Speed Doubly-fed Induction Generator Research Articles

Recovery time and strategy of DFIG speed based on a high-proportional hydropower grid

Large-scale wind power access reduces the equivalent inertia of a power system. The wind turbine has a considerable reserve of rotational inertia, which is important for reducing the risk of low-inertia operation in a grid and for improving frequency reliability. In this work, a doubly fed induction generator (DFIG) inertia frequency regulation strategy is studied. The secondary dip in frequency caused by speed recovery after the wind turbine participates in frequency regulation is an important factor that limits regulation capability on the DFIG. To address such problem, the power dip and recovery generated by the DFIG after active speed recovery is analyzed and modeled. Then, a linearized frequency response model of a hydropower single machine is established to investigate the secondary disturbance effect of wind turbine active speed recovery moment on the frequency of the high-ratio hydropower system.A conclusion is drawn that the moment that corresponds to the first recovery of system frequency to the quasi-steady-state frequency is the optimal recovery moment of the turbine in the linear SFR model. Combined with the frequency control characteristics of the frequency limit control(FLC)’s “large dead zone, ” an active speed recovery strategy of the turbine based on frequency deviation judgment is designed.

Transient stability analysis of DFIG-based wind farm-integrated power system considering gearbox ratio and reactive power control

Nowadays, integration of large-scale wind farms (WFs) into power systems is experiencing rapid growth. As this rapid integration can affect transient stability significantly, employing doubly fed induction generator (DFIG)-based wind turbines, which have shown better behavior regarding system stability, has attracted much attention. This research contributes to the literature by investigating the transient stability of the power system with increasing penetration of DFIG-based WFs. In the proposed framework, the current-balance form is utilized for the network equations, and in this way, transient stability is performed using time-domain simulation. According to the simulation results, when the rate of wind power generation exceeds 0.7 per-unit, the increasing trend of the critical clearing time (CCT) is reversed and the CCT decreases greatly with the increased wind power penetration. In addition, the reactive power compensation by DFIG, the gearbox ratio, the power system strength, and DFIG parameters are comprehensively investigated as effective parameters on transient stability. Since the rated rotor speed of DFIG significantly impacts the electrical torque and machine currents, the reduction of the rated rotor speed due to the change of the gearbox ratio has been investigated as one of the effective factors to improve the transient stability. The simulation results demonstrate the effectiveness of the proposed approach in improving power system transient stability.

Adaptive reinforcement learning interval type II fuzzy fractional nonlinear observer and controller for a fuzzy model of a wind turbine

This article presents an innovative control technique for doubly fed induction generator (DFIG) based wind turbine. A DFIG equation is initially presented using fractional order Takagi and Sugeno interval type II fuzzy modeling. Moreover, fault and disturbance are considered in the DFIG model. Then, to design an observer that estimates both fault and disturbance, the fuzzy model of DFIG is decoupled into three subsystems using a different linear coordinate transformation: the state subsystem without disturbance and fault, the subsystem of fault without disturbance, and the subsystem of disturbance without fault. In the following, using the state subsystem, an adaptive reinforcement learning TS interval type II fuzzy fractional sliding mode observer is introduced. The observer gains are adjusted by applying proximal policy optimization, which is a new application of reinforcement learning. Robustness against uncertainties, fast convergence speed of estimation, high degree of freedom and precise estimation are among features of the suggested observer. In addition, only by utilizing the estimated state subsystem and obtained mathematical relationships, the actuator faults and disturbance are estimated. A fractional order sliding mode controller is proposed for the DFIG speed controller. Low chattering characteristics and accurate response are achieved owing to the fractional speed controller. Also, to compensate the influence of disturbances and faults, an adaptive interval type II fuzzy fractional sliding mode controller is used for rotor current regulations, which employs the estimated fault and disturbance. High robustness, fast response time, low tracking error and chattering are some of the advantages of the proposed fault tolerant controller (FTC). The closed-loop stability of controllers is proved by using the Lyapunov theorem. Simulations is accomplished to evaluate the effectiveness of the proposed method. The proposed controller is compared with other controllers. Robustness against parameter uncertainty is also investigated.

