Greg Asher

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.

Hybrid rotor position observer for wide speed-range sensorless PM motor drives including zero speed

This paper addresses the problem of wide speed-range sensorless control of a surface-mount permanent-magnet (SMPM) machine including zero-speed operation. A hybrid structure integrating a flux observer and signal-injection techniques is proposed, which results in a rotor position signal independent of motor parameters at low and zero speed. Although the SMPM machine typically has a very low geometric saliency, the injection technique is effective in tracking the saturation-induced saliency produced by the stator flux. Experimental results are presented showing an excellent performance for both the sensorless speed and position control using an off-the-shelf SMPM machine.

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 of Offshore DFIG-Based Wind Farm Grid With Line-Commutated HVDC Connection

The paper considers a control solution for integration of large offshore doubly fed induction generator based wind farms with a common collection bus, controlled by a static compensator, into the main onshore grid, using line-commutated high-voltage direct current connection. The papers main focus is a mathematically grounded study of the power system interactions. That study produced an appropriate plant model for formal control design. A design procedure is described and the controlled system is validated using power systems computer-aided design/electromagnetic transient program simulations, which confirm the high performance of the proposed control strategy in both normal operation and fault conditions

Improved Waveform Quality in the Direct Torque Control of Matrix-Converter-Fed PMSM Drives

Despite the ability of matrix converters (MCs) to generate a higher number of voltage vectors compared with standard voltage-source inverters, most of the applications reported in the literature utilize only those having larger amplitudes. This paper investigates the use of MC input voltages with different amplitudes in order to reduce the inherent torque ripple that appears when direct torque control (DTC) is used to drive ac machines, particularly permanent-magnet synchronous motors, as its stator inductance is typically half of that of an induction machine of similar ratings. Utilizing a wider range of input voltage vectors for the MC, but not using the rotating vectors, a new lookup table that distinguishes between small and large torque errors is developed, leading to an enhanced MC-fed DTC. The enhancement enables a reduction in the electromagnetic torque ripple and output-current total harmonic distortion. Furthermore, the proposed control strategy improves the MC voltage transfer ratio up to 86.6% compared with 50% achieved by the conventional DTC using MCs at the expense, however, of slightly decreasing the input power factor control capability. The proposed enhanced MC DTC was tested experimentally, and results comparing its performance with MC DTC using an adapted lookup table are shown.

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.

Comparative analysis of experimental performance and stability of sensorless induction motor drives

This paper compares the experimental performance of three flux and speed observers for speed-sensorless induction motor drives and discusses the cause of their differences. The small signal analysis using the linearized model is carried out to analyze stability. Three methods are generally accepted to be representative candidates for high sensorless performance, namely: 1) rotor-flux model reference adaptive system (MRAS); 2) torque-current MRAS; and 3) adaptive nonlinear flux observer. Many other sensorless methods improved these methods. The paper discusses baseline conditions for the experiments and the stability analysis, which include matched load inertia, specified speed estimator dynamics, and sensorless operation within a speed control loop. For the comparison tests in the paper, the speed estimation dynamics of the methods are the same; this is important for parameter sensitivity. The paper concentrates on the low-speed performance, and all results shown are under sensorless speed control.

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.

Large Offshore DFIG-Based Wind Farm With Line-Commutated HVDC Connection to the Main Grid: Engineering Studies

The paper considers a solution for integration of large offshore doubly fed induction generator-based wind farms with a common collection bus controlled by a STATCOM into the main onshore grid using line-commutated high-voltage dc connection. A design procedure is described and the controlled system is validated using PSCAD/EMTDC simulations confirming high performance of the proposed control strategy in both normal operation conditions and faults. Engineering issues related to STATCOM capacitor sizing and reduction of STATCOM rating are considered and their effectiveness is confirmed.

Frequency Control Design for Offshore Wind Farm Grid With LCC-HVDC Link Connection

This paper considers the formal design for the grid frequency control in an offshore wind farm connected with line-commutated converter high voltage dc (HVDC) link. The control paradigm is based on using the grid frequency control to regulate the HVDC rectifier firing angle or dc-link current and hence control the power flow in the system. The dynamic behaviors of the system are verified by comparing the response from derived transfer functions and PSCAD simulations; hence the grid frequency controllers are designed. The control system performance has been validated by simulations of normal operation and fault regimes. The work provides a good basis for wider research investigation into wind farm operation.