Rotor-side Inverter Research Articles

Automated Identification of Failures in Doubly-Fed Induction Generators for Wind Turbine Applications

Remote, automated monitoring of wind generators is crucial considering that wind turbines are prone to failure due to the harsh operating environment, and difficult to access or test due to the remote location of wind turbines. The demand for condition monitoring of wind generators is increasing with the rapid growth in wind power generation. In this work, a new inverter-embedded off-line test concept for automated testing of doubly-fed induction generators (DFIG) is proposed for detection and classification of defects in the 1) slip ring-brush contact, 2) rotor winding turn insulation, and 3) stator core inter-laminar insulation. The main idea is to use the rotor side inverter for injecting test signals into the rotor winding for detecting asymmetry in the machine, whenever the DFIG is at standstill. The pattern of asymmetry in the equivalent impedance as a function of angle is analyzed for identifying the presence and type of DFIG fault at an early stage for efficient scheduling of maintenance. Experimental verification is given to show that the proposed test provides a simple means of identifying DFIG faults with high sensitivity and reliability without additional hardware.

Implementation and analysis of mono inverter-battery configuration for wind driven doubly fed induction generator

ABSTRACT This paper investigates the controller implementation and analysis of Doubly Fed Induction Generator (DFIG) for powering isolated loads in rural electrification. A simple speed sensorless algorithm-assisted controller for isolated power supply of DFIG has been proposed using a digital controller and voltage source converter at the rotor terminals of the generator. The controller can automatically correct the voltage and frequency of the rotor side inverter pulse to obtain a desired voltage and frequency at stator side despite the variations in the wind speed and load. A hardware prototype of the system has been built and tested on a prototype machine, which is driven by an emulated wind turbine under sub-synchronous and super-synchronous speed operations. The detailed results of both simulated and hardware waveforms are provided in the paper to demonstrate the working of the proposed topology and embedded control scheme. Further, the performance characteristics of DFIG are modeled and verified experimentally. A good agreement exists between theoretical and experimental results.

Inverter-Embedded Detection of Rotor Winding Faults in Doubly Fed Induction Generators for Wind Energy Applications

Automated and remote testing of wind turbines is highly desirable since they are prone to failure with the harsh operating environment, and are difficult to test due to the remote location and limited accessibility. The turn insulation of the rotor winding is susceptible to failure due to operating stresses. Unlike turn faults in the stator, rotor turn faults do not cause immediate failure due to the low induced voltage in the shorted loop at low slip frequency, and therefore, are difficult to detect during operation. Although this fault progresses slowly, it must be detected to prevent performance degradation due to rotor asymmetry. In this article, a new concept for inverter-embedded testing of doubly fed induction generators (DFIG) used for wind energy applications is presented. The main idea is to inject test voltages into the DFIG rotor winding from the rotor side inverter whenever the wind turbine is stopped, to extract information on the rotor asymmetry. The proposed test concept is a simple off-line test that can be implemented in DFIGs without additional hardware to perform automated and sensitive detection of rotor faults. Experimental results show that faults in the rotor winding can be detected with higher sensitivity compared to on-line methods for improving the reliability of wind turbines.

Comparative Study of Synergetic Controller with Super Twisting Algorithm for Rotor Side Inverter of DFIG

Comparative Study of Synergetic Controller with Super Twisting Algorithm for Rotor Side Inverter of DFIG

Model predictive direct power control of three-level T-type inverter-fed doubly-fed induction generator for wind energy system

The paper proposes a simplified direct power control strategy of a doubly-fed induction generator fed by a three-level T-type inverter based on finite control set model predictive control. A mathematical model based on grid voltage orientation was employed to determine the predictive values of the stator flux, rotor current, and capacitor voltages for all feasible rotor-side inverter output voltages. The active and reactive powers were calculated by using the grid voltage and the rotor current. A cost function was applied to track the active and reactive powers, maintain the balance of capacitor voltages, and reduce the common-mode voltage. The best switching control input was chosen by minimizing the cost function and implemented to the inverter. Different operating conditions of wind turbine systems were studied with Matlab/Simulink environment. The simulation results validate the improved performance of the proposed method compared with the classical control in terms of transient response and steady-state conditions.

