Control Strategy Of Doubly Fed Induction Generator Research Articles

Adaptive transient overvoltage control strategy for wind farm based on data‐driven

AbstractTo achieve the transient overvoltage suppression of doubly fed induction generator (DFIG) terminals and the point of common coupling (PCC) in the wind farm (WF), an adaptive overvoltage control strategy based on data‐driven approach is proposed. Firstly, the PCC reactive power demand and DFIG overvoltage threshold are derived, respectively. Secondly, three regression models are developed through surface fitting to determine three variables: the maximum overvoltage of each DFIG, the PCC maximum overvoltage, and the reactive power demand of each DFIG. Subsequently, the K‐means clustering method is adopted to group all DFIGs, and then the reactive power requirement of the PCC is proportionally distributed to each group of DFIGs. Lastly, a coordinated control strategy of DFIGs and static var generator (SVG) is designed based on the values of three regression models, which can adaptively adjust the reactive power absorbed by each DFIG and SVG. In particular, the de‐load control is adopted to enlarge the maximum reactive power capacity of DFIG. The simulation results show that the proposed control strategy can timely and effectively restore the transient overvoltage of the wind farm to the normal range during the gird failure, ensuring the stable operation of the wind farm.

Control strategy of DFIG and SVG cooperating to regulate grid voltage of wind power integration point

To better solve doubly-fed wind farms' voltage stability control problem with static var generator (SVG), this paper proposes and designs a reactive power regulation strategy based on SVG and doubly-fed wind turbines. To stabilize the voltage of the wind farm, SVG is preferred for reactive power compensation when the voltage fluctuates. After the voltage is stable, the reactive power output of SVG and the doubly-fed induction generator (DFIG) is adjusted according to the system state. The DFIG can output enough reactive power and meet the specified power factor conditions. The control mode adopts a real-time tracking mode to monitor the voltage of the control bus. In the model, comprehensive optimization indexes consider the maximum voltage offset, the time-consuming voltage to enter the stable range, the steady-state voltage deviation, and the average voltage offset in voltage fluctuation. The genetic algorithm optimizes the proportional-integral (PI) controller parameters of DFIG and SVG reactive reference values. DFIG and SVG have better response effects to voltage fluctuation than before optimization. Finally, the grid model of DFIG is built, and the simulation results verify the effectiveness and correctness of the proposed method. The cooperative control strategy of SVG and DFIG can adjust the voltage quickly and avoid the long-term reactive power flow between them.

Advanced control strategy of DFIG during symmetrical grid fault

<p>This article proposes a novel scheme to improve the doubly-fed induction generator (DFIG) behavior during grid fault. The DFIG’s are sensitives to voltage variations when abrupt variations of the wind velocity arrive. For enhancing DFIG behavior, protecting the converters, and smoothing the fluctuations power output of the DFIG under sag voltage; a novel hybrid energy storage system scheme and its controller are proposed. The main advantages of our approach are a faster response and suppressing overvoltage on DC bus and globally less stress in the storage system. The control structure decreases the tiredness on the battery and restores the DC bus voltage rapidly, globally the battery system operating time increases. The results obtained by simulations in MATLAB validate the benefits of the suggested control.</p>

Reactive Power Control for Wind Turbine using Exact Linearization Feedback

The quality of service and the respect of contractual characteristics of the voltage is a major issue of the transmission system operator. In this context, the code requires that the wind turbine system participate in the regulation of the voltage. Any variation in voltage must be compensated by producing or absorbing reactive power. This article studies the modelling and control strategies of a wind energy conversion system based on a doubly fed induction generator. It presents a linear quadratic regulator with integrator (LQI) and an exact linearization feedback (ELF) controller for a doubly fed induction generator (DFIG). In the ELF technique, the nonlinear model of DFIG is linearized and the reference of active and reactive powers values are calculated by using reactive power management algorithm.The simulation results show that the decoupling control strategy for DFIG is satisfactory and the predefined operating conditions are respected.

