Sachin T. Navalkar

Wind Tunnel Testing of Subspace Predictive Repetitive Control for Variable Pitch Wind Turbines

An increasing number of wind turbines implement individual blade pitch control (IPC) to reduce turbine dynamic loading, and thereby, to reduce the capital and operational costs associated with energy production. The aim of this paper is to demonstrate IPC on a wind turbine prototype, in a model-free data-driven manner and with reduced pitch activity. For this, subspace predictive repetitive control (RC) is used, which combines online system identification with the continuous implementation of RC to form a fully adaptive control law. The controller is tested on a scaled two-bladed wind turbine with active pitchable blades, placed in an open-jet wind tunnel. Substantial load reductions to an extent of 68% are observed, and strict control over actuator signal frequency content is achieved. The control law also demonstrates the ability to adjust to changes in system dynamics while maintaining a high degree of load alleviation.

Sizing and Control of Trailing Edge Flaps on a Smart Rotor for Maximum Power Generation in Low Fatigue Wind Regimes

An extension of the spectrum of applicability of rotors with active aerodynamic devices is presented in this paper. Besides the classical purpose of load alleviation, a secondary objective is established: optimization of power capture. As a first step, wind speed regions that contribute little to fatigue damage have been identified. In these regions the turbine energy output can be increased by deflecting the trailing edge (TE) flap in order to track the maximum power coefficient as a function of local, instantaneous speed ratios. For this purpose, the TE flap configuration for maximum power generation has been using blade element momentum theory. As a first step, the operation in non-uniform wind field conditions was analyzed. Firstly, the deterministic fluctuation in local tip speed ratio due to wind shear was evaluated. The second effect is associated with time delays in adapting the rotor speed to inflow fluctuations caused by atmospheric turbulence. The increase in power generation obtained by accounting for wind shear has been demonstrated with an increase in energy production of 1%. Finally, a control logic based on inflow wind speeds has been devised and the potential of enhanced power generation has been shown by time-domain simulations. Copyright c

Experimental wind tunnel testing of linear individual pitch control for two-bladed wind turbines

In this paper Linear Individual Pitch Control (LIPC) is applied to an experimental small-scale two-bladed wind turbine. LIPC is a recently introduced Individual Pitch Control (IPC) strategy specifically intended for two-bladed wind turbines. The LIPC approach is based on a linear coordinate transformation, with the special property that only two control loops are required to potentially reduce all periodic blade loads. In this study we apply LIPC to a control-oriented small-scale two-bladed wind turbine, equipped with, among others, two high- bandwidth servomotors to regulate the blade pitch angles and strain gauges to measure the blade moments. Experimental results are presented that indicate the effectiveness of LIPC.

Individual blade pitch for yaw control

Individual pitch control (IPC) for reducing blade loads has been investigated and proven successful in recent literature. For IPC, the multi-blade co-ordinate (MBC) transformation is used to process the blade load signals from the rotating to a stationary frame of reference. In the stationary frame of reference, the yaw error of a turbine can be appended to generate IPC actions that are able to achieve turbine yaw control for a turbine in free yaw. In this paper, IPC for yaw control is tested on a high-fidelity numerical model of a commercially produced wind turbine in free yaw. The tests show that yaw control using IPC has the distinct advantage that the yaw system loads and support structure loading are substantially reduced. However, IPC for yaw control also shows a reduction in IPC blade load reduction potential and causes a slight increase in pitch activity. Thus, the key contribution of this paper is the concept demonstration of IPC for yaw control. Further, using IPC for yaw as a tuning parameter, it is shown how the best trade-off between blade loading, pitch activity and support structure loading can be achieved for wind turbine design.

