Stability Of Doubly Fed Induction Generator Research Articles

Impact of Short-Circuit Ratio on Control Parameter Settings of DFIG Wind Turbines

This work deals with doubly fed induction generator (DFIG) modeling and stability when connected to weak AC grids. A detailed state-space model that includes the phase-locked loop (PLL) is developed. This work aims to determine the influence of the network’s strength on DFIG stability through the short-circuit ratio (SCR). The critical values of the proportional control parameters of the grid-side and rotor-side converters (RSC and GSC), as well as PLL, which make the system unstable, are calculated for different SCR values. Finally, PSCAD/EMTDC dynamic simulations are used to validate the critical control parameters obtained by studying the eigenvalues of the DFIG state-space model regarding system stability.

Magnitude-phase equivalent circuit and steady-state power-angle characteristic analysis of DFIG

Circuit model representing generator characteristics is essential in stability analysis and control system design of doubly-fed induction generator (DFIG). This paper develops the relationship between generated voltage and magnetic field in DFIG, and establishes an equivalent circuit model considering rotor motion. The novel magnitude-phase equivalent circuit (MPEC) model is established based on vector magnitude and angle. Furthermore, the paper analyzes phasor diagrams, calculates the power expression under the MPEC, and obtains steady-state power-angle characteristic of the DFIG. In the proposed power-angle characteristic, the DFIG's output incorporates variations in slip ratio and power-angle, which combines the characteristics of asynchronous and synchronous machines. Based on the proposed MPEC and power-angle characteristic, the steady-state maximum power of the DFIG is determined at the critical stable power-angle of π/2. Finally, simulation and experimental results are given to validate the effectiveness of the proposed equivalent circuit model and power-angle characteristic of DFIG. The results show that the MPEC model is capable of analyzing the DFIG stability from the power-angle motion perspective, and the system exhibits a potential for instability when the power-angle exceeds 1.5.

An improved control strategy using TCSC with grey wolf-optimized DSTATCOM for efficient LVRT of DFIG-based WECS

Doubly fed induction generator (DFIG)-based wind energy conversion systems (WECSs) are widely used for its advantage over other schemes, as it enables the connection to the grid even during faults and provides reactive power compensation. However, transient stability of DFIG and low voltage ride through (LVRT) remained an area of improvement. In this paper, a hybrid approach is proposed to improve the efficiency of the DFIG WECS using grey wolf-optimized distribution static compensator (DSTATCOM) with thyristor-controlled series capacitor (TCSC). Extensive simulation analysis was performed considering design parameters as an objective function. The optimal values of proportional–integral (PI) coefficients were found to minimize the objective function. Using MATLAB, time-domain simulations were carried to confirm the efficiency of the optimized control system. Simulation results further confirm that the combination of TCSC and DSTATCOM may be considered as a promising scheme to enhance fault ride through (FRT) capability of WECS during the normal and unstable conditions.

Power Optimization Control Scheme for Doubly Fed Induction Generator Used in Wind Turbine Generators

Scientists and researchers are exploring different methods of generating and delivering electrical energy in an economical and reliable way, enabling them to generate electricity focusing on renewable energy resources. All of these possess the natural property of self-changing behavior, so the connection of these separate independent controllable units to the grid leads to uncertainties. This creates an imbalance in active power and reactive power. In order to control the active and reactive power in wind turbine generators with adjustable speed, various control strategies are used to allay voltage and current variations. This research work is focused on the design and implementation of effective control strategies for doubly fed induction generator (DFIG) to control its active and reactive power. A DFIG system with its control strategies is simulated on MATLAB software. To augment the transient stability of DFIG, the simulation results for the active and reactive power of conventional controllers are compared with three types of feed forward neural network controllers, i.e., probabilistic feedforward neural network (PFFNN), multi-layer perceptron feedforward neural network (MLPFFN) and radial basic function feedforward neural network (RBFFN) for optimum performance. Conclusive outcomes clearly manifest the superior robustness of the RBFNN controller over other controllers in terms of rise time, settling time and overshoot value.

An improved virtual inductance control method considering PLL dynamic based on impedance modeling of DFIG under weak grid

Impedance-based method is an effective method to analyze the system stability. Based on the impedance stability theory, an improved virtual inductance control method considering PLL dynamic is presented for the stability of doubly-fed induction generator (DFIG) under weak grid condition. Firstly, equivalent input admittance model of the DFIG system including all its controller is developed in the synchronous reference (d-q) frame. Secondly, the generalized Nyquist criterion (GNC) proves that the stability of the DFIG system can be improved by increasing the rotor inductance. Then, a virtual inductance term is introduced into the RSC current controller to emulate additional inductance in the rotor. However, the virtual inductance method may be ineffectual due to the influence of the PLL dynamic under weak grid condition. Therefore, an improved virtual inductance control method considering the PLL dynamic is implemented to improve the stability of the DFIG system under weak grid condition, and the simulation is used to verify the effectiveness of the improved virtual inductance control strategy.

