Doubly Fed Induction Generator Wind Turbine Research Articles

Modelling and dynamic state estimation of a doubly fed induction generator wind turbine

PurposeThis paper aims to propose an 18th-order nonlinear model for doubly fed induction generator (DFIG) wind turbines. Based on the proposed model, which is more complete than the models previously developed, an extended Kalman filter (EKF) is used to estimate the DFIG state variables.Design/methodology/approachState estimation is a popular approach in power system control and monitoring because of minimizing measurement noise level and obtaining non-measured state variables. To estimate all state variables of DFIG wind turbine, it is necessary to develop a model that considers all state variables. So, an 18th-order nonlinear model is proposed for DFIG wind turbines. EKF is used to estimate the DFIG state variables based on the proposed model.FindingsAn 18th-order nonlinear model is proposed for DFIG wind turbines. Furthermore, based on the proposed model, its state variables are estimated. Simulation studies are done in four cases to verify the ability of the proposed model in the estimation of state variables under noisy, wind speed variation and fault condition. The results demonstrate priority of the proposed model in the estimation of DFIG state variables.Originality/valueEvaluating DFIG model to estimate its state variables precisely.

Estimation based enhanced maximum energy extraction scheme for DFIG-wind turbine systems

In this paper, an enhanced control strategy is presented for the doubly fed induction generator (DFIG) based variable speed wind turbine system, to maximize the energy extraction from the variable wind. The idea behind the additional energy capture is to enhance the maximum power point tracking (MPPT) by modifying the reference electrical torque. The proposed reference torque is derived from some unobservable states and wind speed which have been estimated using the unscented Kalman filter (UKF). Thus, the realization of the proposed control strategy requires no additional sensors. The design of the parameters of the modified reference torque has been made using the modal analysis, amount of additional energy extracted and transients in the active power output. The differential–algebraic-equation (DAE) model of the DFIG wind turbine system with all its conventional controls along with the proposed control approach has been modeled first. Subsequently, its equivalent third-order model is described which further has been used for the estimation using UKF. The proposed approach of control acts during the transient time only and the modification of the reference torque with estimation does not require any alteration in the existing control system. Thus, the proposed control can be accepted for the real-life implementation with not much additional cost. The efficacy of the suggested control strategy has been tested on various wind conditions including turbulent wind obtained from the TurbSim tool by NREL.

Wind turbine doubly-fed induction generator defects diagnosis using rotor currents lissajous curves

Nowadays, inverters are power electronics devices used in the renewable energy conversion systems. Therefore, the continuity of service of power converter has been a major concern. In this paper, a technique for diagnosing multiple open switch faults in three phase voltage inverters feeding Doubly Fed Induction Generator DFIG is presented. It is based on the so-called the Lissajous curves of DFIG rotor currents. For this purpose, the modeling of a non-defected wind conversion system based on mathematic model created in Matlab Simulink is firstly presented, after that, an indirect stator field vector-oriented control is applied to obtain the wind system performance. Finally, rotor currents Lissajous curves are analyzed in case of a non-defected and defected inverter. Simulation results show the effectiveness of the proposed method

Output impedance modeling and grid-connected stability study of virtual synchronous control-based doubly-fed induction generator wind turbines in weak grids

Virtual synchronous control (VSynC) technology that mimics the behavior of a conventional synchronous generator can be applied in wind turbines (WTs), where it can effectively improve equivalent inertia, damping coefficient, and wind power permeability. Given the unique partial-scale converter structure of a doubly-fed induction generator (DFIG) and its complicated DFIG-grid coupling effect, the grid-connected VSynC-based DFIG WTs have gained little attention. Moreover, under a weak grid condition, some unstable modes of VSynC can interact with the wind power system, which impacts the DFIG’s dynamic characteristics and system stability. To bridge the gap, this study proposes an output impedance model for a VSynC-based DFIG to analyze the grid-connected stability in weak grids. First, the output impedance mathematical model for a VSynC-based DFIG is derived and explained. On the basis of this novel model, the low-frequency characteristics and vital unstable factors of a VSynC-based DFIG are comprehensively investigated. The impact of different VSynC parameters and grid strengths are extensively studied. Finally, the proposed output impedance model is validated by direct current disturbing theory, frequency scanning method, generalized Nyquist criterion, and time-domain simulation (MATLAB/Simulink). All the obtained results are consistent with each other and prove the effectiveness and accuracy of the proposed impedance model.