Fault-Tolerant Performance of the Novel Five-Phase Doubly-Fed Induction Generator

The article presents the concept of a new design of a multiphase doubly-fed induction generator (DFIG). The innovative design approach is based on the use of a five-phase power supply from the rotor side of generator with a three-phase classic stator power supply. Modern three-phase doubly-fed induction generators are the dominant choice for Wind Energy Conversion Systems (WECS). Solutions of this type are sensitive to the loss of at least one phase of the power supply from the rotor side due to the failure of the rotor side inverter. The proposed design solution in the form of a multi-phase power supply in the rotor circuit is aimed at extending the range of possible failure-free operation of the generator system, and thus reducing system downtime as a result of failure of power electronic systems. The correctness of the adopted conceptual assumptions was confirmed by the results of laboratory tests. The main contribution is to prove that the use of five phase rotor winding significantly improves the overall reliability of the proposed electrical energy generation system.

Analysis of Electromagnetic Transient Characteristics of Doubly-Fed Induction Generator Under Grid Voltage Swell

The electromagnetic transient characteristics (ETC) of doubly-fed induction generator (DFIG) under grid voltage dip have been extensively studied theoretically, and various control strategies have been proposed. However, the ETC of DFIG under grid voltage swell are less studied. In order to study the influence on the ETC of DFIG under grid voltage swell and the corresponding control strategy, this paper compares and analyzes the steadystate and transient components of the stator flux linkage, rotor induced voltage and rotor current of DFIG under grid voltage swell and dip. It is deduced that DFIG is less prone to rotor overcurrent under grid voltage swell so the crowbar protection circuit is not needed, and the grid voltage amplitude changes little, the rotor side inverter is less prone to overmodulation. An experimental setup and simulation model is implemented with high-voltage ride-through of 1.5 MW doubly-fed wind Turbine. Simulation in addition to experimental results verifies the correctness and effectiveness of the theoretical analysis.

A Novel Control Strategy of DFIG Based on the Optimization of Transfer Trajectory at Operation Points in the Islanded Power System

A novel control strategy based on the optimization of transfer trajectory at operation points for DFIG is proposed. Aim of this control strategy is to reduce the mechanical fatigue of DFIG caused by the frequent adjustment of rotating speed and pitch angle when operating in the islanded power system. Firstly, the stability of DFIG at different operation points is analyzed. Then an optimization model of transfer trajectory at operation points is established, with the minimum synthetic adjustment amount of rotating speed and pitch angle as the objective function and with the balance of active power and the stability of operation points as the constraint conditions. Secondly, the wind speed estimator is designed, and the control strategy of pitch system is improved to cooperate with the indirect stator flux orientation control technology for rotor-side inverter control. Then by the coordination control of its rotating speed and pitch angle, an operation trajectory controller is established to ensure the islanded operation of DFIG along the optimal transfer trajectory. Finally, the simulation results show that the proposed control strategy is technical feasibility with good performance.

Integrated DFIG‐based wind turbine model with direct power control

This paper is focused on the model development of doubly fed induction generator (DFIG)‐based wind turbines. The model integrates mechanical and electrical systems. In the mechanical system, the aerodynamic properties are calculated in every section and every angle of attack by using a finite element analysis software, and the dynamic structure analysis is performed by using the Fatigue, Aerodynamics, Structures, and Turbulence (FAST) code. On the other hand, the electrical system includes DFIG modeling, the grid side converter (GSC) with DC bus voltage control, and the rotor side inverter (RSI) with direct power control (DPC). By using the direct power control strategy, the active power, reactive power, and power factor from the DFIG can be controlled to meet the requirements of the grid code. Moreover, the actual measured wind speed data are used in this work to simulate the outputs from the DFIG in order to confirm the feasibility of the wind turbine modeling. According to the simulation results, the proposed model is useful and can provide a reference for further studies about wind turbine technology. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Air-Gap Power-Based Sensorless Control in a DFIG Connected to a DC Link

