Abstract—Wind energy generation with Doubly Fed Induction Generator is dominant one in wind farms. High penetration of DFIG is creating more instability in rotor side and grid side. Applying a set of optimized control parameter, the stability can be further enhanced. In this paper variable load condition rotor angle stability of the DFIG is improved by Grid Side (GSC) and Rotor Side Converter (RSC) control. In DFIG, the frequency of the rotor excitation input, real and reactive powers are modified based on rotor speed and load variation. The simulation result shows the effectiveness and performance of the DFIG with GSC- RSC converter control techniques.
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Wind power generations have been growing rapidly all over the world and have become one of the most promising renewable generation technologies. Among the different types of the wind energy conversion system (WECS), doubly fed induction generator (DFIG) based wind energy conversion systems have gained the increasing proportion due to the tremendous benefits, which include the variable speed constant frequency operation, four-quadrant active and reactive power capabilities, smaller converters rating (around 30% of the generator rating), lower cost and power losses, compared with any fixed-speed induction generators and synchronous generators. Normally, DFIG-based WECSs are supplied by the rotor with back to back converter. Back to back converter consists of rotor side converter and grid side converter , . The two level rotor side converter feeds rotor of DFIG. The control of rotor side converter along with DC link is emphasized in this paper.
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Abstract: The presence of voltage swells over the DC connection of the successful rotor side converter of a Doubly Fed Induction Generator (DFIG) is natural because of vulnerability in twist vitality and in addition the variety of rotor precise speed. This can weaken the execution of the consecutive converter associated on the rotor side of the DFIG. Subsequently, the principle goal of this paper is to plan a criticism linearization procedure to dispose of the dc-interface voltage swell and additionally acquire solidarity control factor. In this paper, the dynamic demonstrating of DFIG alongside the viable rotor side converter is performed. The criticism linearization strategy controls the inward elements of the successful rotor side converter by considering the rotor q-pivot current and DC connect voltage. The MATLAB recreation results portray the viability of the voltage control strategy, through the varieties of rotor side channel, DC interface capacitance and vulnerabilities in the DC connect voltage.
The mechanical equation is expressed as follow: C = J + fΩ + C (9) Where J is the inertia, Ω is the generator speed, f is the mechanical damping coefficient and p is the number of pole pairs. R , R are the stator and rotor phase resistances respectively. ω , ω are respectively the synchronous angular speed of the generator and the angular speed of the rotor . L , L , are respectively the stator and rotor inductances and M is the magnetizing inductance. C. Rotor Side Converter Control The stator flux is set aligned with the d axis and we suppose that the grid is assumed to be strong and stable so Ѱ is constant. Moreover, the stator resistance of the DFIG is neglected . Since the stator flux is aligned with the d axis, we can write Ѱ =Ѱ and Ѱ =0. Hence, equations (3) to (8) become respectively: u ≈ 0 (10)
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environmental and economical considerations, conventional methods of generation of electric power have been replaced by the renewable energy sources. Among renewable energy sources Wind Energy Conversion System is the emerging source of power. This paper presents the enhancement of rotor angle stability of the power system which contains both synchronous generators and double fed induction generators (DFIG). A Power System Stabilizer (PSS) is included in the reactive power control loop of Rotor Side Converter (RSC) of DFIG which damps out the rotor angle oscillations which is controlled by a fuzzy logic controller. The proposed control technique is implemented in MATLAB/Simulink software.
To protect the rotor side converter from tripping due to over-currents in rotor circuit and overvoltage in DC link during grid voltage dips a crowbar is installed in conventional DFIG wind turbines. It is a resistive network connected to the rotor windings of the DFIG. The crowbar limits the voltages to provide a safe route for the currents by bypassing the rotor by a set of resistors. When the crowbar is activated the rotor side converters pulses are disabled and the machine behaves like a squirrel cage induction machine directly coupled to the grid. The magnetization of the machine that was provided by the RSC in nominal condition is lost and the machine absorbs a large amount of reactive power from the stator and thus from the network .