Speed control of a DFIG-based wind turbine using a new generation of ADRC

ABSTRACT This paper represents a new generation of adaptive disturbance rejection control (ADRC) which is more robust against disturbances than conventional ADRC. The non-derivability of the fal function employed in the traditional ADRC has negative impacts on its operation, so alternate functions will be used which are derivable at all segments. In this regard, odd hyperbolic and trigonometric functions were employed. The performance of the proposed structure was investigated to control the doubly fed induction generator (DFIG) speed. To enhance the efficiency of the suggested ADRC, fractional-order calculations and fuzzy logic were utilized simultaneously. The results in MATLAB platform indicate that: 1) With constant wind speed and changing reference speed, using new fal functions in fuzzy fractional-order ADRC (FFOADRC) has improved the performance compared to the PI regulator and FFOADRC with default fal function. 2) During wind speed variations and using new fal functions, the DFIG speed reaches the final steady amount as over damping, while this condition is critical damping when using FFOADRC with default fal function. 3) During network voltage reduction, using new FFOADRCs leads to fewer oscillations in the stator flux, DC-bus voltage, and DFIG speed, which shows their better performance than PI regulator and traditional ADRC.

Integral Backstepping Control Based on High Gain Observer for DFIG-Based Wind Energy Conversion System

In this paper, a nonlinear output-feedback controller is designed for the complete wind energy conversion system (WECS), formed by a wind turbine (WT) interfaced to a utility grid (UG) by using a doubly fed induction generator (DFIG) and an AC/DC/AC converter connected with an RL filter. The main objective is to ensure that the output-feedback controller keeps the DFIG speed at the optimum value provided by the MPPT strategy while ensuring control of the stator reactive power and the DC link voltage. To achieve this objective, the integral backstepping approach is used. The DFIG speed and mechanical torque are observed by using a high gain observer. The proposed controller capabilities are verified by numerical simulations using a 2-MW DFIG WT under a variable wind speed profile using MATLAB/SIMULINK/SimPowerSystems environment.

An Invasive Weed Optimization for Sensor Less Control of Grid Integrated Wind Driven Doubly Fed Induction Generator

The article presents the design and development of invasive weed optimization (IWO) for the sensor less speed control of doubly fed induction generator (DFIG) under balanced and unbalanced conditions. A healthy condition represents the balanced condition while unbalancing condition is characterized by unhealthy conditions like three-phase fault, single line to ground fault, the line-to-line fault, and double line to ground fault. The DFIG is driven by wind and integrated with the grid. The advantages associated with IWO technique are a simple mathematical approach and less data computation. Normally, DFIG consists of two back-to-back converters namely grid side converter (GSC) and rotor side converter (RSC). The GSC is an uncontrolled converter while the RSC is a controlled converter. The existing methods have poor performance parameters like settling time, peak overshoot for balanced conditions, and poor power quality parameters like total harmonic distortion (THD) for unbalanced conditions. An IWO technique has been applied to overcome such limitations. The effectiveness of the sensor less speed control is also analyzed with other techniques like Adaptive Neuro-Fuzzy Interference System (AN-FIS) & artificial neural network (ANN). The design of ANN is based on the feed-forward method using back propagation delay and the design of ANFIS is based on adaptive control and state space control strategy. It is observed that performance parameters like peak overshoot and settling time for the sensor less speed of DFIG are found to be more profound with IWO in comparison to ANFIS, ANN, and other existing techniques for balanced conditions. Similarly in the unbalanced condition, faulty current approaches are quite closer to their healthy state with the IWO method in comparison to other methods. In addition to this, minimum distortion (%THD) for the grid current under unbalanced conditions is also attained with IWO in comparison to ANFIS, ANN, and other existing techniques. Such application of IWO makes the system highly efficient and robust.

