Transition from Islanded to grid-connected mode of microgrids with voltage-based droop control

Microgrids are able to provide a coordinated integration of the increasing share of distributed generation (DG) units in the network. The primary control of the DG units is generally performed by droop-based control algorithms that avoid communication. The voltage-based droop (VBD) control is developed for islanded low-voltage microgrids with a high share of renewable energy sources. With VBD control, both dispatchable and less-dispatchable units will contribute in the power sharing and balancing. The priority for power changes is automatically set dependent on the terminal voltages. In this way, the renewables change their output power in more extreme voltage conditions compared to the dispatchable units, hence, only when necessary for the reliability of the network. This facilitates the integration of renewable units and improves the reliability of the network. This paper focusses on modifying the VBD control strategy to enable a smooth transition between the islanded and the grid-connected mode of the microgrid. The VBD control can operate in both modes. Therefore, for islanding, no specific measures are required. To reconnect the microgrid to the utility network, the modified VBD control synchronizes the voltage of a specified DG unit with the utility voltage. It is shown that this synchronization procedure significantly limits the switching transient and enables a smooth mode transfer

Load-adaptive zero-phase-shift direct repetitive control for stand-alone four-leg VSI

This paper deals with a dedicated load adaptive phase compensation algorithm to be used in Repetitive Control based stand-alone 4-leg VSI. The plant model is achieved, its inherent modifications according to the operating point are highlighted and used to properly adapt the Repetitive Control structure. Modification of the repetitive control parameters is described to obtain the desired phase compensation capabilities achieving a Zero-Phase-Shift condition at each harmonic. This allows to increase the gain of the Repetitive Controller at high order harmonics thus yielding a better VSI output voltages with strongly reduced THD and faster dynamic response. As a consequence, the VSI output voltages are almost independent from the loads to be fed and the 4-leg VSI with the proposed Zero-Phase-Shift Direct Repetitive Control is an ideal candidate to supply sensitive loads in microgrid, in particular for stand-alone applications

Modeling and control of stand-alone AC microgrids: centralized and distributed storage, energy management and distributed photovoltaic and wind generation

El aumento de la penetración de energías renovables en la red eléctrica es necesario para el desarrollo de un sistema sostenible. Para hacerlo posible técnicamente, se ha planteado el uso de microrredes, definidas como una combinación de cargas, generadores distribuidos y elementos de almacenamiento controlados gracias a una estrategia global de gestión energética. Además, las microrredes aumentan la fiabilidad del sistema puesto que pueden funcionar en modo aislado en caso de fallo de red. Esta tesis se centra en el desarrollo de microrredes AC en funcionamiento aislado. El objetivo principal es el diseño y la implementación de estrategias de gestión energéticas sin utilizar cables de comunicación entre los distintos elementos, lo que permite reducir los costes del sistema y aumentar su fiabilidad. Para ello, se abordan los siguientes aspectos: • Gestión energética de una microrred AC con generador diesel, almacenamiento centralizado y generación renovable distribuida • Diseño de técnicas de control “droop” para repartir la corriente entre inversores conectados en paralelo • Gestión energética de una microrred AC con almacenamiento distribuido y generación renovable distribuida • Control de la etapa DC/DC de inversores fotovoltaicos con pequeño condensador de entrada en el seno de una microrred • Control de extracción de máxima potencia sin sensores mecánicos para sistemas minieólicos en el seno de una microrred.The introduction of distributed renewable generators into the electrical grid is required for a sustainable system. In order to increase the penetration of renewable energies, microgrids are usually proposed as one of the most promising technologies. A microgrid is a combination of loads, distributed generators and storage elements which behaves as a single controllable unit for the grid operator. Furthermore, microgrids make it possible to improve the system reliability because they are capable of standalone operation in case of grid failure. This thesis is focused on the development of AC microgrids under stand-alone operation. Its main objective is to design and implement overall control strategies which do not require the use of communication cables, thereby reducing costs and improving reliability. For this purpose, the following aspects are tackled: • Energy management of an AC microgrid with diesel generator, centralized storage and distributed renewable generation • Design of droop methods so that the current is shared among parallel-connected inverters • Energy management of an AC microgrid with distributed storage and distributed renewable generation • Control of the DC/DC stage in photovoltaic inverters with small input capacitors within a microgrid • Sensorless MPPT control for small wind turbines within a microgrid.Programa Oficial de Doctorado en Energías Renovables (RD 1393/2007)Energia Berriztagarrietako Doktoretza Programa Ofiziala (ED 1393/2007

