Implementation of wind turbine controllers

Three of the important, generic, implementation issues encountered when developing controllers for pitch-regulated constant-speed wind turbines are considered, namely, (1) accommodation of the strongly nonlinear rotor aerodynamics; (2) automatic controller start-up/shut-down; and (3) accommodation of velocity and acceleration constraints within the actuator. Both direct linearisation and feedback linearisation methods for accommodating the nonlinear aerodynamics are investigated and compared. A widely employed technique for accommodating the nonlinear aerodynamics, originally developed on the basis of physical insight, is rigorously derived and extended to cater for all wind turbine configurations. A rigorous stability analysis of controller start-up is presented for the first time and novel design guidelines are proposed which can significantly reduce the power transients at controller start-up. The relation to anti-wind-up is noted and several aspects of an existing wind-turbine controller start-up strategy are observed to be novel in the anti-wind-up context. Restrictive position, velocity and acceleration constraints may all be present in wind turbines and the dynamic behaviour of the actuator cannot be neglected. A novel, and quite general, anti-wind-up method, based on the startup strategy, is proposed which caters for all these circumstances. The separate strategies for resolving the implementation issues are combined to achieve an elegant controller realisation which accommodates all the implementation issues in an integrated manner. The importance of adopting an appropriate controller realisation is considerable and is illustrated for a 300 kW wind turbine. The implementation issues encountered in this paper are, of course, not confined to wind turbines but are of wider concern

Appropriate Realisation of MIMO Gain-Scheduled Controllers

The dynamic characteristics of a controller designed by the gain-scheduling approach can be strongly dependent on the realisation adopted; that is, the manner in which the local linear controller designs are combined to obtain a nonlocal controller. The purpose of the present paper is to investigate the choice of appropriate realisations for general MIMO gain-scheduled controllers. An extended local linear equivalence condition for MIMO gain-scheduled nonlinear controllers is proposed which minimises the controller nonlinearity. It is shown that, with few exceptions, it is possible to realise all gain-scheduled controllers as nonlinear controllers satisfying the extended local linear equivalence condition and requiring the controller to do so is not at all restrictive

Performance enhancement of wind turbine power regulation by switched linear control

Power regulation of horizontal-axis grid-connected up-wind constant-speed pitch-regulated wind turbines presents a demanding control problem with the plant, actuation system and control objectives all strongly nonlinear. In this paper a novel switched linear approach is devised. Conventional linear control and a nonlinear controller which, in some sense, optimises performance across the operating envelope provide benchmarks against which the switched control strategy is compared. In comparison with conventional linear control, the switched linear strategy reduces the peak power excursions experienced and the time spent at high power levels, with a consequent reduction in drive-train loads. It achieves very similar performance to the more complex nonlinear controller; that is, the performance is near optimal over the operational envelope. Moreover, in contrast to nonlinear control it admits straightforward, rigorous analysis and permits direct exploitation of the knowledge and experience accumulated with linear control. Hence, switched linear control is more suited for application to wind turbines than the nonlinear control strategy. The improvement in performance, in comparison to conventional linear control, is substantial

Appropriate realisation of gain-scheduled controllers with application to wind turbine regulation

Power regulation of horizontal-axis grid-connected up-wind constant-speed pitch-regulated wind turbines presents a demanding control problem with the plant, actuation system and control objectives all strongly nonlinear. In this paper, a novel nonlinear control strategy is devised which, in some sense, optimises performance across the operating envelope. In comparison with linear control, the nonlinear strategy achieves a substantial improvement in performance. The realisation adopted is crucial in attaining the required performance. An extended local linear equivalence condition is introduced which provides a basis for the selection of an appropriate realisation. This is an important, and general, issue in the design of gainscheduled systems and generic realisations, which satsify the extended local linear equivalence condition, are derived for SISO systems scheduled upon an internal plant or controller variable. For the wind turbine nonlinear controller, realisations which satisfy the extended local linear equivalence condition provide a substantial improvement in performance in comparison to linear control and realisations which do not satisfy this condition

Gain-Scheduled Controller Design: An Analytic Framework Directly Incorporating Non-Equilibrium Plant Dynamics

