Figure 11. DFIG based system response on pure reactive load pickup at 50% level of excitation.
3.1.3. Power Quality Assessment
The 2 common issues related to power quality are flicker and harmonics. Flicker is defined as ‘an impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time’ [31]. Harmonics, on the other hand, is defined as ‘sinusoidal voltages or currents having frequencies that are integer multiple of the frequency at which the supply system is designed to operate’ [31].
Flicker assessment methodology has been described in IEC 61000-4-15 [32]. Two common metrics are used: Short term flicker taken over a maximum 10-minute period and long-term flicker taken as a cubic mean of the short-term flicker values over a 2-hour period. These metrics are normalized to between 0 and 1 where 1 means ‘voltage fluctuations sufficient to cause noticeable flicker to 50% of a sample observing group ’ [32]. Flicker was investigated by assuming a ±2m/s wind speed range within a period of 5 seconds. Fig. 12 shows the network response.