7.4.
2 Working Principles of the Inverter
The inverter is a conversion device to convert the DC power to AC power via making/
breaking operation of the switchgear. The output waveform of the inverter can
be divided into rectangular wave and sinusoidal wave with sinusoidal wave output
more popular. The inverter can change its output pulse sequence width to adjust the
output sinusoidal wave characteristics, which is the so-called pulse width modulation
(PWM) technology. The PV system grid-connected technology has two related
technologies: One is the sinusoidal wave pulse width modulation (SPWM), that
is, the making/breaking time and switching sequence of the control power switch
can be controlled to make the inverter voltage track the given sinusoidal reference
waveform and achieve the sinusoidal wave of the inverter output voltage; and the other
is selective harmonic elimination (SHE-PWM) technology. In the PWM technology,
the making/breaking of the inverter switch is controlled to make the inverter output
the electrical pulse sequence and then control the width of each pulse in the pulse
sequence to make the inverter voltage track any given reference waveform and realise
control over the output power, enabling it good dynamic performance.
Below will introduce the PWM technology for the single-phase inverter. Figure 7.21
shows a single-phase half-bridge voltage inverter circuit. The switches on the two arms
of the inverter (S+ and S−) can be switched on/off to regulate the output voltage V0.
PV Systems and Grid-Connected Technologies 283
Figure 7.21 PWM technologies with
single-phase inverter.
The making/breaking status of the switch tube on each arm of the voltage inverter is
determined by the comparison result of the modulation signal Vm(t) (i.e., the given reference
waveform signal output by the desired inverter) and the triangle carrier wave
Vc(t). When Vm(t)>Vc(t), the switching tube S+: making; the switching tube S−: breaking;
and vice versa. Figure 7.22 shows the process where the comparison between the
modulation signal Vm(t) and the triangle carrier signal Vc(t) is carried out to convert the
DC input to sinusoidal wave AC output.
Here the following definition is introduced: amplitudemodulation indexma =Vm/ Vc,
where Vm is the amplitude of the modulation signal Vm(t), and Vc is the amplitude of
Figure 7.22 Process where the
comparison between Vm(t) and Vc(t) is
carried out to convert the DC input to
sinusoidal wave AC output.
284 Technology, Manufacturing and Grid Connection of Photovoltaic Solar Cells
Vc(t) triangle carrier signal (generally, the triangle carrier signal amplitude is constant
while themodulation signal amplitude is adjustable in the control design).Then, another
definition is introduced: frequency modulation index mf =fc/ fm, wherefc
is the frequency
of the triangle carrier signal and fm is themodulation signal frequency (generally,
the carrier signal frequency is much larger than the modulation signal). Figure 7.22
shows the inverter output voltage is the pulse sequence with pulse width variation, and
the pulse width varies with change of transient value of the sinusoidal wave modulation
signal.The SPWMcontrol technology can make the inverter output power become one
mostly with sinusoidal wave fundamental component. The output signal, however, also
has resonant wave component in addition to the sinusoidal wave component where the
frequency of the sinusoidal wave component is that of the modulation signal.The resonant
wave in the output voltage Vo occurs around the frequency of the carrier and its
times, that is, H = mi±k, where i = 1,2,3,…
Further, the SHE-PWMtechnology can be used, that is, the making/breakingmoment
of the switchgear can be worked out in an accurate manner by themathematicalmethod,
to eliminate the resonant wave of the specific order so as to get a near-sinusoidal AC
output.
Figure 7.23 shows the structural diagramme of three-phase full-bridge voltage inverters
and the sinusoidal wave output after PWM.
It shall be pointed out, in the three-phase DC-AC inverter control, the three vectors
of the three phases can be expressed by space vector. As a result, in the three-phase
voltage inverter, the space vector modulation technology has become the PWMcontrol
technology with widest applications. It is also realised by selection of the appropriate
switching status and calculation of the switching time of the power switching
device.
Figure 7.23 Schematic diagramme
of three-phase full-bridge voltage
inverters and output waveform of
three-phase SPWM sinusoidal wave.
PV Systems and Grid-Connected Technologies 285