In this paper, a velocity-based linearisation framework is employed to develop a novel rigorous approach to gain-scheduling design. The proposed approach enables knowledge concerning the plant dynamics at non-equilibrium operating points to be incorporated directly into the controller design. Since the velocity-based linearisation framework supports the analysis of the transient response, performance considerations can be accommodated. The approach retains continuity with linear methods, which is central to the existing conventional gain-scheduling methodology, and, since a single type of linearisation is employed throughout, the design procedure is both straightforward and conceptually appealing

Counter-example to a common LPV gain-scheduling design approach

The shortcomings of a popular LPV gain-scheduling design approach are demonstrated by a simple counterexample. It is shown that, for a very general class of nonlinear systems, such an ad hoc design approach is unnecessary since soundly-based methods exist for transforming the plant dynamics into LPV/quasi-LPV form

Survey of Gain-Scheduling Analysis & Design

The gain-scheduling approach is perhaps one of the most popular nonlinear control design approaches which has been widely and successfully applied in fields ranging from aerospace to process control. Despite the wide application of gain-scheduling controllers and a diverse academic literature relating to gain-scheduling extending back nearly thirty years, there is a notable lack of a formal review of the literature. Moreover, whilst much of the classical gain-scheduling theory originates from the 1960s, there has recently been a considerable increase in interest in gain-scheduling in the literature with many new results obtained. An extended review of the gainscheduling literature therefore seems both timely and appropriate. The scope of this paper includes the main theoretical results and design procedures relating to continuous gain-scheduling (in the sense of decomposition of nonlinear design into linear sub-problems) control with the aim of providing both a critical overview and a useful entry point into the relevant literature

Gain-Scheduled & Nonlinear Systems: Dynamic Analysis by Velocity-Based Linearisation Families

A family of velocity-based linearisations is proposed for a nonlinear system. In contrast to the conventional series expansion linearisation, a member of the family of velocity-based linearisations is valid in the vicinity of any operating point, not just an equilibrium operating point. The velocity-based linearisations facilitate dynamic analysis far from the equilibrium operating points and enable the transient behaviour of the nonlinear system to be investigated. Using velocity-based linearisations, stability conditions are derived for both smooth and non-smooth nonlinear systems which avoid the restrictions, to trajectories lying within an unnecessarily, perhaps excessively, small neighbourhood about the equilibrium operating points, inherent in existing frozen-input theory. For systems where there is no restriction on the rate of variation, the velocity-based linearisation analysis is global in nature. The analysis techniques developed, whilst quite general, are motivated by the gain-scheduling design approach and have the potential for direct application to the analysis of gain-scheduled systems

Appropriate Realisation of MIMO Gain-Scheduled Controllers

The dynamic characteristics of a controller designed by the gain-scheduling approach can be strongly dependent on the realisation adopted; that is, the manner in which the local linear controller designs are combined to obtain a nonlocal controller. The purpose of the present paper is to investigate the choice of appropriate realisations for general MIMO gain-scheduled controllers. An extended local linear equivalence condition for MIMO gain-scheduled nonlinear controllers is proposed which minimises the controller nonlinearity. It is shown that, with few exceptions, it is possible to realise all gain-scheduled controllers as nonlinear controllers satisfying the extended local linear equivalence condition and requiring the controller to do so is not at all restrictive

Performance enhancement of wind turbine power regulation by switched linear control

Power regulation of horizontal-axis grid-connected up-wind constant-speed pitch-regulated wind turbines presents a demanding control problem with the plant, actuation system and control objectives all strongly nonlinear. In this paper a novel switched linear approach is devised. Conventional linear control and a nonlinear controller which, in some sense, optimises performance across the operating envelope provide benchmarks against which the switched control strategy is compared. In comparison with conventional linear control, the switched linear strategy reduces the peak power excursions experienced and the time spent at high power levels, with a consequent reduction in drive-train loads. It achieves very similar performance to the more complex nonlinear controller; that is, the performance is near optimal over the operational envelope. Moreover, in contrast to nonlinear control it admits straightforward, rigorous analysis and permits direct exploitation of the knowledge and experience accumulated with linear control. Hence, switched linear control is more suited for application to wind turbines than the nonlinear control strategy. The improvement in performance, in comparison to conventional linear control, is substantial