Unformatted text preview: ty at the required voltage and frequency.
Figure 8-9 The 25kVA power converter used in conjunction with a
20kW synchronous generator wind turbine at MUERI All electric power utilities have certain standards concerning the quality of power that may be supplied to the
grid. Concerning wind generated power, these regulations are mainly focused on voltage and frequency
variations caused by wind speed variations and possible surges and voltage drops drawn by induction
generators coming on-line. The utility may also specify regulations limiting harmonics that may be
introduced by turbines using inverters.
As the capacity of wind generated power increases relative to the grid capacity (a concept referred to as wind
penetration), power quality concerns and power regulation become increasingly important.
8.4.1 Voltage Fluctuations or Flicker
Flicker is the term used to describe the effect of voltage fluctuations on an electricity grid. The term flicker
stems from the effect these transients have on incandescent lamps – light intensity increases with voltage.
The effect of flicker becomes a concern in weak grids where there is an unsteady demand from a large power
user or conversely an unsteady power generator, such as a wind turbine.
Fixed speed turbines are well known to introduce dynamic voltage variations into the connected grid
network. These occur due to cyclic power transients resulting from wind shear and the tower shadow effect
as the blades pass through the affected region. The effect of voltage fluctuations is more pronounced where a
single or small number of turbines are connected, as the effect of a multiple turbine installation tends to
cancel out these effects. The use of full or partial variable speed turbines greatly reduce the effect of flicker,
as the energy transients are absorbed as rotor speed variations.
Flicker may also be caused by the current surge as a wind turbine connects to the grid. To reduce the effect of
this, turbines in wind farms are usually ‘turned on’ individually to minimise flicker and use soft starting
circuitry to smooth out transitions.
8.4.2 Power Factor
Wind turbines using asynchronous generators draw reactive power (VARs) from the grid to magnetise the
stator. The utility may impose penalties for drawing reactive power hence steps are taken to ensure voltage
and current are as closely as possible in phase with one another – termed power factor correction. Capacitor
banks are often used to provide reactive power for the generator and the control system is used to switch
banks of capacitors in and out as needed to keep the power factor near unity (or within the utilities
requirements) at the point of grid interconnection. It is important to have insufficient capacitance to self
excite the generator if the grid goes down (islanding). For more information on power factor see Topic 7.2.3.
Wind turbines using inverters may introduce harmonics to the grid in which they are connected if insufficient
filtering is not used. Harmonics may be defin...
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- Three '11
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