Performance Of Doubly Fed Induction Generator Research Articles

Voltage sensorless based virtual flux control of three level NPC back-to-back converter dfigunder grid fault

In this paper, a harmonic elimination of grid and stator currents of doubly fed induction generator (DFIG) in case of grid fault without line voltage sensors is proposed . This can be achieved by compensating power based on virtual flux voltage sensorless technique. Direct power control with space vector modulation (DPC-SVM) is used to control both grid-side (GSC)and rotor-side converters (RSC). To achieve the control objective, compensated active and reactive powers are calculated based on virtual flux technique with balanced and harmonic free current as a control target. A theoretical analysis of active and reactive powers under unbalanced voltage source is clearly demonstrated and the effect of grid fault on the performance of DFIG is profoundly discussed. Simulation results verified the effectiveness of the modified control strategy.

Nonlinear Adaptive Robust Control Strategy of Doubly Fed Induction Generator Based on Virtual Synchronous Generator

Doubly fed induction generator (DFIG) may suffer from huge disturbance owing to the uncertainty of generator parameters and load fluctuation during operation process. Hence, in the context of virtual synchronous generator (VSG) technology and its power-frequency control characteristics, this paper presents a VSG-based nonlinear adaptive robust controller (NARC), aiming to enhance the comprehensive regulation performance of DFIG. Firstly, a nonlinear robust adaptive controller with disturbance rejection and uncertainty adaptive learning ability is designed by establishing an improved dynamic model of the rotor motion equation in VSG, so as to achieve the optimal regulation of DFIG with robustness and adaptability. Secondly, in order to realize the fast dynamic response and autonomous synchronization ability in DFIG, a modified control strategy of the grid side converter (GSC) in DFIG is constructed by voltage and current dual-loop control (DLC) and VSG, which not only retains the significant dynamic response ability of the current inner-loop in DLC, but also improves the inertia and damping of the system. Finally, simulation results indicate that the modified control strategy exhibits satisfied dynamic performance and reliability, and reveals the robustness and adaptability of the control strategy in response to external random interferences and internal uncertain parameters.

Impacts of Grid Frequency Variation on Dynamic Performance of DFIG Based Wind Turbine

Wind energy is one of popular sources of renewable energy that massively used to generate electric power nowadays. Among wind generator technologies, Doubly Fed Induction Generator (DFIG) is placed as the first dominant wind generator type that is installed worldwide since 2004. Its popularity is due to the advantages of its capability to extract more energy compare to the fixed speed type. Moreover, it uses only one-third converter size which reduced the total construction cost compared to its closest rival, Full Converter Wind Turbine Generator Type. Although DFIG widely installed worldwide, it is in fact, very sensitive to the grid disturbances such as frequency variation. Based on IEEE standard for 60 Hz system, a power system should not be allowed to work in less than 59.3 Hz or more than 60.5 Hz. In this paper, DFIG-grid connected is extensively simulated in the condition of frequency variation that is carried out using Matlab/Simulink. The results show that some important parameters such as power output of the generator, voltage at point of common coupling, voltage at DC link and generator speed are experiencing significant rise/drop which in turn might damage the wind turbine generator.

Impact of Superconducting Magnetic Energy Storage Unit on Doubly Fed Induction Generator Performance During Various Levels of Grid Faults

Since 2004, Doubly Fed Induction Generator (DFIG) has become the most admired wind turbine generator type that is widely installed worldwide. The popularity of DFIG is due to its capability to extract more energy than fixed speed wind turbine generator type. Moreover, a DFIG requires only one-third of the full converter size used by type-4 wind turbine generator which in turn reduces the total cost of DFIG installation. The main weakness of DFIG is its vulnerability to grid faults such as voltage dip and swell. Grid faults may have adverse impacts on the overall performance of the DFIG including the voltage at the point of common coupling that, at certain fault levels, may violate the permissible margins of the grid codes established by the transmission line operators. This paper investigates the application of Superconducting Magnetic Energy Storage (SMES) unit to improve the fault ride-through capability of a DFIG during grid faults. Results show that the connection of a SMES unit at the point of common coupling between the DFIG and the grid can enhance the overall performance of the DFIG during such faults events.

