Control Of Doubly Fed Induction Generator Research Articles

High-order sliding mode control of DFIG under unbalanced grid voltage conditions

This paper proposes a novel high-order sliding mode (HOSM) based control methodology for direct power control (DPC) of doubly-fed induction generator (DFIG) wind turbine which is operating under unbalanced grid voltage conditions. Firstly, a set power quality enhancement strategies are presented, incorporating six power compensation schemes corresponding to six selective control targets in both steady and variable speed conditions. Meanwhile, the DFIG output powers are regulated with the proposed HOSM-DPC scheme, wherein a super-twisting algorithm is employed to cope with the chattering phenomenon and simplify the controller structure. The Lyapunov function method is utilized to decide the stability region of the controller parameters. Experiments and simulations are conducted to prove the validity and the performance of HOSM-DPC, where the DPC method with lookup table and the first order sliding mode based DPC are incorporated for comparison purpose. The experimental tests fully reveal the effectiveness of HOSM-DPC in power quality improvement subjected to unbalanced grid voltage conditions, and the superior performance of HOSM-DPC in dynamic response, chattering phenomenon elimination and variable wind speed operation are also validated.

Sensor fault-tolerant control of DFIG based wind energy conversion systems

This paper proposes a fault-tolerant control (FTC) strategy for a doubly-fed induction generator (DFIG) based wind energy conversion system (WECS) with sensor faults. The DFIG system is decomposed into stator, rotor, grid current models, and the sensor fault observer (SFO) is presented to provide accurate estimations of sensor fault components in the presence of model uncertainties. Based on the independent current dynamics of DFIG system, three SFOs are designed in parallel to respectively estimate the stator, rotor, and grid current senor fault signals. During the occurrence of faults, the fault estimations derived from current observers are adopted to compensate the faulty signals, thereby achieving the system reconfiguration of DFIG without requiring any fault diagnosis schemes. Simulation studies are undertaken on a grid-connected WECS under the conditions of stepped and random wind speeds, and single and multiple current sensor faults. Simulation and experimental results reveal that the FTC-controlled DFIG system is resilient from the sensor faults and maintains continuous stable operation.

Functional orientation analysis of primary frequency regulation and inertia control of DFIG

With the penetration of wind power increasing continuously, the inertia and the capability of primary frequency regulation of grid are declining. To deal with this problem effectively, additional primary frequency regulation control and inertia control adopted in wind turbines are necessary. However, at present, the functional orientation of primary frequency regulation control and inertia control is still unclear. It is urgent to clarify the difference between the two kinds of control methods and to make clear the demand of power grid for them. In this paper, based on analysis of main methods of primary frequency regulation and inertia control of doubly-fed induction generators (DFIG), the functional orientation distinction between inertia support function and primary frequency regulation function is studied from perspective of control principles and control results. The different effectiveness of different control functions on the frequency regulation process in different scenarios are analyzed by simulations.

Robust subsynchronous interaction damping controller for DFIG-based wind farms

This paper proposes a robust controller to improve power system stability and mitigate subsynchronous interaction (SSI) between doubly-fed induction generator (DFIG)-based wind farms and series compensated transmission lines. A robust stability analysis is first carried out to show the impact of uncertainties on the SSI phenomenon. The uncertainties are mainly due to the changes in the power system impedance (e.g., transmission line outages) and the variations of wind farm operating conditions. Then, using the µ-synthesis technique, a robust SSI damping controller is designed and augmented to the DFIG control system to effectively damp the SSI oscillations. The output signals of the supplementary controller are dynamically limited to avoid saturating the converters and to provide DFIG with the desired fault-ride-through (FRT) operation during power system faults. The proposed controller is designed for a realistic test system with multiple series capacitor compensated lines. The frequency of the unstable SSI mode varies over a wide range due to the changes in power system topologies and wind farm operating conditions. The performance of the proposed controller is verified through electromagnetic transient (EMT) simulations using a detailed wind farm model. Simulation results also confirm the grid compliant operation of the DFIG.

Research on DFIG-ES System to Enhance the Fast-Frequency Response Capability of Wind Farms

