A Full Order Sensorless Control Adaptive Observer for Doubly-Fed Induction Generator

This paper presents a sensorless control for a Doubly Fed Induction Generator (DFIG) in the context of grid-connected turbine-based wind generation systems. The paper proposes a full order adaptive observer able to track with excellent accuracy the DFIG rotor position even in presence of significant parameters deviations. The developed adaptive observer is coupled with a traditional stator flux based Field Oriented Control (FOC). The novel approach has been validated by an extensive numerical analysis

Flatness-Based Control Methodologies to Improve Frequency Regulation in Power Systems with High Penetration of Wind

To allow for high penetration of distributed generation and alternative energy units, it is critical to minimize the complexity of generator controls and to minimize the need for close coordination across regions. We propose that existing controls be replaced by a two-tier structure of local control operating within a global context of situational awareness. Flatness as an extension of controllability for non-linear systems is a key to enabling planning and optimization at various levels of the grid in this structure. In this study, flatness-based control for: one, Automatic Generation Control (AGC) of a multi-machine system including conventional generators; and two, Doubly fed Induction Machine (DFIG) is investigated. In the proposed approach applied to conventional generators, the local control tracks the reference phase, which is obtained through economic dispatch at the global control level. As a result of applying the flatness-based method, an $n$ machine system is decoupled into n linear controllable systems in canonical form. The control strategy results in a distributed AGC formulation which is significantly easier to design and implement relative to conventional AGC. Practical constraints such as generator ramping rates can be considered in designing the local controllers. The proposed strategy demonstrates promising performance in mitigating frequency deviations and the overall structure facilitates operation of other non-traditional generators. For DFIG, the rotor flux and rotational speed are controlled to follow the desired values for active and reactive power control. Different control objectives, such as maximum power point tracking (MPPT), voltage support or curtailing wind to contribute in secondary frequency regulation, can be achieved in this two-level control structure

Power Converter of Electric Machines, Renewable Energy Systems, and Transportation

Power converters and electric machines represent essential components in all fields of electrical engineering. In fact, we are heading towards a future where energy will be more and more electrical: electrical vehicles, electrical motors, renewables, storage systems are now widespread. The ongoing energy transition poses new challenges for interfacing and integrating different power systems. The constraints of space, weight, reliability, performance, and autonomy for the electric system have increased the attention of scientific research in order to find more and more appropriate technological solutions. In this context, power converters and electric machines assume a key role in enabling higher performance of electrical power conversion. Consequently, the design and control of power converters and electric machines shall be developed accordingly to the requirements of the specific application, thus leading to more specialized solutions, with the aim of enhancing the reliability, fault tolerance, and flexibility of the next generation power systems

A Linear Parameter Varying Controller for Grid-tied Converters under Unbalanced Voltage Network Conditions

This thesis focuses on the development and practical assessment of a contemporary Linear Parameter Varying (LPV) controller for grid-tied converters. The increasing popularity of renewable energy resources necessitates intelligent power converters to interface with utility network. The proposed control methodology can effectively regulate converter powers/currents under highly unbalanced voltage conditions. The methodology can be easily applied to rotating electrical machines that have similar dynamic models. A LPV model of grid-tied converter with filters are derived in synchronous positive and negative rotating frames and a detailed controller design procedure is then carried out using Matrix Linear Inequality technique. The proposed controller uses network frequency as a reference and it has the capability to handle the system frequency variations. Off-line controller design stage is computed by Matlab software while on-line controller calculations are dealt by a Digital Signal Processor (DSP). The highly distorted voltage at the point of common coupling between Voltage Source Inverter (VSI) and utility network may degrade the outputs of the phase locked loop (PLL) module and overall controller performance. An enhanced version of PLL technique is proposed to overcome the voltage distortions and a significant reduction of Total Harmonic Distortion has been recorded. The harmonic issue is successfully treated further with an additional harmonic observer supporting the main controller. To verify the proposed control approach, studies are carried out using Matlab/SIMULINK platform with the code-based simulation. This simulation method can ensure the results close to a real DSP system and enables the user to transfer the simulation studies effectively to the experimental setup without major modifications. A prototype of a three phase VSI with a DSP controller is then investigated using dSPACE DS1104 development board. Experimental results from this system validate the proposed control technique and its benefits

Improved Wind Turbine Control Strategies for Maximizing Power Output and Minimizing Power Flicker

For reducing the cost of energy (COE) for wind power, controls techniques are important for enhancing energy yield, reducing structural load and improving power quality. This thesis presents the control strategies studies for wind turbine both from the perspectives of both maximizing power output and reducing power flicker and structural load, First, a self-optimizing robust control scheme is developed with the objective of maximizing the power output of a variable speed wind turbine with doubly-fed induction generator (DFIG) operated in Region 2. Wind power generation can be divided into two stages: conversion from aerodynamic power to rotor (mechanical) power and conversion from rotor power to the electrical (grid) power. In this work, the maximization of power generation is achieved by a two-loop control structure in which the power control for each stage has intrinsic synergy. The outer loop is an Extremum Seeking Control (ESC) based generator torque regulation via the rotor power feedback. The ESC can search for the optimal torque constant to maximize the rotor power without wind measurement or accurate knowledge of power map. The inner loop is a vector-control based scheme that can both regulate the generator torque requested by the ESC and also maximize the conversion from the rotor power to grid power. In particular, an ∞ controller is synthesized for maximizing, with performance specifications defined based upon the spectrum of the rotor power obtained by the ESC. Also, the controller is designed to be robust against the variations of some generator parameters. The proposed control strategy is validated via simulation study based on the synergy of several software packages including the TurbSim and FAST developed by NREL, Simulink and SimPowerSystems. Then, a bumpless transfer scheme is proposed for inter-region controller switching scheme in order to reduce the power fluctuation and structural load under fluctuating wind conditions. This study considers the division of Region 2, Region 2.5 and Region 3 in the neighborhood of the rated wind speed. When wind, varies around the rated wind speed, the switching of control can lead to significant fluctuation in power and voltage supply, as well as structural loading. To smooth the switch and improve the tracking, two different bumpless transfer methods, Conditioning and Linear Quadratic techniques, are employed for different inter-region switching situations. The conditioning bumpless transfer approach adopted for switching between Region 2 maximum power capture controls to Region 2.5 rotor speed regulation via generator torque. For the switch between Region 2.5 and Region 3, the generator torque windup at rated value and pitch controller become online to limit the load of wind turbine. LQ technique is posed to reduce the discontinuity at the switch between torque controller and pitch controller by using an extra compensator. The flicker emission of the turbine during the switching is calculated to evaluate power fluctuation. The simulation results demonstrated the effectiveness of the proposed scheme of inter-region switching, with significant reduction of power flicker as well as the damage equivalent load