Fault ride through strategy of DFIG using rotor voltage direct compensation control under voltage phase angle jump

Wind power has developed rapidly in recent years, and large-scale wind power facilities connected to power grids will bring many new challenges. Some new operation characteristics of power grids with doubly-fed induction generator (DFIG) may exhibit, for example voltage phase angle jumps (VPAJ). VPAJ can negatively impact the fault ride through (FRT) performance of DFIG. This paper firstly investigates the physical mechanism and the operation characteristics of DFIG with VPAJ. It is noted that the current control strategies designed for voltage amplitude changes are not suitable for VPAJ. Secondly, the paper develops an FRT optimization control strategy under VPAJ which optimizes the DFIG operation characteristics. Finally, simulations of a 250 MW wind farm are presented which validate the proposed FRT strategy.

Dynamic performance comparison of DFIG and FCWECS during grid faults

Among several types, variable speed-based wind turbine generator (WTG) is the most popular type installed worldwide. This type of WTG is able to extract 5% more energy from wind speed compared to the fixed speed WTG. There are two kinds of variable speed based WTG; Doubly Fed Induction Generator (DFIG) and Full Converter Wind Energy Conversion System (FCWECS). DFIG and FCWECS are placed at the first and second topWTG installation worldwide since 2004. However, both of them are very sensitive to the grid dip fault and may violate the allowable margins identified by various international Fault Ride Through (FRT) codes. This paper aims to investigate the responses of DFIG and FCWECS during certain level of grid dips and compare their performanceunder such event. Results show some differences of the performance of DFIG and FCWECS during voltage sag event, however the voltage profile at the point of common coupling is much better in case of DFIG. Results also recommend that DFIG can be effective when connected to weak grids whilst FCWECS is preferably to be connected to strong grids.

Efficient real-time estimation for DFIG—Performance and reliability enhancement of grid/micro-grid connected energy conversion systems

This paper deals with an efficient real-time estimation (ERES) method for Doubly Fed Induction Generator (DFIG) parameters like resistances and inductances. The developed estimation technique is a sensorless method and is robust regarding measurement noises. Moreover, it estimates the rotating speed and the fluxes and their dynamics. The proposed estimation technique can easily serve to enhance the performances and the robustness of the related controllers. This technique can be employed for grid/micro-grid connected generation systems like DFIG-based Wind Energy Conversion Systems (WECSs) or other Energy Conversion Systems (ENCSs). The new approach is developed based on a DFIG state-space model and the recursive least squares algorithm. Simulations verify the viability of the proposed approach for a 1.5 MW DFIG-based WECS in the presence of measurement noises and parameter variations. A comparative study is also provided to show the superiority of the ERES-based adaptive sensorless sliding mode control approach compared to the classical one. It is observed that the motor drive ensures driving with a reduced number of sensors, lower tracking errors, and lower number of voltage fluctuations in the rotor.

Comparison of rotor electrical fault indices owing to inter‐turn short circuit and unbalanced resistance in doubly‐fed induction generator

Doubly-fed induction generator (DFIG) is the dominant technology in the wind energy market. Rotor inter-turn short-circuits (RITSCs) and unbalanced rotor resistance (URR) are the main types of rotor electrical asymmetries in DFIG. The URR has already been considered as an electrical fault or asymmetry in the rotor of DFIG. Although the RITSC introduces URR into the rotor circuit its consequences are not similar due to the structure of the machine and presence of current controllers in the DFIG system. In this study, both RITSC and URR are proposed and compared, and the detection of these faults was performed using appropriate indices in the stator current, reactive power, and rotor modulating voltage signals, which are available in the control system of the DFIG. Furthermore, it is supposed that the discrimination between these two types of faults is feasible by utilising proper fault indices at various operating regions of the wind turbine. The performance of the defined fault indices, for different fault severities, is verified using an experimental setup with the DFIG operating under several conditions such as different power injection into the grid and different rotor speeds, including sub-synchronous and super-synchronous operation.

