Generator and Electrical System - Small Wind Turbines: Analysis, Design, and Application - page 239

Fig. 11.11 A low-pass LC
filter employed to attenuate
the switching frequency
voltage component of the
inverter bridge output
resulting in a low distortion
near-sinusoidal voltage
across the load

L

V load

V bridge

C

square-wave type bridge output voltage is filtered by the output inductor to within
a specified rms current for feeding the grid (the rms current value is determined by
the available power from the wind turbine or by the system controller in the case of
a battery-fed inverter). The THD of the current waveform is typically 5% or less,
depending on the appropriate standards for grid connection, for example IEEE
519-1992 for general information and IEEE 1547.2-2008 which specifically covers
the interconnection of distributed generators to the grid. The application guide for
this standard has valuable background information including ''tips, techniques, and
rules of thumb''.
For off-grid inverter operation, an ''LC filter'' may be added between the bridge
output and the load, as shown in Fig. 11.11 . The objective is to produce a near
sinusoidal voltage across the load where now the load voltage waveform is typi-
cally 5% or less, as determined by consumer product standards such as the German
standards association 3 or Underwriters Laboratories Inc in the USA. 4
Regardless of whether the PWM operated inverter is grid-connected or not, the
voltage waveform across the output side of the bridge itself (prior to filtering) is a
chopped square wave as illustrated in Fig. 11.12 . The width of the pulses is such
that the filtered waveform will be a good approximation to a sine wave. Note that
the waveform shown in the figure is for illustration purposes only as the switching
frequency of the bridge devices is just three times the power frequency (for
example 180 Hz for a power frequency of 60 Hz). Practical single-phase PWM
inverters have a switching frequency of 15-150 times the power frequency, where
the optimal value is a trade off between lower switching losses for the low end and
smaller less expensive LC components at the upper end.
An alternative to designing and building an inverter is to purchase one from a
number of companies that make inverters especially for small wind turbines.
Probably the best known of these are the ''Windy Boy'' range from SMA. 5

3

http://www.din.de/

4

http://www.ul.com/

5

http://www.sma.de/en_US/products/wind-power-inverters/