With the increasing penetration of wind power generation, the frequency regulation burden on conventional synchronous generators has become heavier, as the rotor speed of doubly-fed induction generator (DFIG) is decoupled with the system frequency. As the frequency regulation capability of wind farms is an urgent appeal, the inertia control of DFIG has been studied by many researchers and the energy storage (ES) system has been installed in wind farms to respond to frequency deviation with doubly-fed induction generators (DFIGs). In view of the high allocation and maintenance cost of the ES system, the capacity allocation scheme of the ES system—especially for fast-frequency response—is proposed in this paper. The capacity allocation principle was to make the wind farm possess the same potential inertial energy as that of synchronous generators set with equal rated power. After the capacity of the ES system was defined, the coordinated control strategy of the DFIG-ES system with consideration of wind speed was proposed in order to improve the frequency nadir during fast-frequency response. The overall power reference of the DFIG-ES system was calculated on the basis of the frequency response characteristic of synchronous generators. In particular, once the power reference of DFIG was determined, a novel virtual inertia control method of DFIG was put forward to release rotational kinetic energy and produce power surge by means of continuously modifying the proportional coefficient of maximum power point tracking (MPPT) control. During the deceleration period, the power reference smoothly decreased with the rotor speed until it reached the MPPT curve, wherein the rotor speed could rapidly recover by virtue of wind power so that the secondary frequency drop could be avoided. Afterwards, a fuzzy logic controller (FLC) was designed to distribute output power between the DFIG and ES system according to the rotor speed of DFIG and S o C of ES; thus the scheme enabled the DFIG-ES system to respond to frequency deviation in most cases while preventing the secondary frequency drop and prolonging the service life of the DFIG-ES system. Finally, the test results, which were based on the simulation system on MATLAB/Simulink software, verified the effectiveness of the proposed control strategy by comparison with other control methods and verified the rationality of the designed fuzzy logic controller and proposed capacity allocation scheme of the ES system.

Assessing the Impact of DFIG Synthetic Inertia Provision on Power System Small-Signal Stability

Synthetic inertia provision through the control of doubly-fed induction generator (DFIG) wind turbines is an effective means of providing frequency support to the wider electrical network. There are numerous control topologies to achieve this, many of which work by making modifications to the DFIG power controller and introducing additional loops to relate active power to electrical frequency. How these many controller designs compare to one-another in terms of their contribution to frequency response is a much studied topic, but perhaps less studied is their effect on the small-signal stability of the system. The concept of small-signal stability in the context of a power system is the ability to maintain synchronism when subjected to small disturbances, such as those associated with a change in load or a loss of generation. Amendments made to the control system of a large-scale wind farm will inevitably have an effect on the system as a whole, and by making a DFIG wind turbine behave more like a synchronous generator, which synthetic inertia provision does, may incur consequences relating to electromechanical oscillations between generating units. This work compares the implications of two prominent synthetic inertia controllers of varying complexity and their effect on small-signal stability. Eigenvalue analysis is conducted to highlight the key information relating to electromechanical modes between generators for the two control strategies, with a focus on how these affect the damping ratios. It is shown that as the synthetic inertia controller becomes both more complex and more effective, the damping ratio of the electromechanical modes is reduced, signifying a decreased system stability.

Nonlinear SSR Damping Controller for DFIG Based Wind Generators Interfaced to Series Compensated Transmission Systems

This paper details a nonlinear Sliding Mode Control (SMC) based Sub-Synchronous Resonance (SSR) mitigation method for Doubly Fed Induction Generator (DFIG) based wind turbines interconnected to series compensated transmission systems. The proposed method is used to control the rotor side converter to mitigate SSR while ensuring decoupled torque and reactive power control ability of the DFIG is maintained. The proposed method is implemented on the detailed DFIG wind turbine model provided in the RTDS platform for validation via simulations. Results obtained indicate the controller is successful at damping SSR when tested under varying compensation levels and wind speeds.

Fuzzy sliding mode control of doubly-fed induction generator driven by wind turbine

<span lang="EN-US">This paper present a hybrid nonlinear control based on fuzzy sliding mode to control wind energy conversion system using a doubly fed induction generator (DFIG). Consiting of coupling fuzzy logic control and sliding mode control this technique is introduced to avoid the major disadvantage of variable structure systems, namely the chattering phenomenon. Effectiveness and feasibility of the proposed control strategy are verified by simulation results in Matlab Simulink.</span>

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.

Simulation and Analysis of DFIG System with Wind Turbine Implementing Fuzzy Logic Control

A doubly-fed induction generator (DFIG) applied to wind power generation driven by wind turbine is under study for low voltage ride-through application during system unbalance. Use of DFIG in wind turbine is widely spreading due to its control over DC voltage and active and reactive power. Conventional dq axis current control using voltage source converters for both the grid side and the rotor side of the DFIG are analyzed and simulated. An improved control and operation of DFIG system under unbalanced grid voltage conditions by coordinating the control of both the rotor side converter (RSC) and the grid side converter (GSC) is done in this thesis. Simulation and analysis of DFIG system with wind turbine using Fuzzy logic controller for RSC and GSC under unbalanced condition is presented in the positive synchronous reference frame. The common DC-link voltage is controlled by grid side converter and control of DFIG’s stator output active and reactive power is controlled by rotor side converter. The steady-state operation of the DFIG and its dynamic response to voltage sag resulting from a remote fault on the 120-kV system is shown in this thesis using controllers. Modeling of DFIG system under Fuzzy logic controller to control voltage and active-reactive powers is done using MATLAB/SIMULINK.