An Adaptive fuzzy-PD inertial control strategy for a DFIG wind turbine for frequency support

With increasing levels of wind generation in power systems, guaranteeing continuous power and system’s safety is essential. Frequency control is critical which requires a supplementary inertial control strategy. Since wind power generation depends directly on wind conditions, this creates an immense challenge for a conventional inertial controller with parameters suitable for all power grid operations and wind speed conditions. Therefore, tuning the controller gains is absolutely critical for an integrated conventional/renewable power system. Here, a fuzzy-logic adaptive inertial controller scheme for online tuning of the proportional-derivative-type (PD) inertial controller parameters is proposed. The proposed controller adapts the control parameters of the supplementary inertial control of the doubly fed induction generator (DFIG) wind turbine so that with any disturbance such as load changes, the active power output can be controlled to mitigate the frequency deviation. Simulation results indicate that the proposed adaptive controller demonstrates a more consistent and robust response to load changes compared to a conventional controller with fixed parameters.

Integration of wind turbines in distribution systems and development of an adaptive overcurrent relay coordination scheme with considerations for wind speed forecast uncertainty

The integration of renewable energy (RE)-based distributed generation (DG) with electric power distribution systems (DSs) in association with smart grid technology has numerous benefits, however, due to their intermittent nature, the utilisation of RE-based DG may pose threats to the proper operation of conventional overcurrent (OC) protection schemes. The resulting threats, malfunctions, and non-selective actions could occur by relays within the DS, and the development of efficient protection schemes is necessary. The objective of this study is to propose and simulate an adaptive OC protection scheme in DSs in the presence of doubly-fed induction generator (DFIG) wind turbines based on wind speed forecast data with considerations for wind speed forecast uncertainty and a low-voltage ride-through requirement for wind turbines. In each time interval and possible operating mode, the output power of DFIG-based wind turbines and relay setting parameters are determined offline. The results show that the proposed protection scheme prevents malfunctions and non-selective operations of relays, and also provides faster fault isolations, as compared to the conventional OC protection scheme.

MPC-based DC-link voltage control for enhanced high-voltage ride-through of offshore DFIG wind turbine

To deal with the DC-link overvoltage during high-voltage ride-through (HVRT) of the wind turbine, a coordinated DC-link voltage control (CDVC) scheme is proposed for enhancing the HVRT performance of offshore doubly-fed induction generator (DFIG) wind turbines equipped with the supercapacitor energy storage system (SESS). This scheme is formulated as a two-stage control problem depending on the depth of grid voltage swell, which can prevent damaging the converter resulting from the insufficient reactive support in the first stage, and achieve efficient coordinated control under various magnitude and timescale of grid voltage swell. In the first stage, considering the dynamic behavior of the DFIG during HVRT, a voltage-dependent reactive current control (VRCC) scheme is designed to regulate the current reference of the wind turbine. The first stage aims to supply fast reactive current support without the DC chopper. In the second stage, once the DC-link voltage exceeds the predefined threshold, a coordinated control scheme considering the SESS and the wind turbine operation is implemented using model predictive control (MPC), aiming to regulate the excessive active power by tracking the current reference from the first stage. The proposed control scheme is validated in PSCAD/EMTDC under various operational scenarios. Case study results show that the proposed scheme can effectively mitigate the DC-link overvoltage and enhance HVRT capability under asymmetrical and symmetrical voltage swells.

Analysis of low frequency interactions of DFIG wind turbine systems in series compensated grids

Recent field events and subsequent studies have indicated that doubly-fed induction generator (DFIG) based wind farms might be susceptible to adverse interactions with series compensated systems. This paper focuses on the analysis of the interaction phenomenon. A new benchmark network is proposed to address the investigation of low frequency oscillations between wind generation and series compensated systems based on realistic system parameters. Nonlinear components and detailed representation of wind turbines are taken into account. An analytical model for a generic DFIG based wind farm is developed considering all inner and outer loops as well as its mechanical parts. Participation factor analysis is applied to the model to identify states most contributing to the critical modes and to address suitable mitigation. Sensitivity analysis is used to support redesign of control parameters. Investigations indicate that resonance can be avoided with proper control tuning. Finally, the capabilities of a screening technique for SSR risk assessment suitable for early project stages are also investigated and its results compared to the modal analysis. All conclusions are verified with detailed electromagnetic transient (EMT) simulations.

