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Historically electric grids were mostly formed by spinning generators. Using their electromagnetic components they produce nice sinewaves of voltage and current, synchronous to the spinning of the generator.

Nowadays however, more and more solar, wind and battery sources are added to the grid. Especially solar and battery sources produce direct current. They need to be fitted with an inverter to be able to feed power into the grid.

I imagine a pure sine wave inverter is more complex and therefore more expensive to produce than an inverter that simply chops of the flow and creates rectangular waves.

I would like to understand the following:

On a grid level, why is it bad to have rectangular waves compared to sine waves?

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    \$\begingroup\$ Given that a reasonably powerful microcontroller can be deployed for a couple of dollars, and a switch-mode ASIC for even less, the idea that a sine wave switcher is more expensive is questionable. In short (and I might post this as an answer in the unlikely event that nothing better is submitted) a rectangular wave contains many harmonics which are undesirable in a power line; the lines themselves tend to be highly inductive which could lead to significant wattless current and high transmission losses. Harmonics at the consumer are also undesirable \$\endgroup\$
    – Frog
    Commented Oct 25 at 19:53
  • \$\begingroup\$ …as filtering 50/60Hz is easy but higher harmonics breaking through into an audio circuit (for example) would be obtrusive. \$\endgroup\$
    – Frog
    Commented Oct 25 at 19:55

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Transformer considerations:

  • To handle a rectangular wave, the transformer would need 1.57 times the number of turns than a transformer designed for a sine wave to prevent saturation. Costs more and higher loss. Retrofitting the grid would be a huge undertaking.
  • Harmonics of the rectangular wave will have higher losses due to eddy currents. It may not seem like much, but there are millions of distribution transformers out there.
  • Fast rising edges will cause ringing at the transitions at some unknown frequency which will radiate over a wide area.

Transmission:

  • The waveform will be rectangular near the power station. Long distances away the harmonics will be attenuated and the waveform will start to take on a sinusoidal shape. When connected to the grid, other generators will be trying to supply a rectangular waveform in parallel with a distorted sine wave.
  • The higher frequency harmonics will have higher eddy current losses in the distribution lines.
  • Phase compensation will require a complex network due to the harmonics. A sine wave usually requires a capacitor for phase compensation.
  • Instead of the primary frequency (50 or 60 Hz), now you have a bunch of odd-order harmonics being radiated by the transmission wires. I have enough problems keeping power line hum out of my audio systems.

Generation:

  • It's easy to generate sinusoidal waveforms with rotating generators.
  • Cost effective modern conversion equipment already exists for converting DC to AC.

I'm sure others will come up with more reasons why rectangular waves are a bad idea.

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