Abstract:Due to global concerns about environmental pollution and potential fossil energy shortage, wind energy has become a significant clean energy for electricity generation systems all over the world. In China the total installed capacity of wind power has been over 70 GW in 2012, which makes China become the country with the largest wind power installation capacity worldwide. According to the official plan of Chinese government, the installed capacity of wind power in China is expected to be 150 and 200 GW in 2015 and 2020, respectively. The report from the Global Wind Energy Council has also shown that the total installed capacity of wind power in the US has reached to 60 GW in 2012. In Europe, the European Commission published its Communication "Investing in the Development of Low Carbon Technologies (SET-Plan)" in October 2009, stating that wind power would be "capable of contributing up to 20 % of EU electricity by 2020 and as much as 33 % by 2030". However, due to the intermittence and fluctuation of wind, the increasing penetration of wind power into the transmission grid has imposed many technical challenges on maintaining the secure and stable operation and control of transmission grids. In 2011, there were 193 wind power outages occurred in China. Among them, there were 54 outages with wind power losses ranging from 100 to 500 MW, and 12 outages with wind power losses over 500 MW. Therefore, effective technical solutions associated with the grid integration of large-scale wind power, and safe and reliable operation and control of the transmission grid integrated with large-scale wind power are urgently required. This special issue is dedicated to address the challenges, experiences, and solutions for the development of wind power technologies. Eight original research papers are brought together to report the most recent advances in wind power fields, particularly in wind power forecasting, wind turbine driven generation system, wind power integration, modeling and simulation, and operation, control and dispatch of the transmission grid integrated with large-scale wind power, etc. The issue also includes three reviews about the present state of art of research on wind power technologies and the suggestions for promoting the future research in this field.

Abstract:In recent years, wind power is experiencing a rapid growth, and large-scale wind turbines/wind farms have been developed and connected to power systems. However, the traditional power system generation units are centralized located synchronous generators with different characteristics compared with wind turbines. This paper presents an overview of the issues about integrating largescale wind power plants into modern power systems. Firstly, grid codes are introduced. Then, the main technical problems and challenges are presented. Finally, some possible technical solutions are discussed.

Abstract:This paper discusses a number of ways in which wind power installations can impact the harmonic levels in the power system. Wind turbines are an additional source of harmonic emission, especially when it concerns ‘‘non-characteristic harmonics.’’ Parallel resonances can amplify the emission from individual turbines. A mathematical model is developed to quantify this amplification. Series resonances can result in high currents, driven by the back ground voltage distortion in the transmission grid, flowing into the wind park.Weakening of the transmission grid will increase lower order harmonics but reduce higher order harmonics

Abstract:Wind power has been developing rapidly inmajor countries in the past 10 years. The distinct static and dynamic characteristics of output power compared with conventional generations pose significant challenges on power system adequacy and stability and constraints on the penetration level of wind power in power systems. Based on the uniqueness of wind power versus conventional generations,we discuss its implications on power system adequacy and stability and propose basic solutions for facilitating largescale integrations of wind power into the power system.

Abstract:With a high penetration of wind turbines, the proportion of synchronous generation in the power system will be reduced at times, thus creating operating difficulties especially during frequency events. Therefore, it is anticipated that many grid operators will demand inertia response from wind turbines. In this article, different ways for emulatinginertia response in full-rated power converter-based wind turbines equipped with permanent magnet synchronous generators are considered. Supplementary controlsignals are added to the controller of the wind turbine toextract stored energy from the rotating mass and DC-link capacitors. Simulations in MATLAB/Simulink show that the inertia response is improved by adding a term proportional to the rate of change of frequency and by extracting the stored energy in the DC-link capacitors.

Abstract:Multi-terminal high-voltage DC (MTDC)technology is a promising way to transmit large amounts of offshore wind power to the main grids. This paper proposesa hybrid MTDC scheme to integrate several offshore wind farms into the onshore power grids at different locations. Ahybrid four-terminal HVDC system comprising twoonshore line commutated converters (LCCs) and two voltage source converters (VSCs) connecting an offshorewind farm is constructed in PSCAD/EMTDC. A coordination control scheme based on the VSCs’ AC voltage control and the LCCs’ DC voltage droop control is designed to ensure smooth system operation and proper power sharing between onshore AC grids. The operationalcharacteristics of the system are analyzed. In addition, a black start-up method without any auxiliary power supplyfor the VSCs is proposed. The transmission scheme is tested through simulations under various conditions,including start-up, wind speed variation, and the disconnectionof one VSC or of one LCC.