Hybrid virtual impedance‐based control strategy for DFIG in hybrid wind farm to disperse negative sequence current during network unbalance

When a hybrid wind farm based on doubly fed induction generator (DFIG) and fixed speed induction generator (FSIG) is connected to an unbalanced network, most of the negative sequence current caused by the unbalanced grid will be centralised at the FSIG for its inherent small negative sequence output impedance (NSOI). The centralised negative sequence current can cause significant torque pulsation in the drive train of FSIG, which may go beyond the pressure endurance of the drive train and cause some damages. Meanwhile, losses and risks of overcurrent and grid code violation can also be increased. This study presents a control strategy for DFIG in hybrid wind farm during network unbalance, which can disperse the negative sequence current and the consequent pressure among the three parallel power sources without communication, i.e. among the DFIG stator, the grid side converter (GSC), and the FSIG, rather than centralising the pressure at any of them. In order to achieve this target, a novel hybrid virtual impedance method is proposed to flexibly control the NSOI of the DFIG stator and the GSC from zero to infinity. Theoretical analysis and simulation results are provided to verify the operation performance of the proposed strategy.

Improved Continuous Fault Ride Through Control Strategy of DFIG-Based Wind Turbine During Commutation Failure in the LCC-HVDC Transmission System

The commutation failure fault usually occurs in the line-commutated-converter-based high-voltage direct current transmission system. When commutation failure fault occurs, the voltage of sending alternate current (ac) system changes rapidly, and the connected doubly fed induction generator (DFIG)-based wind turbine may be tripped. Thus, the fault ride through (FRT) control strategy of DFIG should be investigated for enhancing the stability of the sending ac system. However, the voltage of the sending ac system during commutation failure is not changed in rectangular in shape, besides, the voltage presents the “first reduce then rise” characteristic, which is not considered in the existing FRT control strategies. In order to realize the continuous FRT of DFIG during commutation failure, the stator flux and electromotive force when the stator voltage changes continuously have been analyzed for the first time in this article. Furthermore, based on the analysis results, an improved continuous FRT control strategy is proposed. The simulation and experiment results validate the effectiveness of the proposed method. The proposed control strategy is not only suitable for the commutation failure condition, but also for the scenario with continuous voltage variation during grid fault, which indicates that the proposed method is general.

DFIG Model Suitable for SSR Research

The simulation model of doubly fed induction generator (DFIG) is the basis of the research on subsynchronous resonance (SSR) of DFIG. In this paper, the conditions that the model of DFIG is suitable for the research of subsynchronous oscillation (SSO) are analyzed firstly. Then, the drive system, doubly fed induction motor, AC / DC / AC power converter and control strategy of DFIG are modeled, and these models are unified as electromechanical model. At last, the validity of the model is verified by the example of SSR when the DFIG is sent out through series compensated transmission line.

Fast Frequency Support From Wind Turbine Generators With Auxiliary Dynamic Demand Control

Wind penetration is expected to reach substantially higher levels of penetration in the near future. A variable speed wind turbine generator, such as, the doubly-fed induction generator (DFIG) or the permanent magnet synchronous generator, does not naturally provide rotating inertia due to the converter interface, which leads to a reduction of the overall system inertial response. The replacement of the conventional generators which have governors also reduces primary frequency control capability. Multiple control strategies have been proposed to enable a DFIG to provide emulated inertial response through reducing rotor speed that can release some kinetic energy. However, this has no effect on primary settling frequency and may even bring a secondary frequency dip to the system due to rotor speed recovery. Moreover, the rotor speed is not guaranteed to stay in an allowable range during the reduction period. This paper proposes a new control strategy for DFIG to provide adequate inertial response with guaranteed rotor security. To mitigate the potential of the secondary frequency dip and improve the performance of primary frequency control, a dynamic demand control strategy is proposed to coordinate with DFIG control. The coordination strategy can provide fast frequency support from the time-scale of inertial response through primary frequency control. The proposed control can be realized in both small-scale and large-scale power systems.

