Stator Side Converter Research Articles

Synergistic and Sliding Mode Controls of Wind Energy Conversion System

This paper presents the control scheme of a wind energy conversion system connected to the electrical grid. With the increasing of electrical energy demand, renewables energies, in particular, Wind Energy becomes one of the most used sources and plays an important role. The system is composed by a three blade horizontal wind turbine, a permanent magnet synchronous generator (PMSG) which is connected to the network through two converters the Stator Side Converter (SSC) by Direct Current bus and The Grid Side Converter (GSC). The aim is to optimize the extracted energy from the wind. To overcome conventional PI limitations in term of dynamic performances; therefore, both new approaches are applied to the wind turbine. The first is the sliding mode control (SMC) when the second is based on synergetic control. Vector control is applied to the PMSG and integrated to the grid through two converters. Active and reactive power, are controlled by a classical PI controller. Simulation results under Matlab\Simulink confirmed the performance of the proposed SMC and synergetic control in dealing with nonlinear systems with attractive features such as simplicity and good performances.

Coordinated control strategy for doubly‐fed induction generator with DC connection topology

With the development of wind power generation system and requirement of large-scale and long-distance power transmission, it is greatly important for wind farm to achieve the high effective DC grid connection technology. This study presents a novel grid connection topology for doubly-fed induction generator (DFIG) connected to a DC-link, in which a stator side converter and rotor side converter fed by a common DC bus are employed. Coordinated control method for the converters is proposed to regulate the electromagnetic torque directly based on the air-gap flux orientation technique. Compared with the traditional grid connection system, the proposed system has a lot of advantages such as simpler structure, lower requirement for the converter capacity and higher power transmission efficiency. The power distribution between stator side converter and rotor side converter is analysed based on the proposed grid connection topology. Finally, DFIG-based experimental system is developed to validate the correctness and availability of the proposed DC grid connection topology and control strategy.

Applications of Nonlinear Control for Fault Ride-Through Enhancement of Doubly Fed Induction Generators

Applications of several nonlinear control laws, including differential geometric approach and the robust variable structure system (VSS) approach, will be investigated for the fault ride-through (FRT) enhancement of the doubly fed induction generator wind power system. In differential geometric approach, both input-output feedback linearization and state feedback linearization will be studied for rotor-side converter and stator-side converter control. In robust VSS approach, supplementary control generated by the complete models is achieved for FRT enhancement. Two objectives, maximizing the active power output as well as regulating the terminal voltage, will be utilized for designing the sliding manifold. Since the proposed methods are based on the general nonlinear system theory, the FRT capability can be indeed improved even under severe fault conditions. Simulations on a one-machine infinite bus system and a two-area four-machine system are presented to validate this result. Performance comparisons among several existing FRT enhanced control will also be examined to show the effectiveness of these nonlinear control schemes.

Optimal Design of PID Controller for Doubly-Fed Induction Generator-Based Wave Energy Conversion System Using Multi-Objective Particle Swarm Optimization

This paper presents the complete modeling and simulation of Wave Energy Conversion System (WECS) driven doubly-fed induction generator with a closed-loop vector control system. Two Pulse Width Modulated voltage source (PWM) converters for both rotor- and stator-side converters have been connected back to back between the rotor terminals and utility grid via common dc link. The closed-loop vector control system is normally controlled by a set of PID controllers which have an important influence on the system dynamic performance. This paper presents a Multi-objective optimal PID controller design of a doubly-fed induction generator (DFIG) wave energy system connected to the electrical grid using Particle Swarm Optimization (PSO) and Genetic Algorithm (GA). PSO and GA are used to optimize the controller parameters of both the rotor and grid-side converters to improve the transient operation of the DFIG wave energy system under a fault condition as compared with the conventional methods to design PID controllers.

Harmonic Compensation With Zero-Sequence Load Voltage Control in a Speed-Sensorless DFIG-Based Stand-Alone VSCF Generating System

Doubly fed induction generator (DFIG)-based stand-alone variable-speed constant-frequency systems reported so far cannot supply nonlinear unbalanced load current containing cophasor components. This paper presents three topologies of a stand-alone DFIG which can handle this type of load. The harmonic/imbalance component of the load current in each case is supplied by the stator-side converter. In the first option, a Δ/ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Y</i> transformer with neutral connection supplies the load. However, the harmonic current circulating in the delta winding still distorts the load phase voltage beyond the acceptable limit. As a solution, the zero-sequence load voltage is actively controlled through the stator-side converter with the load neutral connected to the midpoint of the dc-link capacitor. However, this configuration also fails when the load current contains dc component as in half-wave rectifiers. Finally, a configuration with an additional inverter leg connected to the load neutral is proposed in conjunction with the zero-sequence stator voltage controller. Simulation and experimental results from this last configuration demonstrate its excellent load voltage regulation property while supplying various nonlinear and unbalanced loads. The control algorithm is implemented without any speed/position sensor.

