Types of inverter

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

1-Voltage 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 series-parallel 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.                                                                                                                     

                                                                                                                      

2-Current 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 line-to-line 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)

1-Square wave

              This is one of the simplest waveforms an inverter design can produce and is best suited to low-sensitivity applications such as lighting and heating. Square wave output can produce "humming" when connected to audio equipment and is generally unsuitable for sensitive electronics.

2-Sine 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.

3-Modified sine wave

              A "modified sine wave" inverter has a non-square 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 modified-sine-wave inverters is a square wave with a pause before the polarity reversal, which only needs to cycle back and forth through a three-position switch that outputs forward, off, and reverse output at the pre-determined 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

•       Single-phase inverters
•    Three-phase 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
• Open-circuit losses, constant
• Voltage-drop losses, current-proportional
• 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)
•      Anti-islanding
•       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 long-term operation of a PV system. Further reductions of cost, size, and weight are required for the diffusion of PV systems.