A simple high-performance current control strategy for V2G three-phase four-leg inverter with LCL filter

Electric vehicles (EVs) can behave as distributed energy storage devices for providing on-demand smart grid support service, that is an emerging Vehicle-to-Grid (V2G) technology. A high-performance and easy-implementation current control strategy for V2G Three-phase four-leg inverter with LCL filter is proposed. It consists of a deadbeat (DB) controller and a paralleled repetitive controller (RC). The DB controller is based on weighted average inductor current (WAIC) scheme, which simplifies the third-order LCL filter to be an equivalent 1st order L filter. The stability of the DB controlled inverter with the unmodelled system time delay is analyzed. DB controller is of very fast response and easy implementation, but is not immune to system time delay and various uncertainties. To overcome the disadvantages, a plug-in RC is added to reinforce the DB controller to remove harmonic distortion from the feed-in current in the presence of parameter uncertainties. A lab prototype of 10kW grid-connected three-phase four-leg inverter has been built up to validate the proposed current control strategy. The simulations and experiments are provided to demonstrate the validity of the proposed control strategy

Increasing voltage utilization in split-link, four-wire inverters

Three-phase four-wire inverters, with either three-leg or four-leg topology, are useful for interfacing distributed generation to networks of unbalanced loads, but neither of the available circuit topologies is ideal. The split-link three-leg topology (with six switches) suffers from poor DC voltage utilization compared with the four-leg topology (with eight switches). The four-leg topology has an electromagnetic compatibility (EMC) difficulty because it imposes large-amplitude high-frequency voltages between the DC-link busbars and ground. To obtain both good dc voltage utilization and good EMC performance, it is proposed to use a split-link inverter with an active balancing circuit (also eight switches). The balancing circuit is used to modulate the DC busbar offset voltage to make better use of the available DC-link voltage. The optimum voltage term is established to be a third harmonic term, and the DC voltage utilization is improved. A deadbeat controller supplemented with a repetitive controller is designed to give good tracking and good disturbance rejection for the busbar offset voltage. System performance is studied through an experimental test rig

Control of high performance single phase DC-AC inverter

Master'sMASTER OF ENGINEERIN

Uncertainty and disturbance estimator design to shape and reduce the output impedance of inverter

Power inverters are becoming more and more common in the modern grid. Due to their switching nature, a passive filter is installed at the inverter output. This generates high output impedance which limits the inverter ability to maintain high power quality at the inverter output. This thesis deals with an impedance shaping approach to the design of power inverter control. The Uncertainty and Disturbance Estimator (UDE) is proposed as a candidate for direct formation of the inverter output impedance. The selection of UDE is motivated by the desire for the disturbance rejection control and the tracking controller to be decoupled. It is demonstrated in the thesis that due to this fact the UDE filter design directly influences the inverter output impedance and the reference model determines the inverter internal electromotive force. It was recently shown in the literature and further emphasized in this thesis that the classic low pass frequency design of the UDE cannot estimate periodical disturbances under the constraint of finite control bandwidth. Since for a power inverter both the reference signal and the disturbance signal are of periodical nature, the classic UDE lowpass filter design does not give optimal results. A new design approach is therefore needed. The thesis develops four novel designs of the UDE filter to significantly reduce the inverter output impedance and maintain low Total Harmonic Distortion (THD) of the inverter output voltage. The first design is the based on a frequency selective filter. This filter design shows superiority in both observing and rejecting periodical disturbances over the classic low pass filter design. The second design uses a multi-band stop design to reject periodical disturbances with some uncertainty in the frequency. The third solution uses a classic low pass filter design combined with a time delay to match zero phase estimation of the disturbance at the relevant spectrum. Furthermore, this solution is combined with a resonant tracking controller to reduce the tracking steady-state error in the output voltage. The fourth solution utilizes a low-pass filter combined with multiple delays to increase the frequency robustness. This method shows superior performance over the multi-band-stop and the time delayed filter in steady-state. All the proposed methods are validated through extensive simulation and experimental results

