Network integration of wind power - Wind Power Integration: Connection and System Operational Aspects - page 117

is similar to a notch pass filter. It will serve both utilities and generators well if
there is a record of power system harmonic performance at a range of nodes and a
model validated against known performance. Harmonic correction retrofit equip-
ment can be very expensive, whereas capacitors rated as part of detuning networks
can provide reactive compensation and harmonic detuning as a single unit. In
theory, full speed range devices will emit more harmonics than DFIG devices, since
the converter must transform the full output as against about 20% needed for rotor
power in a DFIG.
Where several converters are located side by side, there is also an opportunity
for fractional interval harmonic beats (inter-harmonics) and internal resonances to
occur. The detailed design of a wind farm should have regard to these phenomena.

4.5.3 Flicker
Flicker is hard to define, but is generally taken to be a discernible regular increase
and decrease in the luminescence of incandescent luminaires connected to the
system. Some wind farms exhibit a phenomenon known as 3P, i.e. a power oscil-
lation at three times the blade turning speed. The 3P frequency is typically about
1 Hz. The oscillation is thought to be due to the wind shielding effect of each blade
of a three-blade turbine as it passes the tower. If the shielding effect were to reduce
the blade torque output to zero, then the wind turbine output would decrease by one
third at these times. The effect seems less than the theoretical maximum, resulting
in a reduction of about 20%. The physical mechanism is an impressed torque
oscillation that is transmitted through the gearbox without frequency change.
Reactive power demands will vary during this duty cycle. Where many turbines
exist on a site, or many sites are connected to the system before the system supplies
customers, diversity will reduce the impact of these oscillations on customers. If the
oscillations cause significant distortion of customer voltage waveform, utilities are
likely to receive complaints. It is incumbent upon utilities to manage power quality.
For this reason, and because oscillations of this periodicity may excite small-signal
instabilities in the power system, utilities will seek to ensure that wind farms
minimize 3P. Measurement shows that DFIGs smooth 3P oscillations whereas
ancillary energy storage equipment may be necessary to smooth the power oscil-
lations of fixed-speed WTGs.

4.6 Transient system performance

4.6.1 Frequency performance and dynamic response
Network providers and operators seek to be reassured that the power system will
remain stable under all conditions.
Utilities carry out studies and tests to ensure that the loss of any infeed does
not cause instability of other generation and that remaining generation provides a
response in proportion to the decreasing system frequency. Plant must therefore be
capable of operating within a frequency range and should be equipped with a
control mechanism capable of adjusting the output in response to the frequency.