Author：管理員 Released in：2018-05-03 11:00:53 Written words：【Big】【In the】【Small】
Abstract：Inverters principle of operation and parameters
Inverter is a key
system element that is used for power conditioning. Almost any solar systems of
any scale include inverter of some type to allow the power to be used on site
for AC-powered appliances or on grid. Different types of inverters are shown in
Figure 11.1 as examples. The available inverter models are now very efficient
(over 95% power conversion efficiency), reliable, and economical. On the
utility scale, the main challenges are related to system configuration in order
to achieve safe operation and to reduce conversion losses to a minimum.
Inverters: small-scale inverter box for residential use (left) and Satcon utility-scale
The three most
common types of inverters made for powering AC loads include: (1) pure sine
wave inverter (for general applications), (2) modified square wave inverter
(for resistive, capacitive, and inductive loads), and (3) square wave inverter
(for some resistive loads) (MPP Solar, 2015). Those wave types were briefly
introduced in Lesson 6 (Figure 11.2). Here, we will take a closer look at the
physical principles used by inverters to produce those signals.
Different types of AC signal produced by inverters.
The process of
conversion of the DC current into AC current is based on the phenomenon of electromagnetic
induction. Electromagnetic induction is generation of electric potential
difference in a conductor when it is exposed to varying magnetic field. For
example, if you place a coil (spool of wire) near a rotating magnet, electric
current will be induced in the coil (Figure 11.3).
Schematic illustration of electromagnetic induction
Next, if we
consider a system with two coils (Figure 11.4) and pass DC current through one
of them (primary coil), that coil with DC current can act analogously to the
magnet (since electric current produces magnetic field). If the direction of
the current is reversed frequently (e.g., via a switch device), the alternating
magnetic field will induce AC current in the secondary coil.
Inverter cycles. During the 1st half cycle (top), DC current from a DC source -
solar module or battery - is switched on through the top part of the primary
coil. During the 2nd half cycle (bottom), the DC current is switched on through
the bottom part of the coil.
two-cycle scheme shown in Figure 11.4 produces a square wave AC signal. This is
the simplest case, and if the inverter performs only this step, it is a
square-wave inverter. This type of output is not very efficient and can be even
detrimental to some loads. So, the square wave can be modified further using
more sophisticated inverters to produce a modified square wave or sine wave
To produce a
modified square wave output, such as one shown in the center of Figure 11.2,
low frequency waveform control can be used in the inverter. This feature allows
adjusting the duration of the alternating square pulses. Also, transformers are
used here to vary the output voltage. Combination of pulses of different length
and voltage results in multi-stepped modified square wave, which closely
matches the sine wave shape. The low frequency inverters typically operate at
~60 Hz frequency.
To produce a sine
wave output, high-frequency inverters are used. These inverters use the
pulse-width modification method: switching currents at high frequency, and for
variable periods of time. For example, very narrow (short) pulses simulate a
low voltage situation, and wide (long pulses) simulate high voltage. Also, this
method allows spacing the pulses to be varied: spacing narrow pulses farther
apart models low voltage (Figure 11.5).
Pulse-width modulation to approximate the true sine wave by high frequency
In the image
above, the blue line shows the square wave varied by the length of the pulse
and timing between pulses; the red curve shows how those alternating signals
are modeled by a sine wave. Using very high frequency helps create very gradual
changes in pulse width and thus models a true sine signal. The pulse-width
modulation method and novel digital controllers have resulted in very efficient
inverters (Dunlop, 2010).
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