Decoupled digital control of a three-phase four-leg inverter to feed balanced and unbalanced loads

[ES] Este trabajo presenta una estrategia de control digital para inversores trifásicos de cuatro piernas, para cumplir con el requerimiento de normas internacionales de calidad de energía para UPS cuando se alimentan cargas equilibradas y desequilibradas. La estrategia de control propuesta se desarrolla en el marco de referencia dq0 en tiempo discreto, posee un lazo interno para controlar la corriente de los inductores y un lazo externo para controlar la tensión de salida. Se demuestra que utilizando controladores convencionales pueden desacoplarse las variables de los ejes d y q, lo que permite diseñar los controladores del lazo externo de control de tensión como sistemas SISO independientes, cumpliéndose los requerimientos de régimen transitorio y de régimen permanente impuestos por norma. Para mejorar el desempeño del inversor cuando se alimentan cargas desequilibradas, se propone utilizar un controlador Proporcional-Resonante únicamente en el lazo interno del eje 0, lográndose una estrategia de control simple y de bajo costo computacional. Se presentan resultados de simulación y experimentales que permiten validar la estrategia de control propuesta.[EN] This paper presents a digital control strategy for three-phase four-leg inverters to meet the requirements of international power quality standards for UPS, when balanced and unbalanced loads are feeding. The proposed control strategy is developed in the discrete-time dq0-reference frame. It has an inner current loop to control the inductors currents and an outer voltage control loop. The analysis of this paper demonstrates that using conventional controllers, the voltages of d and q axes can be decoupled, into the whole operating range, which allows the outer voltage controllers to be designed as independent SISO systems, meeting the requirements of the transient and permanent regimes imposed by Standards. To improve the performance when the inverter feeds unbalanced loads, this paper proposes to use a Proportional-Resonant controller only in the inner loop of the axis 0, achieving a simple control strategy and, consequently with a low computational cost. Simulation and experimental results allow validating the proposed control strategy.Este trabajo fue soportado por la Secretaría de Ciencia y Técnica de la Universidad Nacional de Río Cuarto (SeCyT, UNRC), la Agencia Nacional de Promoción Científica y Tecnológica (FONCyT) a través del proyecto PICT 1663/2016 y la Red MEIHAPER CYTED.Oggier, E.; Botterón, F.; Oggier, G.; García, G. (2020). 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New limiting algorithms for space vector modulated three-phase four-leg voltage source inverters. IEE Procedings.-Electronics Power Applications, 150(6). https://doi.org/10.1049/ip-epa:20030913Botteron, F., and Pinheiro, H. ,2007. A three-phase UPS that complies with the standard IEC 62040-3. IEEE Transactions on Industrial Electronics, 54(4), 2120-2136. https://doi.org/10.1109/TIE.2007.894782Carballo, R. E., Botterón, F., Oggier, G. G., and García, G. O. ,2016. Design approach of discrete-time resonant controllers for uninterruptible power supply applications through frequency response analysis. IET Power Electronics, 