Alfeu J. Sguarezi Filho

Model-Based Predictive Control Applied to the Doubly-Fed Induction Generator Direct Power Control

This paper proposes a model-based predictive controller for doubly-fed induction generator direct power control. The control law is derived by optimization of an objective function that considers the control effort and the difference between the predicted outputs (active and reactive power) and the specific references, with predicted outputs calculated using a linearized state-space model. In this case, the controller uses active and reactive power loop directly for the generator power control. Because the generator leakage inductance and resistance information were required for this control method, the influence of the estimation errors for these parameters was also investigated. Simulation results are carried out to validate the proposed controller.

Performance of a Direct Power Control System Using Coded Wireless OFDM Power Reference Transmissions for Switched Reluctance Aerogenerators in a Smart Grid Scenario

This paper presents a performance analysis of a wireless direct power control system for switched reluctance aerogenerators. Aiming at a smart grid scenario, the utilization of wireless technologies for transmitting control information requires a powerful modulation and error-correction coding schemes to avoid any serious problems to the energetic plant. These transmission errors can cause permanent damages in the components of the turbine and converters, and they can also compromise the quality of the energy delivered to the grid. The performance of the proposed system is investigated in a frequency-selective fading channel, thus enabling a deeper study of the impact of the use of wireless communications for reference signal transmissions. This research demonstrates the operational viability of wireless systems for this type of application, when an appropriate digital modulation and coding techniques are applied.

Direct torque control for squirrel cage induction generator based on wind energy conversion system with battery energy storage system

This paper presents a new configuration for a wind energy conversion system (WECS) using a squirrel cage induction generator (SCIG) with battery energy storage system (BESS). The proposed configuration utilizes power converters, operates with maximum power point tracking algorithm to maximum wind energy extraction. The SCIG is connected to the power grid through back to back voltage source converters, and the WECS is reinforced by BESS in order to keep the voltage and frequency of the grid constant, during the variation of wind speeds and the power system. The DC-DC converter (buck-boost) is connected with the back to back converter to control the battery bank and to maintain DC-link voltage constant. The SCIG is controlled using the direct torque control strategy and, the active and reactive power of grid side converter is controlled by means of simple vector control. The vector control of the SCIG side and power grid side converters were implemented in SimPowerSystems® of Matlab/Simulink®. The simulation results showed the effectiveness of this approach.

A study on the rotor side control of DFIG-based wind turbine during voltage sags without crowbar system

The connection of wind farms to the electrical power system has increased very fast in the last years requiring the ability of the wind turbines to assist network during voltage sags. Most of the modern wind turbines are based on doubly fed induction generator with a back-to-back power converter connecting the rotor windings to the grid. One limitation of doubly fed induction generator is the operation during grid faults. The rotor side converter can be damage by high currents induced by the stator windings. This article investigates the behavior of the wind farm during voltage sags using 2 different control strategies. The results have indicated that the strategy which the reference currents are calculated to compensate the stator flux variation during voltage sags can avoid overcurrents in the rotor windings, in some cases, without the use of a crowbar system.

Protection strategies for rotor side converter of DFIG-based wind turbine during voltage dips

Modern wind turbines operate with variable speed and most of them are based on doubly fed induction generators (DFIG), with a back-to-back power converter. During voltage dips, the stator terminals can cause overcurrents in the rotor windings, which could threaten the converter integrity. The use of crowbar systems is the most common technique to avoid this situation. The crowbar activation disables the control of the rotor side converter during a short period of time. Once the rotor side converter is blocked, the DFIG operates like a typical induction generator and thus consumes reactive power. In this paper, the performance of the well-known crowbar protection is compared with a current compensation strategy. Results show that the current compensation strategy can have positive effects on the terminal voltages during the fault without disconnection of the rotor side converter.

A sequence components estimation technique applied for distributed generation

This paper proposes to apply an instantaneous sequence estimation technique to enhance the control systems applied in distributed generation. It relies on a recursive least-square estimation of the αβ voltage components which are directly linked to the sequence components. The method was originally proposed to serve a dynamic voltage restorer controller. Here, this technique has been expanded to improve it robustness with regards to harmonic distortions, presented on the point of common coupling (PCC) between the mains and the distributed generator. The technique has been tested on synthetic signals and on signals produced through a simulation of a grid-connected inverter. The results endorse the proposed technique.

Experimental results of sliding-mode power control for doubly-fed induction generator

This paper proposes a decoupled control of active and reactive power for doubly-fed induction generators (DFIG) through the rotor currents using sliding mode control (SMC). In order to decouple the active and reactive power generated, stator-flux-oriented vector control is applied. The sliding mode control strategy proposes is based on two sliding modes plus PI controllers whose main advantage is the easy implementation. Simulation and experimental results are presented to validate the proposed control scheme for a 2 kW DFIG during stator active and reactive power steps and speed variation. During transient operation it is checked good dynamic response.

A Three-phase to three-phase matrix converter prototype

This paper presents some implementation details of a three-phase to three-phase matrix converter prototype. The bidirectional semiconductor switches were built using discrete IGBTs and fast diodes. Design aspects such as protection against overvoltage and short-circuit, commutation process of bi-directional switches, and input filter are addressed in this paper.

A predictive direct power control of doubly-fed induction generator

This paper proposes a model based predictive controller for direct power control of DFIG. The control law is derived by optimization of an objective function that considers the control effort and the difference between the predicted outputs (active and reactive power) and the specific references, with predicted outputs calculated using a linearized state-space model. Because the generator leakage inductance and resistance information were required for this control method, the influence of the estimation errors for these parameters was also investigated. Simulation results are presented for validation the proposed controller.

Analysis of the Doubly Fed Induction Generator performance on frequency support of microgrids

Doubly Fed Induction Generators (DFIG), represent one the of the most employed technologies, for wind conversion systems. Due to the intermittent nature of wind, the conventional control used by these generators has the objective to operate at a point that ensures maximum power extraction of the wind turbine. However, using only this control philosophy, these generators do not participate in frequency control of the power system, since the mechanical system is decoupled from the electrical system due to the action of the back-to-back converter. Consequently, the conventional DFIG does not present inertia response. However, in power systems with high penetration of wind power generation, or systems that operate islanded, as microgrids, the wind power generator inertia response allows a smother behavior of the electrical frequency. In this context, this work studies, analyzes and compares the Doubly Fed Induction Generator behavior contributing to the frequency support of a microgrid formed due an unintentional islanding through the implementation of three control methods in the rotor side converter of the DFIG. In order to evaluate this behavior, computer simulations were performed simulating the occurrence of islanding, where a part of the distribution network shall operate islanded with power supplied by a DFIG and by a synchronous generator. The results allows analyzing the performance of DFIG in the grid frequency support considering different methodologies.