Maximum power production operation of doubly fed induction generator wind turbine using adaptive neural network and conventional controllers

Production of maximum power based on control of the Rotor Side Converter (RSC) of Doubly Fed Induction Generator (DFIG) wind turbine direct axis current is necessary to accomplish fast reaching to the maximum power point and protect the working parts in RSC from high overshoot in the current. An assessment study between adaptive Neural Network (NN) and conventional Proportional Integral (PI) controllers for control of the RSC direct axis current is introduced in this study. NN controller-based Levenberg-Marquardt backpropagation (LMBP) is designed and is trained to mainly control RSC direct axis current. Also, RSC direct axis current is extracted based on PI controller which used to control the speed of DFIG according to the optimum tip speed ratio obtained by genetic algorithm. The simulation results show that the RSC based NN controller is better than RSC-based conventional speed regulator in protecting RSC parts from high overshoot in the current.

Improved Hydroturbine Control and Future Prospects of Variable Speed Hydropower Plant

Hydroturbine control system (HTCS) is a nonlinear, nonminimum phase system comprising of speed/frequency sensors, hydraulic servo motors, wicket gates, gate position feedback mechanism, and proportional integral derivative (PID) controller (governor). Governor controls the guide vane opening (GVO) corresponding to the input (speed/frequency). Nowadays, HTCS becomes more intricate insight of variable speed doubly-fed induction generator for compensating wide variation in water head. Hence, robust control techniques are necessary for the optimal utilization of hydropower potential. In this article, a brief review of HTCS is given and a complimentary sliding-mode controller (CSMC) is designed for GVO serving a 250 MW hydrogenerating unit. The CSMC control law is derived and the stability is verified through the Lyapunov function. Simulation is carried out through the MatLab Simulink environment for a 250 MW HTCS. Then, the performance of CSMC for GVO and its influence on the generator characteristics are analyzed and compared with the traditional PID controller. The simulation results demonstrate the efficacy of the controller during the startup process, subsynchronous, and super synchronous mode of operations. It is inferred that the proposed CSMC has three times faster settling time than that of the traditional PID controller. Experimental validation is carried out with a scaled-down laboratory prototype. Furthermore, future scope and design challenges associated with the hydropower control system are illustrated.

Stability Enhancement of Small-Scale Power Grid with Renewable Power Sources by Variable Speed Diesel Power Plant*

This paper proposes a power control method to improve a stability of a small-scale power grid with renewable energy sources. In an isolated small- scale power grid such as an island, diesel power plant is main power source which has an environmental burden and expensive running cost due to high priced fossil fuel. Thus, expanding installation of the renewable energy sources such as a wind power is strongly desirable. Such fluctuating energy sources, however, harm power quality of the small-scale power grid, and in addition, conventional power plant in the small-scale power grid cannot, in general, stabilize the grid system with such fluctuating power sources. In this study, Variable Speed Doubly-Fed Induction Generator (VS-DFIG) is proposed to be in-stalled at a diesel power plant instead of a conventional Fixed Speed Synchronous Generator (FS-SG), because quick control of a power balance in the small-scale power grid can be achieved by using the inertial energy of VS-DFIG. In addition, utilization of a Battery Energy Storage System (BESS) is also considered to assist cooperatively the VS-DFIG control. As a result of the simulation analysis by using the proposed method, it is verified that frequency fluctuations due to renewable energy source can be effectively reduced by quick power control of the VS-DFIG compared to the conventional FS-SG, and further control ability can be obtained by utilizing BESS. Moreover, the transient stability of a small-scale power grid during a grid fault can also be enhanced.