Power Stability Enhancement of SCIG and DFIG Based Wind Turbine Using Controlled-SMES

Several installed electrical power systems have a traditional type of wind power (WP) generators called squirrel cage induction generator (SCIG). However, SCIG is not the best type of WP in which it has a harmful impact on the stability of power systems. Therefore, with the increase in the loads especially at transient events, the SCIG will lead to system instability. Hence, it is necessary to increase the penetration level of WP by using more efficient generators, such as the doubly-fed induction generator (DFIG). At transient events such as increasing in the loads causes fluctuation of frequency and voltage of power systems. So, it is necessary to use controlled energy storage units to improve the stability of WP. The system used for studying consists of a grid connected to 25 kV bus through a step-down transformer and transmission line supplied initially by SCIG-based WP then supported by DFIG to increase penetration level and covers the increase in the loads. The superconducting magnetic energy storage (SMES) units are proficient in response, efficiency, and lifetime in contrast to other categories of energy storage systems, which make it a favored option for renewable energy. If SMES is combined with DFIG and SCIG, it increases the energy capacity of the total system. The SMES-controller is based on fuzzy logic control (FLC). The models of SMES, SCIG-based WP, and DFIG-based WP are demonstrated with MATLAB/SIMULINK, and the results illustrate the success of the proposed control to minimizing the voltage and frequency fluctuations during the transient events. Also, the model illustrates that the performance of DFIG on voltage and frequency stability is better than that of SCIG.

Impact of Grid Impedance on LVRT Performance of DFIG System With Rotor Crowbar Technology

Nowadays, most of the doubly fed induction generators (DFIGs) are equipped with rotor crowbar for the requirement of low voltage ride through (LVRT). The crowbar resistance is an important parameter, and it is selected taking rotor overcurrent and dc link overvoltage limits into consideration. However, the impact of grid impedance on the LVRT performance of DFIG and crowbar resistance is not adequately researched. This paper proposed an improved method to analyze the LVRT performance of DFIG system with rotor crowbar taking the influence of the grid impedance to fill this gap. The impedance of the present grid would be decreased in the future due to the installation of more wind farms to the grid. As a consequence, the performance of DFIG under LVRT with rotor crowbar is degraded. So the design rules of the crowbar resistance need to be reconstructed. Additionally, the analytical expression of the crowbar resistance is derived considering the grid impedance. The effectiveness of the proposed method is validated through theoretical analysis and MATLAB simulations.

Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review

Modern electrical grids face serious challenges to maintain healthy operational conditions. The nominal voltage and frequency, for instance, are affected by huge load changes and renewable energy integration. Renewable energy conversion systems are traditionally installed to provide active power. Fortunately, advanced renewable energy conversion systems have the ability to provide other facilities such as reactive power control and harmonic mitigation. In fact, recent grid codes obligate renewable energy systems to serve other duties such as generating reactive power rather than just simply supply active power, participate in fault ride through, for instance. This paper presents a comprehensive study on the doubly fed induction generator (DFIG)-based wind energy conversion system (WECS), which provides ancillary services to the grid along with performing its regular duties. These services include reactive power control, voltage ride through, power quality improvement, frequency control, and power oscillation damping. The literature survey and comparative studies show that the DFIG-based WECS has the ability to support electrical grid during normal and transient conditions, and it can also help in the large-scale renewable energy penetration to the power grid. The effect of different industrial and nonlinear controllers on DFIG performance in extracting grid ancillary services is highlighted.

Effect of PLL on transient performance of wind turbines generator under voltage phase jump

With the increasing proportion of wind power installed capacity, the influences of transient characteristics of wind turbines generator on safety and stability of power grid is becoming more and more important. The rotor side of doubly fed induction generator (DFIG) is connected to the grid through a back-to-back inverter, and vector control based on phase-locking synchronisation is used generally. Phase-locked loop (PLL) drive internal potential phase changes to achieve synchronous operation with the grid. Therefore, PLL has great influence on transient characteristics of DFIG. A dynamic model of PLL is built in this study. Phase-locking performance of PLL under voltage phase jump is studied by changing its parameters, and its impact mechanism on transient characteristics of DFIG is proposed. Simulation models are built with DIgSILENT/PowerFactory. The influence of parameters of PLL on phase-locking performance of DFIG under voltage phase jump is simulated.

