Modeling and LVRT analysis of DFIG wind power system

This paper presents the development of the wind power system and low voltage ride through (LVRT) techniques for doubly -fed induction generator (DFIG) wind energy system. In recent years, wind power has been developing rapidly. As the wind power penetration from large scale wind turbines has been increasing rapidly in the transmission system, there is a higher requirement for the frequency stability and power generation efficiency of the wind power generation system, and the power grid puts forward higher continuous operation requirements for the grid connected wind turbine in the power grid fault and demand that the wind power generator has to ride through the grid faults, especially the grid voltage dips. This overview mainly introduces DFIG and low voltage ride through technology in the wind power generation system. This paper introduces respectively their development, control strategies and future trend.

The wind power plant is one of the fastest growing electrical power sources. The integration of wind turbine into the power grid originates various power quality problems. Low-voltage ride through (LVRT) capability improvement is impregnating exigency in recent power quality issue, which is acquainted with various renewable power generation resources like solar PV and wind power plant. Dynamic voltage restorer (DVR) is a custom power device (CPD), which is connected in series with the electrical system, and it is a contemporary and efficient CPD expended in the distribution system to curb the power quality problems. In this paper, LVRT capability of doubly fed induction generator (DFIG) wind turbine system connected to a power grid is enhanced by means of DVR. LVRT capability is improved by the introduction of neural network controller. This controller fulfils the various grid code requirements. Thus, the performance of the DVR becomes far superior by the robust control technique. The simulation results were compared with conventional PI controller and the proposed artificial neural network (ANN) controller. It is proved that the proposed system raises the reliability of grid-connected DFIG wind turbine system.

The Vector controlled doubly fed induction generator (DFIG) wind energy system in a multi-machine power system is very sensitive to faults on transmission lines and thus affects the stability of the system. The performance of vector controlled DFIG highly depends on PI controller parameters. The objective of this paper is to optimize the performance of vector controlled DFIG in multi-machine power systems under faulty conditions by tuning the parameters using advanced differential evolution algorithm. An objective function is formulated, which drives the system with good voltage profile, reduced power oscillations under faulty conditions. The parameters of PI controller are tuned using Composite Differential Evolution algorithm. Simulations are performed on a two area benchmark problem in Matlab/Simulink. The simulation results of the optimized DFIG wind energy system are compared with the actual existed system. The simulation results show that the overall performance and stability of the optimized system are enhanced.

Due to the latest grid codes, wind energy conversion systems (WECSs) are required to remain connected to grid under grid voltage sags and supply reactive power into the grid. So, this paper proposes an enhanced scheme to improve low-voltage ride through (LVRT) capability of doubly fed induction generator (DFIG)-based WECSs under both balanced and unbalanced grid voltage sags. The proposed scheme is composed of active and passive LVRT compensators. The active compensator is performed by controlling the rotor- and grid-side converters of the DFIG to decrease the stator flux oscillations and inject reactive power into the grid. The passive compensator is based on a three-phase stator damping resistor (SDR) located in series with the stator windings. The proposed scheme decreases the negative effects of grid voltage sags in the DFIG system including the rotor over-currents, electromagnetic torque oscillations, and DC-link over-voltage and also injects reactive power into grid to support the grid voltage. So, the LVRT capability of DFIG is enhanced and new grid code requirements are addressed. Simulation results on a 1.5-MW DFIG-based WECS using MATLAB/Simulink demonstrate the effectiveness of the proposed LVRT scheme under both balanced and unbalanced grid voltage sags.

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 paper presents a model and a control strategy for a doubly-fed induction generator (DFIG) wind energy system in an unbalanced microgrid based on instantaneous power theory. The proposed model uses instantaneous real/reactive power components as the system state variables. In addition to the control of real/reactive powers, the controllers use the rotor-side converter for mitigating the torque and reactive power pulsations. The control scheme also uses the grid-side converter for partial compensation of unbalanced stator voltage. The main features of the proposed control method are its feedback variables are independent of reference frame transformations and it does not require sequential decomposition of current components. These features simplify the structure of required controllers under an unbalanced voltage condition and inherently improve the robustness of the controllers. A power limiting algorithm is also introduced to protect power converters against over rating and define the priority of real/reactive power references within the control scheme. The performance of the proposed strategy in reducing torque ripples and unbalanced stator voltage is investigated based on the time-domain simulation of a DFIG study system under unbalanced grid voltage.

