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Inverter design basics & waveforms

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Inverter Architectures
First type inverter: step-up and chop
Second type inverter: high voltage in, only chop
Third type inverter: chop and transform
Output Wave Forms
First waveform: square wave
Second waveform: modified square wave
Third waveform: sine wave

Inverter Architectures

When designing an inverter there are three basic schemes to convert the Solar module's DC energy into AC. This AC may then be fed into the grid or can be used for stand-alone operation of 230V appliances. (The European wall outlets give 230V, but there is no principle difference for the USA at 110V.)

First type inverter: step up and chop

This type converts the low voltage into a high voltage first with a square-wave step-up converter and then converts the high-voltage DC into the wanted AC waveform (for waveforms:see below). Advantage of this architecture: insulation between input and output, easy dimensioning of the input converter, therefor it is often used in small and cheap square-wave inverters. Efficiency may be up to 95% for square-wave, slightly lower for sine-wave inverters.

Second type inverter: high voltage in, only chop

This type requires the input voltage to be higher than the output voltage and converts it directly into the wanted AC waveform (for waveforms: see below). The advantage of this is the high efficiency of the inverter, typical 96%. The main disadvantage is the lack of insulation between the Solar modules and the grid voltages. Also the input voltages always require a large number of modules.

Third type inverter: chop and transform

This type converts the low voltage DC into a low voltage AC first and then converts the low-voltage AC into the wanted AC voltage. This is the architecture I chose for my inverter. The advantages are the low-voltage (=safe) operation, the insulation from the grid after the inverter, the ease with which it makes sine-wave which feeds into the transformer and the most important in many aspects: reliability due to the low number of semiconductors in the power path. Disadvantage is the slightly lower efficiency of the inverter, typically 92%. Also some hum can be generated by the transformer under load.

Output Wave Forms

First waveform: square wave

This is the form of the output voltage from a cheap inverter. Basically it switches its output on and off. This is no problem for heaters and light bulbs, but electronic equipment always has a power supply with a capacitor for energy storage. During the steep rise of voltage in a square-wave, the input current to charge the capacitor will destroy the power supply components.

Second waveform: modified square wave

To combat the problems with square wave there have been several changes, one is depicted here: the voltage rises in smaller steps, keeping the current more within rated limits and more closely approximating the sine wave form. Other approaches have added filters to square wave outputs, to make the rising and falling edge less steep (more trapezium-shape). Still electronic equipment will not work properly or get too hot on these type of signals.

Third waveform: sine wave

The waveform shown here is a good approximation of a sine wave, all type of equipment will run on this signal. The sine wave is approximated by a high-frequency chopping plus filtering (note that the chopping frequency is much higher than depicted here for readibility: typically 75KHz). This chopping is also known as PWM (Pulse Width Modulation). This is the waveform delivered by the inverter I am designing. This is the only waveform allowed to be grid-connected, when the inverter is capable of synchonisation to the grid.