Coordinated control strategy for the short-term frequency response of a DFIG-ES system based on wind speed zone classification and fuzzy logic control

In a power system with a high penetration rate of wind power generation, the rotor speed of a doubly fed induction generator (DFIG) is decoupled from the system frequency; thus, DFIG has no short-term frequency response capability, and the system frequency stability is significantly affected compared to that in a power grid with more conventional synchronous generators. For this reason, a novel fuzzy theory-based coordinated control strategy is presented for improving the short-term frequency response capability of a DFIG-energy storage (ES) system. Firstly, to make the short-term frequency response capability of the DFIG-ES system adaptive for various wind speed conditions, based on the analysis of the DFIG operation characteristics under different wind speeds, four wind speed zones are classified, and accordingly different operation strategies are designed for each wind speed zone. Afterwards, two kinds of fuzzy logic controllers (FLCs) are designed and deployed in the DFIG-ES control system. Specifically, according to the fuzzy logic rules, frequency deviation and DFIG operation state are treated as input parameters, and correspondingly the power support from the combined system and the participation coefficient of the DFIG for short-term frequency support are determined. Moreover, the realization of the presented control strategy is detailedly described. Finally, case studies have been conducted to demonstrate the performance of the proposed control strategy via the comparison between different control strategies, and the simulation results illustrate that the coordinated control strategy can effectively improve the short-term frequency response capability of the DFIG-ES system under various wind speeds.

Enhancement of DFIG performance at high wind speed using fractional order PI controller in pitch compensation loop

Doubly fed induction generator (DFIG) has been widely applied in wind farms. Meanwhile, DFIG control at high wind speeds has become an important issue. In this regard, the main task of pitch angle control loop is too important by the wind speed increase above the middle speeds (e. g. 12 m/s) to limit DFIG active power and rotor speed and to control the machine performance. In this study, a fractional order PI controller (FOPI) is designed and applied for the pitch angle compensation control loop to improve the DFIG performance at the high wind speed of 18 m/s. Due to using the fractional order coefficient as an extra control variable in the FOPI controller function in comparison with the PI controller, a preferable control scheme would be realized in the pitch angle control by the wind speed increase to high values. In this regard, the various simulation results for monitoring the active power and rotor speed are obtained in the multi machine test system including the DFIGs considering high wind speed operating mode via using MATLAB/SIMULINK software. In addition, different kinds of transient faults are considered to evaluate the performance of the proposed FOPI controller, instead of the conventional PI controller. The results validate the advantage of using the FOPI control scheme in the pitch angle control loop for promoting the DFIG active power and rotor speed profiles.

Grid-Voltage-Oriented Sliding Mode Control for DFIG Under Balanced and Unbalanced Grid Faults

Doubly fed induction generators (DFIGs) are widely used in variable-speed wind turbines. Despite the well-accepted performance of DFIGs, these generators are highly sensible to grid faults. Hence, the presence of grid faults must be considered in the design of any control system to be deployed on DFIGs. Sliding mode control (SMC) is a useful alternative for electric machinery control since SMC offers fast dynamic response and less sensitivity to parameter variations and disturbances. Additionally, the natural outputs of SMC are discontinuous signals allowing direct switching of power electronic devices. In this paper, a grid-voltage-oriented SMC is proposed and tested under low voltage grid faults. Unlike other nonmodulated techniques such as direct torque control, there is not a necessity of modifying the controller structure for withstanding low depth voltage dips. For stator natural flux cancelation, the torque and reactive power references are modified to inject a demagnetizing current. Simulation results demonstrate the demagnetization of the natural flux component as well as a robust tracking control under balanced and unbalanced voltage dips.

