Selênio Rocha Silva

Reactive power control in doubly-fed induction generators for wind turbines

High penetration level of wind power generation in the interconnected electrical network and the increasing rated power of the wind energy converter units imposes new technical challenges to engineers. The first one is to guarantee grid stability despite the stochastic nature of wind while the second one is to limit the power losses since electrical machines efficiency do not augment proportionally to the rated power increase in the MW-class. Attending to the new requirements of the power companies on wind turbines concerning the contribution to network stability through reactive power exchange with the mains supply, this work proposes a reactive power control scheme for a doubly-fed induction generator drive that minimises the system power losses. This drive topology enables besides the control of active power the independent control of reactive power with both inverters. This degree of freedom is used in order to share the reactive currents resulting on the reactive power required on the network and increasing the efficiency. The steady-state power losses are modeled and the optimal reactive power share working points are found by an iterative procedure. The control structure design is presented and discussed as well as implementation issues. Experimental results are presented in order to prove the theoretical assumptions.

Behavior of doubly-fed induction generator during symmetrical voltage dips — Experimental results

Wind energy conversion systems using the doubly-fed induction generator (DFIG) are the most commercialized topology worldwide, because this technology uses converters dimensioned for ±30% of the system rated power, thus reducing the equipment cost. The direct connection of the DFIG stator to the electrical system increases its sensitivity to grid disturbances. In this context, this paper analyzes the DFIG behavior during balanced voltage sags using the machine classical model for the theoretical development and analyzing experimental results of a 2.2 kW test bench equipped with a crowbar device. The obtained results are useful in the development of strategies to improve the DFIG supportability during grid disturbances.

Regenerative PWM source for power transformer loading tests

Most of power transformers, before being commercialized, must undertake a series of tests with the objective of analyzing their operational behavior under real operating conditions. Such tests sometimes demand a high amount of power increasing significantly their costs. The paper presents a proposed system which allows testing in closed loop, reducing the energy consumption to the natural losses of the equipment, thus constituting a regenerative system. A VSI-based test environment for power transformers where controlled active and reactive power can be supplied is designed, simulated and realized as a 50KVA- prototype. Special care is taken concerning the ac-filters in order to keep the harmonics within the limits set by standards valid for testing of power transformers. The simulation and experimental results show that the prototype fulfills the given requirements.

Minimum motor losses point tracking for a stand-alone photovoltaic pumping system

Solar photovoltaic energy has been, for many years now, a promising alternative in the diversification of global energetic sources. Although photovoltaic energy has many advantages, it has not yet become commercially popular, because it is still an expensive alternative. In fact, studies have already shown that the investment in such systems are paid back, but it is still hard to find people willing to wait five or six years to have their money back. The aim of this work is to reduce the payback time of photovoltaic stand-alone pumping systems, optimizing not only the efficiency of the photovoltaic conversion, using a maximum power point tracking algorithm, but also minimizing the losses in the induction motor. Based on the design procedures described in this paper, a prototype has been developed and experimental results with the optimizing strategy are shown.

Online neo-fuzzy-neuron state observer

An algorithm for a state observation based on a neo-fuzzy-neuron (NFN) with real time training is presented. Some useful theorems are promptly demonstrated and used to aid the design of the observer. Two applications of this state observer are shown: an induction machine rotor flux observer and an induction machine speed observer. Digital simulation and experimental results show the good performance of the observer.

Discrete-time sliding mode control: a new design strategy applied to induction motor drives

In this paper, a robust discrete-time sliding mode speed controller for induction motor drives is proposed. A nonlinear sliding surface is used to achieve fast response with low overshoot. A very simple control law is proposed in order to avoid torque oscillations inherent to usual discrete-time sliding mode design techniques. With the proposed design methodology, discrete-time sliding mode existence conditions are ensured. Simulation and experimental results show the effectiveness of the proposed algorithm.

Modeling and control of a flexible photovoltaic array simulator

Photovoltaic (PV) systems have gained prominence in the current context of distributed generation systems. The fundamental element of a PV grid-connected systems is the power converter. Power converters must be tested and certified according to the international standards, before being commercialized. Therefore, a flexible equipment to test the converters is necessary. This paper presents the modeling and control structure of a solar array simulator (SAS) of 10 kW for testing power converters. The limitations of conventional structures are pointed out and an improved structure based on two converter stages is proposed. The feasibility of the proposed SAS is shown through simulations using the Matlab/Simulink. The simulator is tested for the system connected to resistive loads and to a photovoltaic converter model. The dynamical behavior is evaluated through solar irradiance variations and partial shadowing tests. The results of the proposed structure show flexibility and good levels of power quality.

Stabilization of Back-to-Back converter in wind generation system connected to the grid: IDA-PBC versus PI Control

Wind energy generation systems connected to the grid have become an important research subject concerning stability and energy optimization. This article addresses the control of a Back-to-Back converter connecting a Permanent Magnet Synchronous Generator to the grid. The system uses an Interconnection and Damping Assignment Passivity-Based Control (IDAPBC). Comparison of classical PI approach with the proposed approach is made. Experimental results are provided.

Analysis of direct power control strategies applied to doubly fed induction generator

This paper presents the results of an investigation into several direct power control (DPC) strategies for the rotor side converter (RSC) of a doubly fed induction generator (DFIG) based wind energy generation system. The schemes investigated DPC of 2-level hysteresis, DPC of 3-level hysteresis and DPC using Discrete Space Vector Modulation (DSVM), with a 5-level hysteresis. The results of Matlab-Simulink simulations and a comparison between the control schemes are presented, regarding variations of active and reactive power, machine parameters, and wind speed.

A Stator Flux Oriented Induction Motor Drive with Deadbeat Direct Torque and Flux Control

A stator field oriented control scheme for induction motor drives is proposed. As a deadbeat controller, the algorithm calculates the voltage vector to be supplied by the frequency converter in order to eliminate the flux and torque errors each switching period. The strategy is a DTC scheme in the sense that there are no current controllers with the advantage of a fixed switching frequency. Simulation and experimental results are presented to show the effectiveness of the proposed method and the quality of torque and flux transient responses.