Reactive power allocation for loss minimisation in a stand alone variable speed constant frequency double output induction generator

Vector control algorithms reported so far for stand alone operation of variable speed constant frequency double output induction generators propose to supply the full magnetisation current of the machine in addition to the torque producing component from the rotor side converter. As the rotor of a normal induction machine is not designed for this additional load this approach leads to underutilisation of the machine torque capacity. Moreover, supplying the entire magnetisation current from the rotor side is not optimal from the point of view of the machine losses. Since the load voltage is regulated by manipulating the magnetising current through a slow acting flux control loop undesirable fluctuations in the load voltage waveforms are observed during fast load transients. An improved stator flux oriented control strategy for this type of generators, proposed in this study, eliminates undesirable load voltage transients by directly regulating the stator flux through the stator side converter. The total reactive power demand of the system is dynamically distributed between the two converters according to an optimum allocation program so as to minimise the losses without disturbing the stator flux. The effectiveness of the proposed algorithms is verified experimentally on a laboratory prototype.

Unbalance and harmonic voltage compensation for a stand‐alone variable speed constant frequency double‐output induction generator supplying non‐linear and unbalanced loads

This study presents an improved control algorithm for a stand-alone double-output induction machine-based variable speed constant frequency generator. The algorithm ensures sinusoidal load voltage and machine stator current for all types of loads such as balanced, unbalanced, linear and non-linear in any arbitrary combination. This is achieved by incorporating harmonic and unbalanced load compensation function to the stator side converter. Unlike other control strategies reported in the literature for this purpose, the proposed algorithm does not require extraction or compensation of individual harmonic/unbalance components of the load voltage/stator current. Harmonic compensation performance of the algorithm is thoroughly analysed theoretically and design guidelines provided. The experimental results obtained from a laboratory prototype demonstrate excellent load voltage regulation property during severe transient operating conditions. Total harmonic distortion and unbalance of the load voltage and stator current even with large unbalanced non-linear loads are also found to be much smaller compared with similar systems reported earlier in the literature.

Indirect vector control of a squirrel cage induction generator wind turbine

The paper deals with a squirrel cage induction generator connected to the grid through a back-to-back converter driven by vector control. The stator-side converter controls the generator torque by means of an indirect vector control scheme. In order to reduce the system dependance from the mechanical system behavior, a torque loop is used in the current reference calculations. Moreover, a torque control is included in the stator-side control in order to overcome a voltage dip fault. The grid-side converter controls the DC bus voltage and the reactive power in order to accomplish the grid codes. The proposed control strategy is evaluated by the simulation of different scenarios such as variable wind speed and different levels of voltage sags by means of Matlab/Simulink®.

Active & Reactive Power Control Of A Doubly Fed Induction Generator Driven By A Wind Turbine

Wind Energy is gaining interest now-a – days as one of the most important renewable sources of energy due to its ecofriendly nature. But the major disadvantage lies in variable speed wind generation and this paper gives a study on control of Wind driven doubly fed Induction Generators. The speeds above and below Synchronous speeds are obtained using a bidirectional power flow converter. By using this reactive power is controlled and hence the overall Power factor of system can be kept at unity under varying load conditions. . This paper presents simulation results of a Grid-connected DFIG. A switch-by-switch representation of the PWM converters with a carrier-based Sinusoidal PWM modulation for both rotor- and stator-side converter has been proposed. Stator-Flux Oriented vector control approach is deployed for both stator- and rotor-side converters to provide independent control of active and reactive power and keep the DC-link voltage constant. A 7.5 KW generator is designed and its effectiveness in controlling is verified in different operating conditions i.e. above and below synchronous speeds.

Parallel Operation of DFIGs in Three-Phase Four-Wire Autonomous Wind Energy Conversion System

This paper deals with a control algorithm for two parallel-operated doubly fed induction generators (DFIGs) driven by wind turbines in a three-phase four-wire autonomous system feeding local loads. The proposed autonomous wind energy conversion system (AWECS) is using back-to-back-connected pulsewidth-modulated insulated-gate-bipolar-transistor-based voltage source converters with a battery energy storage system at their dc link. The system utilizes separate rotor-side converters for each DFIG for maximum power tracking (MPT) through its rotor speed control. However, a common dc bus and a battery bank and stator-side converter are used for voltage and frequency control at the stator terminals of the DFIGs. A delta–star transformer is connected between the stator-side converter and the stator terminals of DFIGs for optimizing the voltage of dc bus, and the load-side neutral is connected to the neutral of the star side of the transformer. The proposed electromechanical system is modeled and simulated in MATLAB using Simulink and Sim Power Systems set toolboxes. The performance of the proposed AWECS is presented to demonstrate its capability of MPT, stator voltage and frequency control, harmonic elimination, load balancing, and load leveling.

