Coordinated Control of Energy Storage in Networked Microgrids under Unpredicted Load Demands

In this paper a nonlinear control design for power balancing in networked microgrids using energy storage devices is presented. Each microgrid is considered to be interfaced to the distribution feeder though a solid-state transformer (SST). The internal duty cycle based controllers of each SST ensures stable regulation of power commands during normal operation. But problem arises when a sudden change in load or generation occurs in any microgrid in a completely unpredicted way in between the time instants at which the SSTs receive their power setpoints. In such a case, the energy storage unit in that microgrid must produce or absorb the deficit power. The challenge lies in designing a suitable regulator for this purpose owing to the nonlinearity of the battery model and its coupling with the nonlinear SST dynamics. We design an input-output linearization based controller, and show that it guarantees closed-loop stability via a cascade connection with the SST model. The design is also extended to the case when multiple SSTs must coordinate their individual storage controllers to assist a given SST whose storage capacity is insufficient to serve the unpredicted load. The design is verified using the IEEE 34-bus distribution system with nine SST-driven microgrids.Comment: 8 pages, 10 figure

Solid state transformer technologies and applications: a bibliographical survey

This paper presents a bibliographical survey of the work carried out to date on the solid state transformer (SST). The paper provides a list of references that cover most work related to this device and a short discussion about several aspects. The sections of the paper are respectively dedicated to summarize configurations and control strategies for each SST stage, the work carried out for optimizing the design of high-frequency transformers that could adequately work in the isolation stage of a SST, the efficiency of this device, the various modelling approaches and simulation tools used to analyze the performance of a SST (working a component of a microgrid, a distribution system or just in a standalone scenario), and the potential applications that this device is offering as a component of a power grid, a smart house, or a traction system.Peer ReviewedPostprint (published version

Balance control of grid currents for UPQC under unbalanced loads based on matching-ratio compensation algorithm

Abstract In three-phase four-wire systems, unbalanced loads can cause grid currents to be unbalanced, and this may cause the neutral point potential on the grid side to shift. The neutral point potential shift will worsen the control precision as well as the performance of the three-phase four-wire unified power quality conditioner (UPQC), and it also leads to unbalanced three-phase output voltage, even causing damage to electric equipment. To deal with unbalanced loads, this paper proposes a matching-ratio compensation algorithm (MCA) for the fundamental active component of load currents, and by employing this MCA, balanced three-phase grid currents can be realized under 100% unbalanced loads. The steady-state fluctuation and the transient drop of the DC bus voltage can also be restrained. This paper establishes the mathematical model of the UPQC, analyzes the mechanism of the DC bus voltage fluctuations, and elaborates the interaction between unbalanced grid currents and DC bus voltage fluctuations; two control strategies of UPQC under three-phase stationary coordinate based on the MCA are given, and finally, the feasibility and effectiveness of the proposed control strategy are verified by experiment results

Dual-Active-Bridge Model and Control for Supporting Fast Synthetic Inertial Action

This article proposes a dual-active-bridge control to support the fast synthetic inertial action in DC microgrids. First of all, the selection of the isolated DC/DC converter to link an energy storage system with the DC bus in a microgrid is analyzed and the advantages of the dual-active-bridge converter controlled by a single-phase shift modulation justify its selection. An active front-end can be then adapted to connect the DC bus with an AC grid. Secondly, this paper presents the design of a discrete PI controller for supporting fast synthetic inertial action. In particular, a discrete dual-active-bridge model based on the transferred power between both converter bridges, which overcomes the approximations of the output current linearization model, is proposed. Moreover, the article introduces a novel equation set to directly and dynamically tune discrete PI parameters to fulfill the design frequency specifications based on the inversion formulae method. In this way, during the voltage/power transients on the DC bus, the controller actively responds and recovers those transients within a grid fundamental cycle. Since the developed set of control equations is very simple, it can be easily implemented by a discrete control algorithm, avoiding the use of offline trial and error procedures which may lead to system instability under large load variations. Finally, the proposed control system is evaluated and validated in PLECS simulations and hardware-in-the-loop tests

