Zero Energy
 

Solar Knowledge: Inverter


Introduction

Alternating current is easier to transport over long distance with low losses as compared to DC and it has become electrical standard. As a result most of the common appliances are designed to operate on alternating current. PV array generates only direct current and in addition batteries can store energy DC power. Alternating current and Direct current are fundamentally different and incompatible. Therefore “bridge” - inverter is required between them.

The main purpose of inverter is to convert DC power from PV array and batteries to alternating current electricity to power alternating current load.

With the advent of complicated integrated circuit, FET and high frequency transformer has made it possible to create efficient inverters that produce waveform closer to a true sine wave.

Inverter types:

There are basically two categories of inverter
    1. Synchronous or line tied inverter
    2. Stand-alone or static inverter
Stand-alone inverters are appropriate for stand-alone photovoltaic system. Some inverters might have both types to facilitate future utility connection option.

Inverters are also classified based on the output waveform they produce. Three most common waveforms are
    1. square wave
    2. modified sine wave
    3. sine wave
Square wave inverters convert the DC input into square alternating current output. They provide little output voltage control, limited surge capability and considerable harmonic distortion. Therefore this type of inverter is used for small appliances, incandescent lamps only. They are inexpensive and should not be used for devices like motor.

Modified sine wave inverter uses FET or SCR to convert direct current input to alternating current output. They can handle large surges and produce less harmonic distortion. This type of inverter can be used to operate wide range of load like motors, lights, electronics appliances like television.

Sine wave inverter is used to operate high sensitive electronic devices that require high quality waveform. They have high surge capabilities with little harmonic distortion.

Figure: Output waveform of inverters