Power Management Strategies for a Wind Energy Source in an Isolated Microgrid and Grid Connected System

This thesis focuses on the development of power management control strategies for a direct drive permanent magnet synchronous generator (PMSG) based variable speed wind turbine (VSWT). Two modes of operation have been considered: (1) isolated/islanded mode, and (2) grid-connected mode. In the isolated/islanded mode, the system requires additional energy sources and sinks to counterbalance the intermittent nature of the wind. Thus, battery energy storage and photovoltaic (PV) systems have been integrated with the wind turbine to form a microgrid with hybrid energy sources. For the wind/battery hybrid system, several energy management and control issues have been addressed, such as DC link voltage stability, imbalanced power flow, and constraints of the battery state of charge (SOC). To ensure the integrity of the microgrid, and to increase its flexibility, dump loads and an emergency back-up AC source (can be a diesel generator set) have been used to protect the system against the excessive power production from the wind and PV systems, as well as the intermittent nature of wind source. A coordinated control strategy is proposed for the dump loads and back up AC source. An alternative control strategy is also proposed for a hybrid wind/battery system by eliminating the dedicated battery converter and the dump loads. To protect the battery against overcharging, an integrated control strategy is proposed. In addition, the dual vector voltage control (DVVC) is also developed to tackle the issues associated with unbalanced AC loads. To improve the performance of a DC microgrid consisting wind, battery, and PV, a distributed control strategy using DC link voltage (DLV) based control law is developed. This strategy provides simpler structure, less frequent mode transitions, and effective coordination among different sources without relying on real-time communication. In a grid-connected mode, this DC microgrid is connected to the grid through a single inverter at the point of common coupling (PCC). The generated wind power is only treated as a source at the DC side for the study of both unbalanced and balanced voltage sag issues at a distribution grid network. The proposed strategy consists of: (i) a vector current control with a feed-forward of the negative-sequence voltage (VCCF) to compensate for the negative sequence currents; and (ii) a power compensation factor (PCF) control for the VCCF to maintain the balanced power flow between the system and the grid. A sliding mode control strategy has also been developed to enhance the overall system performance. Appropriate grid code has been considered in this case. All the developed control strategies have been validated via extensive computer simulation with realistic system parameters. Furthermore, to valid developed control strategies in a realistic environment in real-time, a microgrid has been constructed using physical components: a wind turbine simulator (WTS), power electronic converters, simulated grid, sensors, real-time controllers and protection devices. All the control strategies developed in this system have been validated experimentally on this facility. In conclusion, several power management strategies and real-time control issues have been investigated for direct drive permanent magnet synchronous generator (PMSG) based variable speed wind turbine system in an islanded and grid-connected mode. For the islanded mode, the focuses have been on microgrid control. While for the grid-connected mode, main consideration has been on the mitigation of voltage sags at the point of common coupling (PCC)

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

Modeling and Control of Diesel-Hydrokinetic Microgrids

A large number of decentralized communities in Canada and particularly in Québec rely on diesel power generation. The cost of electricity and environmental concerns suggest that hydrokinetic energy is a potential for power generation. Hydrokinetic energy conversion systems (HKECSs) are clean, reliable alternatives, and more beneficial than other renewable energy sources and conventional hydropower generation. However, due to the stochastic nature of river speed and variable load patterns of decentralized communities, the use of a hybrid diesel- hydrokinetic (D-HK) microgrid system has advantages. A large or medium penetration level has a negative effect on the short-term (transient) and long-term (steady-state) performance of such a hybrid system if the HKECS is controlled based on conventional control schemes. The conventional control scheme of the HKECS is the maximum power point tracking (MPPT). In the long-term conditions, the diesel generator set (genset) can operate at a reduced load where the role of the HKECS is to reduce the electrical load on the diesel genset (light loading). In the short-term, the frequency of the microgrid can vary due to the variable nature of water speed and load patterns. This can lead to power quality problems like a high rate of change of frequency or power, frequency fluctuations, etc. Moreover, these problems are magnified in storage-less DHK microgrids where a conventional energy storage system is not available to mitigate power as well as frequency deviations by controlling active power. Therefore, developing sophisticated control strategies for the HKECS to mitigate problems as mentioned above are necessary. Another challenging issue is a hardware-in-the-loop (HIL) platform for testing and developing a D-HK microgrid. A dispatchable power controller for a fixed-pitch cross-flow turbine-based HKECS operating in the low rotational speed (stall) region is presented in this thesis. It delivers a given power requested by an operator provided that the water speed is high enough. If not, it delivers as much as possible, operating with an MPPT algorithm while meeting the basic operating limits (i.e., generator voltage and rotor speed, rated power, and maximum water speed), shutting down automatically if necessary. A supervisory control scheme provides a smooth transition between modes of operation as the water speed and reference power from the operator vary. The performance of the proposed dispatchable power controller and supervisory control algorithm is verified experimentally with an electromechanical-based hydrokinetic turbine (HKT) emulator. The permanent magnet synchronous generator (PMSG) is preferred in small HKECSs. So, a converter-based PMSG emulator as a testbed for designing, analyzing, and testing of the generator’s power electronic interface and its control system is developed. A 6-switch voltage source converter (VSC) is used as a power amplifier to mimic the behaviour of the PMSG supplying linear and non-linear loads. Technical challenges of the PMSG emulator are considered, and proper solutions are suggested. Finally, an active power sharing control strategy for a storage-less D-HK microgrid with medium and high penetration of hydrokinetic power to mitigate: 1) the effect of the grid frequency fluctuation due to instantaneous variation in the water speed/load, and 2) light loading operation of the diesel engine is proposed. A supplementary control loop that includes virtual inertia and frequency droop control is added to the conventional control system of HKECS in order to provide load power sharing and frequency support control. The proposed strategy is experimentally verified with diesel engine and HKT emulators controlled via a dSPACE® rapid control prototyping system. The transient and steady-state performance of the system including grid frequency and power balancing control are presented

