Coordinated Control of a DFIG-Based Wind-Power Generation System with SGSC under Distorted Grid Voltage Conditions

In this paper, a new control method for rotor side converter (RSC) and grid side converter (GSC) of doubly fed induction generator (DFIG) is proposed. Using the main principle of vector control (VC) and direct power control (DPC) results in a new control strategy for the RSC. Instead of using DPC strategy for the GSC, a direct current control is applied. Under the unbalanced and distorted grid voltage conditions, the new control strategy injects a pure sinusoidal and balanced stator and GSC currents without any need of extracting the negative sequence and 5 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> and 7 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> voltage harmonics. Only a modified phase locked loop (PLL) to extract the positive sequence of the fundamental phase angle, is added to this situation. The accuracy of the proposed control strategy for 2 MW DFIG under normal and unbalanced and distorted voltage conditions is evaluated in MATLAB/SIMULINK environment and results are compared with DPC strategy.

This paper proposes a Co-ordination control of doubly fed induction generator (DFIG) under generalized distorted grid voltage conditions. The system behavior of DFIG during supply imbalance and voltage distortion from grid side is investigated. To improve the fault ride-through operation capability of DFIG, the rotor side converter (RSC) and grid side converter (GSC) are appropriately regulated to eliminate the torque oscillations and ensure constant active power output from overall DFIG generation system. In this simulation model co-ordination control technique is proposed to achieve simultaneous regulation of the positive / negative-sequence currents of DFIG, discrete PID controller is implemented in the positive reference frame. The Matlab simulation results demonstrate the correctness of the developed technique and the usefulness of the proposed control strategy for DFIG based wind turbine system under such adverse grid conditions.

This paper proposes and describes the controllers for doubly-fed induction generators (DFIG) with series grid side converter. It is demonstrated that with series grid side converter instead of a conventional shunt grid side converter the DFIG stator voltage can be kept balanced even under heavy unbalance grid voltages. Therefore, in this way it is avoided the undesired effects such as torque oscillations and high stator and rotor currents presents under unbalance voltages. In addition, it is demonstrated that it is possible with a conventional back-to-back three phase PWM converter to satisfy the grid code active and reactive power requirements. The design and control of the series grid-side converter with a LC-filter will also be presented. An H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">infin</sub> controller is designed to achieve robustness as well as good tracking and disturbance rejection performance for the series voltage converter. Furthermore, the DC-link, rotor currents and active and reactive power controllers are as well described, where the controllers transfer functions are selected to ensure good performance even in the presence of grid voltage unbalance. Finally, simulation results of a 2.27 MVA wind turbine generator are presented to demonstrate the good performance of the DFIG with the proposed controllers.

With steadily increasing wind turbine penetration, regulatory standards for grid interconnection have evolved to require that wind generation systems ride-through disturbances such as faults and support the grid during such events. Conventional modifications to the doubly fed induction generation (DFIG) architecture for providing ride-through result in compromised control of the turbine shaft and grid current during fault events. A DFIG architecture in which the grid side converter is connected in series as opposed to parallel with the grid connection has shown improved low voltage ride through but poor power processing capabilities. In this paper, a unified DFIG wind turbine architecture which employs a parallel grid side rectifier and series grid side converter is presented. The combination of these two converters enables unencumbered power processing and robust voltage disturbance ride through. A dynamic model and control structure for this architecture is developed. The operation of the system is illustrated using computer simulations.

A nonlinear adaptive neuro-fuzzy inference system (ANFIS) based controller is proposed for power electronic systems (PES) of grid connected unified doubly-fed induction generators (DFIG). The unified DFIG utilizes an additional series grid-side converter (SGSC) connected in series with the stator winding of the DFIG to inject voltage into the grid for compensation purposes. The SGSC is useful in tolerating voltage disturbances at the point of common coupling, compared to the conventional DFIG. The conventional proportional integrator (PI) controller of the PES of a unified DFIG is replaced with a nonlinear adaptive neuro-fuzzy inference system (ANFIS) based controller applied for adaptive, fast, and efficient control of power electronic systems. The performance of the conventional PI and ANFIS based controllers are compared on a test system. The transient response of ANFIS based controllers is found to be better compared to the conventional PI controllers.