Analysis of Doubly Fed Induction Generators Participating in Continuous Frequency Regulation with Different Wind Speeds Considering Regulation Power Constraints

Wind turbines (WTs) participate in frequency regulation, which is one of the means to solve the problem of inadequate regulation capacity in power systems with a high proportion of renewable energy. The doubly fed induction generator (DFIG) can reserve part of power to achieve bidirectional regulation capability through rotor over-speed and increasing pitch angle. In this paper, it is pointed out that the available bidirectional regulation power of the WT is constrained by the maximum regulation power under the rotor speed regulation. The regulation power constraints under the pitch regulation considering the time scale are calculated. The adjustment coefficient of WT participating in frequency regulation is designed. Considering the regulation power constraints, the frequency difference interval in which the WT can provide the regulation power according to the adjustment coefficient is analyzed. The rotor speed and pitch coordinated control strategy of DFIG with different wind speeds is designed. Based on 24-hour measured data from a wind farm, the power constraints and their effects of WTs in the wind farm participating in frequency regulation are verified by simulation. The regulation power of the wind farm, frequency quality, and wind power utilization under the different control strategies are analyzed. The results show that the effects of bidirectional power constraints must be taken into account when evaluating the effectiveness of WTs in continuous frequency regulation.

Power oscillation coordination damping control strategy of DFIG for fault ride through after clearing the fault

Wind turbines should have fault ride through capability, and also it can provide damping for the power grid by improving the control strategy. However, the currently corresponding control strategy is designed independent for the fault voltage through control and damping control. If the fault ride through control and damping control of a converter is uncoordinated under the grid fault, the output of the two control branches can be superimposed, which will affect the output power dynamic performance of doubly fed induction generator (DFIG) after clearing the fault. At the same time the overcurrent and other issues may occur in serious cases, and even lead to DFIG trip off. Based on the dynamic process analysis of DFIG after clearing the fault, an additional damping controller in the rotor side converter control of DFIG is designed, which damps the system low-frequency oscillation by modulating the rotor active current component. A coordination damping control strategy of DFIG based on a fault ride through control and damping control is proposed for damping the power oscillation after clearing the fault, the correctness of the proposed control method has been verified through time-domain simulation.

SSR Analysis of DFIG-Based Wind Farm With VSM Control Strategy

The wind turbine is usually integrated into the power grid via back to back converter, and the inertia is decreased. Virtual synchronous machine (VSM) control was proposed to improve inertia of the wind farm. While for the doubly-fed induction generator (DFIG)–based wind farm with traditional control strategy, sub-synchronous resonance (SSR) has become a significant problem when it is connected to series compensation transmission lines. Therefore, the impacts of VSM control strategy for DFIG on SSR need to be investigated. In this paper, a detailed small-signal stability analysis model of the VSM control strategy is first established and eigenvalue analysis is then carried out. SSR in the wind farm with the VSM control under different series compensation levels, controller parameters and wind speeds is analyzed, and it is also compared to that in the wind farm with the conventional vector control strategy (VC). Simulations are performed in PSCAD/EMTDC to verify the result of SSR analysis.

Virtual inertia control strategy of doubly-fed induction generator with additional inertia and damping torque

Doubly-fed induction generator (DFIG) with dual Pulse-Width Modulation (PWM) converter cannot maintain the active power balance or voltage stability of the power grid self-synchronously because of lacking mechanical inertia and mechanical damping. The virtual synchronous generator (VSG) technology can help solve this problem through simulating the mechanical equation and electromagnetic equation of synchronous generator to control the grid-connected converter. However, under the common first-order virtual inertia link used in the VSG control scheme, while DFIG providing active power support, there exists second frequency dip and the rotor speed recovers slowly, which is not conducive for DFIG to provide continuous and effective virtual inertia support. In order to overcome the drawbacks as well as avoid second frequency dip, this paper puts forward a virtual inertia control strategy of DFIG with additional inertia and damping torque. The additional torque is achieved by proportional-derivative (PD) controller with rotor slip as an input signal. Simulation results on DIgSILENT/powerfactory platform show that the proposed control strategy is effective to reduce frequency deviation and recover the rotor speed faster without second frequency dip phenomenon.