This paper deals with the sensorless vector control of a doubly fed induction generator (DFIG) whose stator is connected by a rectifier to a dc link, which also feeds the rotor-side inverter. A noticeable reduction of the costs can be achieved by this topology, because only a single inverter and a diode rectifier are needed instead of the traditional back-to-back converter. A sensorless stator-flux-oriented control is implemented by using an observer, which directly computes the necessary slip angle based on of the air-gap power and without the need of the stator flux estimation. The direct-axis component of the rotor current is adjusted to regulate the stator frequency near the rated value. In order to track the actual stator frequency, the phase-locked-loop approach is used. The control method is validated by simulations and experimental results obtained using a small-rating laboratory setup.

A Scheme for the Power Control in a DFIG Connected to a DC Bus via a Diode Rectifier

This paper deals with a new conversion topology for doubly-fed induction generators (DFIGs) suitable for wind energy conversion systems integrated in microgrids. It consists of a DFIG which is fed by a pulse width modulation converter on the rotor and with the stator connected to a dc grid through a diode rectifier. In this configuration, the stator diode rectifier and the rotor-side inverter share the same dc bus, so that the conventional grid-side inverter is avoided. Since only a diode rectifier designed for the full power and a reduced power inverter are required, this layout allows a cheap and effective integration of the DFIG with other generating and storage systems connected to the same dc bus. A simple control technique suitable to regulate the power delivered to the dc grid is proposed. The scheme is based on the regulation of the amplitude of a suited fraction of the rotor flux linkage: the optimal value of this fraction is theoretically deduced in order to minimize the DFIG derating due to the current harmonics. The effectiveness of the proposed control is proven by simulations and experimental tests.

Detection of rotor electrical asymmetry in wind turbine doubly‐fed induction generators

This study presents a new technique for detecting rotor electrical faults in wind turbine doubly-fed induction generators (DFIGs), controlled by a stator field-oriented vector control scheme. This is a novel method aimed at detecting and identifying rotor electrical asymmetry faults from within the rotor-side inverter control loop, using the error signal, to provide a future method of generator condition monitoring with enhanced detection sensitivity. Simulation and experimental measurements of the proposed signals were carried out under steady-state operation for both healthy and faulty generator conditions. Stator current and power were also investigated for rotor electrical asymmetry detection and comparison made with rotor-side inverter control signals. An investigation was then performed to define the sensitivity of the proposed monitoring signals to fault severity changes and a comparison made with previous current, power and vibration signal methods. The results confirm that a simple spectrum analysis of the proposed control loop signals gives effective and sensitive DFIG rotor electrical asymmetry detection.

Detecting Faults in Doubly Fed Induction Generator by Rotor Side Transient Current Measurement

The doubly fed induction generator (DFIG) is one of the main technologies at variable speed power generation systems. Reliability and efficiency are key factors to realize the maximum energy output of the renewable resources. Detecting generator faults enables the reduction of risk for unexpected outages and thus high economic losses. Stator winding insulation faults count to one of the most frequent failures in electric machines. Common fault detection methods are based on several additional sensors and hardware what makes the system complex, expensive and also fault-prone. In this paper, a method is proposed and investigated to detect stator winding faults based only on measured signals available from inverter build-in sensors. By rotor-side inverter switching the generator is excited by transient voltage pulses and the current response provides the possibility to extract a fault indicator through a specific signal processing. Measurements on a DFIG test stand prove the methods applicability and accuracy.

