Structured Control of LPV Systems with Application to Wind Turbines

Abstract — This paper deals with structured control of linear parameter varying systems (LPV) with application to wind turbines. Instead of attempting to reduce the problem to linear matrix inequalities (LMI), we propose to design the controllers via an LMI-based iterative algorithm. The proposed algorithm can synthesize structured controllers like decentralized, static output and reduced order output feedback for discrete-time LPV systems. Based on a coordinate decent, it relies on a sufficient matrix inequality condition extended with slack variables to an upper bound on the induced L2-norm of the closed-loop system. Algorithms for the computation of feasible as well as optimal controllers are presented. The general case where no restrictions are imposed on the parameter dependence is treated here due to its suitability for modeling wind turbines. A comprehensive numerical example of a gain-scheduled LPV controller design with prescribed pattern for wind turbines illustrate the utilization of the proposed algorithm. I

Nonlinear predictive generalized minimum variance LPV control of wind turbines

More advanced control strategies are needed for use with wind turbines, due to increases in size and performance requirements. This applies to both individual wind turbine controls and for the total coordinated controls for wind farms. The most successful advanced control method used in other industries is predictive control, which has the unique ability to handle hard constraints that limit system performance. However, wind turbine control systems are particularly difficult in being very nonlinear and dependent upon the external parameter variations which determine behaviour. Nonlinear controllers are often complicated to implement. The approach proposed here is to use one of the latest predictive control methods which can be used with linear parameter varying (LPV) models. These can approximate the behaviour of nonlinear wind turbines and provide a simpler control structure to implement. The work has demonstrated the feasibility and benefits that may be obtained

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