Control strategy of DFIG in hybrid micro‐grid using sliding mode frequency controller and observer

A doubly fed-induction generator (DFIG) has a significant advantage of fast dynamic frequency regulation. However, conventional virtual inertia control (VIC) design only takes frequency into account and may face secondary frequency drop during rotor speed restoration. To solve these problems, the novel decentralised control strategy is proposed for a hybrid micro-grid. Firstly, the load disturbance is considered to enhance control system accuracy, which is estimated by using the state observer based on the dynamic mathematical model. Then, the sliding mode VIC controller is designed by taking advantage of the observed disturbance value and frequency deviation for DFIG output dynamic adjustment according to the conventional frequency control (VIC) model. Furthermore, during DFIG rotor speed restoration, pitch angle control based on the mathematical model is applied for reserve capacity, and a new triggering condition according to the conventional condition is designed, so as to release reserve power at an appropriate time. Finally, the proposed strategy is tested in a typically isolated micro-grid with diesel and DFIG through a simulation and experiment.

A Sliding-Mode Direct Power Control Strategy for DFIG Under Both Balanced and Unbalanced Grid Conditions Using Extended Active Power

This paper proposes a sliding-mode direct power control (SMDPC) strategy for doubly fed induction generator (DFIG) under both balanced and unbalanced grid conditions using extended active power. When the traditional power theory is used under unbalanced grid condition, the control strategies usually need to be modified and become more complicated. Therefore, an extended active power is proposed in this paper, which is effective under both balanced and unbalanced grid conditions with a simple control strategy. Based on the extended active power, elaborated analysis of the mathematical model of DFIG is obtained. Furthermore, an SMDPC strategy using the extended active power is proposed, which can obtain sinusoidal stator currents and restrain electromagnetic torque ripples under unbalanced grid condition without the need of decomposition process and phase-locked loop (PLL). Comparative experimental studies of the SMDPC using the extended and traditional active powers for DFIG are conducted to validate the effectiveness of the proposed strategy under both balanced and unbalanced grid conditions. In addition, the dynamic performance and robustness of the proposed SMDPC are also proved to be satisfying by the experimental results.

DVR based reactive power control strategy of DFIG for wind energy generation during unbalanced grid voltages

This document offers a whole new one on direct power control (DPC) technique for a doubly fed induction generator (DFIG)-based the wind vitality era program. This tactic relies on the one on one supervision regarding system productive and reactive strength by simply deciding on applicable voltage vectors on the rotor feature. This strategy works by using the mechanical system flux that take away the complications linked to rotor flux. This guidelines with this strategy rectangular calculate delineated intimately with this document. The one unit parameter needed by the projected DPC technique is usually which the mechanical system resistance as their effect on the system effectiveness is located to get minimal. Simulation final results on the 1.5 MW DFIG program exhibits the effectiveness and robustness in the projected supervision tactic in the course of versions regarding unit details. DVR is usually given to management the facility excellent issues and thereby management the active and reactive power.

Resonant-Based Backstepping Direct Power Control Strategy for DFIG Under Both Balanced and Unbalanced Grid Conditions

This paper proposes a resonant-based backstepping direct power control (BS-DPC) strategy for doubly-fed induction generator (DFIG) under both balanced and unbalanced grid conditions. Proper formulae for the BS-DPC strategy are obtained based on the elaborated analysis of the mathematical model of DFIG. The influence of the unbalanced grid voltage on the normal BS-DPC is analyzed. Furthermore, a resonant-based improved strategy is proposed to achieve different control targets under unbalanced grid condition without the need of decomposition of positive- and negative-sequence components. Comparative experimental studies of the resonant-based BS-DPC and the normal BS-DPC for DFIG are conducted to validate the effectiveness of the proposed strategy under both balanced and unbalanced grid conditions. In addition, the experimental results also prove that the proposed resonant-based BS-DPC can achieve a satisfying transient performance.