Many wind turbine generators (WTGs) are installed in remote, rural areas, where the power grids are usually weak, characterized by unbalanced voltage conditions. If the voltage unbalance is not taken into account in the control system, it will cause poor power quality and poor operating performance of the WTG systems. This paper proposes a novel control scheme to improve the operating performance of a wind turbine equipped with a doubly fed induction generator (DFIG) under unbalanced network voltage conditions. The rotor-side converter (RSC) and grid-side converter (GSC) of the DFIG are controlled in a positive (dq)+ reference frame as well as in a negative (dq)- reference frame. Control of the RSC and GSC in the (dq)+ reference frame is the same as the regular control under balanced network conditions. The supplementary control of the RSC in the (dq)- reference frame minimizes the electromagnetic torque pulsations of the DFIG caused by the unbalanced network voltage; the supplementary control of the GSC in the (dq)- reference frame contributes to balance the total output currents of the DFIG. The proposed control scheme is implemented in PSCAD/EMTDC on a 3.6 MW DFIG wind turbine connected to a power network with unbalanced voltage. Results show that it improves the operating performance of the DFIG wind turbine system.

The harmonic voltages in a power system are generated by the nonlinear voltage source or loads. This chapter analyzes the performance of the doubly fed induction generator (DFIG) wind power system under harmonic distorted grid. The influence of the grid harmonic voltage on the grid‐side converter (GSC) current, stator current, rotor current, DC‐bus voltage, active/reactive power, as well as the torque fluctuations are analyzed. For different power‐scale DFIGs, the different influences of the grid harmonic voltage disturbance on the stator, rotor current, as well as the GSC grid current and electromagnetic torque may be caused by several reasons. The challenge for the DFIG wind power system operating under harmonic distorted grid voltage is evaluated. The chapter shows the influence of the harmonic distorted grid voltage on the DFIG in two aspects: their influence on the DFIG together with the rotor‐side converter (RSC) and their influence on the GSC.

The study presents an optimised parameter design of the proportional integral and resonant (PIR) current regulator for doubly fed induction generator (DFIG) under distorted grid voltage conditions. Based on the mathematical model of DFIG under distorted grid voltage conditions, practical PIR current regulator under synchronous frame is developed to implement the harmonic control target of sinusoidal stator current. The optimised parameter design of PIR regulator is investigated concerning the control system gain margin and phase margin, as well as variation of the resonant bandwidth. The influence of DFIG parameters and grid frequency deviation on PIR design is also discussed. Correctness of the optimised parameter design technique of PIR regulator is validated by the MATLAB simulation and experiment results.

The paper analyzes the capability of a wind power plant, based on doubly-fed induction generators (DFIGs) with an improved control system, to fulfill low voltage ride-through (LVRT) capability of the wind turbines providing the reactive power support. According to grid code requirements, tripping of wind generators under grid faults is not allowed. The sensitivity of DFIGs to grid faults requires the use of crowbar protection for not damaging the electronic equipment. In this paper, the model of a wind power plant based on DFIG using active crowbar protection is developed in Matlab/Simulink. Nevertheless, the results indicate that DFIGs equipped with crowbar protection can protect the electronic equipment, but the rotor side converter is blocked due to crowbar activation and the reactive power control is lost. Therefore, for improving the LVRT, a coordinated control system of the grid side converter, active crowbar protection, and rotor side converter is implemented and proposed, The grid side converter of DFIG is controlled similar to a STATCOM, considering also the control parameters of the crowbar protection. The simulation results show that the proposed coordinated control grid side converter'active crowbar-rotor side converter can supply the reactive power in order to answer the grid codes requirement for LVRT. Therefore, this control system can be useful to improve the LVRT capability of wind power plants based on DFIG.

This work proposes an enhanced Low-Voltage Ride Through (LVRT) nonlinear control of Doubly Fed Induction Generator (DFIG)-based wind turbines under symmetric three-phase voltage dips in stiff grid conditions. Most of the control schemes that use rotor currents control for LVRT act on the stator flux by injecting reactive power into the grid so to re-arise the stator voltage to accepted values. Indeed, when the grid at the DFIG coupling point is powerful enough, the stator flux is no more controllable. This investigation first gives a mathematical model for the wind generation system besides a Backstepping control scheme for grid-connected DFIG powers flow control. Then, effects of a three-phase symmetric voltage dip are analysed and the reasons of rotor current surges, due to the voltage dip, and factors influencing it are inferred. As the stator flux is imposed by the power grid, this work proposes a Backstepping control for both converters to keep DFIG connected to grid within accepted values during faults. The rotor side converter keeps DFIG currents at nominal values and limits transients. The grid side converter keeps the voltage at DC-link stable. Results of these theoretical findings are verified by MATLAB/SIMULINK simulation on a 300 kW wind generation system.

