Power Quality in Grid-Connected Wind Turbines (original) (raw)

Electric power is a very unusual product. It must be constantly available for the customers but its massive storage is not possible. Therefore, electric power needs to be generated as it is used. Furthermore, it is usually generated far from the customers. All these factors make it difficult to control and assure the quality of electric power. There is no procedure that allows the removal of the poor electric power. Consequently, it is necessary to define preventive and monitoring procedures that guarantee a minimum level of power quality and hence the correct behavior of the equipment connected to the power distribution system. Perfect power quality means that the voltage is continuous and sinusoidal with a constant amplitude and frequency. Low-frequency conducted disturbances are the main defects that could compromise power quality. These are classified in the following groups: harmonics and interharmonics, voltage dips and short supply interruptions, voltage unbalance, power frequency variations and voltage fluctuations or flicker. In the last years the use of wind farms and other distributed power generation systems has drastically increased. The question that needs to be raised is how those new generation systems will affect to the whole grid. A portion of the answer must be obtained from the impact that they have on the power quality. In principle, wind energy can be considered a risky source in terms of power quality. Moreover, when wind turbines are part of the grid the power, quality seems to be a complex issue which highly depends on the interaction between the grid and the wind turbines. The main impact on the grid by the wind turbines, concerning power quality, is related to voltage changes and fluctuations, harmonic content, power peaks and flicker. The presence of these disturbances is determined by meteorological conditions and by the technical features of the wind turbine: continuously variable output power due to wind shear, tower shadow or turbulences; performance of electrical components such as generators and transformers; aerodynamic and mechanical behavior of the rotor.. . The power quality of the wind turbines must be certified on the basis of measurements performed according to international standards and guidelines. On one hand, the IEC 61400-21 standard is the reference normative for the certification of the power quality of wind turbines (IEC-61400-21 Ed. 2.0, 2008). The first edition was published in 2001 and it specifies the main relevant features of power quality that should be measured in a wind turbine. This standard establishes the procedures for the measurement and assessment of power quality characteristics of grid-connected wind turbines. According to it, measurements should be performed for harmonic content, flicker, voltage drops as well as active and reactive power, during normal and switching operations. To obtain those characteristics, the measurements should be made on the basis of long time-series of current and voltage, always depending on 24 www.intechopen.com 2.1 Current harmonics, interharmonics and higher frequency components Voltage and current harmonics are usually present on the utility network. Non-linear and electronic loads, rectifiers and inverters, are some sources which produce harmonic content. The effects of the harmonics include overheating, faulty operation of protections, equipment failures or interferences with communication systems. The standard specifically defines different procedures to assess the harmonics, interharmonics and higher frequency components for a wind turbine working under continuous conditions and operating with reactive power as close as possible to zero. This means that, if applicable, 548