Incandescent light bulbs can cause annoyance by flickering due to rapid voltage variations. The human eye is particularly sensitive to flicker at around 8 Hz. Sensitive electronic equipment may also be affected. Such voltage variations are generally caused by rapid variations of active or reactive power flows within the network. The classic causes of unacceptable flicker are sawmills in rural areas and arc furnaces. Distributed generators can also contribute to flicker. A flicker meter measures voltage variations of this type and gives figures for short and long term severity.
Wind turbines can contribute to flicker because tower shadow and turbulent winds can cause rapid variations in active and reactive power output. Fixed speed pitch-regulated turbines are generally worse than stall-regulated ones at high wind speeds. This is because the pitch regulation action in response to, say, a gust is slow to take effect due to limited pitch actuator capability and the inertial mass of the blade, while the aerodynamic action of stalling a blade is very fast indeed. However modern multi-megawatt wind turbines are designed to operate at variable speeds up to the rated wind speed but also to allow marginal speed variations between rated and cut-out wind speeds. This results in smoothing of the power delivered, especially around rated power, and a substantial reduction in flicker.
The degree of flicker caused by a given generator is highly dependent on the network characteristics, in particular the fault level and X/R ratio at the point of common coupling. Thus, a generator that is perfectly acceptable at one point on a network may be unacceptable elsewhere. In general, this dependence is the same as that for a steady state voltage rise; thus, a wind farm that causes a significant voltage rise may also cause significant flicker. Flicker is more of an issue for single turbines or small clusters on low voltage distribution networks. As the power fluctuations from individual wind turbines are not correlated, the effect of flicker is less pronounced for large wind farms.
Switching a generator, either on or off, can cause a step change in the voltage at the PCC due to an associated abrupt change in active or reactive power flow. If the power changes associated with switching can be made more gentle, then such voltage steps can be minimized, at least during planned switching operations.
Direct on-line starting of an induction generator can cause a dip in the voltage, due to large starting currents, sometimes called inrush currents. For large induction generators, it is normal to use a soft-starter to limit these starting currents. In wind farms the practice is to connect turbines one at a time. Both steps and dips, if repetitive or cyclic, may contribute to flicker. With the increasing use of power electronic interfaces between the renewable energy generator and the grid, steps and dips during switching are virtually eliminated.
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