An input–output linearization algorithm‐based inter‐area damping control strategy for DFIG

A novel input–output linearization algorithm‐based inter‐area oscillation damping control strategy for the doubly fed induction generator (DFIG) is proposed in this paper. The dynamic model of the study system, a classical two‐area power system for inter‐area oscillation study integrated with a DFIG‐based wind farm, is modeled first. Then, the suitable input and output variables are chosen according to the control objective, which are the variables need to be linearized. The feedback control law, which can linearize the selected system input to system output, is obtained via the Lie derivative of the output function and state function according to the input–output linearization algorithm. Therefore, the control parameters can be designed to utilize the linear system control algorithm to improve the system dynamic response. The proposed control strategy can deal with the nonlinearity existing in the damping control loop, and thus the performance of the damping control can be improved. The simulation carried on the MATLAB/Simulink platform was employed to investigate the performance of the proposed damping control strategy for DFIG. Simulation results are compared with those of a system with no additional damping controller and with a conditional damping controller. It shows that proposed method can improve the system damping obviously. Compared to the conventional power system stabilizer (PSS) control, the proposed damping controller can damp the swing in power more quickly. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

A flexible power control strategy for rotor-side converter of DFIG under unbalanced grid voltage sags

Doubly fed induction generator (DFIG) is widely applied in variable-speed wind energy conversion system. The disconnection of a substantial amount of DFIG may arouse the instability problem of power system, thus wind power generators have to remain connected during short-time grid faults. As a result, the voltage sag will lead to overcurrent and overvoltage in the rotor winding of a DFIG, moreover the unbalanced voltage sags will also cause serious fluctuations in its electromagnetic torque and output power. This paper studies the relationship of the stator instantaneous powers with the three-phase stator voltage and rotor current of a DFIG under unbalanced grid voltages. A generalized formula of current reference for the rotor-side converter of DFIG is constructed by introducing continuous adjustment coefficients. Meanwhile, the analytical equations of rotor peak current, stator active and reactive power fluctuations are derived to characterize the operating performance of DFIG. The impacts of adjustment coefficients on DFIG control performance and the feasible region of coefficients restrained by the rotor current constraint are discussed. In consideration of the rotor current limit, the flexible power control strategy for DFIG in unity power factor mode (UPFM) and reactive power supporting mode (RPSM) is presented. The correctness of proposed method is verified by simulation and experiment tests of single-DFIG and multi-DFIG systems.

Adaptation virtual inertia control strategy of DFIG and assessment of equivalent virtual inertia time constant of connected power system

As the mechanical parts of doubly fed induction generator (DFIG) are decoupled from electrical part, the total equivalent rotary inertia of power system are decreased with growing continuously of wind power penetration, which will bring pressure to frequency stability of system. On the basis of analysing operation and control characteristics of DFIG, the equivalent virtual inertia time constant of DFIG, wind farms based on DFIG, and connected power system was calculated. A virtual inertia control strategy of DFIG using rotor current direct control based on equivalent virtual inertia time constant was proposed. The proposed virtual inertia control strategy of DFIG includes traditional control function module, status assessment module, and additional virtual inertia control module. The time domain simulation was carried out on the basis of theoretical analysis, the simulation results show that the proposed virtual inertia control strategy of DFIG can provide efficacious frequency support to the power grid at all kinds of operating conditions, and the frequency stability of power system with large-scale wind power was improved.

Reactive power control strategy of DFIG‐based wind farm to mitigate SSO

Wind farms connected with series compensated transmission network suffer from the risk of sub-synchronous oscillation (SSO). Moreover, the strong active and reactive power control ability of doubly fed induction generator (DFIG) exerts non-negligible influence on SSO. In this study, the impacts of adjustable parameters on SSO are studied based on eigenvalue analysis. The results indicate that increasing inductive reactive power produced by DFIG can provide positive damping to the system; furthermore, the universality of this rule is verified in the whole operating region of DFIG. According to the rule, a reactive power control strategy of DFIG is proposed for SSO suppression. The reactive power is suggested to increase gradually once the SSO is detected and stop until SSO is suppressed or the reactive power limit is reached. The proposed control method is a low-cost method, which only need to adjust the reactive power reference value of DFIG to damp SSO. Finally, the time-domain simulation results illustrate the validity and adaptability of the proposed SSO reactive power control strategy.