Abstract —— Due to network events and power system emergencies, doubly-fed induction generator (DFIG) based wind turbines can suffer from low voltage ride through which may result to the disconnection of the DFIG. Before connecting to the electricity grid again, DFIG has to be synchronized to minimize the transient effects and ensure smooth connection to the power grid. In this paper, a direct voltage control based synchronization technique is proposed for DFIG based wind turbines. Additional control features are added to original direct voltage control method for implementation of the proposed synchronization strategy. A grid connection logic is designed and presented for soft connection of DFIG to the grid. The proposed technique is implemented in a DFIG based wind turbine model and tested through simulations with various practical scenarios. Voltage transients, frequency analysis, voltage phase and system stability are analyzed in details and simulation results presented. From the results, it can be revealed that the proposed synchronization technique functions effectively and efficiently. It is also shown that by using the proposed technique, the process of synchronization and grid connection do not introduce any changes into the system, which may cause protection devices to trip or mal-operation.
2021-08-26 14:08:26
1.03MB
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Abstract —— This paper presents a mathematical model of a doubly fed induction generator (DFIG) in the positive synchronous reference frame under distorted grid voltage conditions. The oscillations of the DFIG’s electromagnetic torque and the instantaneous stator active and reactive powers are fully described when the grid voltage is harmonically distorted. Four alternative control targets are proposed to improve the system responses during grid harmonic distortions. A new rotor current control scheme implemented in the positive synchronous reference frame is developed. The control scheme consists of a proportional–integral (PI) regulator and a harmonic resonant (R) compensator tuned at six times the grid frequency. Consequently, the fundamental and the fifth- and seventh-order components of rotor currents are directly regulated by the PI–R controller without sequential-component decompositions. The feasibility of the proposed control strategy is validated by simulation studies on a 2.0-MW wind-turbine-driven DFIG system. Compared with the conventional vector control scheme based on standard PI current controllers, the proposed control scheme leads to significant elimination of either DFIG power or torque oscillations under distorted grid voltage conditions.
2021-08-26 14:08:25
1.82MB
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Abstract ——This article deals with a phase-locked loop (PLL)- based novel control for wind turbine driven doubly fed induction generator interfaced to utility grid with a battery energy storage (BES) connected at the dc link. The control of grid-side converter (GSC) is modified to export/import constant power to/from the grid. The state of charge of BES helps in deciding the reference export power to the grid apart from the manual selection using averaged wind power in a particular period of time. An off maximum power point tracking logic is incorporated in the rotor-side converter (RSC) control to operate the BES within its constraints and, moreover, to feed constant power to the grid. In addition, the energy management scheme of the system is presented in the form of flowchart for both exporting and importing power to/from the grid. The RSC and GSC have taken care of unity power factor operation at stator terminals and to mitigate harmonics and grid currents balancing, respectively. The system performance is found robust as the PLL response is not affected even under grid voltages with dc offset. The system is modeled and simulations are carried out in MATLAB using SimPowerSystems tool box. Moreover, the control scheme performance is compared with conventional control algorithms both in terms of PLL and converter controls. To validate the effectiveness of the control scheme, a prototype of the system is developed. Test results demonstrate the satisfactory performance of the system under various operating conditions.
2021-08-26 14:08:25
7.26MB
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Abstract —— The doubly fed induction generator is particularly appropriate to be used in wind power generation systems. However, in weak grids its stator connection to unbalanced or distorted voltages imposes low frequency and high amplitude rotor current harmonics. Also, the grid side converter must deal with the same voltage conditions. The paper addresses the first issue presenting a voltage harmonics controller in the dq reference frame to be included in the high dynamics rotor current controller. Simulation and experimental results demonstrate the performance of the proposed controller imposing a sinusoidal input current.
2021-08-26 14:08:23
3.46MB
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Abstract —— Grid connected wind turbines may produce flicker during continuous operation. This paper presents a simulation model of a MW-level variable speed wind turbine with a doubly fed induction generator developed in the simulation tool of PSCAD/EMTDC. Flicker emission of variable speed wind turbines with doubly fed induction generators is investigated during continuous operation, and the dependence of flicker emission on mean wind speed, wind turbulence intensity, short circuit capacity of grid and grid impedance angle are analyzed. A comparison is done with the fixed speed wind turbine, which leads to a conclusion that the factors mentioned above have different influences on flicker emission compared with that in the case of the fixed speed wind turbine. Flicker mitigation is realized by output reactive power control of the variable speed wind turbine with doubly fed induction generator. Simulation results show the wind turbine output reactive power control provides an effective means for flicker mitigation regardless of mean wind speed, turbulence intensity and short circuit capacity ratio.
2021-08-26 14:08:22
598KB
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