The doubly fed induction generators (DFIG) have been recognized as the dominant technology used in wind power generation systems with the rapid development of wind power. However, continuous operation of DFIG may cause a serious wind turbine generators tripping accident, due to destructive over-current in the rotor winding which is caused by the power system fault or inefficient fault ride-through (FRT) strategy. A new rotor current control scheme in the rotor-side converter (RSC) ispresented to enhance FRT capacities of grid-connected DFIG. Due to the strongly nonlinear nature of DFIG and insensitive to DFIG parameter’s variations, a novel sliding mode controller was designed. The controller combines extended state observer (ESO) with sliding model variable structure control theory. The simulation is carried out to verify the effectiveness of the proposed control approach under various types of grid disturbances. It is shown that the proposed controller provides enhanced transient features than the classic proportional-integral control. The proposed control method can effectively reduce over-current in the RSC, and the transient pulse value of electromagnetic torque is too large under power grid fault.

Doubly fed induction generator (DFIG) needs to get adapted to situations like rapid change in wind speed and sudden requirement for grid disturbances to meet modern grid rules. The paper explains the design of controllers for MPPT algorithm for the turbine, DFIG converters, and sensorless rotor speed estimation. These intelligent controllers are used to maintain equilibrium in rotor speed, generator torque, and stator and rotor voltages. They are also designed to meet desired reference real and reactive power during the turbulences like sudden change in reactive power or voltage with concurrently changing wind speed. The turbine blade angle changes with variations in wind speed and direction of wind flow and improves the coefficient of power extracted from turbine using MPPT algorithm. Rotor side converter (RSC) helps to achieve optimal real and reactive power from generator, which keeps rotor to rotate at optimal speed and to vary current flow from rotor and stator terminals. For tracking reactive power demand from grid and in maintaining synchronism when grid voltage changes due to fault at a very faster and stable way Grid side converter (GSC) is helpful. Rotor speed is estimated using stator and rotor flux estimation algorithm. Parameters like stator and rotor voltage, current, real, reactive power, rotor speed, and electromagnetic torque are studied using MATLAB simulation. The performance of DFIG is compared when there is in wind speed change only; alter in reactive power and variation in grid voltage individually along with variation in wind speed.

Due to stochastic nature of wind, considerably impact the stability of grid and deteriorates power quality. This paper is attributed to the sophisticated control design of the grid-interfaced doubly fed induction generator (DFIG) wind power generation system. The battery storage system is incorporated in the DC-link with the aid of a bidirectional DCDC converter, to sustain the steady DC-link voltage. It also supports grid power smoothing, and power balance even under stochastic wind speed variation. The rotor side converter (RSC) employs an off - maximum power point tracking approach to keep the BESS operating within the limit and deliver consistent power to the grid. An improved phase lock loop control is designed for DFIG-based WES that suppresses dc offset at the input and allows RSC and grid side converter (GSC) to perform appropriately even under numerous grid disturbance conditions. The presented system is modeled and simulated in MATLAB/Simulink, and evaluated on the real-time simulator (RTS) test bench. The satisfactory system response is endorsed under numerous operating conditions.

A quick post fault recovery of wind generator system could be achieved with the help of a series dynamic braking resistor (SDBR). This paper investigates the dynamics, size, optimal location of a series dynamic braking resistor in the doubly fed induction generator (DFIG) structure based on different switching strategy of the braking resistor. Three signals were investigated in switching the SDBR during transient conditions of the DFIG wind generator; the DC-link voltage, rotor current and grid voltage, respectively. Also, three locations were considered in connecting the SDBR in the DFIG system; the stator, rotor and grid side converter, respectively. Furthermore, the optimum size and best insertion time of the SDBR were investigated based on the most effective performance of the SDBR scheme considering its switching signal and position in the DFIG system. Simulations were run in power system computer-aided design and electromagnetic transient including DC (PSCAD/EMTDC). The results show that when the SDBR is connected to the DFIG, improved performance was achieved; however, the stator based SDBR topology in conjunction with a grid voltage actuating signal, fast insertion time and short duration of operation quickly recovered the DFIG wind generator variables during transient conditions.

Regarding doubly fed induction generator (DFIG) operation, unbalanced and harmonically distorted grid voltage conditions have been treated as two separate control problems. This paper reports a solution for the rotor- and grid-side power converters, which allows one to keep the DFIG successfully in operation under both grid voltage conditions. The proposed solution is based on sliding-mode control (SMC). The rotor-side converter is commanded so that the electromagnetic torque and the stator reactive power remain free of fluctuations that arise during grid voltage disturbances. Meanwhile, the grid-side converter ensures both constant DC-link voltage and steady active power output from the overall system. The developed algorithms turn out being robust against parameter variations and of fast dynamic response. In addition, none of the converters need either voltage or current positive and negative sequences extraction. The simulation results presented demonstrate the appropriateness of SMC to face such disturbed scenarios. Finally, the stability proof of both converters' control algorithms is provided in the appendices.