Research on DFIG-ES System to Enhance the Fast-Frequency Response Capability of Wind Farms

With the increasing penetration of wind power generation, the frequency regulation burden on conventional synchronous generators has become heavier, as the rotor speed of doubly-fed induction generator (DFIG) is decoupled with the system frequency. As the frequency regulation capability of wind farms is an urgent appeal, the inertia control of DFIG has been studied by many researchers and the energy storage (ES) system has been installed in wind farms to respond to frequency deviation with doubly-fed induction generators (DFIGs). In view of the high allocation and maintenance cost of the ES system, the capacity allocation scheme of the ES system—especially for fast-frequency response—is proposed in this paper. The capacity allocation principle was to make the wind farm possess the same potential inertial energy as that of synchronous generators set with equal rated power. After the capacity of the ES system was defined, the coordinated control strategy of the DFIG-ES system with consideration of wind speed was proposed in order to improve the frequency nadir during fast-frequency response. The overall power reference of the DFIG-ES system was calculated on the basis of the frequency response characteristic of synchronous generators. In particular, once the power reference of DFIG was determined, a novel virtual inertia control method of DFIG was put forward to release rotational kinetic energy and produce power surge by means of continuously modifying the proportional coefficient of maximum power point tracking (MPPT) control. During the deceleration period, the power reference smoothly decreased with the rotor speed until it reached the MPPT curve, wherein the rotor speed could rapidly recover by virtue of wind power so that the secondary frequency drop could be avoided. Afterwards, a fuzzy logic controller (FLC) was designed to distribute output power between the DFIG and ES system according to the rotor speed of DFIG and S o C of ES; thus the scheme enabled the DFIG-ES system to respond to frequency deviation in most cases while preventing the secondary frequency drop and prolonging the service life of the DFIG-ES system. Finally, the test results, which were based on the simulation system on MATLAB/Simulink software, verified the effectiveness of the proposed control strategy by comparison with other control methods and verified the rationality of the designed fuzzy logic controller and proposed capacity allocation scheme of the ES system.

Direct torque control improvement of a variable speed DFIG based on a fuzzy inference system

In this paper, a fuzzy direct torque control (DTC) for a variable speed doubly fed induction generator (DFIG) based wind turbine is investigated. The DTC has been widely applied to doubly fed induction machines in recent years due to the high performances that it provides in comparison with the classical field oriented control. Meanwhile, it has a major drawback that is the significant torque and flux undulations generated by the hysteresis band controllers. To overcome this problem, the improvement of this technique by removing these controllers is proposed in this paper. The proposed control technique is based on replacing the hysteresis controllers by a fuzzy inference system which will have the same inputs as these controllers plus the sector angle determination so that the look-up table can be replaced by a fuzzy rule-base matrix. The simulation was performed in MATLAB/SIMULINK, and the results obtained make it possible to evaluate the performance of the proposed technique over the classical one.

Modification of DFIG's Active Power Control Loop for Speed Control Enhancement and Inertial Frequency Response

This paper proposes a fuzzy-based speed controller for the doubly fed induction generator (DFIG)-based wind turbines with the rotor speed and wind speed inputs. The controller parameters are optimized using the particle swarm optimization algorithm. To accelerate tracking the maximum power point trajectory, the conventional controller is augmented with a feed-forward compensator, which uses the wind speed input and includes a high-pass filter. The proposed combined speed controller is robust against wind measurement errors and as the accuracy of anemometers increases the speed regulation tends toward the ideal controller. The cutoff frequency of the applied filter is determined considering a compromise between the sensitivity to measurement errors and speed of regulation process. We also design an auxiliary frequency controller to equip the DFIGs with an inertial frequency response. In the proposed controller, two important constraints are taken into account: the feasible rotor speed range during the injection period, and the minimum time to recover the DFIG's speed. The impacts of the proposed controllers are evaluated through extensive time-domain simulations on an IEEE 9-bus test system using the DIgSILENT/PowerFactory software. Results confirm the effectiveness of the proposed controllers in serious transients and load disturbances.

