Page 265 - Fluid Power Engineering

232 Chapter Eleven

voltage is at least 15% of the normal voltage for less than 0.625 s and
voltage recovers linearly to 90% of normal voltage within 3 s. This
helps to improve the reliability of the grid. LVRT is accomplished
by supplying reactive power to the grid during the low-voltage
condition.

Power Quality: Flicker, and Harmonics
Flicker, as the name suggests, is a short-term variation in voltage that
causes a light bulb to noticeable change in intensity. The source of
flicker is (i) change in torque as blade passes the tower during normal
operations, if a single turbine were supplying energy and (ii) startup
and disconnect of turbine(s) from the grid.
The next power quality issue is harmonics. Harmonics are
introduced when power converters are used, for example, in
variable-speed turbines. The source of harmonics is because of the
imperfect sinusoidal wave (sinusoidal is approximated by a step-
function) generated by power converter during the DC–AC stage.
Pulse-width modulation, a method employed by most modern tur-
bines produces harmonics with a frequency of about 2000 Hz
and small amplitude. Such high-frequency and low-amplitude har-
monics can be easily removed by high-frequency filters. Higher-
frequency harmonics, if not filtered, can cause reduction in the
impedance of capacitors. Lower-frequency harmonics seen with
thyristor-based power converters is lower frequency and higher am-
plitude, which causes higher current heating of capacitors and trans-
formers.
IEC-61400-21 11 specifies parameters that are used to character-
ize power quality of a grid-connected wind turbine, procedures for
measuring the parameters, and procedures for assessing compliance
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with power-quality requirements. The IEEE 1547-2003, Standard for
Interconnecting Distributed Resources with Electric Power Systems
provides technical specifications for power quality and other aspects
that projects with generation capacity of 10 MW or less must meet
when connecting to the grid. Each utility uses these standards as a ba-
sis for developing their own interconnection requirements for wind
projects that are 10 MW or less.

Short-Circuit Power
Strength of a grid at the point of common coupling (PCC) is an im-
portant measure of how much power can be injected into the grid.
Short-circuit power, S cc , is the amount of kVA or MVA at the PCC un-
der maximum load. The rated grid voltage is assumed to be constant