of microgrids for its operation in islanded mode, in addition to its large scale integration into the current electrical power system. The unregulated introduction of distributed generation based on renewable energy sources into the power system could cause as many problems as it would solve. The unpredictability of the generated power would introduce large disturbances into the electric system, making it difficult to control, and eventually resulting in an unstable system. To overcome these issues, the paradigm of microgrids has been proposed: a small power system, able to operate islanded from the main grid, which will permit the large scale introduction of renewable energy sources interfaced with power electronic converters together with energy storage systems into the distribution grids. Microgrids¿ ability to allow their users to operate islanded from the utility grid, brings the potential to offer a high quality of service. It is in the islanded operation mode, particularly in microgrids with a high proportion of renewable based generation, where the major technical challenges are found. This thesis focuses in three of the main challenges of islanded and weak electrical grids: the power converter control of electrical storage systems, its decentralized control design, and also the improvement of power quality in grids disturbed by renewable generation. These topics are addressed from a control point of view, that is, to tackle the electrical problems, modelling them and proposing advanced control strategies to improve performance of microgrids. Energy storage system are a vital element to permit the islanded operation of microgrids, either in the long or short term. New control strategies are proposed in this thesis for the improvement of the converters¿ performance. In addition to the control of the converter, the management and control of different energy storage systems for microgrids are also studied. In particular, supercapacitors and batteries have been considered for the short and long term operation, respectively. Then, the control of islanded microgrids is addressed. Typical controls for islanded microgrids are analysed and new tools for designing stable controllers are proposed. Also, methodologies to analytically obtain the operating point (power flow) of droop controlled grids are studied and proposed. The high penetration of renewable energy sources in weak low-voltage grids results in undesirable electrical disturbances. This problematic in power quality is tackled and innovative solutions to mitigate it are proposed. In particular, a novel power smoothing scheme with simultaneous state of charge regulation of the ESS and power filtering. The new power smoothing scheme, along with the proposed control strategies for storage systems have been experimentally validated in a laboratory test bench, using a supercapacitor bank and a high power lithium-ion battery available at IREC's facilities.Les microxarxes són les candidates en un futur a curt termini, a substituir la generació basada en el carbó, de cara a assolir un sistema energètic més respectuós amb el medi ambient i més sostenible. La popularització de l'ús d'energies renovables ha fomentat la millora de les tecnologies per a microxarxes, en particular els sistemes d'emmagatzematge i l'electronica de potència. Desprès de les millores en tecnologies de microxarxes aconseguides durant l'última dècada, hi ha un nou repte al qual fer front: el control i gestió de microxarxes per la seva operació aïllada, a més de la integració a gran escala dins del sistema elèctric actual. La introducció descontrolada de fonts de generació distribuides en el sistema elèctric pot causar tants problemes com els que podria sol·lucionar. La incertesa en la producció elèctrica pot introduir grans pertorbacions al sistema elèctric, fent-lo difícil de controlar, i fins i tot el pot arribar a inestabilitzar. Per tal de fer front a aquestes dificultats, es proposa el paradigma de microxarxa: un petit sistema elèctric capaç d'operar de forma aïlla de la xarxa de distribució elèctrica, el qual hauria de permetre la integració a gran escala d'energies renovables a través de l'electrònica de potència, juntament amb sistemes d'emmagatzematge d'energia, dins de les xarxes de distribució. Les microxarxes permeten als seus usuaris a funcionar aillats de la xarxa elèctrica, donant la possibilitat d'oferir una alta qualitat de servei. És en el mode de funcionament aïllat, particularment en microxarxes amb una altra proporció de generació basada en renovables, on es troben la major part de reptes tecnològics. Aquesta tesi es centra en tres d'aquests reptes de les xarxes aillades i dèbils: el disseny del control per a convertidors de potència per a sistemes d'emmagatzematge elèctric, el control descentralitzat de les microxarxes i també la millora en la qualitat de subministre elèctric en xarxes afectades per generació renovable. Aquestes temes es tracten des d'el punt de vista de la teoria de control de sistemes, aixó significa, abordar el problema elèctric, modelar-lo, i proposar estrategies de control avançades per millorar el funcionament de les microxarxes. Els sistemes d'emmagatzematge són un element vital per permetre l'operació aïllada de les microxarxes, tant a llarg com a curt termini. En aquesta tesi es proposen noves estratègies de control per millorar el funcionament dels convertidors d'electrònica de potència. A més del control del convertidor, també s'estudia la gestió i control de diferents sistemes d'emmagatzematge d'energia per a microxarxes. En particular, supercondensador i bateries s'han considerat per l'operació a curt i llarg termini respectivament. Seguidament, s'enfila el control de microxarxes aïllades. S'analitzen els controls típics per a microxarxes i es proposen noves eines de disseny que permeten garantitzar l'estabilitat. A més a més, metodologies per a obtenir el punt d'operació (el flux de potènica) per a xarxes amb control tipus "droop" també s'estudien i proposen. L'alta penetració de fonts d'energia renovables en xarxes de baixa tensió i febles resulta en pertorbacions elèctriques indesitjables. Aquesta problematica en la qualitat de subministrament s'aborda i es proposen solucions inovadores per mitigar els efectes negatius. En particular, s'ha proposat un nou sistema de suavitzat de potència que regula simltaneament l'estat de càrrega del sistema d'emmagatzematge i filtra la potencia fluctuant. El nou esquema de suavitzat de potència, juntament amb les estrategies proposades per als sistemes d'emmagatzematge elèctric s'han validat experimentalment en un banc de laboratori, emprant superconsadors i una bateria d'alta potència, disponibles a les instal·lacions de l'IREC