A Downsizing Strategy for Combinatorial PMSG Based Wind Turbine and Micro-SMES System Applied in Standalone DC Microgrid

This paper presents a combinatorial standalone permanent magnet synchronous generator (PMSG) based variable speed wind turbine (VSWT) and small-size superconducting magnetic energy storage (SMES) system into the DC microgrid system. The principal purpose of SMES system is to preserve power balance by absorbing power during peak wind generation and to release it during low power generation. This work accomplished by describing the optimized design of the SMES solenoid coil, ensuring the desired energy storage capacity based on the simulated annealing (SA) algorithm. More importantly, the new control technique is developed for bi-directional DC-DC converter to level output power of the wind turbine depending on the demand thereby reducing the capacity of the DC-DC converter system. Detailed simulation studies implemented in PSCAD/EMTDC corroborate the superior robustness and balancing performance of the proposed micro-SMES controller with an optimal coil size under various situations including variable wind speed. This combination will result in “scaling-factors” knowledge through downsizing strategy which will lead to the most efficient system from cost cutting, energy savings, and downsizing viewpoints

Wind Power Frequency Control in Doubly FED Induction Generator Using CFMPC-FOPID Controller Scheme

Because the majority of wind turbines operate in maximum output power tracking mode, power system frequency cannot be supported. However, if the penetration rate of wind power increases, the system inertia related to frequency modulation may decrease. In addition, frequency stability will be severely affected in the event of significant disturbances to the system load. Due to the high penetration of wind power in isolated power systems, this study suggests a coordinated frequency management approach for emergency frequency regulation. In order to prevent the phenomenon of load frequency control in doubly fed induction generators (DFIGs), a unique efficient control scheme is developed. The Cascaded Fractional Model Predictive Controller coupled with Fractional-Order PID controller (CFMPC-FOPID) is developed to provide the DFIG system with an efficient reaction to changes in load and system parameters. The proposed controller must have a robust tendency to respond quickly in terms of minimum settling time, undershoot, and overshoot. Nonlinear feedback controllers are designed using frequency deviations and power imbalances to achieve the reserve power distribution between generators and DFIGs in a variety of wind speed conditions. It makes upgrading quick and easy. In Matlab/Simulink, a simulation model is built to test the viability of the suggested approach

SENSORLESS DIRECT POWER CONTROL FOR THREE-PHASE GRID SIDE CONVERTER INTEGRATED INTO WIND TURBINE SYSTEM UNDER DISTURBED GRID VOLTAGES