Integral sliding mode control for back‐to‐back converter of DFIG wind turbine system

In this study, an integral sliding mode control approach for controlling the power electronics converters of a doubly-fed induction generator (DFIG) wind turbine system is presented. The power electronics interface consists of back-to-back converters. The rotor side converter regulates the active and reactive powers at the DFIG stator through controlling the stator currents. The stator current dynamics, with respect to the rotor voltages, is developed from the conventional equations of the DFIG model. In this control configuration, the knowledge about the rotor currents is not required, which reduces the use of the current measurement sensors. The grid side converter ensures constant dc-link voltage while transferring the power from the DFIG rotor to the grid. The proposed control approach uses a composition of sliding mode and integral parts to improve the overall performance and robustness against parametric variations and uncertainties. A lab-scale DFIG wind turbine system is used to investigate the proposed control approach efficiency under various operating conditions. The experimental results show the effectiveness of the proposed control approach in achieving control objectives to operate the DFIG wind turbine system.

A Novel Supervised Control Strategy for Interconnected DFIG-Based Wind Turbine Systems: MiL Validations

This work presents the overall model of the doubly fed induction generator (DFIG) and proposes a new control design for the grid side converter of the concerned DFIG plant. The system under study consists of two DFIG wind turbines which are connected to the grid by a bi-directional Z-source inverter. In order to ameliorate the DC link voltage regulation of the DFIG generation plant; particularly, a new supervised control method is introduced and is directly applied to the grid side converter (GSC). In this way, initially, by tuning two control coefficients (CCs) which forms the footprint of uncertainty (FOU), a double-input interval type-2 fuzzy logic (DIT2-FL) is designed for the GSC system. The best setting of the control coefficients for the desired performance of the concerned system is recommended and a new DIT2-FL supervised online adaptive neuro fuzzy inference system (ANFIS) is then developed accordingly. By implementing the supervised controller, the dynamic behavior of the tested wind turbines under two distinct operations for low/high wind speeds is investigated. Detailed experimental outcomes have been presented to ascertain the correctness and accuracy of the suggested control methodology. Compared with the prevalent state-of-the-art techniques, the suggested controller leads to remarkable transient improvement of closed-loop DFIG system.

A Cut-Out Strategy for Wind Turbines that Ensures Low-Voltage Ride-Through Capability

To ensure the dynamic stability of the system and improve low-voltage ride-through (LVRT) capability, this study presents a cut-out strategy for doubly-fed induction generator (DFIG) wind turbines that combines reactive power output with asynchronous load reduction. This strategy reduces the risk of the large-scale disconnection of DFIGs in a wind farm by regulating the reactive power output of DFIGs in the wind farm and removing DFIGs in an asynchronous state based on the reduction ratio. Simulation results demonstrate that the implementation of this strategy can help increase the voltage at the point of common coupling and the LVRT time for a wind farm.

Modified switch type fault current limiter for low‐voltage ride‐through enhancement and reactive power support of DFIG‐WT under grid faults

The sudden voltage drop may upsurge the current level and trigger the self-protective system to disconnect the wind turbines that are detrimental for grid constancy. A novel structure of a modified switch type fault current limiter (M-STFCL) is proposed that protects the doubly-fed induction generator wind turbine (DFIG-WT) during the symmetrical and asymmetrical grid faults. The M-STFCL is cost-effective and requires little maintenance during operation. The proposed system maintains the rotor current and DC-link voltage below the maximum acceptable limits, thus, fortify the back-back converters. The M-STFCL is tested with both the sliding-mode control (SMC) and proportional–integral (PI) controller and their results are compared. From the simulation results, it is obvious that SMC has edged over the PI controller and demonstrated superior control over the critical parameters. The performance of M-STFCL is also compared with the conventional switch type fault current limiter and the analytical results clearly validate the significance of the proposed system.

An Improved Dynamic Voltage Restorer Model for Ensuring Fault Ride-Through Capability of DFIG-based Wind Turbine Systems

Renewable energy sources (RES) are being integrated to electrical grid to complement the conventional sources in meeting up with global electrical energy demand. Among the RES, Wind Energy Conversion Systems (WECS) have gained global electricity market competitiveness especially the Doubly Fed Induction Generator (DFIG)-based Wind Turbines (WTs) because of flexible regulation of active and reactive power, higher power quality, variable speed operation, four quadrant converter operation and better dynamic performance. Grid connected DFIG-based WTs are prone to disturbances due to faults in the network which made the utilization of the power generated a major concern. The grid code requirement for integrating the DFIGs to grid specified that they must remain connected and support the grid stability during grid disturbances of up to 1500milliseconds. The ability of the DFIG WT system to uphold to the grid codes requirement is termed the Fault Ride – Through (FRT). This paper presented a 1.5MW grid connected DFIG-based WT model with a Dynamic Voltage Restorer (DVR) for FRT capability enhancement. The design and simulation were performed in MATLAB/Simulink software. The test system was subjected to disturbances leading to Low Voltage Ride – Through (LVRT), Zero Voltage Ride – Through (ZVRT) and High Voltage Ride – Through (HVRT) considering three – phase balanced fault and single line to ground fault. The performance of improved model of DVR shows enhancement over conventional DVR in terms of voltage compensation and fault current mitigation.