An adaptive limiter for unbalanced operation of doubly fed induction generator-based wind energy system

Actual limits on parameters of type-3 wind generation unit, i.e., doubly fed induction generator (DFIG), are in terms of peak values of phase quantities but enforcement of limits in DFIG is achieved by positive and negative sequence controllers. The limits are generally enforced considering the scenario where positive and negative sequence vectors are collinear. However, this restricts the capability regions of the DFIG. The objective of this paper is to implement an adaptive limiter which conforms to all possible variations in phase angle between positive and negative sequence components. This will improve the range of operation irrespective of the level of unbalance of the connected distribution system. An adaptive limiter for voltage and current, considering different pulse width modulation (PWM) techniques for the voltage source converters (VSC), is implemented for unbalanced operation of DFIG and compared with generalised collinear technique.

An adaptive limiter for unbalanced operation of doubly fed induction generator-based wind energy system

Actual limits on parameters of type-3 wind generation unit, i.e., doubly fed induction generator (DFIG), are in terms of peak values of phase quantities but enforcement of limits in DFIG is achieved by positive and negative sequence controllers. The limits are generally enforced considering the scenario where positive and negative sequence vectors are collinear. However, this restricts the capability regions of the DFIG. The objective of this paper is to implement an adaptive limiter which conforms to all possible variations in phase angle between positive and negative sequence components. This will improve the range of operation irrespective of the level of unbalance of the connected distribution system. An adaptive limiter for voltage and current, considering different pulse width modulation (PWM) techniques for the voltage source converters (VSC), is implemented for unbalanced operation of DFIG and compared with generalised collinear technique.

A control strategy for grid-side converter of DFIG under unbalanced condition based on Dig SILENT/Power Factory

The models of doubly fed induction generator (DFIG) and its grid-side converter (GSC) are established under unbalanced grid condition based on DIgSILENT/PowerFactory. According to the mathematical model, the vector equations of positive and negative sequence voltage and current are deduced in the positive sequence synchronous rotating reference frame d-q-0 when the characteristics of the simulation software are considered adequately. Moreover, the reference value of current component of GSC in the positive sequence frame d-q-0 under unbalanced condition can be obtained to improve the traditional control of GSC when the national issue of unbalanced current limits is combined. The simulated results indicate that the control strategy can restrain negative sequence current and the two times frequency power wave of GSC’s ac side effectively. The voltage of DC bus can be maintained a constant to ensure the uninterrupted operation of DFIG under unbalanced grid condition eventually.

Enhancing the HVRT and LVRT Capabilities of DFIG-based Wind Turbine in an Islanded Microgrid

Doubly fed induction generator (DFIG) based wind turbines are very sensitive to grid voltage variations. Therefore, low-voltage-ride-through (LVRT) and high-voltage-ride-through (HVRT) capabilities are employed to improve DFIG performance during grid faults and voltage swell events. In this paper, a superconducting magnetic energy storage (SMES) device with a PWM voltage source converter and a DC-DC chopper is proposed to enhance the DFIG LVRT and HVRT capabilities in an islanded microgrid simultaneously. The simulation results demonstrate that the SMES absorbs or releases energy from/to the microgrid during voltage swell events and fault condition respectively and consequently, improves the DFIG performance and enhances the DFIG LVRT and HVRT capabilities. The effectiveness of the proposed method is validated through detailed simulations in PSCAD/EMTDC.

Fault-tolerant control of doubly-fed induction generators under voltage and current sensor faults

This paper presents a Kalman filter-based fault-tolerant control (KFFTC) strategy for a doubly-fed induction generator (DFIG) under voltage and current sensor faults. Based on the independent time-varying models of stator voltages, stator currents, and rotor currents, six Kalman filters are designed in parallel to estimate voltage and current components in the presence of measurement noise. The sensor faults are detected and isolated based on the residuals calculated from observations obtained by sensors and estimations provided by Kalman filters. The faulty measured signals are then replaced by the estimated signals derived from corresponding Kalman filter to reconfigure the control system of DFIG during the sensor faults. Simulation and experimental studies undertaken on a grid-connected DFIG system reveal that the KFFTC strategy is able to correctly detect the sensor faults and isolate the faulty sensor, and it ensures the fault-tolerant operation of DFIG under the conditions of stator-voltage, stator-current, and rotor-current sensor faults.