9(15), 2871-2879. https://doi.org/10.1049/iet-pel.2015.1059Chen, D., Zhang, J., Qian, Z., 2013. Research on fast transient and 6n±1 harmonics suppressing repetitive control scheme for three-phase gridconnected inverters. IET Power Electronics 6 (3) , 601-610 . https://doi.org/10.1049/iet-pel.2012.0348Corcau, J. I., Grigorie, T. L., Jula, N., Cepisca, C., and Popoviciu, N. ,2010. Dynamics and Control of Three-Phase Four-Leg Inverter. EHAC'10, 26-31.Cosner, C., Anwar, G., and Tomizuka, M. ,1990. Plug in repetitive control for industrial robotic manipulators. Proceedings., IEEE International Conference on Robotics and Automation, 1970-1975. https://doi.org/10.1109/ROBOT.1990.126295Franklin, G. F. P. ,1997. Digital Control of Dynamic Systems ,Addison Wesley (ed.); 3rd ed.Gannett, R. a., Sozio, J. C., Boroyevich, D., 2002. Application of synchronous and stationary frame controllers fornunbalanced and nonlinear load compensation in 4-leg inverters. APEC. Seventeenth Annual IEEE Applied Power Electronics Conference and Exposition (Cat. No.02CH37335) 2 (c) , 1-6 . https://doi.org/10.1109/APEC.2002.989372Houari, A., Djerioui, A., Saim, A., Ait-ahmed, M., and Machmoum, M. ,2017. 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F., Mustafa, M. W., Ghadimi, A. A., and Rezvani, A. ,2016. An Improved Control Strategy for a Four-Leg Grid-Forming Power Converter under Unbalanced Load Conditions. Advances in Power Electronics, 2016. https://doi.org/10.1155/2016/9123747N. Mohan, T. Undeland, W. R. ,2002. Power Electronics: Converters Applications and design 3rd Ed. Wiley.Nasiri, R., and Radan, A. ,2011. Adaptive decoupled control of 4-leg voltage-source inverters for standalone photovoltaic systems: Adjusting transient state response. Renewable Energy, 36(10), 2733-2741. https://doi.org/10.1016/j.renene.2011.03.007Ortega, R., Carranza, O., Sosa, J. C., García, V., and Hernández, R. ,2016. Diseño de controladores para inversores monofásicos operando en modo isla dentro de una microrred. RIAI - Revista Iberoamericana de Automatica e Informatica Industrial, 13(1), 115-126. https://doi.org/10.1016/j.riai.2015.09.010Phillips, C. L., Nagle, H. T., and Chakrabortty, A. ,2015. State Variables. In Digital control system analysis & design (4th ed., pp. 63-71).Pichan, M., and Rastegar, H. ,2017. Sliding-mode control of four-leg inverter with fixed switching frequency for uninterruptible power supply applications. IEEE Transactions on Industrial Electronics, 64(8), 6805-6814. https://doi.org/10.1109/TIE.2017.2686346Pichan, M., Rastegar, H., and Monfared, M. ,2017. Deadbeat Control of the Stand-Alone Four-Leg Inverter Considering the Effect of the Neutral Line Inductor. IEEE Transactions on Industrial Electronics, 64(4), 2592-2601. https://doi.org/10.1109/TIE.2016.2631459Ryan, M. J., De Doncker, R. W., and Lorenz, R. D. ,2001. Decoupled control of a 4-leg inverter via a new 4×4 transformation matrix. IEEE Transactions on Power Electronics, 16(5), 694-701. https://doi.org/10.1109/PESC.1999.789001Thandi, G. S., Zhang, R., Xing, K., Lee, F. C., and Boroyevich, D. ,1999. Modeling, control and stability analysis of a PEBB based DC DPS. 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Study and evaluation of distributed power electronic converters in photovoltaic generation applications