Operation and Control of DC Microgrid

Power harnessing technology from the renewable energy resources has been developed over the past two decades. This technology enabled us to integrate renewable energy-based power generation to the conventional electric power grid. This study aims to improve the dynamic response and the load regulation using improved control strategies of the dc converters used to interface utility and renewable energy-based power generation. The power sharing between multiple dc microgrids/ac-dc microgrids is also investigated

Stability-Centric Design of a Droop-Mounted Adaptive Nonlinear Control for EV Charging in DC Microgrid

This paper presents a streamlined two-layer control system for effective power sharing and switching control in a DC microgrid designed for electric vehicles. The system integrates Energy Storage Systems and advanced converters to ensure a broad operational range and bidirectional power flow. The Dual active bridge topology is used to integrate the EV to DC MG. Hence, the DC MG system has multiple power converter operating simultaneously. The enhanced droop control strategy is advised for the upper layer and the switching controller is derived using nonlinear controls theory embedding the barrier functions. The enhanced droop strategy shares the power considering the individual dynamics of the storage devices while Barrier-based sliding mode control is applied to converters for current/voltage tracking. Mathematical analysis, leveraging Lyapunov's theory, confirms the large signal stability of the system. Demonstrated through MATLAB/Simulink-based simulations, the control system exhibits proficient load power sharing, and the adaptive nonlinear controller showcases robustness against unforeseen disturbances. Moreover, the comparative analysis provides insight into the performance of the proposed control methods concerning traditional methods. Hardware-in-loop tests, utilizing Typhoon HIL 404, authentically validate the real-time performance of the proposed control strategies. Different EV and Constant Power Load scenarios ensure a thorough examination, supporting the efficacy of the system. The study contributes valuable insights into the feasibility and efficiency of these control strategies, paving the way for advancements in sustainable electric mobility

A Novel Three-Level Isolated AC-DC PFC Power Converter Topology with Reduced Number of Switches

The three-level isolated AC-DC power factor corrected (PFC) converter provides safe and more efficient power conversion. In comparison with two-level, three-level PFC converter has the advantages of low total harmonic distortion, low device voltage rating, low di/dt, better output performance, high power factor, and low switching losses at higher switching frequencies. The high frequency transformer (HFT) grants galvanic isolation, steps up or down secondary voltage, and limits damage in case of a fault current. The existing three-level converter based on solid-state transformer (SST) topologies convert ac power from the electrical grid to a dc load while maintaining at least the minimum requirements set by the international standards (i.e., high power factor and low total harmonic distortion). The SST topologies with the capability of controlling intermediate dc-bus and output voltage simultaneously require two full bridges at the primary and secondary side of the HFT. As the power level increases, the number of cascaded bridges increases accordingly, and the price associated with these semiconductor devices becomes highly expensive. As result, the demand of converting high power level led to emphasis on high performance and cost-effective power conversion topology. The aim of this dissertation is to develop a new low-cost and high-performance three-level isolated AC-DC (PFC) converter topology. The proposed topology replaces the conventional three-level inverter in the secondary side of the HFT by only two switches and four diodes while still maintaining the basic functionality of a three-level converter (i.e., regulating the output voltage, controlling the dc-bus voltage to be within desired limits). The advantages of this new topology are: (1) low conduction losses; (2) low-cost; (3) no need to consider the issue of the power backflow; (4) zero-voltage switching (ZVS) and zero-current switching (ZCS) at turn ON are inherently guaranteed without any extra control effort. Two isolated three-level AC-DC power converter topologies are developed and investigated through the dissertation. First topology is based on the neutral point clamping (NPC) converter, and the second topology composed of the T-type converter. Two scale-down prototypes rated at 900-W and 1kW, 200 V are built to test the overall performance of the proposed topologies. The first and second topologies exhibit 94.5 % and 95.8 % efficiency scaled at a nominal power, respectively. The secondary bridge (novel circuit) in both topologies, which consists of two switches and four diodes, has 99.34 % practical efficiency