Performance comparisons of doubly-fed machines

This research project aims at evaluating a conversion system based on the emerging Brushless Doubly Fed Reluctance Machine (BDFRM) through a comparative experimental study with a traditional and well established slip-ring counterpart, the Doubly Fed Induction Machine (DFIM). One of the main objectives is to establish whether this alternative machine is worthy of industrial consideration in variable speed applications with limited speed ranges (e.g. wind turbines, pump-like drives etc.) in terms of control, reliability, efficiency and power factor performance as major criteria. Such kind of work has not been reported in the open-literature to date and represents the main contribution of the project being undertaken. A conventional and widely used parameter-independent vector control (VC) scheme has been selected for the operation of both the machines using a shaft-position sensor. The VC algorithm has been simulated and implemented in real-time on state-of-the-art eZdsp development platform based on the TMS320F28335 Digital Signal Controller (DSC). The control code has been derived from a programme written in C++ using the corresponding compiler, the Code Composer Studio (CCS). Comprehensive computer simulations have been done in Matlab/Simulink using the parameters obtained by off-line testing of the DFIM and BDFRM prototypes, which have been built in the same stator frame for comparison purposes. The simulation results have been experimentally verified on two identical test rigs where a commercial 4-quadrant cage induction machine V/f drive has been used as a prime mover or load for either the DFIM or the BDFRM subject to their operating mode. The preliminary experimental results on two small-scale prototypes have shown that the BDFRM can achieve competitive performance to the similarly rated DFIM and as such should warrant further investigation and increasing interests of both academic and industrial communities as a potential large-scale wind generator or a pump drive

Critical Aspects of AGC Emerging from Optimal Control to Machine Learning Techniques

With the emphasis towards renewable energy lot more advancement has been done in the field of electric energy system and it is expected that future energy system may have wind power dominating control areas or hydro power be it bulk hydro or micro hydro based power generations in order to cater the rising energy demands of the modern society. Hence, automatic generation control (AGC) plays a crucial role in the modern electrical energy system in order to maintain the frequency standards to nominal value besides maintaining the power interchange between the interconnected controls areas in order to distribute value and cost effective power. This paper presents the literature survey related to some of the critical aspects of AGC such as diverse sources power generations, hydro dominating control areas, wind power based power generations and applications of flexible alternating current transmission system (FACTS) in AGC. This paper also discusses the novel control designs based on the concept of optimal control, output vector feedback, model predictive control, robust control and finally the machine learning based AGC designs are explored and the critical gaps among the available research work are well presented and discussed

Islanded Wind Energy Management System Based on Neural Networks

Wind power, as the main renewable energy source, is increasingly deployed and connected into electrical networks thanks to the development of wind energy conversion technologies. This dissertation is focusing on research related to wind power system include grid-connected/islanded wind power systems operation and control design, wind power quality, wind power prediction technologies, and its applications in microgrids. The doubly fed induction generator (DFIG) wind turbine is popular in the wind industry and thus has been researched in this Dissertation. In order to investigate reasons of harmonic generation in wind power systems, a DFIG wind turbine is modeled by using general vector representation of voltage, current and magnetic flux in the presence of harmonics. In this Dissertation, a method of short term wind power prediction for a wind power plant is developed by training neural networks in Matlab software based on historical data of wind speed and wind direction. The model proposed is shown to achieve a high accuracy with respect to the measured data. Based on the above research work, a microgrid with high wind energy penetration has been designed and simulated by using Matlab/Simulink. Besides wind energy, this microgrid system is operated with assistance of a diesel generator. A three-layer energy management system (EMS) is designed and applied in this microgrid system, which is to realize microgrid islanded operation under different wind conditions. Simulation results show that the EMS can ensure stable operation of the microgrid under varying wind speed situations

Wind Farm

During the last two decades, increase in electricity demand and environmental concern resulted in fast growth of power production from renewable sources. Wind power is one of the most efficient alternatives. Due to rapid development of wind turbine technology and increasing size of wind farms, wind power plays a significant part in the power production in some countries. However, fundamental differences exist between conventional thermal, hydro, and nuclear generation and wind power, such as different generation systems and the difficulty in controlling the primary movement of a wind turbine, due to the wind and its random fluctuations. These differences are reflected in the specific interaction of wind turbines with the power system. This book addresses a wide variety of issues regarding the integration of wind farms in power systems. The book contains 14 chapters divided into three parts. The first part outlines aspects related to the impact of the wind power generation on the electric system. In the second part, alternatives to mitigate problems of the wind farm integration are presented. Finally, the third part covers issues of modeling and simulation of wind power system