Generator and Electrical System - Small Wind Turbines: Analysis, Design, and Application - page 238

Fig. 11.10 Modified square
wave load voltage waveform
for a single-phase inverter
without a filter. The vertical
axis is voltage and
the horizontal-axis is phase
relative to the grid frequency

1

0.5

0

-0.5

-1

0 0.2 0.4 0.6 0.8 1

Phase angle/2 π

power applications. Most electrical devices, such as computers, home appliances,
and motors (with a slight derating), can work well on modified-square-wave-
quality AC, but some require very close-to-sinusoidal power. Audio equipment,
for example, will often produce a noticeable hum when powered by poor quality
AC. To obtain a better approximation to a sinusoidal load voltage waveform a
filter could be added and/or a more sophisticated PWM gating algorithm employed
to control the timing of the IGBT switching devices. The four switching devices in
Fig. 11.9 (or six devices in the case of a 3-phase inverter) can be manufactured
individually with or without the feedback diodes; alternatively, the entire set of
IGBTs with feedback diodes can be manufactured in a single module.
There is no standard numbering of the switching devices in a single-phase
bridge inverter. That used in Fig. 11.9 indicates a simple switch timing to produce
the modified square wave load voltage waveform shown in Fig. 11.10 . In this
figure, the first positive pulse is obtained by gating devices 1 and 2 simultaneously,
and the following zero voltage interval is obtained by gating devices 2 and 3. The
negative pulse is then obtained by the gating of devices 3 and 4, followed with a
zero voltage interval obtained by gating devices 4 and 1. The process is repeated to
produce the second positive pulse and so forth.
For the modified square wave of Fig. 11.10 , the rms voltage is

q

2h op p

V load ; rms ¼ V bat

ð 11 : 5 Þ

where h op may be chosen to minimise, for example, the total harmonic distortion,
THD, of the modified square wave (in which case h op corresponds to 67 o ). The
THD measures the deviation of the actual voltage waveform from the nominally
pure sine wave of the grid electricity.
The full-bridge inverter without a filter is very popular for low power inverter
systems, However, the application of PWM to the switching devices and adding
filtering can make the inverter output nearly sinusoidal. There are two general
approaches. For grid-tied inverters, a small inductor may be inserted between the
positive inverter output terminal and the grid system voltage (i.e. the inductor is in
a series with the grid voltage). The controller is usually designed such that the