Measurement of Voltage Flicker: Application to Grid-connected Wind Turbines

Abstract

Electric power is an essential commodity for most industrial, commercial and domestic processes. As a product, electric power must be of an acceptable quality, to guarantee the correct behavior of the equipment connected to the power distribution system. Low-frequency conducted disturbances are the main factors that can compromise power quality. The IEC 610002-1 standard classifies low-frequency conducted disturbances in the following five groups: harmonics and interharmonics, voltage dips and short supply interruptions, voltage unbalance, power frequency variations and voltage fluctuations or flicker. Voltage fluctuations are defined as cyclic variations in voltage with amplitude below 10% of the nominal value. Most of the connected equipment is not affected by voltage fluctuations, but these fluctuations may cause changes in the illumination intensity of light sources, known as flicker. Flicker may produce a very unpleasant visual sensation, leading to complaints from utility customers. The annoyance level depends on the type of lamp and amplitude, frequency and duration of the voltage fluctuations. Its precise quantification is a complex task that must be statistically approached to characterize adequately the perception of a large number of people. A flickermeter must characterize the behavior of the lamp-eye-brain set that represents most people and must provide an indication of the discomfort, or flicker severity. In 1986, The International Electrotechnical Commission (IEC) published the first standard describing the functional and design specifications for the measurement of flicker. The main sources of flicker are large industrial loads, such as arc furnaces, or smaller loads with regular duty cycles, such as welding machines or electric boilers. However, from the point of view of power generation, flicker as a result of wind turbines has gained attention in recent years. Rapid variations in wind speed produce fluctuating power, which can lead to voltage fluctuations at the point of common coupling (PCC), which in turn generate flicker. The IEC 61400-21 standard establishes the procedures for measuring and assessing the power quality characteristics of grid-connected wind turbines. The section dedicated to flicker proposes a complex model for calculating the flicker coefficient that characterizes a wind turbine. This coefficient must be estimated from the current and voltage time series obtained for different wind conditions. The wind turbine being tested is usually connected to a medium-voltage network, having other fluctuating loads that may cause significant voltage fluctuations. In addition, the voltage fluctuations imposed by the wind turbine depend on the characteristics 15