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This paper presents a coordinated control method for a doubly-fed induction generator (DFIG)-based wind-power generation system with a series grid-side converter (SGSC) under distorted grid voltage conditions. The detailed mathematical models of the DFIG system with SGSC are developed in the multiple synchronous rotating reference frames. In order...
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... the stationary αβ reference frame, the voltage and current vectors can be represented as the combinations of the fundamental positive-sequence vector and the harmonic fifth-and seventh-order vectors. In order to simplify the analysis, the multiple synchronous rotating reference frames are adopted, as shown in Figure 2. For the positive (dq) + reference frame, the d + -axis is aligned with the positive-sequence grid voltage vector. ...
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In this paper, we focus on the modeling and control of a wind power system based on a double-fed induction generator DFIG. We proposed a technique of active and reactive power control to improve the performance and dynamics of variable speed wind system. The objective of the modeling is to apply the direct and indirect vector control stator flux or...
A grid connected doubly-fed induction generator (DFIG) system, driven by variable speed wind turbine is considered in this research to satisfy grid code requirements. Remaining grid synchronized and stable under voltage sag and voltage swell, obtaining power control through dc link voltage control, and providing unity power factor at grid terminals...
Citations
... One of the most important power quality issues is to control the voltage and current distortion in the PCC as determined by the standards [10,11]. Many control strategies have been developed to mitigate harmonics in VSWTs [12,13]. The work in [14] proposes a mathematical model and control strategy for DFIG under unbalanced and distorted grid voltage conditions. ...
... The EFR electrical model ( Figure 2) consists of a similar set of squirrel cage induction machines. Equation (12) in the fundamental positive synchronous reference frame (dq + ) considering the relative electrical speed of the EFR armature (ω s ) [13]. The superscript + indicates the referential frame related to the fundamental positive frequency ( f i ) of the voltage applied to armature V + dq . ...
The power quality analysis is an essential issue in the integration of distributed energy resources to the grid. Recent standards regulate the harmonics disturbances due to the increasing penetration of intermittent energy sources interconnected with the grid employing power converters. This paper aims to analyze the power quality of an interconnected wind turbine system based on a Squirrel Cage Induction Generator (SCIG) driven by an Electromagnetic Frequency Regulator (EFR). The steady state of the EFR harmonic model is developed in the stationary frame based on the conventional induction generator modeling, which allows the study of the harmonic disturbances in the electrical and mechanical variables due to the PWM inverter of the EFR’s armature voltage. There is no electrical connection between the EFR and SCIG, and the results show that the inherent system inertia contributes to the mitigation of the harmonic content at the grid side generated by the switching. In addition to the steady-state results, the Total Rated Distortion (TRD), which considers the harmonics and interharmonics components, was computed and presented a good performance compared to the IEEE 1547 standard and real data extracted of a single Doubly Fed Induction Generator (DFIG). Finally, the harmonic performance of the proposed system was evaluated considering the impact of the equivalent Thevenin impedance of the grid at the Point of Common Coupling (PCC).
... Series compensation devices have been widely used to overcome this problem. The SC lines offer many advantages such as increase in power transfer capability, enhancement of transient stability limit, improving the voltage profile and reducing power losses [1][2][3]. However, with the interconnection of bulk power systems, for the line with SC device, the system impedance on the back of the line may be less than that of the traditional line. ...
With the increasing integration of huge-size generators and tighter interconnection among power grids, in internal fault, the fault current of Series Compensated (SC) lines may be reversed. At this time, the differential protection of SC lines may fail to operate due to insufficient sensitivity. In this research analyzes the reasons for the current reverse in SC lines and its effect on differential protection. The operation characteristics of differential protection are studied by focusing on multiple factors, including different series compensation degree, fault types, system operation mode, power angle difference, fault points, and transition resistance. An improved criterion based on the current amplitudes and phases on both sides of the series compensated lines is proposed. Further, to strengthen the operation characteristics of differential protection, the triple-fold line is chosen to improve the sensitivity for internal faults in this paper. The criterion is effective to improve the protection sensitivity of SC lines during internal fault. The PSCAD/EMTDC simulation and the power system dynamic physics simulation demonstrates that protection sensitivity is obviously increased in the improved scheme.
... Renewable energy technologies, especially wind power technology, have won widespread attention and seen rapid development due to the grim energy and environmental issues, and DFIG wind power systems are the most widely used [1][2][3][4][5][6]. DFIG can not only operate at variable speed constant frequency (VSVF), but also adjust a large amount of reactive power. ...