While studying about DFIG based wind power generation system it is observed that current regulators used for the DFIG rotor side converter to control the flux linkage at rotor winding; so in this way the current through rotor winding is controlled by means of indirect method. The controller on rotor-side, therefore, does not have a good tracking capacity for reference-current and the dynamic performance of the current through rotor winding for events like fast transients actually poor. In the wave form high overshoots are observed very clearly, which should be required protection system in the rotor side converter for example connection of a resistive crowbar. In this project a method for developing direct current control method for rotor side converter is explained. The closed loop dynamic system of the DFIG based wind turbine system is studied to detect proper terms called as cross coupling terms; these terms are used for transformation of the flux linkage and this flux is controlled by means of current control. The method used here is successfully studied in presence symmetrical three phase faults and voltage reductions; hence it provides the control configuration of two direct current regulators of rotor. After studying this entire thing we considered the method of direct current control for rotor side controller is used to improve the transient response of DFIG systems during grid faults significantly with control implementation complexity to indirect current regulators.
within the network. DFIGs have the capability to operate in variable speed regions so we have to achieve a smoothened and twice the power than any other conventional generator will produce. In the development of wind turbine techniques, DFIG is becoming more popular because of its unique characteristics such as high efficiency, low cost and flexible control . Most of the wind turbines face a problem of LVRT. One common LVRT solution is to install a crowbar circuit across the rotor terminals. When the rotor over current is detected, the crowbar circuit short circuits the rotor terminals and isolates the converters from the rotor. And thus Rotor Side Converter triggering is blocked. This provides conservative protection to the rotor circuit and the RSC changes the DFIG to a squirrel cage induction machine, which absorbs reactive power from the grid. As a result dynamic VAR compensators, such as static VAR compensators or static synchronous compensators are sometimes installed at the DFIG terminals to provide reactive power support during a grid fault . Unbalanced grid faults degrade the performance of DFIG- based wind turbines. In fact, if voltage unbalance is not taken into account the stator and rotor currents will be highly unbalanced even with a small unbalanced stator voltage. The unbalanced currents will create unequal heating on stator and rotor windings which will produce a complete change in torque and power pulsations of the generator which is twice the line frequency . Several control approaches have been presented for DFIG systems operating with unbalanced grid faults. The rotor-side system is decomposed in two separate models which are represented with positive and negative-sequence components respectively. Two parallel controllers which are expressed in the positive and negative-synchronous reference frame are also presented. The goal of the positive-sequence controller is to regulate the rotor side converter as in the case of normal operating conditions
The operation and control of a Doubly-fed Induction Generator (DFIG) system. This DFIG system is currently the system of choice for multi-MW wind turbines. The aero- dynamic system must be capable of operating over a wide range of wind speeds in order to achieve an optimum aerodynamic efficiency by tracking the optimum tip-speed ratio. Therefore, the rotor of the generator must be able to operate at a variable rotational speed. Back to back converter is a two stage of converter, which converts AC-DC-AC. The AC-DC conversion is rectifier stage this converter is called as grid side converter (GSC) and the DC-AC conversion is inverter stage this converter is rotor side converter (RSC).Both converters are connected “back to back” with a DC link capacitor between them. Dc link capacitor puts voltage at constant for efficient inversion. RSC controls the torque or speed of the DFIG and also controls the power factor of the
Abstract— Now a day penetration of renewable energy sources is increasing day by day and wind energy system is leading in all this. DFIG with two levels AC/DC/AC converter is used with one as rotor side converter and other as the grid side converter. Rotor side converter controls the voltage and frequency of the DFIG whereas the grid side converter is there to control the DC link voltage. The main problem of the DFIG is harmonic as it contain two converter.to control the harmonic constant instantaneous power control strategy is used to control the harmonics.
