Gerardo Tapia

Two Alternative Modeling Approaches for the Evaluation of Wind Farm Active and Reactive Power Performances

Based on the stator-flux-oriented electromechanical model of a doubly fed induction machine (DFIM), this paper presents two different approaches to wind farm modeling. Particularly, the overall control system of the DFIM is modeled in detail, so that tuning equations that allow adjusting the parameters of its proportional-integral (PI) compensators straightforwardly are provided. In addition to validating the wind generator model proposed, experimental tests carried out on a 660-kW doubly fed induction generator (DFIG) prove that such a control system makes it possible to govern separately its stator-side or net active and reactive powers. The two wind farm models proposed, which are assumed to be made up of DFIM type generators exclusively, are mainly devised to assist the design and test of closed-loop control algorithms for the wind farm total power factor. They generally imply significantly different computational loads, so that the second may be regarded as a simplified version of the first one. The net active and reactive power performances of both wind farm models are finally compared via simulation, which allows identifying the stronger and weaker points associated to each model

DFIG Power Generation Capability and Feasibility Regions Under Unbalanced Grid Voltage Conditions

Several strategies have been proposed for operating doubly-fed induction generator (DFIG)-based wind turbine under unbalanced grid voltage conditions. This study focuses on those strategies in which the rotor-side power converter aims at eliminating the oscillations affecting the electromagnetic torque and the stator reactive power. Given the limited size of the DFIGs power converters, and hence their tolerable current and voltage boundaries, this study analyzes which DFIG power generation capability is under unbalanced grid voltage, and therefrom derives rotor current and stator power controllable ranges. Besides, the feasibility regions of the DFIG are also deduced for different types of imbalance. Furthermore, and based on the outcome of previous analysis, a modified rotor current limiter, as also its equivalent stator power limiter, is proposed. In contrast to the conventional ones, the limiters proposed here take into account that imbalances may arise in the grid voltage. As a consequence, systems overall performance is considerably enhanced under unbalanced grid voltage conditions. Finally, simulation results establish the validity of the treated issues.

Methodology for Smooth Connection of Doubly Fed Induction Generators to the Grid

A systematic methodology for smooth connection of wind-turbine-driven doubly fed induction generators (DFIGs) to the grid is presented. Synchronization of the voltage induced in the DFIG open stator to that of the grid, which needs to be accomplished prior to connection, is thoroughly examined. A particular grid-voltage-oriented rotor control scheme is considered for this purpose. Generic tuning equations for the rotor current integral-proportional (I-P) controllers involved in this scheme are also derived. Transition between the control configurations devoted to synchronization and normal operation-active power generation and reactive power interchange with the grid-at the instant of connection is studied in detail. Mainly due to the reference frame selected for synchronization, the greater part of this transition takes place naturally. However, given that the rotor current dynamics vary significantly depending on whether the DFIG stator is connected to the grid or not, the parameters of the I-P controllers involved in both schemes will accordingly be different. Consequently, a ldquobumplessrdquo strategy is provided that preserves the smoothness of the connection. A simple method for initial rotor positioning, required when performing vector control based on an incremental encoder, is also suggested. The resulting overall methodology is validated on a 7-kW DFIG-based laboratory-scale test bench.

Comparison of wind turbine LQG controllers using Individual Pitch Control to alleviate fatigue loads

This paper focuses on the design of Linear Quadratic Gaussian (LQG) controllers for variable-speed horizontal axis Wind Turbines (WT). These turbines use blade pitch angle and electromagnetic torque control variables to meet specified objectives for Full Load (FL) zone. The main control objectives are to reduce structural dynamic loads and to regulate the power of the WT. The controllers are designed in order to optimize a trade-off between several control objectives. Four different LQG using Individual Pitch Control (IPC) are designed, with Wireless-Sensors (WS) placed at the end of the blades for the last one. Their control model is progressively more complex. The first one takes into account a rigid simple behavior, the second control model considers the first mode of the drive-train flexibility, the third model takes into account the drive-train and tower flexibilities and the fourth that of the blades. Likewise, their optimization criteria consider for each controller a new control objective to alleviate fatigue loads in the drive-train, then, also in the tower and finally also in the blades. The evaluation of the fatigue loads affecting the WT components are based on a Rainflow Counting Algorithm (RFC) and the Miners rule. The results indicate a significant reduction of fatigue loads especially in the drive-train and the blades when its flexibility is taken into account in the control models.