Impact of df/dt Inertia Control on Doubly Fed Induction Generator Type Wind Turbine

the supplemental df/dt control is the common method for wind power generation to support system inertial response. This paper investigates the impact of the df/dt inertia controller on doubly fed induction generator (DFIG) type wind turbine. The controller is usually a high pass filter. Two parameters which are the gain and the time constant are considered. By constructing the linearized model of DFIG, the impacts of the two parameters of the controller are discussed. It shows that with different phase locked loop (PLL) parameters and current control parameters, different time constants and gains are required to ensure the DFIG stability. The time domain simulation validates the result.

Improvement of transient stability of DFIG based wind generator by using of resistive solid state fault current limiter

Currently, the application of variable wind speed machines, especially doubly-fed induction generator (DFIG) becomes very effective in the fast growing wind generator (WG) market due to its advantageous property of partial converter ratings and ability to adapt to variable wind speed. The foremost concern for the DFIG based WG system is to maintain the transient stability during fault, as the stator of the DFIG is directly connected to the grid. In this paper, a simple controller based resistive solid-state fault current limiter (R-type SSFCL) is proposed to augment the transient stability of the DFIG based WG system. In simulations, temporary balanced and unbalanced faults were applied in the test system model to investigate the R-type SSFCL transient stability performance. A series dynamic braking-resistor (SDBR) and an LR-type solid-state fault current limiter (SSFCL-LR) are also considered to compare their performance with the proposed R-type SSFCL. These simulations were performed with Matlab/Simulink software. Simulation results clearly indicate that the R-type SSFCL enhances the transient stability of the DFIG system and it works better than the SDBR and SSFCL-LR in every aspect.

Performance analysis of diode-bridge-type non-superconducting fault current limiter in improving transient stability of DFIG based variable speed wind generator

The application of doubly-fed induction generator (DFIG) is very effective in the fast-growing wind generator (WG) market. The foremost concern for the DFIG based WG system is to maintain the transient stability during fault, as the stator of the DFIG is directly connected to the grid. However, according to the grid code, DFIG needs to remain connected to the network even during the fault. Therefore, transient stability enhancement of the DFIG is very important. In this paper, to achieve this goal a diode-bridge-type nonsuperconductor fault current limiter (NSFCL) is proposed to augment the transient stability of the DFIG based WG system. A three-line-to-ground (3LG), double-line-to-ground (2LG) and single-line-to-ground (1LG) faults were applied to one of the double-circuit transmission lines of the test system to investigate the proposed NSFCL transient stability performance. Besides, a DC resistive superconducting fault current limiter (SFCL), bridge-type fault current limiter (BFCL) and series dynamic braking resistor (SDBR) are also considered to compare their performance with the proposed NSFCL. Simulations were carried out in Matlab/Simulink software. Simulation results show that the NSFCL enhances the transient stability of the DFIG system. Moreover, the NSFCL works better than the DC resistive SFCL, BFCL and SDBR in every aspect.

Large Disturbance Stability Analysis of Wind Turbine Implemented with DFIG

As one of the most promising Distributed Generation (DG) sources, wind power technology has been widely developed in recent years. Doubly fed induction generator (DFIG) is currently employed as one of the most common topologies for wind turbine generators (WTGs). This generator operates as a synchronous/asynchronous hybrid generators. Therefore, it is necessary to power engineers find understand the characteristics of the DFIG better. This paper presents an analytical method for analysis of large-disturbance stability of DFIG. The proposed method is based on the concepts of stable and unstable electrical-mechanical equilibrium points, the electrical-mechanical torque versus rotor speed and Critical fault clearance time of DFIG. The results of analysis specify the effective parameters on the large-disturbance stability of DFIG too. By using this method ones can reduce the simulation efforts necessary to assess the large-disturbance stability of DFIG.

Transient stability improvement of doubly fed induction generator based variable speed wind generator using DC resistive fault current limiter

Currently, the doubly fed induction generator (DFIG) is getting wider popularity due to its ability to adapt with variable wind speed and to capture more wind energy. According to the grid code, transient stability enhancement of the DFIG system is very important. Though DFIG has a salient feature of the fault ride through capability, this is not sufficient to preserve the necessity of the grid code when the DFIG system is connected with the grid. To accomplish this goal, a DC resistive superconducting fault current limiter (SFCL) is proposed, as it reduces the system power losses during stable operation of the network with improved system efficiency, in comparison with the conventional SFCL. To verify the performance of the transient stability of the DFIG based wind power system with the DC resistive SFCL, both the symmetrical and asymmetrical faults are considered. The performance of the DC resistive SFCL is compared with that of the series dynamic braking resistor (SDBR) and the crowbar system. Simulations are carried out by using the Matlab/Simulink software. Simulation results clearly indicate that the proposed DC resistive SFCL improves the transient stability of DFIG based variable speed wind generator well. Moreover, the overall performance of the proposed method is better than that of the SDBR, as well as the crowbar system.

