Dynamic stability and sub-synchronous resonance suppression in wind-integrated grids using rotor-side virtual inertia control

To investigate the fault features of doubly-fed induction generator (DFIG) with rotor side converter (RSC) control, an analytic method of calculating the DFIG short-circuit current is proposed in this paper. Based on the space vector model of DFIG with RSC control, the dynamic equation of rotor current reflecting both the machine transient process and the impact of RSC control is derived, and analytic expressions of rotor and stator fault current in time-domain are obtained. The calculation result is certified by being compared with the PSCAD simulation result. According to the derived analytic expressions, the components of the DFIG short-circuit current are analyzed. It indicates that the machine terminal voltage dip of DFIG, the current reference value, the orientation angle, and the proportional-integral value of RSC controller are main factors influencing the short-circuit current characteristics.

This paper presents an independent control of rotor side converter (RSC) and grid side converter (GSC) for a doubly fed induction generator (DFIG). A novel cascade feedback linearization (CFL) technique based on non-linear differential geometry is developed for design of RSC, which leads to decoupled currents. Here, the rotor DC link voltage can be regulated with respect to a square transform on its voltage reference and the d-axis current that satisfies conditions for zero dynamics. The controlled RSC has a capability to track DC link voltage reference faster and also attain global stability. The GSC controller has been designed by incorporating a new incremental hysteresis comparator (IHC) that utilizes the hysteresis band to produce the suitable switching signal to the GSC to get enhanced controllability during grid unbalance. The IHC produces higher duty-ratio linearity and larger fundamental GSC currents with lesser harmonics. The latter can thus achieve fast transient response for GSC. All these features are confirmed through time domain simulation on a 15 KW DFIG Wind Energy Conversion System.

This article presents the sharing of reactive power between two converters of a doubly fed induction generator (DFIG) based wind energy conversion system interacting with the grid. The rotor side converter (RSC) control of DFIG is designed for sharing of reactive power at below rated wind speeds, which essentially reduces the amount of rotor winding copper loss. However, at rated wind speed, the RSC control is designed to maintain the unity power factor at stator terminals and to extract rated power without exceeding its rating. Further, the reduction in rotor winding copper loss due to reactive power distribution is demonstrated with an example. Moreover, the grid side converter (GSC) control is designed to feed regulated power flow to the grid along with reactive power support to DFIG and to the load connected at point of common coupling. Moreover, the GSC control is designed to compensate load unbalance and load harmonics. The battery energy storage connected at dc link of back-to-back converters, is used for maintaining the regulated grid power flow regardless of wind speed variation. The system is modeled and its performance is simulated under change in grid reference active power, varying wind speed, sharing of reactive power, and unbalanced nonlinear load using SimPowerSystems toolbox of MATLAB. Finally, a prototype is developed to verify the system steady state and dynamic performance. Moreover, system voltages and currents are found sinusoidal and balanced, and their total harmonic distortions are as per the IEEE 519 standard.

In order to improve the stability of doubly‐fed induction generator (DFIG)‐based wind turbines under weak grid, previous works have proposed the power synchronization control strategy in the rotor side converter (RSC). However, the potential instability risk of dc‐link voltage control in the grid side converter (GSC) has been neglected. Here, DFIG with power synchronization and phase‐locked synchronization for RSC and GSC respectively, is called as hybrid synchronization mode (HSM)‐controlled DFIG and, the full‐order state space model of HSM‐controlled DFIG are established. Eigenvalue and participation factor analysis shows that the HSM‐controlled DFIG is unstable under extremely weak grid due to the interaction of phase‐locked loop (PLL) and current‐loop in GSC. Furthermore, dominant pole analysis indicates that the HSM‐controlled DFIG exhibits weak damping at the AC side in strong grid. To enhance the stability of HSM‐controlled DFIG under extremely weak grid, an improved hybrid synchronization control strategy is proposed. The improved control strategy utilizes active power and dc‐link voltage to achieve PLL‐free operation of DFIG. The effects of the parameters in dc‐link voltage synchronous loop on control bandwidth, system stability and dynamic coupling are analysed. Finally, the validity of the theoretical analysis is investigated with experiments based on 2MW control‐hardware‐in‐loop platform.

This paper describes a voltage control scheme of a doubly fed induction generator (DFIG) wind turbine that can inject more reactive power to the grid during a fault so as to support the grid voltage. To achieve this, the coordinated control scheme using both rotor side converter (RSC) and grid side converters (GSC) controllers of the DFIG are employed simultaneously. The RSC and GSC controllers employ PI controller to operate smoothly. In the voltage control mode, the RSC and GSC are operated. During a fault, both RSC and GSC are used simultaneously to supply the reactive power into the grid (main line) depending on voltage dip condition to support the grid voltage. The proposed system is implemented for single DFIG wind turbine using MATLAB simulation software. The results illustrate that the control strategy injects the reactive power to support the voltage stability during a fault rapidly. Also, the braking system is designed to protect the wind turbine system from over speed. For this purpose, the braking resistors are being used.