Moving discretized control set model-predictive control for dual-active bridge with the triple-phase shift

Triple-phase shift (TPS) is commonly utilized to enhance the efficiency of the dual-active-bridge (DAB) converters. However, the small-signal model of the circuit varies with operating mode, terminal voltage ratio, and power. In order to address this issue, a control inspired by the finite-control set model-predictive control is proposed. The proposed moving discretized control set model-predictive control (MDCS-MPC) can achieve great control flexibility and good transition performance throughout the power and terminal voltage range with global control parameters. It presents fixed switching frequency with low computational burden due to the utilization of only two prediction horizons. The operating principle of the proposed MDCS-MPC is introduced in development of a cost function that provides stiff load voltage regulation. The steady-state error in MDCS-MPC has also been analyzed and compensated. The application of MDCS-MPC in a multiobjective control scenario has been addressed. Experiments on a 300-V/300-V 20-kHz 1-kW DAB converter are carried out to verify the theoretical claims

Comparison of SPS, DPS and TPS in Dual active bridge converter

LAUREA MAGISTRALEI convertitori DC-DC sono un componente molto importante nel settore dellìelettronica di potenza. La loro applicazione alle fonti di energia rinnovabile, alle microreti CC e ai sistemi di accumulo di energia li ha resi una parte fondamentale per il settore. Tra i convertitori CC-CC, quelli isolati hanno attirato molta attenzione per i loro numerosi vantaggi. Il convertitore dual active bridge (DAB) è un convertitore CC-CC isolato noto per la sua bidirezionalità, la modalità di funzionamento buck-boost, l'isolamento galvanico e l'elevata efficienza grazie.. La modulazione del convertitore DAB è una delle problematiche più attraenti per la ricerca poiché la tradizionale modulazione “single phase shift”presenta alcune limitazioni importanti come una regione di commutazione a tensione nulla limitata. Pertanto, per superare queste limitazioni sono state introdotte la modulazione a doppio sfasamento e triplo sfasamento. Inoltre, un altro aspetto dello studio dei convertitori sono le tensioni di modo comune e l'effetto dei convertitori su di esse. Le tensioni di modo comune sono importanti poiché in molte applicazioni possono dare origine a tanti problemi come scatti indesiderati del sistema di protezione. In questa tesi, dopo aver studiato in dettaglio le tre principali modulazioni a sfasamento, è stato modellato il sistema in Simulink per confrontare le tre tecniche di modulazione in termini di efficienza, regione di commutazione a tensione nulla e tensioni di modo comune. L'effetto di ogni modulazione su ognuna di queste variabili sarà studiato in dettaglio.DC-DC converters are one of the most important parts of the DC systems. Their application in renewable energy sources, DC microgrids, and energy storage systems have made them an inevitable part of the industry. Amon DC-DC converters, the insulated ones have attracted a lot of attention to themselves because of their numerous benefits. The dual active bridge (DAB) converter is a DC-DC insulated converter which is known because of its bidirectionality, buck-boost mode of working, galvanic insulation, and high efficiency because of its soft switching. The modulation of the DAB converter is one of the most attractive area for the researches since the traditional single phase shift modulation has some major limitations such as a limited soft-switching region. Therefore, to overcome these limitations the dual phase shift and triple phase shift modulation have been introduced. Also, another aspect of studying converters is common mode voltages and the effect of converters on them. The common mode voltages are important since in many applications they may give rise to so many problems such as unwanted trips of the system. In this thesis after studying the three major phase shift modulation in details, a prototype in Simulink has been used to compare the three phase shift modulations in terms of efficiency, zero voltage switching region and common mode voltages. The effect of each modulation on each one of these variables will be study in details