An innovative optimal RPO-FOSMC based on multi-objective grasshopper optimization algorithm for DFIG-based wind turbine to augment MPPT and FRT capabilities

Abstract Doubly Fed Induction Generator (DFIG) with consideration of its exceptional capabilities, i.e.: variable speed operation, low mechanical stresses, and excellent power quality in limited-range speed applications is a famous kind of wind turbines. Even so, there is a challenge due to its nonlinear dynamic features and several uncertainties like unknown non-linear disturbances and parameter uncertainties. Hence, this paper proposes a novel Robust Perturbation Observer based Fractional Order Sliding Mode Controller (RPO-FOSMC) for DFIG to extract the maximum power and improve the Fault Ride-Through (FRT) capability. The strong nonlinear aerodynamics of wind turbine, the uncertain dynamic parameters of induction generator and the stochastic characteristics of wind waves are constructed in perturbation term which is estimated using the proposed RPO-FOSMC. Accordingly, the perturbation compensator provides an appropriate robustness concerning different uncertain models and attains an exceptional control capability during stochastic wind waves. Considering the inherent multi-objective nature of the nonlinear control design problem, Multi-Objective Grasshopper Optimization Algorithm (MOGOA) has been implemented to augment the robustness and dynamic performance of RPO-FOSMC. Three distinct conditions are considered to compare and analyse the fast and robust dynamic performance of optimal RPO-FOSMC against other conventional approaches. Eventually, the comprehensive simulation results have revealed and validated the exceptional dynamic capability of the suggested control strategy.

Design and implementation of interval type-2 fuzzy logic-PI based adaptive controller for DFIG based wind energy system

In this paper, an advanced interval type-2 fuzzy logic-proportional integral (PI) controller has been proposed for torque and voltage control loops of rotor side converter of doubly fed induction generator (DFIG). The gains of PI controller are determined and tuned by interval type-2 fuzzy logic method according to system operating condition. Thus, the adaptive nature of type-2 fuzzy logic and robust nature of PI controller are combined eventually, which exhibits good steady state and dynamic responses. The performance of the proposed controller has been evaluated for different operating conditions of DFIG such as severe fault and voltage sag with reference to varying wind speed. Real time simulations are developed for DFIG based wind energy system (WES) to validate the proposed controller using the OPAL-RT digital simulator. The performance of the proposed controller is examined through a comparative analysis with its traditional fuzzy logic-PI counterpart. The transient analysis of DFIG with interval type-2 fuzzy-PI shows the improved results subjected to three-phase fault and voltage sag as desired by the grid codes.

Inter area Stability Enhancement of Interconnected power system using Second-order Sliding Mode Controller

The presence of DFIG (Doubly Fed Induction Generator) affecting the stability of the inter-area interconnected power system due to the lagging response of DFIG under undesirable conditions like faults. This paper addresses a Second-order Sliding Mode controller for DFIG to damp inter-area frequency oscillations of the interconnected power system. The proposed heuristic controller is a sliding mode controller of second-order type and works based on the reactive power capability of DFIG. This novel controller is not affected by system modeling uncertainties and unwanted parameter variations. The proposed controller is designed and implemented on 3-bus interconnected system in MATLAB/Simulink environment and the results obtained compared with the conventional controller under different fault conditions to show the effectiveness. Results demonstrated that the sliding mode controller effectively damping inter-area oscillations compared with a conventional controller.

Adaptive third order terminal sliding mode power control of DFIG based wind farm for power system stabilisation

A Doubly Fed Induction Generator (DFIG) based wind farm (DFIGWF) has been proposed in this paper for improving the stability of wind farm with multi machine power system by implementing of an adaptive third order terminal sliding mode power controller for rotor side converter. In comparison to the conventional one our proposed strategy significantly improves the active power output by DFIGWF at after fault state. Subsequently electrical power output is amended by generators, and this improves the stability of system. The overall improvement in the relative oscillations of the generators present in different areas of a standard IEEE 6 machine 30 bus power system is analyzed by comparing the critical clearing time of the proposed control strategies with conventional one.

Adaptive direct power control for double fed induction generator used in wind turbine

This papers deal with a new Adaptive Direct Power Control for Doubly-Fed Induction Generator of 1.5 MW. The main feature of the proposed strategy is based on the replacement of the fixed switching table by an adaptive one. The online update of the adaptive switching table depends on the reactive power variation and past switching sequences. The proposed adaptive direct power control is compared with Vector Control and Classical Direct Power Control. The robustness of the proposed control scheme against parameter, load and wind speed variations have done with success. The main performance of the Adaptive Direct Power Control strategy is the reduction of powers ripples, thus reduce of torque ripple on the shaft of the turbine.