Improving the transient performance of DFIG wind turbine using pitch angle controller low pass filter timing and network side connected damper circuitry

In wind turbine systems, pitch angle controllers are employed to reduce the aerodynamic power gained during high wind speeds. On the other hand, passive filters are usually used to mitigate disturbances in grid-connected voltage source converters (VSCs). To avoid the risk of instability in the grid-connected VSC, as a result of resonance in the capacitive and inductive components, it is necessary to consider damping. In this study, the low pass filter timing constant of the pitch angle controller of the doubly-fed induction generator (DFIG) wind turbine was varied considering different time constants. The best timing response of the low pass filter was further used to analyse different filter topologies, in addition to a new control strategy that uses two trap passive filters with shunt resistor–capacitor as an active damper, in augmenting the DFIG wind turbine during grid fault. Simulation studies in PSCAD/EMTDC were carried out to compare the performance of the proposed scheme with some other conventional filter solutions for the DFIG wind generator, during a severe bolted three-phase to ground fault. The simulation results demonstrate the improved performance and faster recovery of the wind generator variables after the fault considering the proposed filter scheme.

SSCI mitigation of grid‐connected DFIG wind turbines with fractional‐order sliding mode controller

AbstractTo mitigate subsynchronous control interaction (SSCI) in doubly fed induction generator (DFIG)‐based wind farm, this paper proposes a robust controller for rotor‐side converter (RSC) using fractional‐order sliding mode controller (FOSMC). The proposed FOSMC can improve robustness and convergence properties of the controlled system, thus achieving SSCI damping under various operating conditions. Impedance‐based analysis and time‐domain simulation are performed to check the capability of the designed FOSMC as compared with conventional sliding mode control (SMC) and subsynchronous damping control (SSDC). Simulation results demonstrate that FOSMC can mitigate SSCI within shorter time and effectively reduce the fluctuation range of system transient responses under various operating conditions of wind speeds and compensation levels. Moreover, FOSMC also improves system robustness against parameter uncertainties and external disturbances, which is important for safe operation of realistic wind farms.

Power System Stability Study on Multi Machine Systems having DFIG Based Wind Generation System

Power system stability is related to principles of rotational motion and the swing equation governing the electromechanical dynamic behavior. In the special case of two finite machines the equal area criterion of stability can be used to calculate the critical clearing angle on the power system, it is necessary to maintain synchronism, otherwise a standard of service to the consumers will not be achieved. With the increasing penetration of doubly fed induction generators (DFIGs), the impact of the DFIG on transient stability attracts great attention. Transient stability is largely dominated by generator types in the power system, and the dynamic characteristics of DFIG wind turbines are different from that of the synchronous generators in the conventional power plants. The analysis of the transient stability on DFIG integrated power systems has become a very important issue. This paper is a review of three types of stability condition. The first type of stability, steady state stability explains the maximum steady state power and the power angle diagram. There are several methods to improve system stability in which some methods are explained.

A new hybrid multilevel converter for DFIG-based wind turbines fault ride-through and transient stability enhancement

The increasing participation of wind turbines based on doubly fed induction generator (DFIG) in the power system around the world is motivating the investigation of novel strategies to enhance the reliability of DFIG and power system connection. This paper proposes the use of a new hybrid multilevel back-to-back converter on DFIG wind turbine to improve its fault ride-through capability and power system transient stability. The proposed topology is built adding single-phase full-bridge inverters in series with each phase of traditional three-phase inverter. This converter is able to generate higher voltages in rotor circuit whenever it is required, which can effectively protect power converter and at the same time contribute with grid transient performance. Simulation results, using PSCAD®, are presented for a 2 MW DFIG machine submitted to symmetrical faults in single machine—infinite bus and three-bus system. Results show that the proposal is effective as a protection and works well as an alternative to traditional solutions, limiting currents and maintaining power control during voltage dips. It is also effective in contribution with the improvement of transient stability margin by the possibility of reactive power injection.

