MW Doubly Fed Induction Generator Research Articles

A Low Component Count Series Voltage Compensation Scheme for DFIG WTs to Enhance Fault Ride-Through Capability

Fault ride-through (FRT) operation has been a challenge for the doubly fed induction generator (DFIG) based wind turbines (WTs) as the stator winding is directly connected to the grid. Additionally, several grid codes have been established for the grid interconnection of WTs, which demand the WT to stay connected and provide the predefined reactive current support to the grid during FRT operation. The series voltage compensation (SeVC) based FRT schemes for DFIG WTs outperforms all others in terms of smooth transient performance. However, such FRT schemes require an additional voltage-source converter (VSC) and a bulkier series transformer to provide the SeVC. This paper proposes a low component count SeVC scheme that is applicable to both individual WTs and wind parks to cope up with the recent grid codes requirements. The proposed configuration eliminates the need of a series transformer and an additional VSC for the SeVC operation with the use of three additional IGBTs/switches. Furthermore, a coordinated control strategy is proposed to control the WT that enhance its FRT capabilities. The proposed low component count FRT scheme and coordinated control strategy are validated using the detailed mathematical modeling and simulation of 1.5 MW DFIG WT.

Hierarchical Voltage Control of a Wind Power Plant Using the Adaptive IQ-V Characteristic of a Doubly-Fed Induction Generator

Because wind generators (WGs) in a wind power plant (WPP) produce different active powers due to wake effects, the reactive power capability of each WG is different. This paper proposes a hierarchical voltage control scheme for a WPP that uses a WPP controller and WG controller. In the proposed scheme, the WPP controller determines a voltage error signal by using a PI controller and sends it to a doubly-fed induction generator (DFIG). Based on the reactive current-voltage (IQ-V) characteristic of a DFIG, the DFIG injects an appropriate reactive power corresponding to the voltage error signal. To enhance the voltage recovery capability, the gains of the IQ-V characteristic of a DFIG are modified depending on its reactive current capability so that a DFIG with greater reactive current capability may inject more reactive power. The proposed scheme enables the WPP to recover the voltage at the point of common coupling (PCC) to the nominal value within a short time after a disturbance by using the adaptive IQ-V characteristics of a DFIG. The performance of the proposed scheme was investigated for a 100 MW WPP consisting of 20 units of 5 MW DFIGs for small and larger disturbances. The results show the proposed scheme successfully recovers the PCC voltage within a short time after a disturbance.

Compensation of negative sequence stator flux of doubly‐fed induction generator using polar voltage control‐based direct torque control under unbalanced grid voltage condition

This study proposes a polar voltage control-based direct torque control method to reduce the effects of unbalanced grid voltage on doubly-fed induction generator (DFIG)-based wind turbine system. Under unbalanced grid voltage, the stator flux has a negative sequence component which leads to second harmonic pulsation in torque, stator active power, stator reactive power, stator current and rotor current. In the control scheme, the negative sequence rotor voltage vector is controlled to compensate the negative sequence stator flux by negative sequence rotor flux. Simulation study is carried out on a 2 MW DFIG system using MATLAB/SIMULINK. Feasibility of the proposed control strategy is experimentally verified on a 1.5 kW DFIG system.

Fuzzy logic control of grid connected DFIG system using back-to-back converters

This paper deals with the power generation control in variable speed wind turbines. In this context, the wind energy conversion system is equipped with a doubly-fed induction generator (DFIG) and two back-to-back PWM voltage source converters in the rotor circuit. A vector control of the rotor side converter (RSC) provides an independent control of stator active and reactive power and optimal speed tracking for maximum energy capture. A vector control scheme for the grid side converter (GSC) gives an independent control of slip active and reactive power exchanged between the grid and the GSC, in order to ensure sinusoidal supply currents and to maintain DC-link voltage constant. In this paper, an intelligent fuzzy inference system is proposed as alternative of the conventional PI controller to overcome any disturbance, three fuzzy controllers are used in the RSC for the speed, active and reactive power control, another fuzzy inference system is proposed as PI gain tuner for the DC-link voltage control loop. Performances have been tested on 1.5 MW DFIG by using Matlab/Simulink software environment.

Fuzzy logic based power management strategy of a multi-MW doubly-fed induction generator wind turbine with battery and ultracapacitor

Integrating energy storage systems (ESS) with wind turbines results to be an interesting option for improving the grid integration capability of wind energy. This paper presents and evaluates a wind hybrid system consisting of a 1.5 MW doubly-fed induction generator (DFIG) wind turbine and double battery-ultracapacitor ESS. Commercially available components are used in this wind hybrid system. A novel supervisory control system (SCS) is designed and implemented, which is responsible for setting the active and reactive power references for each component of the hybrid system. A fuzzy logic controller, taking into account the grid demand, power generation prediction, actual DFIG power generation and state-of-charge (SOC) of the ESSs, sets the active power references. The reactive power references are proportionally delivered to each element regarding their current limitations in the SCS. The appropriate control of the power converters allows each power source to achieve the operation defined by the SCS. The wind hybrid system and SCS are assessed by simulation under wind fluctuations, grid demand changes, and grid disturbances. Results show an improved performance in the overall response of the system with the implementation of the SCS.