Operation and design issues of a doubly fed induction generator stator connected to a dc net by a diode rectifier

The operation of a doubly fed induction generator (DFIG) connected to a dc net is considered in this study. The dc net is connected to the stator of the DFIG by a diode rectifier and simultaneously feeds the rotor-side inverter, which is used to control the machine. When the dc net operates at constant voltage, this solution preserves the possibility to work with a low flux at low speed, and, at the same time, reduces the cost of the power electronics. In fact, the only needed electronic converter is the rotor inverter designed for a fraction of the machine power. After introducing a simple control technique, the study proposes an analytic model for the steady-state operation and discusses the design specifications of the DFIG for given values of the dc voltage and of the power of the wind turbine. The most appropriate stator and rotor rated powers, rated stator voltage and turns ratio of the DFIG are deduced. Simulations and experimental results are presented in order to support the theory.

Sensorless Control of Synchronous Machine With an Inverter Integrated Rotor

This paper proposes a new sensorless method for a synchronous machine with an inverter integrated rotor (SMIIR). The SMIIR is a newly developed machine based on the wound rotor synchronous machine (WRSM) but has no brushes and slip-rings. The conventional high-frequency signal injection sensorless methods are based on a physical magnetic saliency. Therefore, the conventional methods cannot be applied to the SMIIR since it has no adequate physical magnetic saliency. The proposed sensorless method suggests a virtual resistance saliency created by the rotor-side inverter, of which the saliency ratio and axes can be arbitrarily set. Based on this virtual saliency, the rotor position information can be included in the relationship between the high-frequency stator voltage and stator current independent of the machine parameters or operating conditions. Because the SMIIR inherently injects a high-frequency voltage for the power transfer, there is no need to inject additional voltage for the proposed high-frequency sensorless control. The initial d-axis detection algorithm using the rotor-side inverter without magnetic saturation is also proposed. The feasibility of the proposed sensorless method was verified by the experimental results of the prototype SMIIR.

Integrated Mechanical and Electrical DFIG Wind Turbine Model Development

This paper focuses mainly on the development of a completed typical doubly fed induction generator (DFIG) wind turbine model. It includes the interaction between mechanical and electrical parts. In the mechanical part, the aerodynamic of the wind turbine blade was analyzed by the XFOIL. With the properties of the blade, the model was established by the fatigue, aerodynamics, structures, and turbulence (FAST) in the MATLAB/Simulink. The DFIG electrical model was constructed by using mathematical equations in MATLAB/Simulink that integrates with the FAST. In the DFIG, the space vector is applied in the ac machine to represent the flux, voltage, and current magnitudes. The grid-side converter stabilized the voltage at the dc bus, and the rotor-side inverter directly controls the electric torque in the generator. Currently, most research articles model the mechanical and the electrical parts of the wind turbine separately. In this paper, both systems were discussed in detail and integrated into a complete model for the DFIG-based wind turbine system.

A Doubly Fed Induction Generator Controlled in Single-Sided Grid Connection for Wind Turbines

This paper proposes a configuration of wind turbine using a doubly fed induction generator (DFIG) in single-sided grid connection. The stator of the machine is connected with a direct power grid, and the rotor is controlled by an inverter without any external power source. This is called a single external feeding of DFIG (SEF-DFIG). The conventional DFIG uses an additional grid power converter to regulate the rotor power; but in SEF-DFIG, the external source of a grid power is only connected to the stator windings. Due to this feature, the rotor-side inverter can be integrated on the rotor without any slip ring. In this paper, for applying the proposed scheme to the wind turbine, the performances of the generating operation are analyzed. The experimental results of a 2.4-kW DFIM system are presented to prove the feasibility of the proposed system and method.

The Research of Verifying Low Voltage Ride through of Double-Fed Wind Turbine Generator System and Voltage Dropping Simulation

In order to maintain the stable operation of grid system, the grid company requires wind turbine generator system with low-voltage ride through (LVRT) capability. With this issue, the article builds a double-fed wind turbine generator system model, Crowbar circuit strategy model to protect rotor-side inverter and fault pitch control strategy model to inhibition rotor speed during grid fault. Then all the models and strategies are verified by the simulation of grid low voltage. The simulation results show that these models can complete LVRT.