Small Signal Stability of DFIG Connected Power System and Identification of Oscillatory Modes Using Prony Analysis

This paper investigates the impact of wind power on small signal stability of power system using parameters like; wind power penetration level and Doubly Fed Induction Generator (DFIG) voltage controller gain. The impact of wind penetration has been analysed by replacing conventional synchronous generator with DFIG based wind turbines. For this purpose, standard IEEE 14 Bus system has been selected. Time tested modal analysis has been used to compute the oscillatory modes that occur due to small disturbances. Further, the identification of low frequency oscillations has been done using prony analysis. Prony analysis has been performed in time as well as in frequency domain, and used for extracting hidden frequency information from the waveform of the signal.

A Systematic Approach and Genetic Algorithm Method to Power System Stabilizer Design for Wind Turbine Equipped with DFIG

This paper develops a power system stabilizer (PSS) design for a wind turbine equipped with doubly fed induction generator (DFIG) which is based on vector control to improve the performance and dynamic stability of DFIG under fault conditions. The proposed PSS design is combined with genetic algorithm to obtain the higher-fitness answer as a strong optimization technique to the design of PSS parameters. A study network containing a wind farm equipped with DFIG was employed and all simulations will be carried out using MATLAB. It is shown that the employment of a proposed PSS can substantially enhance the contribution of a DFIG-based wind farm to network damping and dynamic stability.

Improvement of the Low-Voltage Ride-through Capability of Doubly Fed Induction Generator Wind Turbines

So far, active crowbars are a preferred technique for doubly fed induction generator (DFIG) wind turbines, which is used to protect the power converter against over-current and undesirably high dc link voltage during voltage dip. However, its main drawbacks are that (1) the DFIG absorbs reactive power from the grid during grid voltage dips, (2) the crowbar activation increases the acceleration of the rotor and so, deteriorates the dynamic stability of DFIG, and (3) the control is not flexible for long-time voltage sags. In the paper, three different initiating logic control methods of crowbar protection are compared, and how low-voltage ride-through (LVRT) characteristics of DFIG wind turbines with active crowbar are affected by different switching logic control modes of the crowbar are investigated. According to the comparison results, an improved crowbar switching control strategy is proposed to reduce its operation time and improve the LVRT capability of DFIG wind turbines. In addition, an emergency pitch blade angle control scheme to reduce the acceleration of the rotor and prevent the over-speeding of rotor is presented in detail, and as a result, the LVRT capability of DFIG wind turbines is enhanced even during long-time voltage sags. Finally, the presented control strategies are validated in simulation tool Matalab/Simulink for a 1.5MW generator.

Enhanced Fault Ride-through Scheme for Mitigating Rotor Speed Instability of Doubly Fed Induction Generator Based Wind Farms

This paper analyzes and proposes a novel scheme with suitable control strategy to increase the probability of successful fault ride-through (FRT) as well as to enhance the rotor speed stability of doubly fed induction generator (DFIG) based wind farms. The proposed FRT scheme consists of an uncontrolled rectifier, two sets of IGBT switches, a diode and an inductor connected between the rotor circuit and dc link capacitor, in parallel with the rotor side converter of DFIG. In the proposed scheme, during a fault event, the input mechanical energy of the wind turbine is stored as electromagnetic energy in the inductor. Consequently, torque balance between the electrical and mechanical quantities is maintained and the rotor speed deviation is thereby reduced. This results in reduced reactive power requirement and rapid reestablishment of terminal voltage on fault clearance. Moreover, the stored electromagnetic energy in the inductor is transferred into the dc link capacitor on fault clearance; thereby relieving the grid side converter from charging the dc link capacitor. The proposed FRT scheme is validated through an extensive time domain simulation study employing PSCAD/EMTDC software. Simulation results demonstrate the effectiveness of the proposed FRT scheme in achieving successful fault ride-through and rotor speed stability of DFIG based wind farms.

Stability of doubly-fed induction generator under stator voltage orientated vector control

The stability of a doubly-fed induction generator (DFIG) under vector control in stator voltage orientation (SVO) is investigated. Prior art has tended to assume that the inner current loop dynamics can be neglected when an SVO is employed. As a result, the poorly damped poles of the DFIG system were considered unaffected by the inner current loop tuning. The state-space model of the machine including the inner current closed loop dynamics is developed for schemes where different feed-forward compensation terms are used. The interaction between inner current loop dynamics and damping of the critical poles of the system is illustrated through analysis and simulation. The main outcome of the analysis is that the stability of the machine system in an SVO depends solely on the parameters of the proportional-integral controllers. Erroneous tuning can lead to instability, irrespective of the particular feed-forward compensation scheme, which could cause the disconnection of the machine as a result of rotor current oscillations of unacceptable magnitude in an actual case. The main contribution is to provide the necessary methodology in order to ensure the stable operation of a DFIG under SVO vector control.