Abstract
Attention has been paid by many researchers to address the various challenges of grid connection of DFIG-based Wind Energy Conversion Systems (WECS). This chapter focuses on the design of a robust H8 controller for the power flow between the stator of the Doubly-Fed Induction Generator (DFIG) and the grid. The robust H8 controller design is formulated as a mixed-sensitivity problem. A mathematical model of the DFIG written in an appropriate d-q reference frame is established to carry out simulations. The proposed power control scheme is elaborated and compared with a conventional Proportional-Integral (PI) controller based on vector control technique. The results show interesting performance of the controlled system in terms of the power reference tracking (the active and reactive power) and robustness against parameter variations compared with the conventional PI controller.
Top1. Introduction
Due to depletion of fossil fuels and increase of polluted emissions, renewable energy production is rapidly growing. Wind energy is one of the most promising renewable energy sources due to the fact that it is more efficient, availability and low cost (Shafiullah et al., 2013). It has several advantages, such as: is the most environment-friendly, 100% clean energy resource, energy-efficient, cost-efficient. Hence, wind energy has begun to be used as the panacea for solving the global warming problem. Energy of the wind has been used for thousands of years for water pumping, grinding grain, and other low-power applications. There were several early attempts to build large-scale wind powered systems to generate electrical energy (Gálvez-Carrillo & Kinnaert, 2011).
Squirrel cage induction machine is used in several wind energy conversion systems. This machine has proven its efficiency when it is directly connected to the grid due to qualities such as robustness, low cost and simplicity (Pena et al., 1996). However, the wind turbine must be designed to keep the machine’s speed constant near the synchronous speed. All this constraint reduces the possibility to increase the electrical energy produced for high wind speeds. A converter can be used between the stator of the machine and the grid but it is crossed by the full power and must be correctly cooled (Schreiber, 2001).
Among the existing aerogenerators, Doubly Fed Induction Generators (DFIGs) are the most widely employed in wind power systems (Babu & Mohanty, 2009; Hansen et al., 2004). DFIG-based wind turbines are the most widely employed in wind power systems, they are used for large-scale wind power generation systems due to their interesting characteristics such as, their ability to operate at variable speed, the capacity to control the active and reactive powers into four quadrants by means of field orientation, besides, they have controllable power factor, improved system efficiency and reduced converter rating, which is typically 30% of the generator rating and, hence, decreases the cost and power loss (Hansen et al., 2007; Rothenhagen et al., 2009; Nian et al., 2011). The power control of DFIG in wind turbine is traditionally based on stator-voltage oriented (Hopfensperger et al., 2000), stator-flux-oriented (Chowdhury & Chellapilla, 2006).
In Leonard (2001), explains the vector control technique used for the independent control of torque and excitation current. The converter design and control technique are well explained in (Mohan et al., 1989). The DFIG with PI controllers and its performance under normal operation conditions has been discussed in a number of publications (Carrasco et al., 2006; Petersson et al., 2005; Andreas & Thiringer, 2004; Xu et al., 2009; Arbi et al., 2009). In Yamamoto & Motoyoshi (1990), a control approach has been proposed and applied using a rotating reference frame fixed on the gap flux of the generator and the control active and reactive powers is carried out independently, the results have been proved by experiment. A detailed design of the DFIG using back-to-back PWM voltage source converters in the rotor circuit and they also validated the system experimentally considering a grid connected system has been studied in Pena et al. (1996). In Roncero-Sanchez et al. (2005), a comparative study in discrete time to achieve a decoupled control of the active and reactive powers and to obtain a deadbeat system with two control methods (a classical PI controller and a predictive-integral controller).
Key Terms in this Chapter
Reactive Power: Such as inductors and capacitors dissipate zero power, yet the fact that they drop voltage and draw current gives the deceptive impression that they actually do dissipate power.
Vector Control: Also called field-oriented control (FOC), is a variable frequency drive (VFD) control method which controls three-phase AC electric motor output by means of two controllable VFD inverter output variables: (Voltage magnitude, Frequency).
Robuste H8: (i.e. “H-infinity”) methods are used in control theory to synthesize controllers achieving stabilization with guaranteed performance.
Doubly Fed Induction Generator: Are electric generators that have windings on both stationary and rotating parts, where both windings transfer significant power between shaft and electrical system. Usually the stator winding is directly connected to the three-phase grid and the three-phase rotor winding is fed from the grid through a rotating or static frequency converter.
Active Power: The product of the voltage across a branch of an alternating current circuit and the component of the electric current that is in phase with the voltage.
Robust Controller: Is such that its properties do not change much if applied to a system slightly different from the mathematical one used for its synthesis. This specification is important: no real physical system truly behaves like the series of differential equations used to represent it mathematically. Typically a simpler mathematical model is chosen in order to simplify calculations; otherwise the true system dynamics can be so complicated that a complete model is impossible.
Grid: (Also referred to as an electricity grid or electric grid) is an interconnected network for delivering electricity from suppliers to consumers.
PI Controller: A PI Controller (proportional-integral controller) is a special case of the PID controller in which the derivative (D) of the error is not used.