A Novel Position-Sensorless Algorithm for Field-Oriented Control of DFIG With Reduced Current Sensors

Position-sensorless field-oriented control of doubly fed induction generator (DFIG) necessitates at least 4–6 current sensors. In this paper, a novel rotor position and current estimation algorithms are proposed, which make the control of DFIG feasible using only three current sensors. The elimination of one current sensor improves hardware reliability by reducing the redundancy of physical components to be used, thereby reducing the complexity and also the system cost. The proposed algorithms employ two rotor current sensors and one stator current sensor. The current estimation algorithm extracts the components of $\vec{{i}_{s}}$ in ${{d}^{s}}{{q}^{s}}$ frame using the sensed value of one stator current and the reference values of $\vec{{i}_{r}}$ components in ${{d}^{e}}{{q}^{e}}$ frame. The proposed algorithms reduce mathematical burden and dependency on machine parameters. The effectiveness of proposed algorithm is validated using the real-time control hardware-in-loop, which comprises OPAL-RT and STM32F407VG microcontroller with ARM Cortex-M4 32 bit MCU. Simulation and experimental results on 2 MW DFIG-based WECS are presented under steady-state and dynamic wind speed conditions. The performance of proposed algorithm is found to be satisfactory under different operating conditions.

Modifying the control design of DFIG‐based wind turbines in order to inhibit power system oscillations

Providing additional damping for system electromechanical oscillations through doubly fed induction generator (DFIG)-based wind turbines is investigated in this study. In this regard, first an objective function is developed to tune the DFIG control gains, aim to maximise the wind farm (WF) contribution to system oscillation damping, but it is shown that it causes a decrease in the damping of DFIG stator mode. In other words, with the current DFIG control design, it is not possible to improve both the small signal stability of the power system and the dynamic stability of WF together. Afterwards, two high-pass filters are suggested to be employed in the control design of DFIG. It is shown that with the modified control design, it is possible to improve the system oscillation without spoiling the dynamic stability of the WF itself. Eigenvalue analysis is performed to validate the effectiveness of the modified control design.

Pole Placement Control for Doubly-Fed Induction Generators Using Compact Representations in Complex Variables

The paper presents three algorithms to synthesize a dynamic controller for a doubly-fed induction generator (DFIG). In each case, the pole placement approach is used to compute the parameters of a feedback controller that regulates the active and reactive powers produced using the rotor voltages. Due to the special structure of the DFIG's model, the analysis and design of the feedback control loop are simplified by transforming its representation into an equivalent system with complex coefficients and half the number of states. This approach yields analytic solutions of the problem with remarkable simplicity. The complex framework facilitates the direct placement of the DFIG system poles in the left half-plane, which ensures stability and performance of the closed-loop system. Additionally, this framework can be used to evaluate the robustness properties of the closed-loop systems. The ability of the synthesized controllers to provide a desirable dynamic and steady-state response is investigated through experiments on a laboratory testbed.

Improved Rotor Braking Protection Circuit and Self-Adaptive Control for DFIG during Grid Fault

This paper introduces an improved rotor braking protection circuit configuration and the corresponding self-adaptive control strategy to enhance the low voltage ride-through (LVRT) capability of the doubly-fed induction generator (DFIG). The proposed protection circuit consists of a crowbar circuit and a series rotor braking resistor array, which guarantees the safe operation of wind generators under the LVRT. Moreover, to adapt the proposed protection and further enhance the performance of the improved configuration, a corresponding self-adaptive control strategy is presented, which regulates the rotor braking resistor and protection exiting time automatically through calculating the rotor current in the fault period. The LVRT capability and transient performance of the DFIG by using the proposed method is tested with simulation. Compared with the conventional crowbar protection or the fixed rotor braking protection, the proposed protection and the control strategy present several advantages, such as retaining the control of the rotor side converter, avoiding repeated operation of the protection and accelerating the damping of stator flux linkage during a grid fault.

Tuning a model predictive controller for doubly fed induction generator employing a constrained genetic algorithm

This study presents a model predictive control (MPC) for a doubly fed induction generator (DFIG) power control using a state-space prediction model. Genetic algorithms (GAs) have demonstrated their potential in finding good solutions for complex problems. However, GA in its original form lacks a mechanism for handling constraints. In this way, a method for tuning the MPC based on a novel constrained GA is proposed. In this way, the method permits a good solution for the weighing matrices with predetermined maximum requirements, such as maximum overshoot, just using the DFIG control simulation. Finally, experimental results are presented to endorse the proposed theory.