This paper proposes a coordinated control of the rotor side converters (RSCs) and grid side converters (GSCs) of doubly-fed induction generator (DFIG) based wind generation systems under unbalanced voltage conditions. System behaviors and operations of the RSC and GSC under unbalanced voltage are illustrated. To provide enhanced operation, the RSC is controlled to eliminate the torque oscillations at double supply frequency under unbalanced stator supply. The oscillation of the stator output active power is then cancelled by the active power output from the GSC, to ensure constant active power output from the overall DFIG generation system. To provide the required positive and negative sequence currents control for the RSC and GSC, a current control strategy containing a main controller and an auxiliary controller is analyzed. The main controller is implemented in the positive (dq)+ frame without involving positive/negative sequence decomposition whereas the auxiliary controller is implemented in the negative sequence (dq)- frame with negative sequence current extracted. Simulation results using EMTDC/PSCAD are presented for a 2 MW DFIG wind generation system to validate the proposed control scheme and to show the enhanced system operation during unbalanced voltage supply.

Grid-connected doubly-fed induction generator (DFIG) based wind energy conversion system (WECS) is widely used in harnessing wind power. The paper attempts to characterize a 15-kW grid-connected DFIG based WECS operating under variable speed and loading conditions. Back to back converter topologies are utilized consisting of rotor side converter (RSC) and grid side converter (GSC) controlled in the synchronous reference frame coordinates. The design, modeling, and control of various system components are deliberated. The interaction of the grid with the proposed DFIG-WECS is analyzed where the regulation of the machine active power is achieved through the control of RSC. The current quality, voltage regulation is achieved by the control of GSC. Case studies involving constant wind speed, variable wind speed, and variable loading are undertaken for the characterization analysis. The performance characterizations of the entire system is validated in Matlab/Simulink simulation environment where the critical performance parameters like DC link voltage, generator torque, DFIG stator current, active and reactive power delivered by the DFIG to the grid, PCC side voltage/ current profile and the PCC current Total harmonic distortion (THD) are rigorously monitored, presented and discussed for each case studies.

The impedance-based model of Doubly Fed Induction Generator (DFIG) systems, including the rotor part (Rotor Side Converter (RSC) and induction machine), and the grid part (Grid Side Converter (GSC) and its output filter), has been developed for analysis and mitigation of the Sub- Synchronous Resonance (SSR). However, the High Frequency Resonance (HFR) of DFIG systems due to the impedance interaction between DFIG system and parallel compensated weak network is often overlooked. This paper thus investigates the impedance characteristics of DFIG systems for the analysis of HFR. The influences of the rotor speed variation, the machine mutual inductance and the digital control delay are evaluated. Two resonances phenomena are revealed, i.e., 1) the series HFR between the DFIG system and weak power grid; 2) the parallel HFR between the rotor part and the grid part of DFIG system. The impedance modeling of DFIG system and weak grid network, as well as the series HFR between DFIG system and parallel compensated weak network has been validated by experimental results.

This paper deals with the mitigation of sub‐synchronous resonance (SSR) in doubly‐fed induction generator (DFIG)‐based wind farms using a sub‐synchronous resonance damping controller (SSRDC). The performance of the SSRDC depends on its input control signal and the location of its output control signals. Hence, this paper presents an algorithm to select the best location for applying the SSRDC. The DFIG parameters are used as the inputs of this algorithm. Also, the participation factors analysis is employed as this algorithm's main core. The output of this algorithm determines that the control signal of SSRDC can be applied either in the grid‐side converter (GSC) and/or in the rotor‐side converter (RSC). The best input location in the GSC is the DC‐link voltage and the best input location in the RSC is the q‐component of the rotor voltage. The accuracy of this algorithm was evaluated by investigating the effect of various input signal locations on the SSR using the eigenvalue analysis. This analysis indicated that the dc‐link voltage and the q‐components of the rotor voltage are the most effective signals on the sub‐synchronous oscillatory modes. Moreover, this paper introduces a new SSRDC using these two signals. The performance of this controller is validated through the eigenvalue analysis and a time domain simulation.

This paper deals with the smooth transition between grid connected mode to an islanded mode and the stator terminals of the DFIG synchronization. The system considers battery, photovoltaic (PV) array and doubly fed induction generator (DFIG). In an islanded mode, battery-based grid forming converter (GFC) controls the voltage and frequency at the common connection point (CCP) or point of common coupling (PCC). The grid side converter (GSC) of the DFIG is connected to the CCP via the interfacing inductors. The DC-link between the GSC and rotor side converter (RSC) is regulated by the GSC control algorithm. The RSC control of the DFIG is divided into two parts. In the first part, the RSC control develops the controlled stator terminal voltage and frequency at the stator windings, which is connected to the CCP via a solid-state switch (STS). The second part of the RSC control provides the essential quantity of reactive power demand by DFIG and also extracts the peak wind power. The maximum PV array power is extracted and fed to the CCP via a voltage source converter (VSC) and the interfacing inductors. An advanced control complex band pass filter-based FLL (CBPF-FLL) control is used to control the battery-based VSC during the grid mode of operation. This control provides improved power quality like unity power factor and lower harmonics in grid current as per the IEEE-519 standard.

