Advanced Materials Research Vols. 805-806

Abstract: Aim to solve the harmonic problems in microgrid, parallel operation of APFs (active power filter) is studied. With the comparison and study of many strategies about current sharing and parallel operation, a new parallel method and control strategy of APF is proposed in microgrid. And the implementation method of parallel operation and current sharing is studied. Finally, both the simulation and experimental results demonstrates that the method is correct and effective, and the good dynamic performance and compensation effect have been proved.

Abstract: At present, the research on wind turbine low voltage ride through (LVRT) behavior is one of hotspots in wind power industry. With increasing share of wind power in the grid, it is important to enhance the LVRT performance of wind turbines to guarantee the safety and reliability of the grid. The phenomenon of voltage double-dip is one of cases during the voltage dips. In this paper, the voltage double-dip phenomenon is described, and the stator flux attenuation of DFIG wind turbine in the voltage dip is analyzed. The results show that when the second voltage dip occurs, the initial value of the stator flux should be considered. Finally, by on-site testing results, the impact of voltage double-dip to the wind turbine is verified, if the initial value of the stator flux is not considered, and the interval of two voltage dips is different.

Abstract: With the rapid development of wind power, electric grid faces significant challenges from the variable nature and anti-peak-regulation characteristic of wind power. In order to mitigate the impact of wind power, large capacity electrochemical energy storage is proposed to solve this problem. This study establishes a unit commitment (UC) model with large capacity electrochemical energy storage given the specific characteristics. Meanwhile, wind forecast deviation and curtailment are considered. Case studies with modified IEEE 39-bus system are employed to validate the proposed method. The impact of electrochemical energy storage system on economics, peak load shifting and accommodating wind power is analyzed.

Abstract: With the penetration of wind energy is becoming higher and higher in power grid, it is very important to investigate the impact of wind generations on small signal stability. In this paper, a complete small signal model of wind turbine with direct-drive permanent magnet generator is built to study the impact of large-scale wind farms on the small signal stability of power system. By means of simulation and eigenvalue analysis, an actual power system is investigated, and the damping characteristic of power grid under different wind power penetration is discussed.

Abstract: This paper showed that the doubly-fed induction generator (DFIG) was sensitive to the faults of power grid. Through theoretic analysis for active power characteristic of DFIG, it was recognized that the conventional PI-based vector control strategy was likely to cause the excitation system overregulated due to lack of additional protection technique. On this condition, a DFIG risks damaged to its power converter due to overcurrent or overvoltage. At the basis of prior art, this paper developed a novel control scheme, which focused on solving overregulated problems caused by faults of power grid. The proposed control strategy was tested on an illustration DFIG-based power system modeled by MATLAB/SIMULINK. The results demonstrate that the proposed control scheme can effectively resist overcurrent under the conditions of both symmetrical and unsymmetrical grid faults.

Abstract: When transmitting power to the grid through fixed series compensated lines, the sub-synchronous oscillations (SSO) may occur in doubly-fed wind farms. This paper analyzes the mechanism of SSO in doubly-fed wind farms, establishes the TCSC model and analyzes its equivalent positive resistance and reactance at sub-synchronous frequency so that TCSC can be used to mitigate SSO. A simulation model of the doubly-fed wind farms connected to the network by the fixed series compensated and TCSC is established in MATLAB/SIMULINK, and the appropriate parameters of TCSC is set. The simulation results verified the mitigation of SSO in doubly-fed wind farms with TCSC.

Abstract: In this paper, the variable speed wind turbine model based on doubly-fed induction generator (DFIG) and a practical power grid model to be studied are implemented in DIgSILENT / PowerFactory. The transient voltage stability and the static voltage stability of the power grid with high wind power penetration will be studied.

Abstract: The paper builds up a wind farm simulation model by DIgSILENT software, and analyses the short-circuit current (SCC) characteristics of doubly-fed induction generators (DFIGs) and wind farm. Simulation analysis shows that DFIGs have different SCC characteristics compared with synchronous generators. The wind farm only provides little fault current when symmetrical fault occurs in the grid; while under asymmetrically grounding fault, the SCC in wind farm side mainly includes zero-sequence current, which shows that the wind farm has weak power characteristic. After integrating the wind farm into the power grid, sensitivity of current-based protection, transformer differential protection as well as phase selection element will be influenced at some extent. Some countermeasures to reduce the impacts of wind power integration on protection are proposed based on the analysis of the impacts of wind farm on protection.

Abstract: The study undertook various calculations of the turbulent wind flow around a body in close proximity to neighboring obstacles, with the aim of gaining an understanding of the wind velocity and the surface-pressure variations with respect to the azimuth angle of wind direction and the gap distance between the obstacles. This paper presents the effects of wind flow interference among three parallel-aligned wall-mounted cubes for azimuth angles of Φ = 0°, 15°, 30°, and 45° and gap distances of G = 0.5h, 1.0h, 1.5h, and ∞ (i.e., a single cube), where G is the gap distance and h is the cube height. A transient detached eddy simulation (DES) was carried out to calculate the highly complicated wind flow domain around the three cubes to observe the surface-pressure, velocity, vortex and spectra characteristics. The results indicate that an increasing wind azimuth angle can even change the mean surface pressure coefficient on the side face of the center cube from negative to positive value. In addition, because of the interference effects, the velocity and pressure distributions around the center cube also show a substantial change depending on the gap distance.

Abstract: Along with the concern about environmental pollution and global warming, the development of wind energy has rapidly progressed over the last decade by the improving in the technology and the provision of government energy policy. In view of the intermittent property of wind energy causing variability, unpredictability and uncertainty, this paper analyzes the related technical features of wind energy, e.g., power curve, wind speed, wind power and energy, to provide the further reference for analyzing the impacts of wind energy on power system in depth. First of all, wind turbine, the key part of wind energy, is discussed, including its components and power curve. Second, wind speed, the key factor for calculating wind power and energy, is analyzed and derived in detail. On the basis of wind speed distribution, two types of wind speed are calculated, i.e., the arithmetic mean wind speed and the cubic root cube wind speed. Then, wind power and energy are presented and calculated. Finally, the related conclusions are drawn.