Introduction
A power inverter, or inverter, is an
electronic device or circuitry that changes direct current (DC) to alternating
current (AC)
The input voltage, output voltage
and frequency, and overall power handling depend on the design of the specific device or circuitry. The
inverter does not produce any power; the power is provided by the DC source
A power inverter
can be entirely electronic or may be a combination of mechanical effects (such
as a rotary apparatus) and electronic circuitry. Static inverters do not use moving
parts in the conversion process
Types of
inverter
Types of inverter according to the input
1Voltage source inverters (VSI)
The inverter is called voltage
source inverter (VSI), if the input dc is a voltage source. The VSI circuit has
direct control over output ac voltage. Shape of voltage output waveforms by an
ideal VSI should be independent of load connected at the output.
The simplest dc voltage
source may be a battery bank, which may consist of several cells connected in
seriesparallel combination. Solar photovoltaic cells can be another voltage
source. An ac voltage supply, after rectification into dc, will also qualify as
a dc voltage source. A voltage source is called stiff, if the source voltage
magnitude does not depend on load connected to it. All voltage source inverters
assume stiff voltage supply at the input.
2Current source inverters (CSI)
Current source inverters, a DC
source is connected to an inverter through a large series inductor Ls .the
inductor of Ls is sufficiently large that the direct current is constrained to
be almost constant.
The switch current output
waveform will be roughly a square wave, since the current flow is constrained
to be nearly constant.
The linetoline voltage will
be approximately triangular. It is easy to limit over current in this design
but the output voltage can swing widely in response to changes in load.
The frequency of both current
and voltage source inverters can be easily changed by changing the firing
pulses of the gates of the switches, so both inverters can be used to drive ac
motor at variable speeds.
Types of inverter according to the shape of
an AC output
An inverter can produce a square wave, modified sine wave, pulsed sine
wave, pulse width modulated wave (PWM) or sine wave depending on circuit
design. The two dominant commercialized waveform types of inverters as of 2007
are modified sine wave and sine wave.
output waves (pure sine wave, modified sine wave and square wave)

1Square wave
This is one
of the simplest waveforms an inverter design can produce and is best suited to
lowsensitivity applications such as lighting and heating. Square wave output
can produce "humming" when connected to audio equipment and is
generally unsuitable for sensitive electronics.
2Sine wave
A power
inverter device which produces a multiple step sinusoidal AC waveform is
referred to as a sine wave inverter. To more clearly distinguish
the inverters with outputs of much less distortion than the "modified sine
wave" (three step) inverter designs, the manufacturers often use the
phrase pure sine wave inverter. Almost all consumer grade inverters
that are sold as a "pure sine wave inverter" do not produce a smooth
sine wave output at all, just a less choppy output than the square wave (one
step) and modified sine wave (three step) inverters. In this sense, the phrases
"Pure sine wave" or "sine wave inverter" are misleading to
the consumer. However, this is not critical for most electronics as they deal
with the output quite well.
Where power inverter devices substitute for standard line power,
a sine wave output is desirable because many electrical products are engineered
to work best with a sine wave AC power source. The standard electric utility
power attempts to provide a power source that is a good approximation of a sine
wave.
3Modified sine wave
A
"modified sine wave" inverter has a nonsquare waveform that is a
useful rough approximation of a sine wave for power translation
purposes.
Most
inexpensive consumer power inverters produce a modified sine wave rather than a
pure sine wave.
The
waveform in commercially available modifiedsinewave inverters is a square
wave with a pause before the polarity reversal, which only needs to cycle back
and forth through a threeposition switch that outputs forward, off, and
reverse output at the predetermined frequency. Switching states are
developed for positive, negative and zero voltages as per the patterns given in
the switching Table 2. The peak voltage to RMS voltage ratio does not
maintain the same relationship as for a sine wave. The DC bus voltage may be
actively regulated, or the "on" and "off" times can be
modified to maintain the same RMS value output up to the DC bus voltage to
compensate for DC bus voltage variations.
The
ratio of on to off time can be adjusted to vary the RMS voltage while
maintaining a constant frequency with a technique called Pulse Width
Modulation (PWM). The generated gate
pulses are given to each switch in accordance with the developed pattern to
obtain the desired output. Harmonic spectrum in the output depends on the width
of the pulses and the modulation frequency. When operating induction motors,
voltage harmonics are usually not of concern; however, harmonic distortion in
the current waveform introduces additional heating and can produce pulsating
torques.
Numerous
items of electric equipment will operate quite well on modified sine wave power
inverter devices, especially loads that are resistive in nature such as
traditional incandescent light bulbs.
Types of inverter according to the AC
load
 Singlephase inverters
 Threephase inverters
Inverter Efficiency
 Modern PV inverter has conversion efficiency from DC to AC of more than 90% over a wide power range including low partial load. In a PV inverter three types of losses occur
 Opencircuit losses, constant
 Voltagedrop losses, currentproportional
 Resistance losses, proportional to the current square
Characteristics
of solar inverter
 High efficiency
 It works at maximum power point tracking (MPPT)
 Tolerant the weather conditions
 Low harmonics in other word (pure sine wave)
 Antiislanding
 Lifespan which some types of inverters has age 10 or 12 years
Conclusions
PV grid interconnection inverters have fairly good performance. They have high conversion efficiency and a power factor exceeding 90% over a wide operational range, while maintaining current harmonics THD less than 5%. Cost, size, and weight of a PV inverter have been reduced recently, because of technical improvements and advances in the circuit design of inverters and integration of required control and protection functions into the inverter control circuit. The control circuit also provides sufficient control and protection functions such as maximum power tracking, inverter current control, and power factor control. There are still some subjects as yet unproven. Reliability, life span, and maintenance needs should be certified through longterm operation of a PV system. Further reductions of cost, size, and weight are required for the diffusion of PV systems.
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