This paper primarily focuses on an advance control strategy to enhance the low voltage ride through (LVRT) capability in doubly fed induction generator (DFIG) based wind energy conversion system (WCES). In the proposed control strategy, the captured wind energy during grid faults circumstances is stored timidly in the rotor’s inertia kinetic energy. Though a minimal amount of energy is available in the grid, stator current and DC-link voltage are set beneath the perilous levels. However, both the required stator voltage and stator current are kept within a tolerable range of rotor side converter (RSC), through state feedback linearization technique for maintaining the accurate control to suppress the overvoltage and overcurrent. Furthermore, stator current oscillations are significantly suppressed during fault transient. The input mechanical energy from the wind turbine can be resumed after the fault clearance. In spite of being dissipated in the resistors of crowbar circuit, as in the conventional LVRT assemblies, torque balancing among electrical and mechanical measures is attained; DC-link voltage instabilities and rotor speed inconsistencies are substantially reduced. As a result, a noticeable reduction in the requirement of reactive power and swift restoration of terminal voltage on fault clearance is acquired successfully. Correspondingly, several tests are conducted to validate the effectiveness and enhancement in the performance of the DFIG based wind farms, when the proposed control strategy is implemented over it during numerous fault ride-through circumstances.

The grid-connected Doubly Fed Induction Generator (DFIG) system is exposed to severe grid faults. The DFIG is sensitive to grid disturbances, which lead to consideration in the controller design. This paper proposed the Modified Super Twisting (MST) algorithm for the Low Voltage Ride Through (LVRT) enhancement under voltage sag condition. This proposed algorithm is implemented using the 2nd Order Sliding Mode (SOSM) to control the DFIG based wind generator. The higher-order switching functions are introduced in the SOSM for sliding surface control. Moreover, the Lyapunov analysis for the MST algorithm brings down the chattering amplitude. The advantages of the proposed algorithm are that it reduces the system uncertainties, chattering effect and improves the settling period of the system parameters. The performance of the proposed algorithm is compared with existing algorithms in the literature with the help of MATLAB/SIMULINK. The Hardware-In-Loop (HIL) is used to validate the simulation results, which have been performed on the OPAL-RT setup. In addition, the proposed algorithm is also tested on an equivalent model of the practical Wind Farm (WF). Based on the studies, it is found that the proposed algorithm enhanced the LVRT performance of the single Wind Turbine (WT)-DFIG system as well as the practical WF under transient conditions.

Electromagnetic transients resulting from voltage dips complicate the low-voltage ride-through (LVRT) of doubly fed induction generator (DFIG) based wind turbines (WTs). To study the transients deeply, an analytical method to solve the dynamic response is presented in this paper. Although some attempts have been made in literature, the accuracy cannot be ensured, or the physical significance implied in the transients is not feasible to be revealed without a proper method to this complex process. To clarify the response, a concept of equivalent fault source is defined in the stator and rotor circuit respectively and the overall behavior is decomposed into a stator-fault-sourced response and a rotor-fault-sourced response. The former corresponds to the dips of the stator voltage, while the latter deals with the variations in the rotor resistance and the terminal voltage due to activation of the crowbar. Adding these two zero-state responses to the healthy pre-fault conditions, the overall response can be constructed accurately and each of the terms has its apparent significance. With the analytical solution, the behavior is discussed and some important features are presented. The accuracy of the solution is verified by close agreement with the simulative results from a MATLAB/Simulink model of a 2MW DFIG and the experimental records on an 11kW laboratory-scale test rig.

For doubly-fed induction generator (DFIG)-based wind power systems, when crowbar-less control methods are used for low voltage ride through (LVRT), the LVRT capabilities are different depending on the specific method. However, because of the limited voltage output of rotor side converter (RSC), there is a limit for crowbar-less control methods. In order to analyze the limit under asymmetrical voltage dips, an optimal crowbar-less control method is proposed to minimize the rotor current during faults. Based on the proposed method, the control limit under asymmetrical faults can be obtained, and then the LVRT capabilities for asymmetrical faults, with crowbar-less control methods, can be evaluated. Finally, a case study of a typical 1.5 MW DFIG-based wind power system is carried out, and from the results, it can be seen that among the three types of asymmetrical faults, the phase-to-phase faults are the most difficult to ride through, and the single-phase-to-ground faults are the easiest to ride through.

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.

This paper presents details comparisons between the stator-flux oriented and the voltage oriented control strategies for the doubly fed induction generator (DFIG) wind energy system. This paper develops a dynamic model of DFIG in the rotating reference frame, which is used to obtain the rotor current components into the generator torque and stator reactive power references using the both control strategies. Comparisons are made between both control strategies in terms of the stator flux and voltage position and also the stator power control. Finally, simulation studies are carried out on a 1-kW DFIG wind energy system. The validity of the performance and efficiency of the control strategies for variable wind speed are confirmed by the simulation results.

