The spectrum of power from wind turbines

This paper presents system modeling and control design for fast load voltage regulation using static compensators (STATCOMs). The modeling strategy gives a clear representation of load voltage magnitude and STATCOM reactive current on an instantaneous basis. The particular coordinate transformation employed here also facilitates extraction of linearized system dynamics in conjunction with circuit simulators. It is rigorously shown that the control problem of load voltage regulation using reactive current is nonminimum phase. Linear and nonlinear controllers for the regulation problem are designed and compared via simulation results. Internal dynamics of the STATCOM are modeled using the same strategy. Lyapunov based adaptive controllers are designed for controlling the STATCOM reactive current while maintaining its dc bus voltage. Simulation results of the controlled STATCOM integrated with the load bus voltage controller are presented to show efficacy of the modeling and control design.

Voltage regulation with STATCOMs: modeling, control and results

The increased use of power electronic controlled equipment, such as variable speed drives, automated production lines, personal computers and non-linear electronic devices in power systems has given rise to a type of voltage and current waveform distortion called as ‘harmonics’. Harmonic can be defined as the undesirable components of a distorted periodic waveform whose frequencies are the integer multiples (non-integer multiples in case of inter-harmonics, and the frequency less than fundamental frequency in case of sub-harmonics) of the fundamental frequency. Presence of these harmonics results in increased losses, equipment heating and loss-of-life, and interference with protection, control and communication circuits as well as customer loads. The research has been underway since very beginning for control of power system harmonics and to supply consumers with reliable and ‘clean’ fundamental-frequency sinusoidal electric power that does not represent a damaging threat to their equipment. This paper, therefore, reviews the progress made in power system harmonics research and development since its inception. Attempts are also made to highlight the current and future issues involved in the development of quality and reliable electric power technology for future applications. A list of 145 research publications on the subject is also appended for a quick reference. Copyright © 2007 John Wiley & Sons, Ltd.

Power system harmonics research: a survey

In order to fully study the electrical, mechanical, and aerodynamic aspects of a wind turbine with a doubly fed induction generator, a detailed model that considers all these aspects must be used. A drawback of many works in the area of wind turbine simulation is that either a very simple mechanical model is used with a detailed electrical model, or vice versa. Hence, the effects of interactions between electrical and mechanical components are not accurately taken into account. In this paper, three simulation programs - TurbSim, FAST, and Simulink - are used to model the wind, mechanical and electrical parts of a wind turbine, and its controllers. Simulation results obtained from the model are used to observe the interaction of all three factors affecting the operation of a wind turbine system. For example, it is shown how an electrical disturbance can cause dangerous tower vibrations under high speed and turbulent wind conditions, which may not be feasible using a simple model of the wind and wind turbine.

Simulation of a Wind Turbine With Doubly Fed Induction Generator by FAST and Simulink

This paper presents a control scheme for grid-connected pulsewidth-modulated voltage-source inverters (VSIs) featuring fast load-voltage regulation and effective mitigation of unbalanced voltage disturbances. To ensure perfect regulation of the voltage at the point of common coupling (PCC) and provide means for rejecting fast and dynamic voltage disturbances, the frequency modes of the disturbances to be eliminated should be included in the stable closed-loop system. Toward this, a hybrid voltage controller combining a linear with variable-structure control element is proposed for an inverter-based distributed-generation interface to regulate the voltage at the PCC. The proposed voltage controller can embed a wide band of frequency modes through an equivalent internal model. Subsequently, a wide range of voltage perturbations, including capacitor-switching disturbances, can be rejected. To account for unbalanced voltage disturbances, a dual-sequence voltage controller is proposed. To provide accurate and robust tracking of the generated active and reactive current trajectories, a newly designed deadbeat current control algorithm is proposed. The controller is designed under the practical considerations of inherent plant delays, which are associated with the digital implementation of the control algorithm, and the uncertain nature of the current dynamics. Theoretical analysis and comparative evaluation tests are presented to demonstrate the effectiveness of the proposed control scheme.

A Control Scheme for PWM Voltage-Source Distributed-Generation Inverters for Fast Load-Voltage Regulation and Effective Mitigation of Unbalanced Voltage Disturbances

Wind energy conversion systems (WECS) produce fluctuating output power, which may cause voltage fluctuations and flicker. Flicker assessment in networks may be difficult since its evaluation requires long computing time and special procedures to calculate the flicker severity index, Pst. In this paper, a frequency domain method to study flicker propagation is presented. This method is based on propagation of frequency components from WECS output currents throughout the grid. In this way, a fast flicker analysis in a network of any size can be performed. Also, an algorithm for flicker measurement in the frequency domain, which allows Pst calculation, is proposed. Several study cases have been performed, and results have been compared with time domain simulations, showing good agreement between them.

Propagation of flicker in electric power networks due to wind energy conversions systems

A hybrid method in the time and frequency domains has been developed for the analysis of the harmonic distortion in power systems. This method is more complete than others reported in the literature and it allows both balanced and unbalanced systems to be considered. The linear elements of a network are modelled in the frequency domain while the nonlinear elements are modelled in the time domain, which permits consideration of all topologies and configurations. Acceleration techniques are used for finding the steady state in time-domain methods. Linear and nonlinear loads can be specified in terms of power by integrating a three-phase load-flow algorithm for the fundamental frequency with a hybrid algorithm for the harmonic interaction analysis. The proposed method tries to solve the present problems to study harmonic-distortion propagation in a power system. The integration of two complex and different algorithms is done here for the first time and offers great possibilities for power-systems analysis.

J. Usaola Garcia

Papers

Propagation of flicker in electric power networks due to wind energy conversions systems

Three-phase harmonic load flow in frequency and time domains