This research project has proposed a new modulation technique called “Local Carrier Pulse Width Modulation” (LC-PWM) for MMCs with different cell voltages, taking into account the measured cell voltages to generate switching sequences with more accurate timing. It also adapts the modulator sampling period to improve the transitions from level to level, an important issue to reduce noise at the internal circulating currents. As a result, the new modulation LC-PWM technique reduces the output distortion in a wider range of voltage situations. Furthermore, it effectively eliminates unnecessary AC components of circulating currents, resulting in lower power losses and higher MMC efficiency.Departamento de Tecnología ElectrónicaDoctorado en Ingeniería Industria

Control of power electronic interfaces in distributed generation.

Renewable energy has gained popularity as an alternative resource for electric power generation. As such, Distributed Generation (DG) is expected to open new horizons to electric power generation. Most renewable energy sources cannot be connected to the load directly. Integration of the renewable energy sources with the load has brought new challenges in terms of the system’s stability, voltage regulation and power quality issues. For example, the output power, voltage and frequency of an example wind turbine depend on the wind speed, which fluctuate over time and cannot be forecasted accurately. At the same time, the nonlinearity of residential electrical load is steadily increasing with the growing use of devices with rectifiers at their front end. This nonlinearity of the load deviates both current and voltage waveforms in the distribution feeder from their sinusoidal shape, hence increasing the Total Harmonics Distortions (THD) and polluting the grid. Advances in Power Electronic Interfaces (PEI) have increased the viability of DG systems and enhanced controllability and power transfer capability. Power electronic converter as an interface between energy sources and the grid/load has a higher degree of controllability compared to electrical machine used as the generator. This controllability can be used to not only overcome the aforementioned shortfalls of integration of renewable energy with the grid/load but also to reduce THD and improve the power quality. As a consequence, design of a sophisticated controller that can take advantage of this controllability provided by PEIs to facilitate the integration of DG with the load and generate high quality power has become of great interest. In this study a set of nonlinear controllers and observers are proposed for the control of PEIs with different DG technologies. Lyapunov stability analysis, simulation and experimental results are used to validate the effectiveness of the proposed control solution in terms of tracking objective and meeting the THD requirements of IEEE 519 and EN 50160 standards for US and European power systems, respectively

Power Conditioning for Plug-In Hybrid Electric Vehicles

Plugin Hybrid Electric Vehicles (PHEVs) propel from the electric energy stored in the batteries and gasoline stored in the fuel tank. PHEVs and Electric Vehicles (EVs) connect to external sources to charge the batteries. Moreover, PHEVs can supply stand-alone loads and inject power to the grid. Such functionalities have been defined as Vehicle to House (V2H) and Vehicle to Grid (V2G) and promoted by national and international policies such as the Energy Independency and Security Act (EISA) of 2007, enacted by the United States Congress. Exchanging energy between the vehicle and external sources is performed by the vehicular power conditioner (VPC). This dissertation proposes a design procedure for VPCs. The research mainly focuses on the VPC’s power converter design. A conceptual design approach is proposed to select the proper power converter topologies according to the determined power conditioning needs. The related standards and previous works are reviewed to determine the design guidelines. A set of specifications are introduced for a three port onboard VPC. This VPC is a reference for designs, simulations and experiments. The reference VPC is implemented with a modular three-stage isolated topology that utilizes voltage source ac-dc converters as the power conditioning stages. The multiport extension of this topology extends the vehicular power conditioning concept into a novel vehicular integrated power system. All the vehicle’s electric sources and loads can exchange energy in the described multiport integrated power system. Novel design methods are proposed for the power converter, filters, magnetic circuit and control of the VPC. The practical challenges of the VPC development are analyzed. The major contributions of this dissertation include a pioneer grounding scheme for VPC considering the vehicular standards, a novel modeling approach for the Snubberless Dual Active Bridge (DAB) commutation, an innovative integrated ac inductor, and a new experimental modeling method for multiwinding transformers. The contributions are supported by analyses, simulations, and practical experiments

New Control Algorithms for the Distributed Generation Interface in Grid-Connected and Micro-grid Systems

