Roberto Cárdenas

Overview of Control Systems for the Operation of DFIGs in Wind Energy Applications

Doubly-Fed Induction Generators (DFIGs), often organized in wind parks, are the most important generators used for variable speed wind energy generation. This paper reviews the control systems for the operation of DFIGs in wind energy applications. Control systems for connections to balanced or unbalanced grids, and sensorless control.

Vector controlled induction machines for stand-alone wind energy applications

This paper describes the system and control structures for vector controlled induction generators used for variable speed, wind energy conversion (WEC) systems. The paper focuses on WEC systems feeding an isolated load or weak grid since for such systems the generated voltage and power flow must be regulated by the WEC system itself and the control structures are not trivial. The structures for both cage induction and doubly-fed induction generators are described and their performance and complexity compared. Experimental results for both systems feeding an isolated load are given and show that both systems are capable of good output voltage regulation whilst tracking the optimal speed for energy capture.

Control of a Doubly Fed Induction Generator via an Indirect Matrix Converter With Changing DC Voltage

This paper presents a control strategy for a doubly fed induction generator (DFIG) using an indirect matrix converter, which consists of an input side matrix converter and an output side voltage source converter (VSC). The capability of the input converter to generate different “virtual dc link” voltage levels is exploited. The commutation of the VSI with reduced voltage is illustrated for operating points where the output voltage demand is low without any deterioration of the current control performance. The proposed method leads to a reduction in the commutation losses in the output converter and reduced common-mode voltage. For the input converter, soft switching commutation is obtained by synchronizing the input and output converter pulsewidth-modulation patterns. This modulation strategy is particularly applicable in DFIG applications because the required rotor voltage decreases when the DFIG speed is close to the synchronous speed. The complete control strategy is experimentally validated using a 2-kW rig.

Vector Control of Front-End Converters for Variable-Speed Wind–Diesel Systems

This paper presents a novel power-balance control method for a wind-diesel generation feeding an isolated grid. The system is based on a variable-speed wind energy conversion system (WECS) connected to an ac load using a power converter. An energy storage system (ESS), connected to the ac load using an additional converter, is used to balance the power generated by the WECS with the load. In this paper, the vector control systems for both interfacing power converters are discussed; the control uses the WECS converter to regulate the ac load voltage and the ESS converter to regulate the power flow to achieve a power balance. A small signal model is used to design the control systems. Finally, the proposed control is implemented in a 2-kW experimental prototype and the experimental results are fully analyzed and discussed in the paper

Secondary Control Strategies for Frequency Restoration in Islanded Microgrids With Consideration of Communication Delays

One of the well-known methods to share active and reactive power in microgrids (MGs) is droop control. A disadvantage of this method is that in steady state the frequency of the MG deviates from the nominal value and has to be restored using a secondary control system (SCS). The signal obtained at the output of the SCS is transmitted using a communication channel to the generation sources in the MG, correcting the frequency. However, communication channels are prone to time delays, which should be considered in the design of the SCS; otherwise, the operation of the MG could be compromised. In this paper, two new SCSs control schemes are discussed to deal with this issue: (1) a model predictive controller (MPC); and (2) a Smith predictor-based controller. The performance of both control methodologies are compared with that obtained using a conventional proportional integral-based SCS using simulation work. Stability analysis based on small signal models and participation factors is also realized. It is concluded that in terms of robustness, the MPC has better performance.

The Application of Resonant Controllers to Four-Leg Matrix Converters Feeding Unbalanced or Nonlinear Loads

Matrix converters (MC) have some advantages when compared to conventional back-to-back pulsewidth modulation voltage-source converters. The MC may be considered more reliable and is smaller because the bulky dc capacitor is eliminated from the topology. Therefore, when MCs are used in ac-ac power conversion, the size and weight of the whole generation system is reduced. To interface a MC-based generation system to an unbalanced three-phase stand-alone load, a four-leg MC is required to provide an electrical path for the zero-sequence load current. Moreover, to compensate for the voltage drops in the output filter inductances, nonlinearities introduced by the four-step commutation method and voltage drops in the semiconductor devices, closed-loop regulation of the load voltage is required. In this paper, the design and implementation of a resonant control system for four-leg MCs is presented. The application of this control methodology when the four-leg MC is feeding, a linear/nonlinear unbalanced load is also presented in this study. High-order resonant controllers are also analyzed. Experimental results, obtained from a small prototype, are discussed.

A Cascade Multilevel Frequency Changing Converter for High-Power Applications

A novel frequency changing conversion scheme using three cascade multilevel converters in a Π topology is presented. The scheme resembles a direct frequency converter using the cascade converter in its simplest form (series strings of H-bridge modules equipped with a dc link capacitor) as the building block of the overall converter. This yields a highly modular implementation approach which may be attractive for large power applications such as intertie connections and variable speed drives. Frequency conversion takes place in a cascade converter which connects the input and output ports. Two other converters are placed, respectively, in parallel to the input, to remove unwanted current components from the input, and the output to regulate output voltage. Operation of this topology is explained and a scheme to control all the converters is developed, including control of converter currents, capacitor voltages, and output voltage. Experimental results, using a low-power prototype, confirm the foundations of the topology and verify its overall performance operating as a power supply at typical output frequencies (25 Hz, 162/3 Hz and dc) while being fed from a 50-Hz system. Additionally, PowerSIM simulations demonstrate that the topology may be suitable for implementing high-performance, high-power ac drive systems using vector control techniques.

Control strategies for voltage control of a boost type PWM converter

This paper addresses the nonlinear problem of regulating the DC link voltage, in a vector controlled three-phase boost type PWM converter with particular emphasis on applications in wind energy systems. The AC side current control is performed in a synchronously rotating reference frame. An analysis of the system is provided and several alternative DC link voltage controller designs are investigated and compared. Fuzzy controllers augmented using a feedforward compensation technique are found to provide excellent performance. A 3 kW experimental set up has been built and experimental results are provided to validate the control designs.

Analysis and Experimental Validation of Control Systems for Four-Leg Matrix Converter Applications

Matrix Converters (MCs) have some advantages when compared to conventional back-to-back PWM voltage source converters. The converter may be considered more reliable and it can be smaller because the bulky dc capacitors are eliminated from the topology. For ac to ac power conversion, the size and weight of the whole generation system can be much reduced when back-to-back converters are replaced by MCs. To supply electrical energy to an unbalanced 3Φ stand-alone load, a fourth leg is required to provide a path for the zero-sequence load current. To regulate the load voltage, closed-loop control is required. In this paper, the application of d -q controllers and resonant controllers to four-leg MCs is addressed. The design and performance issues of the controllers, for operation with balanced, highly unbalanced loads and nonlinear loads are discussed in this paper. Experimental results obtained from a small prototype are presented and analysed in detail.

Control system for grid generation of a switched reluctance generator driven by a variable speed wind turbine

This paper presents a novel control system for the operation of a grid connected switched reluctance generator driven by a variable speed wind turbine. The SRG is connected to a power converter which is controlled to drive the wind energy conversion system (WECS) to the point of maximum aerodynamics efficiency using closed loop control of the power output. A second power converter, a vector controlled voltage source PWM inverter, is used to interface the machine to the grid. An experimental prototype was constructed in which the wind turbine was emulated by an induction machine drive. Experimental result obtained with this prototype are presented and discussed in this paper.