Sub-Synchronous Resonance Damping Control Strategy for DFIG Wind Turbines

Doubly-fed induction generator (DFIG) wind turbines connected to capacitive series-compensated transmission lines are prone to exhibit oscillatory behavior. The phenomena is called sub-synchronous resonances (SSRs), as these oscillations occur at frequencies below the fundamental component. This paper first develops a modeling methodology for DFIG wind turbines, based on impedance matrices, that is applied to model a real wind farm where SSRs were reported. The stability analysis performed shows how the interaction between the grid-side converter and the rotor-side converter contribute to the instability of DFIG wind energy conversion systems connected to series compensated grids. With this model, we propose a simple sub-synchronous resonance control strategy based on an orthogonal proportional action applied to the rotor currents, and a variable gain in the PI controller adjusted as a function of the DFIG rotational speed. This control strategy depends only on the rotor currents, which are local and already measured variables in any DFIG wind turbine, and is implemented in the rotor side converter, so it does not imply an additional cost at wind farm or wind turbine level and can be applied to any DFIG wind energy conversion system (WECS). Additionally, it proves to be robust for any line impedance series compensation level, and it does not need real-time information concerning the grid at which the wind turbine is connected, or its parameters. A real case study is considered, where the sub-synchronous resonance damping strategy presented in this work is able to stabilize the system for every possible line impedance compensation level.

A Subsynchronous Oscillation Suppression Method Based on Self-Adaptive Auto Disturbance Rejection Proportional Integral Control of Voltage Source Converter Based Multi-Terminal Direct Current System with Doubly-Fed Induction Generator-Based Wind Farm Access

A subsynchronous oscillation suppression strategy based on self-adaptive auto disturbance rejection proportional integral controller is proposed for doublyfed induction generator-based wind farm integrated into grid through voltage source converter based multi-terminal direct current. In this strategy, the nonlinear PI controller is constructed by fal function to replace the traditional linear PI controller, and then the tracking differentiator is used to arrange the appropriate transition process in combination with the idea of active disturbance rejection control, and the self-adaptive auto disturbance rejection proportional integral controller is designed. By applying the controller to the inner loop of the converter on the rotor side of the doubly-fed induction generator, the adaptability of the control parameters of the inner loop to the change of operating conditions of the system can be improved, and the dynamic performance of the system can be improved. The simulation results on PSCAD/EMTDC show that, compared with SSDC, when the wind speed is 7.5 m/s, 8.5 m/s and 9.5 m/s, the convergence time can be shortened by 0.2 s, 0.1 s and 0.25 s, respectively. When the number of grid-connected doubly-fed induction generator wind turbines is 200 and 220, the convergence time is shortened by 0.1s. When the self-adaptive auto disturbance rejection proportional integral controller and the multi-channel variable-parameter additional subsynchronous damping controller work together, the convergence time under the above three wind speeds are 2.7 s, 2.7 s and 2.3 s, respectively. When the number of grid-connected doubly-fed induction generator wind turbines is 200 and 220, the convergence time is 2.65 s and 2.75 s, respectively. It can be concluded that the self-adaptive auto disturbance rejection proportional integral controller can realize the effective suppression of the subsynchronous oscillation under different operating conditions of the wind farm via the comparison with the additional subsynchronous damping control of doubly-fed induction generator. Besides, subsynchronous oscillation will converge faster and the stability of the system can be enhanced when the self-adaptive auto disturbance rejection proportional integral controller and the multi-channel variable-parameter additional subsynchronous damping controller work together.

The fuzzy logic controllers synthesis method in the vector control system of the wind turbine doubly-fed induction generator

The article is devoted to the improvement of control systems for wind turbines by developing fuzzy controllers with higher transient characteristics and low computational costs of identification in comparison with the applied PI controllers. Based on the self-organization method, a fuzzy speed controller of the doubly-fed induction generator (DFIG) of a wind turbine was synthesized, which uses a zero-order Sugeno fuzzy inference system and is made in the form of a block-oriented Wiener model. This regulator is an element of the vector control system of the transistor converter on the rotor side. The results of simulation modeling of the fuzzy controller showed that it provides a lower transition time compared to the PI controller, by 53.59% during acceleration and by 79.76% during braking, and 23.81% less error speed deviations from the reference signal. Such indicators can minimize losses while maintaining the maximum output power point of the power plant. The implementation of the developed system on wind turbines contributes to increasing the efficiency of wind farms, reducing the cost of electricity production, reducing the payback period of equipment, and the sustainable development of alternative energy in general.