Modeling and control of isolated hybrid system using wind driven DFIG and hydro driven SCIG

This paper deals with design and control of a new isolated wind-hydro hybrid generation system employing a doubly fed induction generator (DFIG) driven by a variable speed wind turbine and a constant speed squirrel cage induction generator (SCIG) driven by a constant power hydro turbine feeding three-phase four-wire local loads. The proposed hybrid energy conversion system is using back to back connected pulse width modulated (PWM) insulated gate bipolar transistors (IGBTs) based voltage source converters (VSCs) with a battery energy storage system (BESS) at their dc link. The ac terminals of one VSC (rotor side converter) are connected to the rotor terminals of DFIG for the maximum power tracking (MPT) through its rotor speed control. The ac terminals of the other VSC (stator side converter) are connected to the stator terminals of DFIG and SCIG and load terminals through a delta-star transformer. The stator side converter is used for the voltage and frequency control (VFC) at the stator terminals of the DFIG and SCIG. The proposed electro-mechanical system is modeled and simulated in MATLAB using simulink and sim power systems (SPS) set toolboxes. The performance of the proposed wind-hydro hybrid system is presented to demonstrate its capability of MPT, stator voltage and frequency control, harmonic elimination, load balancing, and load leveling. Copyright © 2010 John Wiley & Sons, Ltd.

Negative sequence compensation within fundamental positive sequence reference frame for a stiff micro-grid generation in a wind power system using slip ring induction machine

An isolated wind power generation scheme using slip ring induction machine (SRIM) is proposed. The proposed scheme maintains constant load voltage and frequency irrespective of the wind speed or load variation. The power circuit consists of two back-to-back connected inverters with a common dc link, where one inverter is directly connected to the rotor side of SRIM and the other inverter is connected to the stator side of the SRIM through LC filter. Developing a negative sequence compensation method to ensure that, even under the presence of unbalanced load, the generator experiences almost balanced three-phase current and most of the unbalanced current is directed through the stator side converter is the focus here. The SRIM controller varies the speed of the generator with variation in the wind speed to extract maximum power. The difference of the generated power and the load power is either stored in or extracted from a battery bank, which is interfaced to the common dc link through a multiphase bidirectional fly-back dc–dc converter. The SRIM control scheme, maximum power point extraction algorithm and the fly-back converter topology are incorporated from available literature. The proposed scheme is both simulated and experimentally verified.

Control strategy for a Doubly-Fed Induction Generator feeding an unbalanced grid or stand-alone load

In this paper, the control systems for the operation of a Doubly-Fed Induction Generator (DFIG), feeding an unbalanced grid/stand-alone load, are presented. The scheme uses two back-to-back PWM inverters connected between the stator and the rotor, namely the rotor side and stator side converters respectively. The stator current and voltage unbalances are reduced or eliminated by injecting compensation currents into the grid/load using the stator side converter. The proposed control strategy is based on two revolving axes rotating synchronously at ± ω e. From these axes, the d– q components of the negative and positive-sequence currents, in the stator and grid/load, are obtained. The scheme compensates the negative-sequence currents in the grid/load by supplying negative-sequence currents via the stator side converter. Experimental results obtained from a 2-kW experimental prototype are presented and discussed in this work. The proposed control methodology is experimentally validated for stand-alone and weak grid-connected conditions and the results show the excellent performance of the strategy used.

Dynamic modeling of doubly fed induction generator wind turbines

It is now recognized that many large wind farms will employ doubly fed induction generator (DFIG) variable speed wind turbines. A number of such wind farms are already in operation and more are planned or under construction. With the rising penetration of wind power into electricity networks, increasingly comprehensive studies are required to identify the interaction between the wind farm(s) and the power system. These require accurate models of doubly fed induction generator wind turbines and their associated control and protection circuits. A dynamic model has been derived, which can be used to simulate the DFIG wind turbine using a single-cage and double-cage representation of the generator rotor, as well as a representation of its control and protection circuits. The model is suitable for use in transient stability programs that can be used to investigate large power systems. The behavior of a wind farm and the network under various system disturbances was studied using this dynamic model. The influence of the DFIG control on the stability of the wind farm was also investigated by considering different control gains and by applying network voltage control through both stator side and rotor side converters.

Vector Control of a Variable Speed Doubly-Fed Induction Machine for Wind Generation Systems

SummaryControl techniques suitable for a doubly fed induction generator applied to a wind generation system are presented. A Scherbius scheme using two back-to-back voltage-fed current-regulated inverters in the rotor circuit is employed allowing subsynchronous and supersynchronous operation with low distortion currents. A vector control scheme using a reference frame oriented along the stator flux vector is used for the machine. This leads to independent control of the electrical torque and the rotor excitation current. A vector scheme aligned with the stator voltage vector is used to control the stator-side converter; this gives independent control over the active and reactive power flow between the supply and the stator-side converter. The control strategies are tested with the machine working as a variable speed constant frequency generator driven by a wind turbine emulator. Two approaches for optimum speed tracking of the wind turbine are implemented and experimental results for a prototype system ar...