What is the waveform of a grid tie inverter? Is it a pure sine wave or a modified one?

A question answered on Quora July 30, 2018

It doesn’t make sense to talk about the waveform of a grid-tie inverter in the way you do a portable inverter, because they operate on different principles.

Portable inverters are voltage controlled power sources, and therefore we talk about their output in terms of a voltage waveform. This is important because many devices depend on a somewhat sinusoidal voltage for proper operation.

Grid-tie inverters (GTIs) however are current controlled power sources, and effectively have no control over their output voltage. They depend on the grid connection to dictate the system voltage, therefore the voltage waveform is exactly as supplied by the grid. They do not have any capability of supplying any sort of waveform, or usable power, in the absence of the grid connection.

GTIs do however attempt to supply a current waveform that somewhat matches the sinusoidal voltage of the grid, but in reality this is not terribly important. At any given point during the AC cycle, as long as the inverter is supplying current in the correct polarity it is doing its job, the magnitude of the current is of little consequence. Indeed, many consuming devices behave in the same way with current waveforms wildly fluctuating from sinusoidal, ie. phase cut controls such as those used in power tools, and anything with a switchmode power supply. The ultra low impedance of the grid ensures its voltage stability with all these fluctuating current sources and sinks.

The following two charts show the voltage and current waveforms of SMA Sunny Boy GTIs, the first operating at 10% of rated capacity, and the second while performing an (automated) grid impedance test. As can be seen, these waveforms are far from sinusoidal (the approximately sinusoidal waves in these charts are grid voltage):

This chart is the same GTI as the top one running at full rated capacity:

With the full voltage available from the PV array, the inverter is able to much more closely follow the voltage waveform, due to the fact that at every point in the cycle adequate power is available from the combination of PV array and the inverter’s capacitor bank. This is not the case when PV output is low, therefore the inverter just does the best it can, producing a somewhat messy output.

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