Active and Reactive Power Control of a Doubly Fed Induction Generator

Doubly fed induction generators (DFIG) are increasingly used in grid interfaced wind energy systems to address voltage regulation and provide adequate reactive power support. This paper presents dynamic modeling and simulation of a doubly fed induction generator based on grid-side and rotor-side converter control. The DFIG, grid-side converter, rotor-side converter, and its controllers ar e performed in MATLAB/Simulink software. Dynamic response in grid connected mode for variable speed wind operation is investigated. Simulation results on a 3.6 MW DFIG system are provided to demonstrate the effectiveness of the proposed control strategy during variations of active and reactive power, rotor speed, and converter dc link voltage.

Study on the Output Performance of DFIG under Unbalanced Grid Voltage

This paper uses dynamic equivalent circuits to study the impact of the unbalanced grid voltage on a Doubly Fed Induction Generator (DFIG) and draws the curves between the voltage unbalance factor (VUF) and the output characteristics of DFIG. A mathematical model of the grid and rotor side converters (GSC and RSC, respectively) is given in the stationary αβ reference frame under unbalanced network voltage conditions, which could be used in the coordinated control of the GSC and RSC. Finally, simulation studies on a 1.5MW DFIG can verify the feasibility of the proposed coordinated control strategy which can be used to improve the performance and stability of the whole wind power generation system.

An analytic electromagnetic calculation method for performance evolution of doubly fed induction generators for wind turbines

An analytic electromagnetic calculation method for doubly fed induction generator (DFIG) in wind turbine system was presented. Based on the operation principles, steady state equivalent circuit and basic equations of DFIG, the modeling for electromagnetic calculation of DFIG was proposed. The electromagnetic calculation of DFIG was divided into three steps: the magnetic flux calculation, parameters derivation and performance checks. For each step, the detailed numeric calculation formulas were all derived. Combining the calculation formulas, the whole electromagnetic calculation procedure was established, which consisted of three iterative calculation loops, including magnetic saturation coefficient, electromotive force and total output power. All of the electromagnetic and performance data of DIFG can be calculated conveniently by the established calculation procedure, which can be used to evaluate the new designed machine. A 1.5 MW DFIG designed by the proposed procedure was built, for which the whole type tests including no-load test, load test and temperature rising test were carried out. The test results have shown that the DFIG satisfies technical requirements and the test data fit well with the calculation results which prove the correctness of the presented calculation method.

Single-Phase DC Crowbar Topologies for Low Voltage Ride Through Fulfillment of High-Power Doubly Fed Induction Generator-Based Wind Turbines

This paper presents two new single-phase crowbar protection topologies, for doubly fed induction generator (DFIG) based wind turbines. These crowbar topologies, can be suitable for fulfilling the low voltage ride through (LVRT) requirements demanded by the grid codes, in next generation of high-power DFIG-based wind turbines, allowing the possibility to use simpler and more cost effective crowbar designs, than the classic three-phase dc crowbar, commonly used among manufacturers. Therefore, this paper compares the differences of the classic three-phase dc crowbar and the new proposed ones, together with the study of the behavior of a prototype of 6.25 MW DFIG, when is affected with severe voltage symmetric and asymmetric voltage dips. Control actions and design ideas are discussed, trying to accomplish the LVRT requirements, concluding by means of simulation results that the proposed new single-phase crowbar topologies can be an attractive alternative crowbar solution, for high-power wind turbines.

A Study on Small Signal Stability of the Large-Scale Wind Farm with DFIGs

The high penetration of doubly-fed induction generators (DFIGs) entails a change in dynamics and operational characteristics of the power system, thus this paper investigates the small signal stability of the large-scale wind farm with DFIGs. The GE 1.5MW DFIG is modeled in power system analysis software package (PSASP), and a large-scale wind farm with DFIGs is established. Then, the two-area test system with four generators is applied to assess the effect of the large wind farm on power system inter-area oscillatory mode in which the penetration and the installation site of the wind farm are considered. Finally, the simulation results indicate that abundant penetration of DFIG-based wind power will improve the inter-area oscillatory, and the integration of wind farms with DFIGs in the receiving area makes the inter-area mode highly damped.

