Roberto Cardenas

Overview of Multi-MW Wind Turbines and Wind Parks

Multimegawatt wind-turbine systems, often organized in a wind park, are the backbone of the power generation based on renewable-energy systems. This paper reviews the most-adopted wind-turbine systems, the adopted generators, the topologies of the converters, the generator control and grid connection issues, as well as their arrangement in wind parks.

Sensorless vector control of induction machines for variable-speed wind energy applications

A sensorless vector-control strategy for an induction generator in a grid-connected wind energy conversion system is presented. The sensorless control system is based on a model reference adaptive system (MRAS) observer to estimate the rotational speed. In order to tune the MRAS observer and compensate for the parameter variation and uncertainties, a separate estimation of the speed is obtained from the rotor slot harmonics using an algorithm for spectral analysis. This algorithm can track fast dynamic changes in the rotational speed, with high accuracy. Two back-to-back pulse-width-modulated (PWM) inverters are used to interface the induction generator with the grid. The front-end converter is also vector controlled. The dc link voltage is regulated using a PI fuzzy controller. The proposed sensorless control strategy has been experimentally verified on a 2.5-kW experimental set up with an induction generator driven by a wind turbine emulator. The emulation of the wind turbine is performed using a novel strategy that allows the emulation of high-order wind turbine models, preserving all of the dynamic characteristics. The experimental results show the high level of performance obtained with the proposed sensorless vector-control method.

MRAS observer for sensorless control of standalone doubly fed induction generators

This paper presents an analysis of a model reference adaptive system (MRAS) observer for the sensorless control of a standalone doubly fed induction generator (DFIG). The analysis allows the formal design of the MRAS observer of given dynamics and further allows the prediction of rotor position estimation errors under parameter mismatch. The MRAS observer analysis is experimentally implemented for the vector control of a standalone DFIG feeding a load at constant voltage and frequency. Experimental results, including speed catching of an already spinning machine, are presented and extensively discussed. Although the method is validated for a standalone generator, the proposed MRAS observer can be extended to other applications of the doubly fed induction machine.

Control of a switched reluctance generator for variable-speed wind energy applications

This paper presents a novel control system for the operation of a switched reluctance generator (SRG) driven by a variable speed wind turbine. The SRG is controlled to drive a wind energy conversion system (WECS) to the point of maximum aerodynamic efficiency using closed loop control of the power output. In the medium and low speed range, the SRG phase current is regulated using pulsewidth-modulation (PWM) control of the magnetizing voltage. For high speeds the generator is controlled using a single pulse mode. In order to interface the SRG to the grid (or ac load) a voltage-source PWM inverter is used. A 2.5-kW experimental prototype has been constructed. Wind turbine characteristics are emulated using a cage induction machine drive. The performance of the system has been tested over the whole speed range using wind profiles and power impacts. Experimental results are presented confirming the system performance.

MRAS Observers for Sensorless Control of Doubly-Fed Induction Generators

This paper addresses the analysis and performance of several model reference adaptive system (MRAS) observers for sensorless vector control of doubly-fed induction machines. Small signal models allow the formal analysis of the observers for a given dynamic. The performance of each MRAS observer is analyzed, considering grid-connected and stand-alone operation. The MRAS observers are implemented in a 3.5 kW experimental prototype composed of a doubly-fed induction generator and a wind turbine emulator. Experimental results validate the predictions of the small signal models and demonstrate the performance of the sensorless methods during both steady state and variable speed wind energy generation.

Control System for Unbalanced Operation of Stand-Alone Doubly Fed Induction Generators

In this letter, a novel control system for the operation of a doubly fed induction generator (DFIG), feeding an unbalanced three-phase load is presented. The load unbalance is compensated using the front-end converter. The control system is based on two revolving axes, rotating at plusmnomegae, which are used to control the positive and negative sequence currents supplied by the front-end converter. Experimental results obtained from a 2-kW experimental prototype are presented and discussed in this letter.

Power smoothing in wind generation systems using a sensorless vector controlled induction Machine driving a flywheel

This paper presents a novel control strategy for power smoothing in generation systems in which power flow variations can occur. These variations are the norm in wind energy generation. The system is based on a sensorless vector controlled induction machine driving a flywheel. The induction machine is controlled to operate in a wide speed range by using flux weakening above rated speed. A speed observer is used to obtain the rotational speed in the whole speed range. In order to tune the speed observer and compensate for the parameter variation and uncertainties, a separate estimation of the speed is obtained from the rotor slot harmonics using an algorithm for spectral analysis. This algorithm can track fast dynamic changes in the rotational speed, with high accuracy. The control strategies have been experimentally verified on a 3.5-kW experimental setup with an induction machine and flywheel. The experimental results show the high level of performance obtained with the proposed sensorless vector control system.

Control of the Reactive Power Supplied by a WECS Based on an Induction Generator Fed by a Matrix Converter

In this paper, a new control system to regulate the reactive power supplied by a variable-speed wind energy conversion system (WECS), based on an induction generator fed by a matrix converter (MC), is presented. The control system discussed in this paper is based on an input current observer, implemented using an estimation of the modulation matrix, and a nonlinear control loop that regulates the displacement angle at the MC input. The reactive power capability of the proposed system is also investigated. The work presented in this paper demonstrates that, for the proposed WECS, the maximum reactive power supplied to the grid is about 40% of the nominal value. Experimental results obtained from an experimental prototype are presented in this paper. The performance of the system using a wind turbine emulator and typical wind profiles is discussed in this paper.

Stability Analysis of a Wind Energy Conversion System Based on a Doubly Fed Induction Generator Fed by a Matrix Converter

In this paper, the performance of a grid-connected wind energy conversion system (WECS), based on a doubly fed induction generator (DFIG) fed by a matrix converter (MC), is presented. The MC replaces the back-to-back converters conventionally used to control a DFIG. The MC is operated with close-to-unity power factor at the grid side. Stability issues related to the operation of the MC in the proposed WECS are discussed. A small signal model is used to investigate the dynamic performance of the two control arrangements discussed in this paper. Experimental results, obtained with a 4-kW prototype, are presented and fully discussed in this paper. The performance of the system for variable speed generation is verified using the emulation of a variable speed wind turbine implemented with a digitally controlled dc machine.

Control strategies for enhanced power smoothing in wind energy systems using a flywheel driven by a vector-controlled induction machine

This paper presents a novel control strategy for power smoothing in wind energy applications, especially those feeding a stand-alone load. The system is based on a vector-controlled induction machine driving a flywheel and addresses the problem of regulating the DC-link system voltage against both input power surges/sags from a wind turbine or sudden changes in load demand. The control is based on a feedforward compensation scheme augmented by a nonlinear controller. Two feedforward compensation schemes are discussed and the limitations and performance of each scheme are analyzed. Experimental results are presented which verify the excellent performance of the feedforward compensation technique.