Harmonic amplification for doubly-fed induction generator wind turbine systems (DFIG WTSs) will occur due to the existence of non-linear loads and reactive power compensation installation, and grid voltage and total grid current at the point of common coupling (PCC) will be distorted. An impedance model is established to analyze the interaction between DFIG WTS, non-linear loads and weak grids. Harmonic current impact factor and harmonic voltage impact factor is proposed to analyze the impact of harmonic current source on total grid current and voltage at the PCC with different control strategies. A virtual harmonic resistor and capacitor method is adopted to reduce the harmonic voltage. An impedance-based analysis method is adopted to analyze the stability of the DFIG system. To achieve optimal control of harmonic voltage and harmonic current, a coordination factor is proposed to adjust the dynamic allocation for harmonic voltage and harmonic current at PCC. The experimental results demonstrate the effectiveness of the proposed control strategy.
... In regards to wind power generation, the double fed induction generator (DFIG) has become the leading type of wind turbines. As a variable speed wind generator, DFIG has many advantages compared with other types, such as its good stability after connecting to the grid, and decoupled control of active and reactive power [9][10][11]. These advantages are realized on the basis of the pulse width modulation (PWM) converter, which is widely used in the DFIG systems for its ability to realize a bidirectional flowing of energy. ...
Harmonic pollution of double fed induction generators (DFIGs) has become a vital concern for its undesirable effects on power quality issues of wind generation systems and grid-connected system, and the double pulse width modulation (PWM)converter is one of the main harmonic sources in DFIGs. Thus the harmonic analysis of the converter in DFIGs is a necessary step to evaluate their harmonic pollution of DFIGs. This paper proposes a detailed harmonic modeling method to discuss the main harmonic components in a converter. In general the harmonic modeling could be divided into the low-order harmonic part (up to 30th order) and the high-order harmonic part (greater than order 30) parts in general. The low-order harmonics are produced by the circuit topology and control algorithm, and the harmonic component will be different if the control strategy changes. The high-order harmonics are produced by the modulation of the switching function to the dc variable. In this paper, the low-order harmonic modeling is established according to the directions of power flow under the vector control (VC), and the high-order harmonic modeling is established by the switching function of space vector PWM and dc currents. Meanwhile, the simulations of harmonic a components in a converter are accomplished in a real time digital simulation system. The results indicate that the proposed modeling could effectively show the harmonics distribution of the converter in DFIGs.
... However, the complexity of adjusting the PI controller coefficients still remains. In [31], the PI plus resonant (PI+R) controllers are used for the SGSC voltage control (tuning) and PGSC current control (tuning), respectively under harmonically distorted voltage conditions in the rotating positive (dq) + reference frame. This method avoids sequential decomposition of currents and voltages which in turn, avoid the slight errors caused by the decomposition procedure. ...
In this paper, the power quality in a wind power generation system is studied. It is shown that adding a Series Grid-Side Converter (SGSC) to the Doubly Fed Induction Generator (DFIG) structure and applying a suitable control algorithm will make the system able to compensate for the adverse effects of the voltage unbalance and, consequently, it can improve the power quality. In order to decrease design complexity and implement the control algorithms in the stationary reference frame, a Sliding-Mode Control (SMC) method for controlling the DFIG system with SGSC is proposed. One of the advantages of the proposed method in this paper is its robustness to parameter variations both in the DFIG and the connected network. Moreover, the designed controller leads to a fast dynamic response. It should be mentioned that a coordinated control is carried out between SGSC and the other DFIG converters. The simulation results approve the validity of the mentioned advantages and the effectiveness of the proposed method.
... In reality, however, asymmetrical faults happen much more frequently than symmetrical faults [11]. Furthermore, it is recognised that both transmission and distribution networks could experience voltage imbalance, which can also occur in a weak power grid even during normal operations [12][13][14][15]. A weak power grid is one where changes in real and reactive power flowing into or out of the network will cause significant changes in the voltage on the network, which is also referred to as having a low short-circuit level or a low fault level [16,17]. ...
This paper analyses the effects of doubly fed induction generator (DFIG) tidal current turbines on a distribution grid under unbalanced voltage conditions of the grid. A dynamic model of an electrical power system under the unbalanced network is described in the paper, aiming to compare the system performance when connected with and without DFIG at the same location in a distribution grid. Extensive simulations of investigating the effect of DFIG tidal current turbine on stability of the distribution grid are performed, taking into account factors such as the power rating, the connection distance of the turbine and the grid voltage dip. The dynamic responses of the distribution system are examined, especially its ability to ride through fault events under unbalanced grid voltage conditions. The research has shown that DFIG tidal current turbines can provide a good damping performance and that modern DFIG tidal current power plants, equipped with power electronics and low-voltage ride-through capability, can stay connected to weak electrical grids even under the unbalanced voltage conditions, whilst not reducing system stability.
... With the tremendously increasing total installed wind power capacity in power systems, large scale wind power integration brings new challenges to the secure and stable operation of these power systems [1,2]. Due to the international academic and industrial perspective that the wind is stronger and more consistent offshore than onshore, the installation of offshore wind farms is growing worldwide. ...
