G.M. Asher

Overview of Control Systems for the Operation of DFIGs in Wind Energy Applications

Doubly-Fed Induction Generators (DFIGs), often organized in wind parks, are the most important generators used for variable speed wind energy generation. This paper reviews the control systems for the operation of DFIGs in wind energy applications. Control systems for connections to balanced or unbalanced grids, and sensorless control.

Sensorless Control of Doubly-Fed Induction Generators Using a Rotor-Current-Based MRAS Observer

This paper presents a new sensorless method for the vector control of doubly-fed induction machines (DFIMs) without using speed sensors or rotor position measurements. The proposed sensorless method is based on the model reference adaptive system (MRAS) estimating the rotor position and speed from the machine rotor currents. The method is appropriate for both stand-alone and grid-connected operation of variable speed DFIMs. To design the MRAS observer with the appropriate dynamic response, a small signal model is derived. The sensitivity of the method for variation in the machine parameters is also analyzed. Speed catching on the fly and synchronization of the doubly-fed induction generator with the utility are also addressed. Experimental results obtained from a 3.5-kW prototype are presented and fully analyzed.

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.

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.

Suppression of saturation saliency effects for the sensorless position control of induction motor drives under loaded conditions

This paper presents an automated commissioning procedure used for the elimination of the saturation saliency effects in the sensorless position control of field-orientated cage induction motor drives. The position control itself is based on extracting a rotor position estimate from a high-frequency signal injection interacting with natural or engineered rotor position saliencies within the machine. The paper shows that this estimate cannot be robustly or accurately obtained if saturation saliencies are present. The paper introduces a method for suppressing the effects of the saturation saliency through information gained in a prior commissioning procedure. The effectiveness of the procedure is demonstrated through experimental results showing both good suppression of the saturation harmonics and true sensorless position control under high load torques.

Switched reluctance generators for wind energy applications

The switched reluctance machine has very good characteristics in term of efficiency, control simplicity and robustness. The rotor is made of solid iron with no electrical excitation and this results in considerable robustness and reliability. The machine is cost competitive with the cage induction motor, has superior efficiency to it and its power converter is cheaper and fundamentally more reliable. Even with these advantages, its use as a generator has not been properly investigated. This paper describes the use of a 7.5 kW switched reluctance machine working as a generator and discusses the control systems and the application of this sort of generator connected in a variable speed wind turbine system.<<ETX>>

The effect of rotor design on sensorless speed estimation using rotor slot harmonics identified by adaptive digital filtering using the maximum likelihood approach

Adaptive digital filtering has been demonstrated as an effective technique for extracting a real-time, sensorless, speed signal from rotor slot harmonics (RSHs) embedded in the line current waveform of induction motor drives. It is known that sensorless speed estimation techniques using RSHs may exhibit poor performance with certain motor designs. This paper examines the reasons for that poor performance which reflects differences in the magnitude of the slot harmonic signals consequent upon rotor design. Experimental results for a 30 kW motor with 6 different rotors are presented. The significant parameters are the number of rotor slots, skew and the accuracy of construction. It is clearly shown how inferior performance can arise. Conversely, improvements in reliability of speed estimation and transient response can be obtained by recognising those aspects which provide an enhanced signal, by minimising the background noise of the inverter/machine, or by enhancing the adaptive filter. The recursive maximum likelihood technique is presented as an improved algorithm for tuning the digital filter which aids transient response and reliability of speed estimation. Real time, experimental transient performance is demonstrated for the different rotors used in this paper and the performance failure of a particular slot combination demonstrated.

Vector controlled induction machines for stand-alone wind energy applications

This paper describes the system and control structures for vector controlled induction generators used for variable speed, wind energy conversion (WEC) systems. The paper focuses on WEC systems feeding an isolated load or weak grid since for such systems the generated voltage and power flow must be regulated by the WEC system itself and the control structures are not trivial. The structures for both cage induction and doubly-fed induction generators are described and their performance and complexity compared. Experimental results for both systems feeding an isolated load are given and show that both systems are capable of good output voltage regulation whilst tracking the optimal speed for energy capture.

Sensorless speed detection of inverter fed induction motors using rotor slot harmonics and fast Fourier transform

A novel approach to sensorless speed detection for adjustable-speed AC drives is described. No a priori knowledge is required about the motor construction, electrical parameters, or load condition. In addition, no external tuning is needed for the system. The technique is based on instantaneous spectral estimation using the fast Fourier transform, whereby the speed-dependent slot ripple harmonic frequency is determined. For the assessment of this technique, an all-digital speed detector has been built around a general-purpose 386 microcomputer. The performance of this detector over a wide range of inverter frequencies and load conditions is discussed.<<ETX>>