For PMSG wind power system with back-to-back PWM converters, in the conventional control method , the generator rotor speed is controlled by the gen- erator- side converter, and the grid-side converter control the DC-Link voltage. Considering the nonlinear rela- tionship between the DC-Link voltage and the generator rotor speed, this paper proposes an improved control method of the power conversion. The DC-Link voltage is maintained within regular range by the generator-side converter which introduces feedback linearization control technology. The generator speed reference ω* is output by the grid-side converter for the maximum power point tracking, and the ultimate value ω w * is got after com-
Interactions between the blades and vortical structures within the wake of a helicopter rotor are a significant source of impulsive loading and noise, particularly in descending flight. Brown’s Vorticity Transport Model has been used to investigate the influence of the fidelity of the local blade aerodynamic model on the accuracy with which the high-frequency airloads associated with blade-vortex interactions can be predicted. The Vorticity Transport Model yields a very accurate representation of the structure of the wake, and allows significant flexibility in the way that the blade loading, and hence the source of vorticity into the wake, can be represented. Two models for the local blade aerodynamics are compared. The first is a simple lifting-line model and the second is a somewhat more sophisticated lifting-chord model based on unsteady thin aerofoil theory. A marked improvement in accuracy of the predicted high-frequency airloads of the HART II rotor is obtained when the lifting-chord model for the blade aerodynamics is used instead of the lifting-line type approach. Errors in the amplitude and phase of the loading peaks are reduced and the quality of the prediction is affected to a lesser extent by the computational resolution of the wake. Indeed, the lifting-line model increasingly over- predicts the amplitude of the lift response to blade-vortex interactions as the computational grid is refined, exposing clearly the fundamental deficiencies in this commonly-used approach particularly when modelling the aerodynamic response of the blade to interactions with vortices that are much smaller than its chord. In comparison, the airloads that are predicted using the lifting-chord model are relatively insensitive to the resolution of the computation, and there are fundamental reasons to believe that properly converged numerical solutions may be attainable using this approach.
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The converter provides a high voltage gain due to the modified Dickson charge pump voltage multiplier circuit. The converter is made up of two stages. The first stage is a two-phase boost converter which outputs a modified squarewave voltage between its output terminals A and B. The second stage is the modifiedDickson chargepumpvoltage multiplier circuit that boosts the square wave voltage (VAB) to provide a higher dc outputvoltage. For the converter to operate normally, both switches S 1 and S 2 must have an overlap time where both switches are ON and also one of the switches
The active stator power of the generator ( ) is compared with the reference point value ( ) which is determined by the wind speed. The difference between these two values will go to a Proportional Integral (PI) controller, which is used to generate the required value of rotor current ( ) . Likewise, a PI controller of the reactive power side is used to generate the required rotor current ( ). The two outputs of both PI controllers are transformed from the frame into the frame to obtain the required value of rotor currents. Then, , and are algebraically summed with , and respectively . The last result is obtained because of generation and demand quantities. The triggering pulses would control the IGBT switches in the rotor-side converter and that will enhance the stability of the entire system by sustaining the frequency and voltage within permissible tolerances  . The rotor-side converter controller is used to control independently the stator voltage (or reactive power) and output active power of the wind turbine  . The generic control loop is illustrated in Figure 5. Since the converter operates in a stator-flux q-d reference frame, the rotor current is decomposed into an active power (q-axis) and a reactive power (d-axis) component. When the wind speed change, the active and reactive (or voltage) power of the generator will also change. On the other hand the role of the grid-side converter is to control the DC- link voltage by maintaining it constant and it is also used to generate or absorb reactive power. The DC link voltage is used as well, with the q-d reference frame oriented along the stator currents and stator voltages, enabling independent control of the active and reactive power flowing between the grid and theconverters.
