Nektarios Karakasis

Efficiency increase in a wind system with Doubly Fed Induction Generator

In this paper, an improved efficiency control strategy for a Wind Energy Conversion System (WECS) with Doubly Fed Induction Generator (DFIG) is presented. The developed control strategy minimizes the electric loss in the DFIG and provides maximum power point tracking (MPPT) control in the wind turbine. The power loss in the DFIG can be decreased by applying flux weakening control at the low wind speeds while it is transferred to nominal flux operation in high wind speeds. The magnetic flux is regulated through control conditions which also govern the transition process from the flux weakening to nominal flux operation and vise-versa. Therefore, increase of the generated electric energy by the wind system is attained and also expansion of the exploitable wind speed range towards the lower wind speed region is accomplished. The proposed control technique is implemented with a system of lower rated power converters and thus, the advantage of DFIG for lower requirements in the power converter capacity against other types of electrical generators (i.e. permanent magnet synchronous generators and squirrel cage induction generators) still holds. Selective simulation results are presented for demonstrating the efficiency increases in a WECS with the suggested control system.

Performance study of start-up control techniques in a Wind Energy Conversion System with induction generator

This paper studies the performance of a Wind Energy Conversion System (WECS) with an induction generator under various start-up control techniques. The capability of self-excitation of the induction generator using three control techniques is examined. The generator is connected to the power grid by means of a fully controlled frequency converter which consists of a pulse-width modulation (PWM) rectifier, an intermediate dc-link circuit and a PWM inverter. Field oriented control is applied and Maximum Power Point Tracking (MPPT) of the wind turbine is achieved by using the Perturb & Observe (P&O) control technique. A squirrel cage induction generator is considered in this paper. The control system has been simulated using the Matlab/Simulink software and several simulation results are presented in order to demonstrate the performance of the WECS under the examined start-up control techniques.

Indirect estimation of the Yaw-Angle misalignment in a horizontal axis wind turbine

This paper deals with the problem of the yaw angle misalignment in a wind turbine of horizontal axis type and proposes a novel technique for indirectly estimating it. Thus, accurate yaw control can be attained because it is not affected by the inaccuracies that are caused by the vortex flow downstream of the blades which highly influences the wind direction measurement obtained by sensors located at the back end of the nacelle. Furthermore, the proposed technique is cost-effective because, it is not needed the more accurate but highly expensive remote sensing devices. Although it is more suitable for large wind turbines because higher power increase can be accomplished by an accurate yaw control, due to the low cost it can be used in small wind turbines. Thus, it can be applied to any type of power range wind turbines and any type of electrical generators. The practicality and effectiveness of the suggested technique have been verified through simulation analysis conducted by Matlab/Simulink and experimentally on a low power scaling real wind turbine. Several simulation and experimental results have been included in the paper in order to demonstrate the above achievements of the proposed yaw control technique.

Discrete-time model predictive control for high performance speed control in an induction motor drive

This paper proposes a discrete-time predictive controller for providing high performance speed control in an induction motor drive. Specifically, the conventional speed PI controller is replaced by a model predictive controller and the control law is derived by the optimization of an objective function that considers the reference and the real speed as well as the acceleration of the induction motor by using the state-space model. Thus, high dynamic performance can be attained at any load and speed conditions, whereas in a conventional speed PI controller, the parameters are optimally tuned only for a specific operating condition of load and speed. Furthermore, since the developed predictive control algorithm is based on the discrete model of the induction motor, it can be more effectively implemented in a motor drive microcontroller. Also, since the acceleration of the induction motor is considered in the control algorithm, fast dynamic response and satisfactory rejection of the load disturbances can be accomplished. The proposed discrete-time model predictive control system has been simulated in the Matlab/Simulink environment. Several simulation results have been presented in order to validate the effectiveness and demonstrate the operational improvements of the proposed control scheme.

Improved monitoring and battery equalizer control scheme for electric vehicle applications

This paper proposes an improved battery management system (BMS) that provides supervisory monitoring and active cell-to-cell equalization in order to protect the battery lifespan in an electric vehicle (EV). This can be accomplished through a Supercapacitor (SC) bank that can store and then recover electric energy to any weak cell of the EV battery bank. Specifically, the proposed control scheme online detects any problematic battery cell through a supervisory monitoring scheme that oversees the main monitoring system of battery pack. Then, a matrix power switches system connects the SC-bank in parallel with the problematic cell, in order to attain voltage equalization with the other battery cells. Moreover, urgent energy assistance is provided to any weak cells in case of high dynamic conditions, by aiming to keep their voltage as close as possible to the voltage of the other series connected cell unites. The SC-bank is connected to a problematic battery cell through a bidirectional non-isolated dc-dc converter. The functionality and the effectiveness of the proposed supervisory monitoring and battery equalizer system in an EV have been verified by simulations with Matlab/Simulink. Thus, selective simulation results are presented in order to validate the operational improvements obtained by the suggested control scheme.