Driven by economic, technical, and environmental reasons, the energy sector is moving into an era where large portions of increases in electrical energy demand will be met through widespread installation of distributed resources or what's known as distributed generation (DG). DG units can operate in parallel to the main grid or in a micro-grid mode. The later is formed by a cluster of DG units connected to a distribution network to maintain the reliability of critical loads, mainly when the grid supply is not available. Distributed resources include variable frequency sources, high frequency sources, and direct energy conversion sources producing dc voltages or currents. The majority of distributed resources are interfaced to the utility grid or to the customer load via dc-ac pulse-width-modulated (PWM) voltage source inverter (VSI) systems. However, these interfaces introduce new issues, such as the absence of the physical inertia, wide-band of dynamics, limited overload capability, susceptibility to parameters variation, and switching harmonics generation. In addition, the uncertain and dynamic nature of the distribution network challenges the stability and control effectiveness of a grid-connected inverter-based DG interface. Generally, difficulties appear in the form of grid impedance and interfacing parameter variations, fast and slow grid-voltage disturbances, grid distortion and unbalance, and interactions between the inverter ac-side filter and the grid. On the other hand, a micro-grid system will be dominated by inverter-based DG units. Unlike conventional power system generators, inverter-based DG units have no physical inertia. This fact makes the micro-grid system potentially susceptible to oscillations resulting from system disturbances. Severe and random disturbances might be initiated in a micro-grid system, due to load changes, the power sharing mechanism of the inverters and other generators, and interactions between the DG interface and the network. Motivated by the aforementioned difficulties, this thesis presents new control algorithms for the DG interface that guarantee stable and high power quality injection under the occurrence of network disturbances and uncertainties, in both the grid-connected and micro-grid systems. The control architecture of the proposed DG interface relies on the following subsystems. First, a newly designed deadbeat current regulation scheme is proposed. The proposed design guarantees high power quality current injection under the presence of different disturbing parameters such as grid voltage distortion, interfacing parameter variation, and inverter system delays. Further, it utilizes the maximum dynamic performance of the inverter in a way that provides a high bandwidth and decoupled control performance for the outer control loops. Different topologies of the ac-side filter are considered for the current control design. Second, a novel adaptive discrete-time grid-voltage sensorless interfacing scheme for DG inverters is proposed. The adaptive interface relies on a new interface-monitoring unit that is developed to facilitate accurate and fast estimation of the interfacing impedance parameters and the grid voltage vector (magnitude and position) at the point of common coupling. The estimated grid voltage is utilized to realize a grid-voltage sensorless interfacing scheme, whereas the interfacing parameters are utilized for the self-tuning control and interface-parameter monitoring. Further, a simple and robust synchronization algorithm and a voltage-sensorless average power control loop are proposed to realize an adaptive voltage-sensorless DG interface. The voltage-sensorless interface positively contributes to the elimination of the residual negative sequence and voltage feed-forward compensation errors, and to the robustness of the power sharing mechanism in paralleled inverter systems, where the power-sharing mechanism is generally based on open-loop controllers. Third, a new voltage control scheme for the DG interface featuring fast load voltage regulation and effective mitigation of fast voltage disturbances is proposed. The proposed voltage control scheme targets the problem of fast and large-signal-based voltage disturbances, which is common in typical distribution feeders. A hybrid voltage controller combining a linear with a variable-structure-control element is proposed for the DG interface. Positive and dual-sequence versions of the proposed voltage controller are developed to address the issue of unbalanced voltage disturbances. The proposed voltage controller successfully embeds a wide band of frequency modes through an equivalent internal model. Subsequently, wide range of balanced and unbalanced voltage perturbations, including capacitor-switching disturbances, can be effectively mitigated. Fourth, to constrain the drift of the low frequency modes in a conventional droop-controlled micro-grid, a new transient-based droop controller with adaptive transient-gains is proposed. The proposed power-sharing controller offers an active damping feature that is designed to preserve the dynamic performance and stability of each inverter unit at different loading conditions. Unlike conventional droop controllers, the proposed droop controller yields two-degree of freedom tunable controller. Subsequently, the dynamic performance of the power-sharing mechanism can be adjusted, without affecting the static droop gain, to damp the oscillatory modes of the power-sharing controller. The overall robust DG interface facilitates a robust micro-grid operation and safe plug-and-play integration of DG units on existing distribution systems; hence increasing the system penetration of DG. The direct result of this development is huge financial saving for utilities by capturing the salient features of deploying DG into existing utility networks. Further, these developments are significant to the industry as they provide the blue print for reliable control algorithms in future DG units, which are expected to operate under challenging system conditions