(the speed of the neap tides varies from 2 to 2.5m/s), which happens when the moon and the sun are at right angles and pull seawater in different directions. Nova Scotia’s Bay of Fundy is characterized by high tides that can reach up to 17 meters. The electrical-side layout and modeling approaches used in tidal in-stream systems are similar to those used for wind and offshore wind systems. The speed of water cur- rents is lower than wind speed, while the water density is higher than the air density and as a result wind turbines operate at higher rotational speeds and lower torque than tidal in-stream turbines which operate at lower rotational speed and high torque [1-5].
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Having in mind that pressure on the discharge chamber is equal to 1 bar; the shorter dashed line shows the pressure difference Δp between rotors and discharge chamber. The pressure data, in the diagrams (c) and (d) of figure 4, can be divided into three zones: Zones a and c where the pressure difference between the chambers is quite small and air flow is simply due to the rotor motion. After the discharge port is open, rotors, like a piston, push the air out of the chamber and force it to accelerate to pass through the small V-section causin strong axial mean velocity gradients, peaks and turbulence variation.
discovered to be very complex and at least three distinct peaks are recognisable. Also, there is a tendency for the peaks to shift to higher velocities as soon as the control volumes move away from the centre of V-section towards its edges on both sides. On the left hand side of diagrams, before the opening line, the flow motion is smoother with higher variations on the main rotor side. Figures 10(a) and (b) represent the axial RMS velocity variation at the outlet of the discharge port and correspond to those of figure 9(c) and (d). In general the RMS profiles follow those of the mean velocity with large fluctuations immediately after the opening of the port.
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In this section, dynamics model of both electrical and mechanical parts of DFIG based wind turbine is pro- vided. The stator winding of DFIG is directly con- nected to the grid, and the rotor winding is coupled via an IMC. The IMC must handle the slip power, i.e. about 30% of the rated power of wind turbine. The speed range of the generator is typically of synchronous speed, thus providing flexibility to operate in both sub and super synchronous modes, depending on the wind conditions. The inverter of the IMC is used to control the active and reactive power of the DFIG. The recti- fier of the IMC is often operated at unity power factor. Depending on the rotor speed, the IMC will either ab- sorb power (sub-synchronous) from grid or inject power (super-synchronous) to the grid. Therefore, the IMC must have the ability of bidirectional power flow to the network.
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The transient approaches are essential to study DFIG dynamic performance in a short time period and steady- state techniques are important to examine DFIG characteristics under different control conditions in a detailed manner. Steady-state approach plays an important role to examine DFIG characteristics under different conditions in a broader spectrum and also in the design and development of highly advanced control schemes. Unlike fixed speed turbines, DFIG delivers power to the grid from both the stator and rotor paths. The stator of the generator is directly connected to grid while the rotor is connected to the grid through PWM power converters.
and diffusive processes within the wake influence the distribution of the vorticity on the length-scale of the vortex core (see, for instance, Ref. 16) and are likely, if incorrectly represented, to result in significant errors in the prediction of BVI. For concentrated vortical structures, accurate resolution of the fine-scale struc- ture is especially important where the miss-distance between the vortex and the blade is very small, but, on the other hand, detailed structural effects wash out quite quickly with increasing miss-distance because of the properties of the Biot-Savart relationship between the velocity and the vorticity. Indeed, given the de- scending flight condition that was modelled during the tests (and as can be inferred from the vortex trajecto- ries shown in Figs. 5 to 8) very few of the BVI events on the HART II rotor are due to close interactions between the vortices and the blades. Where extended vortex structures are involved, however, it is possi- ble that predictions of BVI-induced loading may be adversely affected by poor resolution of the fine-scale distribution of the vorticity in the wake even where miss-distances are significantly larger. Indeed, a pos- sible cause of the discrepancy in resolution of the BVI- induced airloads on the advancing side of the rotor put forward in Kelly et al.’s earlier study of the HART II system (Ref. 5), given the rather unusual character of the tip vortices in the HART II tests, was a possible under-resolution of the process whereby the tip vor- tex of the blades on the advancing side of the rotor is formed, and hence a slight mis-distribution of the vorticity within the trailing vortex structure behind the blades.
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