Enhancement of demagnetization control for low-voltage ride-through capability in DFIG-based wind farm

Low voltage ride through (LVRT) is one of the most popular methods to protect doubly fed induction generator (DFIG) against balanced and unbalanced voltage dips. In this study, a novel LVRT capability strategy is enhanced using forcing demagnetization controller (FDC) in DFIG-based wind farm. Moreover, not only stator circuit but also rotor circuit were developed by electromotor force (EMF) for LVRT in DFIG-based wind farm. The transient stability performances of the DFIG with and without the FDC and EMF were compared for three- and two-phase faults. In addition to variations such as 34.5 kV bus voltage and terminal voltage of DFIG, speed of DFIG, electrical torque of DFIG and d-q axis rotor-stator current variations of DFIG were also evaluated. It was seen that the system became stable within a short time using the FDC and EMF.

Modeling of Wind Power Plant with Doubly-Fed Induction Generato

This paper presents the modeling and simulation of wind turbine driven by doubly-fed induction generator which feeds ac power to the distribution network. A stator flux oriented vector control is used for the variable speed doubly-fed induction generator operation. By controlling the generator excitation current the amplitude of the stator EMF is adjusted equal to the amplitude of the grid voltage. To set the generator frequency equal to the grid one, the turbine pitch angle controller accelerates the turbine/generator until it reaches the synchronous speed. The system is modeled and simulated in the Matlab Simulink environment in such a way that it can be suited for modeling of all types of induction generator configurations. The model makes use of rotor reference frame using dynamic vector approach for machine model. The system is also simulated when a fault occurs in 25 kV grid of distribution system. The results of a single line to ground fault and a symmetrical three-phase ground fault is analyzed. The results show that the wind energy conversion system can normally operate in fault conditions.

A New Robust Decoupled Control of the Stator Active and Reactive Currents for Grid-Connected Doubly-Fed Induction Generators

This paper addresses the grid-connected variable speed doubly-fed induction generator, and proposes a new decoupled control to replace the conventional decoupled active and reactive powers (P-Q) control. The proposed decoupled control is based on decoupling the stator active and reactive currents, in contrast with the conventional decoupled P-Q control, which is based on decoupling the stator active and reactive powers by forcing the stator d- or q-voltage to zero. The proposed decoupled control has all the advantages of the conventional decoupled P-Q control such as constant switching frequency and robustness against slip angle inaccuracy, and it has some additional advantages: The proposed control requires less machine parameters; for the controller design, it requires the stator-to-rotor turns ratio only; for the online calculation, it does not requires any machine parameter. The proposed decoupled control is more flexible and robust since the control is independent of the grid voltage orientation. It is robust against variation in the grid voltage amplitude. Several experiments are carried out using a 1.1 kW doubly-fed induction generator (DFIG), and the results support the proposed decoupled control and demonstrate some of its advantages.

A Novel Observer Design for Sensorless Sampled Output Measurement: Application of Variable Speed Doubly Fed Induction Generator

This paper presents a novel exponentially convergent nonlinear observer design for variable speed doubly fed induction generators (DFIG). The main feature of the proposed observer lies in the use of sampled-data without necessitating mechanical sensors, making the observer more reliable. A main component of the observer is an inter-sampled predictor of the output current vector. The observer exponential convergence is established and analyzed using Lyapunov stability technique and the small gain theorem. The proposed observer combines the advantage of a high-gain structure in terms of convergence speed and the continuity of the estimated state trajectory. One difficulty of the present observer design problem is that the electromagnetic torque is not related to the output by an injective map. The simulation results in variable speed DFIG operation are provided to confirm the theoretical results.