Benchmarking aerodynamic prediction of unsteady rotor aerodynamics of active flaps on wind turbine blades using ranging fidelity tools

Simulations of a stiff rotor configuration of the DTU 10MW Reference Wind Turbine are performed in order to assess the impact of prescribed flap motion on the aerodynamic loads on a blade sectional and rotor integral level. Results of the engineering models used by DTU (HAWC2), TUDelft (Bladed) and NTUA (hGAST) are compared to the CFD predictions of USTUTT-IAG (FLOWer). Results show fairly good comparison in terms of axial loading, while alignment of tangential and drag-related forces across the numerical codes needs to be improved, together with unsteady corrections associated with rotor wake dynamics. The use of a new wake model in HAWC2 shows considerable accuracy improvements.

Integrating robust lidar-based feedforward with feedback control to enhance speed regulation of floating wind turbines

The structural cost of offshore wind energy can be drastically reduced by the development of floating wind turbines. However, the design of a feedback controller for rotor speed control of such turbines faces fundamental bandwidth limitations because of the presence of nonminimum phase zeros. New developments in lidar technology enable turbines to measure the incoming wind and use the measurements for feedforward control. This paper explores the possibility of combining lidar feedforward with feedback control for floating wind turbines to enhance the speed control performance and increase controller bandwidth without affecting stability. Robust stability and performance of the controllers are investigated. The controllers are validated using the high-fidelity simulation environment FAST for floating turbines with lidar, and enhanced control performance is achieved.

Subspace Predictive Repetitive Control with Lifted Domain Identification for Wind Turbine Individual Pitch Control

Individual pitch control (IPC) is gaining increasing acceptance as a method for mitigation of periodic disturbances in wind turbines. This paper aims at formulating a repetitive control (RC) methodology capable of adapting online to changing turbine dynamics. This is achieved by performing system identification in a reduced-dimensional space using basis functions and using the identified parameters to synthesise an RC law to reject periodic disturbances: this methodology is termed Subspace Predictive Repetitive Control (SPRC). The method is tested on an industrial simulation test bench and is able to identify and perform load control on the turbine without affecting its power production.

Nuclear norm-enhanced recursive subspace identification: Closed-loop estimation of rapid variations in system dynamics

For a time-varying plant operating in closed-loop with a stabilising controller, rapid changes in system dynamics can be detected online using recursive subspace identification methods to estimate the open-loop system behaviour. However, these methods usually involve a speed-accuracy trade-off: accurate identification can often only be achieved by slow updates, which increases the lag in the detection of changes in system dynamics. In this paper, a closed-loop, recursive subspace identification algorithm is extended with a convex cost function based on the nuclear norm. The nuclear norm heuristic exploits structure in the algorithm by enforcing a low-rank condition on the state predictor matrix. This condition reduces the variance of the estimates at the price of introducing a bias. The new algorithm is demonstrated for a system where the damping changes from positive to negative, and it is shown to successfully and consistently estimate the onset of open-loop instability, outperforming conventional recursive identification. Further, by tuning the forgetting factor in the estimation algorithm, a favourable speed-accuracy trade-off can be achieved.

Experimental investigation of an autonomous flap for load alleviation

This paper presents an experimental aeroservoelastic investigation of a novel load alleviation concept using trailing edge flaps. These flaps are autonomous units, which are self-powered and self-actuated, using trailing edge tabs, thereby demonstrating advantages in comparison with conventional flap systems in terms of wiring and structural integration. The flaps are free-floating and mass underbalanced, such that they may flutter at operation velocities unless suppressed by their own control system. This makes the system very responsive for turbulence and control action. In the wind tunnel campaign presented in this paper, the limit cycle behavior of autonomous, free-floating flaps was investigated. It has been shown that limit cycle oscillation can be reached either through structural limiters or by control actions of the trailing edge tabs. In the latter case, the amplitude of the limit cycle oscillation is adjustable to the required energy output. An energy balance of harvested power and power consumption for actuators and sensing system was made showing that the vibration energy of limit cycle oscillations can be used to keep the amplitude of the limit cycle constant, while the electric batteries that power the load alleviation system are being charged.

Experimental and Numerical Investigation of an Autonomous Flap for Load Alleviation

This paper presents the design, a numerical aeroservoelastic investigation, and an experimental proof of concept of an autonomous flap system. Autonomous flaps are load-alleviation devices, intende...