Nowadays actual scientific problem of renewable energy is the problem of developing control systems, that ensure the generation of stable voltage in a changing environment. This paper presents a structure and methods of synthesis of control system development, which allow to decide the indicated problem. An electric subsystem of wind power plant is a well-known structure with well-known advantages, consisting mainly of double-fed induction generator (DFIG) and power converter in the circuit of rotor. Operation of the control system based on the original nonlinear mathematical model of DFIG in rotational dq coordinates, taking into account the nonlinear external and internal DFIG disturbances, such as inductances of coil winding, changes of coil winding resistances from temperature, etc. The study received mathematical model, adaptive control algorithms and its computer simulation of the wind turbine with DFIG. Introduction An increasing requirement in energy resources at unstable oil prices stimulate to search the alternative decisions of power problems. One of the most successfully developed types of alternative energy is wind power. Its rapid growth observed in the world, proofing an attractiveness of this technology. It is expressed in the purity, reliability and input speed. So, according to Global Wind Energy Council (GWEC), to the end of 2015 global wind power capacity will reach 609 GW, compared to 237. 4 gigawatts at the end of 2011. The three leading world markets remain the China, U. S. and Europe. In recent years, the rapid growth of installed wind power capacity turned out to be possible through the development and implementation of new technological solutions, one of which is DFIG used as generator in wind power plants(WPP). Such DFIG are widely used in powerful WPP, as they allow to convert the available wind energy more efficiently, especially in periods of weak winds. It is known that the parameters of the generated voltage depends on terms of environment. At the same time, the necessity of quality stable voltage generation sets the problem of technological decisions development, allowing to minimize influence of different factors on output voltage quality. Such a technological solution is adaptive control system (CS) of a WPP with DFIG , permitting high speed reaction to wind speed changes and the rate of the connected electrical load, thus maintaining the stability of the generator output voltage. Synthesizing an adaptive high-speed CS and using the structure of WPP with DFIG it becomes possible to design and build these types of wind turbines. Synthesis of nonlinear CS Nonlinear CS control law. The solution of the indicated problem, offered in this paper, is based on the nonlinear model of DFIG in rotational dq coordinates, as in the conditions of different disturbances, linearizing appears uneffective [1]. Because, coming from the structure of WPP, control circuit is a rotor circuit, then primary control objective is feed on it control voltage of some parameters, at that DFIG output voltage remains unchanging and corresponds to set, without depending on influences. As it applies to a model, it is necessary to get the control values of voltages International Conference on Advances in Energy, Environment and Chemical Engineering (AEECE-2015) © 2015. The authors Published by Atlantis Press 1 on the dq rotor axes dr V and qr V , at the well-known required stator voltages ref ds V and ref qs V . The classic mathematical model of DFIG does not take into account the nonlinear changes of its parameters, such as winding inductances, changes of winding resistances from temperatures etc., accepting their permanent, that is not quite right. Meantime, in real DFIG such changes have an influence on control quality, so the synthesis of control system, which is taking into account such nonlinear disturbances, is an actual task. The disturbances arising up from nonlinear character of the control object are difficult to mathematical description, however valuation of these disturbances with the subsequent bringing of adjustments in work of CS on the basis of such estimation is a fully solvable task. For this purpose, in the standard mathematical model of DFIG we will add some functions of immeasurable disturbances, and (Eq.1), accordingly for the stator d and q axes currents, and next we synthesize control laws.

Crowbar-based control is one of the main low voltage ride through (LVRT) control methods for doubly-fed induction generator (DFIG), but converter is still at risk of being damaged by over-current at rotor side after crowbar exits during fault recovering period. For the issue, a model for fault process analysis based on equivalent dynamic model of DFIG is established. The mechanism of phase angle jumping when fault occurring and grid voltage recovering and its impact on vector orientating precision are analyzed. Based on this, jumping phase angle compensation principle is proposed to improve existing LVRT control strategy. Then the process of control is divided into four periods, i.e., normal operation period, crowbar switching period, crowbar exiting period and grid voltage recovering period. In normal operation and crowbar exiting period, stator flux orientation control of DFIG is applied. In crowbar switching period, IGBT pulses of rotor side converter are locked, but grid side converter maintains normal operation. In grid voltage recovering period, phase angle compensation control is applied. Simulation results show that over-current in grid voltage recovering period can be effectively limited by the proposed control strategy.

This paper deals with the modeling, analysis, control and simulation of a doubly-fed induction generator (DFIG) driven by a wind turbine. This grid connected wind energy conversion system (WECS) is composed of DFIG and two back-to-back PWM voltage-source converters in the rotor circuit. A mathematical model of the machine, derived in an appropriate dq reference frame is established. The grid voltage oriented vector control is used for the grid side converter (GSC) in order to maintain a constant DC bus voltage and to compensate for reactive power at the power network. The stator voltage orientated vector control is adopted in the rotor side converter (RSC) control strategy, providing efficient handling of active and reactive power at the stator, as well as a maximum power point tracking (MPPT) method for the DFIG-based wind turbine. The proposed system is simulated for different operating conditions to illustrate the reliability of the control technique. Corresponding system simulation results under nonlinear load variations and wind speed transients are presented to demonstrate the significance of MPPT in WECS, and the effectiveness of adopted control technique.