Abstract:With the development of wind energy, it is necessary to develop equivalent models to represent dynamic behaviors of wind farms in power systems. The equivalent wind method is investigated for the aggregation of doubly-fed induction generator wind turbines. The detailed procedures for the calculation of equivalent wind are analyzed. The necessity of classifying incoming winds is shown. To improve the performances of the method,incoming winds are classified according to mean wind speeds and positive/negative semi-variances of wind speeds,and a group of turbines with similar incoming winds are aggregated together. The effectiveness of the method is verified through simulations in MATLAB/Simulink.

Abstract:The ability of an energy storage system to improve the performance of a wind turbine (WT) with afully rated converter was evaluated, where the energy storage device is embedded in the direct current (dc) link with a bidirectional dc/dc converter. Coordinated dc voltage control design of the line-side converter and the energy storage dc/dc converters was proposed using a common dcvoltage measurement for smoothing the output power. Atransfer function and Bode diagram were introduced toanalyze the system performance with different control parameters. MATLAB/Simulink simulations are presented to demonstrate the effectiveness of the proposed methods.It was found that the proposed methods smooth the power output from the WT to the grid and thus improve the quality of the generated power.

Abstract:With the rapid growth of grid-connected wind power penetration level, it is necessary to study the impacts of wind power on power system stability. The small-signal stability of power systems with large-scale wind power is explored using the eigenvalue analysis method. A prototype sample system, the two-synchronous-generator system with awind farm, is proposed for the oretical analysis. Then, simplified models of wind turbines (WTs) and the corresponding equivalent models of wind farms are analyzed. Three kinds oftypical WT models, i.e., squirrel cage induction generator,doublyfed induction generator, and permanent magnet synchronous generator are used. Furthermore, based on thesimplified equivalent models, effects of large-scale windfarms on the electromechanical oscillation modes (EOMs) of synchronous systems are discussed. Simulation results indicatethat wind farms of the three kinds of WTs have positive effects on EOMs. However, long transmission lines connecting wind farmto the system may produce negative effectson the small-signal stability of the system.

Abstract:With the technical development of wind power forecasting, making wind power generation schedule in power systems become an inevitable tendency. This paper proposes a new dispatch method for wind farm (WF)cluster by considering wind power forecasting errors. Aprobability distribution model of wind power forecasting errors and a mathematic expectation of the power shortage caused by forecasting errors are established. Then, the total mathematic expectation of power shortage from all WFs is minimized. Case study with respect to power dispatch in aWF cluster is conducted using forecasting and actual wind power data within 30 days from sites located at Gansu Province. Compared with the variable proportion method,the power shortage of the WF cluster caused by wind power forecasting errors is reduced. Along with theincrement of wind power integrated into power systems,the method positively influences future wind power operation.

Abstract:A novel renewable energy intermittency modeland a new midterm dynamic simulation tool in power systems are developed for examining dynamic behavior along the load curve for different combinations of the system operation reserves and renewable portfolio standard (RPS) rates. The system’s import limits are considered. It is concluded that ignoring intermittency and governor effects is an inadequate method to assess intermittency impact. The intermittencymidterm dynamic impact must be studied. For the studied system, the instability is expected to be about 25 %RPS with current reserves. Besides, the most vulnerable peak hour toinstability is the afternoon peak hour when solar begins to drop off. This article stimulates further dynamic intermittencystudies on the issues caused by renewable intermittency.The studies on the issues caused by renewable intermittency have not been revealed because of inadequate/incomprehensive study methodologies so that effective,mitigative solutions can be developed to guarantee the reliability of power grid when incorporating higher RPS if highoperation reserves are impractical.

Abstract:The use of wind power has grown rapidly in recent years. Wind power is a clean source of energy, but can have negative impacts on the distribution grid. The influence of large-scale wind power integration on the safeand stable operation of a power system can not be ignored.It is necessary and urgent to achieve grid adaptability forwind turbines in China. Using a 35 kV/6 MVA grid simulator,the performance of a grid is investigated by simulation.Typical grid disturbances such as voltage deviation,frequency fluctuation, voltage unbalance, and distortion can be simulated. A grid adaptability testing methodologywas developed and applied to a doubly fed wind turbine with a focus on analyzing real test data to as certain itsthree-phase voltage unbalance adaptability, which was successfully demonstrated. The methodology can also beused to guide other grid adaptability tests.