Power
Z. Dehghani Arani; S. A. Taher; M. H. Karimi; M. Rahimi
Abstract
Background and Objectives: The wind turbines (WTs) with doubly fed induction generator (DFIG) have active and reactive power as well as electromagnetic torque oscillations, rotor over-current and DC-link over-voltage problems under grid faults. Solutions for these problems presented in articles can be ...
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Background and Objectives: The wind turbines (WTs) with doubly fed induction generator (DFIG) have active and reactive power as well as electromagnetic torque oscillations, rotor over-current and DC-link over-voltage problems under grid faults. Solutions for these problems presented in articles can be classified into three categories: hardware protection devices, software methods, and combination of hardware and software techniques.Methods: Conventional protection devices used for fault ride through (FRT) capability improvement of grid-connected DFIG-based WTs impose difficulty in rotor side converter (RSC) controlling, causing failure to comply with grid code requirements. Hence, the main idea in this paper is to develop a novel coordinated model predictive control (MPC) for the power converters without need to use any auxiliary hardware. Control objectives are defined to maintain DC-link voltage, rotor current as well as electromagnetic torque within permissible limits under grid fault conditions by choosing the best switching state so as to meet and exceed FRT requirements. Model predictive current and electromagnetic torque control schemes are implemented in the RSC. Also, model predictive current and DC-link voltage control schemes are applied to grid side converter (GSC).Results: To validate the proposed control method, simulation studies are compared to conventional proportional-plus-integral (PI) controllers and sliding mode control (SMC) with pulse-width modulation (PWM) switching algorithm. In different case studies comprising variable wind speeds, single-phase fault, DFIG parameters variations, and severe voltage dip, the rotor current and DC-link voltage are respectively restricted to 2 pu and 1.2 times of DC-link rated voltage by the proposed MPC-based approach. The maximum peak values of DC-link voltage are 1783, 1463 and 1190 V by using PI control, SMC and the proposed methods, respectively. The maximum peak values of rotor current obtained by PI control, SMC and the proposed strategies are 3.23, 3.3 and 1.95 pu, respectively. Also, PI control, SMC and the proposed MPC methods present 0.8, 0.4 and 0.14 pu, respectively as the maximum peak values of electromagnetic torque.Conclusion: The proposed control schemes are able to effectively improve the FRT capability of grid-connected DFIG-based WTs and keep the values of DC-link voltage, rotor current and electromagnetic torque within the acceptable limits. Moreover, these schemes present fast dynamic behavior during grid fault conditions due to modulator-free capability of the MPC method.
Power
H. Berahmandpour; S. M. Kouhsari; H. Rastegar
Abstract
Background and Objectives: Uncertainty and variability are two main specifications of wind generation and the ability of the power system to overcome these challenges is called flexibility. The flexibility index is a measure to evaluate the flexibility level of the power system mainly to achieve the ...
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Background and Objectives: Uncertainty and variability are two main specifications of wind generation and the ability of the power system to overcome these challenges is called flexibility. The flexibility index is a measure to evaluate the flexibility level of the power system mainly to achieve the best level system flexibility.Methods: Flexibility index should show a good view of the ability of the power system and also be easily converted to an equivalent cost to be combined with the operation cost function. So, in this paper by using economic dispatch simulation for the economic trade-off between the generation cost and the cost of flexibility, the best level of system flexibility in the presence of wind farms considering unit constraints and system loss is achieved. Where the difference between flexibility index in the no wind base case and the flexibility index in each time zone with wind incorporation is defined as the flexibility penalty by the suitable penalty factor. The combination of generation cost and flexibility cost makes the main part of objective function.Results: The results on the test system verify the proposed method where by increasing penalty factor, improvement in flexibility index is achieved but the generation cost will be increased. So, it shows a good economic trade-off between generation cost and flexibility value. Also the desired flexibility level can be obtained by changing the penalty factor in each wind power penetration. So, the result of the sensitivity analysis shows the best level of flexibility regarding operation cost.Conclusion: In this paper a new flexibility index is introduced especially for wind power incorporation and for real time operation purpose. This index can be combined by economic dispatch objective function as the penalty (cost) for economic trade-off analysis and to show the best flexibility level of generation system in each operation point.