Cooperated Control Strategy of DFIG Wind Power Generation System under Unbalanced Grid Condition

This paper proposes a cooperated control of the grid side converter (GSC) and rotor side converter (RSC) in doubly-fed induction generator (DFIG) wind power generation system under unbalanced grid condition. Mathematical model of doubly-fed induction generator (DFIG) and GSC under unbalanced grid voltage condition are investigated. Dual-dq current control strategies of the RSC and GSC under unbalanced grid condition are detailed studied. A cooperated control strategy of the GSC and RSC under unbalanced grid condition is proposed to provide enhance operation of DFIG system. The GSC is controlled to remove the total active power fluctuation of the system and RSC is controlled to eliminate the DFIG electromagnetic torque oscillation. Simulation based on Matlab/Simulink of a 1.5 MW DFIG prototype was carried out to validate the proposed cooperated control strategy.

A Novel Power Limiter Based on Newton-Raphson Method for Doubly-Fed Induction Generator under Unbalanced Voltage Drop Conditions

A novel power limiter based on Newton-Raphson method is proposed in this paper for the Doubly-fed Induction Generator (DFIG) under the conditions of unbalanced grid voltage drop. Unlike the existing power limiters, which provide an approximate estimation of the maximal reactive power at the cost of introducing a large error in achieving its control target, ours formulates the design of power limiter into an optimization problem which tries to maximize the DFIG’s reactive power towards its two decisive factors: limited rotor current and rotor side converter’s output voltage. Finally the Newton-Raphson method is exploited to find the global maximum of the proposed optimization problem. Consequently, the proposed power limiter could more accurately determine the DFIG’s maximal reactive power without any loss of the accuracy in achieving its control target. Simulation results on a 1.5MW DFIG shows the efficacy of the proposed power limiter for DFIG under the conditions of unbalanced voltage drop.

Direct power control of wind‐turbine‐driven DFIG during transient grid voltage unbalance

ABSTRACTIn this paper, a direct power control (DPC) of a wind‐turbine‐driven doubly fed induction generators (DFIGs) under unbalanced network voltage conditions is studied. Variations of the stator output active, reactive and electromagnetic powers are fully deduced in the presence of negative sequence supply voltage. The rotor side converter is controlled on the basis of DPC to eliminate the electromagnetic torque oscillations at double supply frequency under unbalanced stator supply. The rotor voltage references estimation requires only simple calculations without any integral operation. The proposed control scheme removes rotor current regulators and the decomposition processing of positive and negative sequence rotor currents. The performance of the proposed and conventional DPC schemes is compared under the same operating conditions. Simulation results using Matlab/Simulink are carried out for a 1.5 MW DFIG wind generation system to show the validity of the proposed scheme during unbalanced voltage supply. Copyright © 2013 John Wiley & Sons, Ltd.

Doubly‐fed induction generator wind turbine modelling for detailed electromagnetic system studies

Wind turbine (WT) technology is currently driven by offshore development, which requires more reliable, multi-megawatt turbines. Models with different levels of detail have been continuously explored but tend to focus either on the electrical system or the mechanical system. This study presents a 4.5 MW doubly-fed induction generator (DFIG) WT model with pitch control. The model is developed in a simulation package, which has two control levels, the WT control and the DFIG control. Both a detailed and a simplified converter model are presented. Mathematical system block diagrams of the closed-loop control systems are derived and verified against the simulation model. This includes a detailed model of the DC-link voltage control - a component which is usually only presented in abstract form. Simulation results show that the output responses from the two models have good agreement. The grid-side converter control with several disturbance inputs has been evaluated for three cases and its dynamic stiffness affected by operating points are presented. In addition, the relation of pitch controller bandwidth and torsional oscillation mode has been investigated using a two-mass shaft model. This model can be employed to evaluate the control scheme, mechanical and electrical dynamics and the fault ride-through capability for the turbine.

Improved LVRT Capability and Power Smoothening of DFIG Wind Turbine Systems

This paper proposes an application of energy storage devices (ESD) for low-voltage ride-through (LVRT) capability enhancement and power smoothening of doubly-fed induction generator (DFIG) wind turbine systems. A grid-side converter (GSC) is used to maintain the DC-link voltage. Meanwhile, a machine-side converter (MSC) is used to control the active and reactive powers independently. For grid disturbances, the generator output power can be reduced by increasing the generator speed, resulting in an increased inertial energy of the rotational body. Design and control techniques for the energy storage devices are introduced, which consist of current and power control loops. Also, the output power fluctuation of the generator due to wind speed variations can be smoothened by controlling the ESD. The validity of the proposed method has been verified by PSCAD/EMTDC simulation results for a 2 MW DFIG wind turbine system and by experimental results for a small-scale wind turbine simulator.