In recent years, the increasing penetration level of wind energy into power systems has brought new issues and challenges. One of the main concerns is the issue of dynamic response capability during outer disturbance conditions, especially the fault-tolerance capability during asymmetrical faults. In order to improve the fault-tolerance and dynamic response capability under asymmetrical grid fault conditions, an optimal integrated control scheme for the grid-side voltage-source converter (VSC) of direct-driven permanent magnet synchronous generator (PMSG)-based wind turbine systems is proposed in this paper. The optimal control strategy includes a main controller and an additional controller. In the main controller, a double-loop controller based on differential flatness-based theory is designed for grid-side VSC. Two parts are involved in the design process of the flatness-based controller: the reference trajectories generation of flatness output and the implementation of the controller. In the additional control aspect, an auxiliary second harmonic compensation control loop based on an improved calculation method for grid-side instantaneous transmission power is designed by the quasi proportional resonant (Quasi-PR) control principle, which is able to simultaneously restrain the second harmonic components in active power and reactive power injected into the grid without the respective calculation for current control references. Moreover, to reduce the DC-link overvoltage during grid faults, the mathematical model of DC-link voltage is analyzed and a feedforward modified control factor is added to the traditional DC voltage control loop in grid-side VSC. The effectiveness of the optimal control scheme is verified in PSCAD/EMTDC simulation software.
... In recent years, wind power generation has become a very attractive source of renewable energy thanks to its unique merits: it is economical, clean and inexhaustible. Thus, more and more wind farms are being connected to power systems [1,2]. However, such large-scale integration of wind power into the power system brings technical challenges for the wind farm operators; for example, the wind turbines should keep continued connection with the power system during a fault and other conditions [3,4]. ...
Unlike the traditional method for power quality improvement and low-voltage ride through (LVRT) capability enhancement of wind farms, this paper proposes a new wind power integrated system by means of an inductive filtering method, especially if it contains a grid-connected transformer, a static synchronous compensator (STATCOM) and fully-tuned (FT) branches. First, the main circuit topology of the new wind power integrated system is presented. Then, the mathematical model is established to reveal the mechanism of harmonic suppression and the reactive compensation of the proposed wind power integrated system, and then the realization conditions of the inductive filtering method is obtained. Further, the control strategy of STATCOM is introduced. Based on the measured data for a real wind farm, the simulation studies are carried out to illustrate the performance of the proposed new wind power integrated system. The results indicate that the new system can not only enhance the LVRT capability of wind farms, but also prevent harmonic components flowing into the primary (grid) winding of the grid-connected transformer. Moreover, since the new method can compensate for reactive power in a wind farm, the power factor at the grid side can be improved effectively.
... Moreover, the cost of the device increases and the reliability decreases. In [11][12][13] the authors have suggested modifying the hardware and installing a series grid side converter (SGSC). However, these can increase the complexity of control systems and the cost of the device itself. ...
An innovative control strategy is proposed for enhancing the low voltage ride-through (LVRT) capability of a doubly fed induction generator based on wind energy conversion systems (DFIG-WECS). Within the proposed control method, the current control loops of the rotor side converter (RSC) are developed based on passivity theory. The control scheme for the grid side converter (GSC) is designed based on a two-term approach to keep the DC-link voltage close to a given value. The first term based on the maximal voltage of GSC is introduced in the GSC control loops as a reference reactive current. The second one reflecting the instantaneous unbalanced power flow between the RSC and GSC is also introduced in the GSC control loops as a disturbance considering the instantaneous power of the grid filter to compensate the instantaneous rotor power. The effectiveness of the proposed control strategy is verified via time domain simulation of a 2.0 MW-575 V DFIG-WECS using PSCAD/EMTP. Simulation results show that the control of the DFIG with the proposed approach can improve the LVRT capability better than with the conventional one.
... The unbalanced currents cause unequal heating in the stator and rotor windings. As a result, DFIGs without unbalanced voltage control may have to be disconnected from the grid when network voltage unbalance is more than 6% [3]. Control schemes to reduce the effects of unbalanced grid voltage on DFIG are well reported in the literature. ...
This study proposes a polar voltage control-based direct torque control method to reduce the effects of unbalanced grid voltage on doubly-fed induction generator (DFIG)-based wind turbine system. Under unbalanced grid voltage, the stator flux has a negative sequence component which leads to second harmonic pulsation in torque, stator active power, stator reactive power, stator current and rotor current. In the control scheme, the negative sequence rotor voltage vector is controlled to compensate the negative sequence stator flux by negative sequence rotor flux. Simulation study is carried out on a 2 MW DFIG system using MATLAB/SIMULINK. Feasibility of the proposed control strategy is experimentally verified on a 1.5 kW DFIG system.
