Environmental Engineering Reference
In-Depth Information
Figure 4.56 Six-pulse Bridge Inverter (B6)
A transformer is integrated into most inverters. This transformer separates
the inverter from the grid and can transform the inverter voltage to a higher or
lower grid voltage. However, a transformer always introduces losses and is not
essential in all inverter designs. For transformerless inverters, more
sophisticated protection methods are used because there is no longer any
galvanic separation between the solar generator and the grid.
The described B2 bridge connection delivers an alternating current to a
single phase of the grid. This can cause asymmetries in the three phases of the
public grid. To overcome this, three-phase generation is preferred for inverter
powers above 5 kW. Chapter 5 (wind power) describes the basics of three-
phase currents. One circuit for the generation of three-phase current is the
six-pulse bridge connection (B6 bridge), shown in Figure 4.56. The thyristors
of this circuit switch in a way that generates three alternating currents and
voltages shifted by 120°.
Besides the described B2 and B6 bridge connections, other circuits such as
the M2 or M3 star connection or other bridge connections are used. However,
the principle of these connections is similar to that of the above-described
variants.
Pulse-width modulation
An inverter that works on the pulse-width modulation (PWM) principle also
uses the above described B2 or B6 bridge circuits. However, the thyristors do
not switch just once per half-wave: multiple switching generates pulses of
different widths as shown in Figure 4.57. The sinusoidal fundamental wave is
obtained after filtering. The quality of the sinusoidal oscillation is much better
compared to rectangle inverters, i.e. PWM inverters have much lower
harmonic content. Therefore, most inverters use the PWM principle today.
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