Dynamic Simulation of a Wind Turbine Driven Doubly Fed Induction Generator under Wind Fluctuation

The wind is the main driving force of wind turbine, which directly affects the dynamic responses of generator. Due to its randomness and intermittence, wind energy has great impact on the quality and security of the operation of power system. The aim of this paper is to study the influence of wind speed fluctuation on the dynamic responses of the doubly fed induction generator (DFIG) connected to a power system. The mathematical models of a 3.6 MW DFIG and different wind speed patterns are first derived. The MATLAB/Simulink software is then utilized to model the DFIG, grid-side converter, rotor-side converter, and the wind speed patterns. Simulation results obtained from these models are used to analyze the effects of wind speed fluctuation on the system dynamic responses.

Wind Power Extraction from DFIG Wind Turbines Using Stator-Voltage and Stator-Flux Oriented Frames

This paper investigates and compares wind power extraction from a DFIG (doubly-fed induction generator) wind turbine using stator-voltage and stator-flux oriented frames. The paper analyzes how wind power extraction control of a DFIG wind turbine is converted to speed control, and then how speed and reactive power control of the wind turbine is converted to generator current control using the two different orientation frames. The paper also investigates what are the differences in developing wind power extraction control strategies of a DFIG wind turbine using the two different orientation frames. Simulation study is conducted for a 1.5MW DFIG wind turbine, in which the turbine driving torque is modeled by considering typical wind turbine aerodynamic characteristics. The study shows that the performance of DFIG wind power extraction is similar using both stator-voltage and stator-flux oriented frames. But, it is found that a conventional wind power extraction approach using the stator-flux oriented frame could deteriorate the power quality of the DFIG system while it is more stable to estimate the position of the stator-flux space vector by simply adding -90 degree to the stator-voltage space vector.

A Novel Vector-Based Hysteresis Current Regulator for Doubly-Fed Induction Generator Wind Turbines Under Non-Ideal Supply Conditions

This paper proposes a novel vector-based hysteresis current regulator (VBHCR) in the rotor-side converter of doubly-fed induction generator (DFIG) based wind turbine to enhance its operation under non-ideal supply voltage conditions. First, the induction generator is modelled using the “Γ-form” equivalent circuit and the stator-voltage-oriented vector control of DFIG is introduced. Then, the principle and practical implementation of the proposed VBHCR in the rotor-side converter of DFIG is presented. This current regulator employs two sets of hysteresis comparators working in three-phase and rotor reference frames, integrated with a switching table. Compared to the commonly-used proportional-integral current regulators, it reveals very fast transient response with a simple control structure. Simulation results, obtained for a 1.5 MW DFIG, show the validity of the proposed control scheme under normal operation conditions, as well as its desirable steady-state performance. Furthermore, the advantages of the proposed current regulator under non-ideal supply voltage conditions (including unbalance and harmonic distortions) are illustrated.

Fault Currents of Doubly-Fed Induction Generator Under Symmetrical Fault Condition

In wind power systems, the fault ride-through capability of doubly-fed induction generators (DFIGs) has become a focus of study. The analysis and computation of fault currents are of great significance for implementing DFIG fault ride-through. This paper systematically analyses dynamic responses of DFIG stator and rotor currents when three-phase short circuit occurs at stator terminals. A new approach, in which the DFIG under fault condition is equivalent to a rotor excited induction machine running in a steady-state and a squirrel-cage induction machine subjected to a three-phase short circuit, is proposed to gain the fault currents mathematical expressions. The two equivalent machines represent the particular solution and the general solution of DFIG differential equations in physical meaning, respectively. Moreover, the method of space vector analysis is introduced to achieve analytical expressions of the fault currents. The maximum rotor and stator short circuit currents, as well as the times when they appear, are demonstrated. Main components of the short circuit currents and their relations are presented. Finally, simulations on a 2 MW DFIG system are carried out, and prove to be in close agreement with results from mathematical analysis.

DFIG wind generation systems operating with limited converter rating considered under unbalanced network conditions – Analysis and control design

This paper presents an analysis and operation of doubly-fed induction generator (DFIG)-based wind power generation systems under unbalanced network voltage conditions. The limited voltage and current ratings, and control capabilities of both grid side and rotor-side converters (GSC&RSC) for unbalanced network voltage compensation are investigated. A current control scheme consisting of a proportional integral (PI) controller plus a resonant (R) compensator is designed. The control scheme is implemented in the positive synchronous rotating reference frame and provides precise current control for both the GSC and RSC without involving the decomposition process of positive and negative sequence currents. By taking into account both the converters’ current ratings, a coordinated control strategy between the GSC and RSC is proposed and adjusted. Finally, simulation studies on a 1.5 MW DFIG generation system and experimental tests on a 1.5 kW prototype are carried out. The correctness of the analytical results as well as the feasibility of the proposed coordinated control scheme are confirmed when the network voltage is unbalanced.