Wind turbines with permanent magnet synchronous generator (PMSG) are widely used as sources of energy connected to a grid. The studied system is composed of a wind turbine based on PMSG, a bridge rectifier, a boost converter, and a controlled inverter to eliminate low-order harmonics in grid currents under disturbances of grid voltage. Traditionally, the grid side converter is controlled by using the control VFOC (Virtual Flux Oriented Control), which decouple the three-phase currents indirect components (id) and in quadratic (iq) and regulate them separately. However, the VFOC approach is dependent on the parameters of the system. This paper illustrates a new scheme for the grid-connected converter controller. Voltage imbalance and harmonic contents in the three-phase voltage system cause current distortions. Hence, the synchronization with the network is an important feature of controlling the voltage converter. Thus, a robust control method is necessary to maintain the adequate injection of the power during faults and/or a highly distorted grid voltage. The proposed new control strategy is to use the direct power control based virtual flux to eliminate side effects induced by mains disturbances. This control technique lowers remarkably the fluctuations of the active and reactive power and the harmonic distortion rate. The estimated powers used in the proposed control approach is calculated directly by the positive, negative, and harmonic items of the estimated flux and the measured current without line sensor voltage.Ветряные турбины с синхронным генератором на постоянных магнитах (PMSG) широко используются в качестве источников энергии, подключенных к сети. Исследуемая система состоит из ветряной турбины на основе PMSG, мостового выпрямителя, повышающего преобразователя и управляемого инвертора для устранения гармоник низкого порядка в токах сетки при возмущениях напряжения сети. Традиционно преобразователь на стороне сети управляется с помощью виртуального потокоориентированного управления VFOC (Virtual Flux Oriented Control), который разделяет трехфазные токи на косвенные компоненты (id) и на квадратичные компоннеты (iq) и регулирует их отдельно. Однако подход VFOC зависит от параметров системы. Данная статья иллюстрирует новую схему для контроллера преобразователя, подключенного к сети. Дисбаланс напряжения и содержание гармоник в трехфазной системе напряжения вызывают искажения тока. Следовательно, синхронизация с сетью является важной особенностью управления преобразователем напряжения. Таким образом, надежный метод управления необходим для поддержания адекватной подачи энергии во время неисправностей и/или значительно искаженного напряжения сети. Предложенная новая стратегия управления заключается в использовании виртуального потока на основе прямого управления мощностью для устранения побочных эффектов, вызванных помехами в сети. Этот метод управления значительно снижает колебания активной и реактивной мощности и уровень гармонических искажений. Оценочные мощности, используемые в предлагаемом подходе к управлению, рассчитываются непосредственно по положительным, отрицательным и гармоническим элементам оцененного потока и измеренного тока без напряжения линейного датчика

Fuzzy logic control of hybrid systems including renewable energy in microgrids

With a growing demand for more energy from subscribers, a traditional electric grid is unable to meet new challenges, in the remote areas remains the extension of the conventional electric network very hard to do make prohibitively expensive. Therefore, a new advanced generation of traditional electrical is inevitable and indispensable to move toward an efficient, economical, green, clean and self-correcting power system. The most well-known term used to define this next generation power system is Micro Grid (MG) based on renewable energy sources (RES). Since, the energy produced by RES are not constant at all times, a wide range of energy control techniques must be involved to provide a reliable power to consumers. To solve this problem in this paper we present a Fuzzy Logic Control of isolated Hybrid Systems (HRES) Including Renewable Energy in Micro-Grids to maintain a stability in voltage and frequency output especially in the standalone application. The considered HRES combine a wind turbine (WT) and photovoltaic (PV) panels as primary energy sources and an energy storage system (ESS) based on battery as a backup solution. Simulation results obtained from MATLAB/Simulink environment demonstrate the effectiveness of the proposed algorithm in decreasing the electricity bill of customer

Small wind turbine incremental current based MPPT with current control for grid-connected DC microgrid application

This paper presents an incremental-type current based Maximum Power Point Tracking (MPPT) system for a small wind turbine (SWT) connected to a grid-connected DC microgrid. The proposed MPPT algorithm calculates the required current, according to the increase or decrease in the DC link voltage, to obtain maximum power transfer to the DC microgrid at all wind speeds. The calculated current value by the MPPT algorithm is used as a reference current for a Proportional Integral (PI) current controller which controls the inductor current in the boost converter connecting the system to the DC microgrid. The proposed MPPT algorithm offers the advantage of requiring less computational calculations, since it uses only the measured DC link voltage, without the need to calculate the DC link power. The proposed MPPT algorithm was simulated on a modelled system consisting of a Permanent Magnet Synchronous Generator (PMSG) driven by a SWT, which was interfaced to the DC microgrid through a rectification stage and a boost converter. The MPPT system, the boost converter, and current control are explained in detail. Simulation results of the system operating under varying wind conditions are presented to highlight the performance of the developed MPPT algorithm.peer-reviewe