An Optimal and Non-Linear Speed Control of OWC Wave Energy Plant with Wells Turbine and DFIG

This paper presents controller scheme to maximize the turbine output power and thereby improve the efficiency in oscillating water column (OWC) wave power plant equipped with Wells turbine and doubly fed induction generator (DFIG). The scheme is based on flow coefficient estimation and controller design for tracking of rotational speed. Initially, a linear reference tracking (LRT) approach is applied to achieve the maximized output power by establishing a linear relationship between reference rotational speed and input pressure drop. This follows implementation of fuzzy theory based maximum power point tracking (FMPPT) approach to provide the optimal speed reference. Then, a backstepping (BS) controller is designed to track reference rotational speed of DFIG so as to improve the output power. The BS controller is derived using Lyapunov stability theorem which ensures the stability of the overall closed loop system. The advantages of the proposed control scheme over conventional PI control and the uncontrolled system is demonstrated for regular waves and irregular waves. Finally, the FMPPT-BS control approach has been validated with JONSWAP irregular wave model.

Voltage stability enhancement using an adaptive hysteresis controlled variable speed wind turbine driven EESG with MPPT

This paper investigates the enhancement in voltage stability achieved while connecting a variable speed wind turbine (VSWT) driven electrically excited synchronous generator (EESG) into power systems. The wind energy conversion system (WECS) uses an AC-DC-AC converter system with an uncontrolled rectifier, maximum power point tracking (MPPT) controlled dc-dc boost converter and adaptive hysteresis controlled voltage source converter (VSC). The MPPT controller senses the rectified voltage (VDC) and traces the maximum power point to effectively maximize the output power. With MPPT and adaptive hysteresis band current control in VSC, the DC link voltage is maintained constant under variable wind speeds and transient grid currents.The effectiveness of the proposed WECS in enhancing voltage stability is analysed on a standard IEEE 5 bus system, which includes examining the voltage magnitude, voltage collapse and reactive power injected by the systems. Simulation results show that the proposed WECS has the potential to improve the long-term voltage stability of the grid by injecting reactive power. The performance of this scheme is compared with a fixed speed squirrel cage induction generator (SCIG), a variable speed doubly-fed induction generator (DFIG) and a variable speed permanent magnet synchronous generator (PMSG).

Passivity-Based Control Strategies of Doubly Fed Induction Wind Power Generator without Speed Sensors

Due to its great potential value in theory and application, nonlinear control strategies of doubly fed induction generator (DFIG), especially passivity-based control (PBC) methods, have been a focus in the literature. A PBC strategy for DFIG with time-varying load torque to track flux and speed trajectories is proposed in this paper. The key point of this method is the identification of terms, known as workless forces, which appear in the dynamic equations of DFIG but have no effect on the energy balance equation of the closed loop. The speed of DFIG is estimated via an ANN estimator avoiding the use of mechanical sensors. PBC combined with ANN speed estimator not only reserves the advantages of PBC such as nonexistence of singularity, but also rejects the flux and speed tracking error caused by stator and rotor resistance variation. This algorithm simplifies the control structure and enhances the robustness of the control system. The feasibility and effectiveness is testified by the experimental results based on dSPACE.

Stabilisation of grid connected wind farms using doubly-fed induction generators with FACTS controllers

The impact of the wind generation on the power systems is no longer negligible if high penetration levels are going to be achieved. It is very important to boost the transfer capability of the transmission system while improving the utilisation of transmission assets to accommodate high penetration of wind power on the network. Hence, more sophisticated control schemes using advanced power electronic-based flexible AC transmission system (FACTS) controllers are required which will enhance voltage security and voltage stability control. This paper presents the impact of FACTS controllers on the dynamic stability of power systems connected with wind energy conversion systems. The wind farm model considered is a variable speed doubly-fed induction generator model. The dynamic simulation results yield information on 1 the impact of faults and wind speed changes on the performance of DFIG-based induction generators 2 the change in controllable parameters of the FACTS controllers following the faults and wind speed changes 3 transient rating of the FACTS controllers for enhancement of rotor speed stability of induction generators and angle stability of synchronous generators.