Adaptive Reference Trajectory for Power Quality Enhancement in Three-Phase Four-Wire Standalone Power Supply Systems with Nonlinear and Unbalanced Loads

International audienc

Power Quality Enhancement in Hybrid Photovoltaic-Battery System based on three–Level Inverter associated with DC bus Voltage Control

This modest paper presents a study on the energy quality produced by a hybrid system consisting of a Photovoltaic (PV) power source connected to a battery. A three-level inverter was used in the system studied for the purpose of improving the quality of energy injected into the grid and decreasing the Total Harmonic Distortion (THD). A Maximum Power Point Tracking (MPPT) algorithm based on a Fuzzy Logic Controller (FLC) is used for the purpose of ensuring optimal production of photovoltaic energy. In addition, another FLC controller is used to ensure DC bus stabilization. The considered system was implemented in the Matlab /SimPowerSystems environment. The results show the effectiveness of the proposed inverter at three levels in improving the quality of energy injected from the system into the grid.Peer reviewedFinal Published versio

Microgrids/Nanogrids Implementation, Planning, and Operation

Today’s power system is facing the challenges of increasing global demand for electricity, high-reliability requirements, the need for clean energy and environmental protection, and planning restrictions. To move towards a green and smart electric power system, centralized generation facilities are being transformed into smaller and more distributed ones. As a result, the microgrid concept is emerging, where a microgrid can operate as a single controllable system and can be viewed as a group of distributed energy loads and resources, which can include many renewable energy sources and energy storage systems. The energy management of a large number of distributed energy resources is required for the reliable operation of the microgrid. Microgrids and nanogrids can allow for better integration of distributed energy storage capacity and renewable energy sources into the power grid, therefore increasing its efficiency and resilience to natural and technical disruptive events. Microgrid networking with optimal energy management will lead to a sort of smart grid with numerous benefits such as reduced cost and enhanced reliability and resiliency. They include small-scale renewable energy harvesters and fixed energy storage units typically installed in commercial and residential buildings. In this challenging context, the objective of this book is to address and disseminate state-of-the-art research and development results on the implementation, planning, and operation of microgrids/nanogrids, where energy management is one of the core issues

DSOGI-PLL based power control method to mitigate control errors under disturbances of grid connected hybrid renewable power systems

The control of power converter devices is one of the main research lines in interfaced renewable energy sources, such as solar cells and wind turbines. Therefore, suitable control algorithms should be designed in order to regulate power or current properly and attain a good power quality for some disturbances, such as voltage sag/swell, voltage unbalances and fluctuations, long interruptions, and harmonics. Various synchronisation techniques based control strategies are implemented for the hybrid power system applications under unbalanced conditions in literature studies. In this paper, synchronisation algorithms based Proportional-Resonant (PR) power/current controller is applied to the hybrid power system (solar cell + wind turbine + grid), and Dual Second Order Generalized Integrator-Phase Locked Loop (DSOGI-PLL) based PR controller in stationary reference frame provides a solution to overcome these problems. The influence of various cases, such as unbalance, and harmonic conditions, is examined, analysed and compared to the PR controllers based on DSOGI-PLL and SRF-PLL. The results verify the effectiveness and correctness of the proposed DSOGI-PLL based power control method

Microgrid Stability Controller Based on Adaptive Robust Total SMC

This paper presents a microgrid stability controller (MSC) in order to provide existing distributed generation units (DGs) the additional functionality of working in islanding mode without changing their control strategies in grid-connected mode and to enhance the stability of the microgrid. Microgrid operating characteristics and mathematical models of the MSC indicate that the system is inherently nonlinear and time-variable. Therefore, this paper proposes an adaptive robust total sliding-mode control (ARTSMC) system for the MSC. It is proved that the ARTSMC system is insensitive to parametric uncertainties and external disturbances. The MSC provides fast dynamic response and robustness to the microgrid. When the system is operating in grid-connected mode, it is able to improve the controllability of the exchanged power between the microgrid and the utility grid, while smoothing the DGs’ output power. When the microgrid is operating in islanded mode, it provides voltage and frequency support, while guaranteeing seamless transition between the two operation modes. Simulation and experimental results show the effectiveness of the proposed approach