Comparison of wind turbine LQG controllers designed to alleviate fatigue loads

This paper focuses on the design of Linear Quadratic Gaussian (LQG) controllers for variable-speed horizontal axis Wind Turbines (WTs). Those turbines use blades pitch angle and electromagnetic torque control variables to meet specified objectives for Full Load (FL) zone. The main control objectives are to reduce the structural dynamic loads and to regulate the power of the WT. The controllers are designed in order to optimize a trade-off between several control objectives. Four different LQG controllers are designed. Their control model is progressively more complex. The first one takes into account a rigid simple behavior, the second control model considers the first mode of flexibility of the drive train, the third model takes into account the tower flexibilities and the fourth that of the blades. In the same manner, their optimization criteria considers for each controller a new control objective to alleviate fatigue loads in the drive train, then, also in the tower and finally also in the blades. The evaluation of the fatigue loads affecting the WT components are based on a Rainflow Counting Algorithm and the Miners rule (RFC). The results indicate a significant reduction of fatigue loads in the drive-train when its flexibility is taken into account in the control model.

VOLTAGE REGULATION OF DISTRIBUTION NETWORKS THROUGH REACTIVE POWER CONTROL

Abstract A procedure for the design and tuning of two control loops of a High Voltage DC system is presented. The controlled variables are direct current on the rectifier side and direct voltage on the inverter side. The dynamics of both control loops interact with each other and are determined by the overall properties of the combined AC/DC system. Constraints on the information available for feedbackin each loop require a decentralised controller structure. Genetic Algorithms are demonstrated to be an efficient tool for the design and tuning of such a decentralised control system.

Sliding-mode control for linear permanent-magnet motor position tracking

Abstract Based on Utkins research work regarding speed control of rotary motors, a position tracking algorithm for linear permanent magnet synchronous motors (LPMSM) is designed making use of multi-variable sliding-mode control and asymptotic observers. It is applicable to LPMSMs with either interior or surface-mounted magnets, no matter whether the armature is stationary or movable. Furthermore, the control signals yielded are able to drive the power transistors of the inverter feeding the motor directly, hence avoiding the employment of techniques such as PWM or SVM. Performance of the control structure proposed is evaluated on the simulation model of a commercially available LPMSM.

Sliding-Mode Control for a DFIG-Based Wind Turbine under Unbalanced Voltage

Abstract This paper reports a first-order sliding-mode control (1-SMC) design for controlling the doubly-fed induction generator (DFIG)-based wind turbines rotor-side power converter. The design is particularly focused on keeping the generator successfully in operation under unbalanced grid voltage conditions, as todays grid codes require. Aside from controlling the stator-side active and reactive powers’ average value, the rotor-side converter is commanded so as to remove the fluctuations affecting the electromagnetic torque and the reactive power during unbalanced voltage. The paper aims to put forward the bases of the proposed design together with the described algorithms stability proof. Finally, the appropriateness of the sliding-mode control to deal with the aforementioned disturbed scenarios is supported by means of simulation results.

Student-tailored final year project on microcontroller-based hardware-in-the-loop speed control of a wind generator

Student-supervisor cooperation was carried out to design the inter-subject final year project reported in this paper. According to the student feedback, this approach allowed gathering his main interests – vector control, microcontroller programming and wind power generation – together, therefore reinforcing his motivation towards his final year project and making him feel particularly responsible for its outcome. Microcontroller-based hardware-in-the-loop emulation made possible to combine the three afore-cited student interests. In this context, the virtual prototype of a current-controlled wind turbine-driven 2-MW permanent-magnet synchronous generator is presented, along with the pseudo-code corresponding to the permanent-magnet synchronous generator speed control algorithm programmed in an 8-bit microcontroller. In addition, tuning formulas are derived for the digital integral-proportional controllers commanding both the permanent-magnet synchronous generator current and speed. Detailed descriptions are provided in order to guarantee reproducibility. Implementation of the hardware-in-the-loop rig is also tackled, supported by illustrative results obtained when running it. The developed hardware-in-the-loop rig is considered suitable for laboratory practices of subjects like digital control and microcontroller programming.