This paper proposes a coordinated control of the rotor-side converter (RSC) and grid-side converter (GSC) of a doubly fed induction generator (DFIG)-based wind-turbine generation system under generalized unbalanced and/or distorted grid voltage conditions. The system behaviors of the RSC and GSC during supply imbalance and distortion are investigated. To enhance the fault ride-through operation capability, the RSC is properly controlled to eliminate the torque oscillations, whereas the GSC is carefully designed to ensure constant active power output from the overall DFIG generation system. To achieve simultaneous regulation of the positive-/negative-sequence currents and fifth-/seventh-order harmonic currents for both the RSC and GSC, a novel current controller, consisting of a conventional proportional-integral regulator and a dual-frequency resonant compensator, is proposed and implemented in the positive ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> ) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> reference frame. The simulation and experiment studies demonstrate the correctness of the developed model and the effectiveness of the suggested control strategy for DFIG-based wind-turbine systems under such adverse grid conditions.

A control strategy for compensating AC network voltage unbalance using doubly fed induction generator (DFIG)-based wind farms is presented. A complete DFIG dynamic model containing both the rotor and grid side converters is used to accurately describe the average and ripple components of active/reactive power, electromagnetic torque and DC bus voltage, under unbalanced conditions. The principle of using DFIG systems to compensate grid voltage unbalance by injecting negative sequence current into the AC system is described. The injected negative sequence current can be provided by either the grid side or the rotor side converters. Various methods for coordinating these two converters are discussed and their respective impacts on power and torque oscillations are described. The validity of the proposed control strategy is demonstrated by simulations on a 30 MW DFIG-based wind farm using Matlab/Simulink during 2 and 4% voltage unbalances. The proposed compensation strategy can not only ensure reliable operation of the wind generators by restricting torque, DC link voltage and power oscillations, but also enable DFIG-based wind farms to contribute to rebalancing the connected network.

The subsynchronous resonance (SSR) of a double-fed induction generator (DFIG) and its suppression method are studied in this paper. The SSR may be aroused by the interaction between the double-fed induction generator and the series-compensated transmission lines. This paper proposes an expression of the electrical damping for assessing the SSR stability based on the complex torque method. The expression is derived by linearizing the DFIG model at the operating point. When the mechanical damping is neglected, the expression can be used to calculate whether the electrical damping is positive or negative to judge the SSR stability. The expression can quantitatively analyze the impact of the wind speed, the compensation degree, and the parameters of the rotor speed controller and the rotor-side converter controller on the SSR stability. Furthermore, a subsynchronous damping control (SDC) strategy is designed to suppress the SSR. The parameters of the SDC are optimized by particle swarm optimization (PSO) based on the electrical damping. Finally, the above research is verified by the PSCAD/EMTDC time-domain simulations. The results show that the stability of SSR decreases with the decrease of wind speed, the increase of series compensation degree, the increase of proportional coefficient, and the decrease of integral coefficient in rotor speed controller and rotor-side converter. The designed subsynchronous oscillation controller can suppress the SSR of a DFIG.

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.

The advent of rising electrical energy consumption gave rise to a steady increase in the demand on power generation. So, in addition to conventional power generation units a number of renewable energy units are increasingly being integrated into the system. A wind electrical generation system is the most cost competitive of all the environmentally clean and safe renewable energy sources in world. The stator of the wind energy conversion systems generally Doubly Fed Induction Generator (DFIG) is directly are connected to the grid through Voltage Source Converters (VSC) to make variable speed operation possible. The stator of the generator is directly connected to the grid while the rotor is connected through a back-to-back converter which is dimensioned to stand only a fraction of the generator rated power. The rotor side converter (RSC) usually provides active and reactive power control of the machine while the grid-side converter (GSC) keeps the voltage of the DC-link constant. In the linear controller for DFIG, a vector-control scheme for the grid-side PWM converter and Instantaneous Reactive Power Theory(IRPT) for rotor-side PWM converter is applied. The proposed model of the linear controller and the conventional linear controller using Synchronous Reference Frame Theory(SRF) are both simulated in MATLAB/Simulink platform. A comparative study is done between the linear controller using SRF theory and the proposed linear controller in order to show how the proposed linear controller improves the performance of the grid connected Doubly Fed Induction Generator (DFIG).