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The electric company was working on the power lines for four days out in the woods, so they put our street on huge diesel generators (each was the size of a small car) that ran constantly to supply us with power. Everything worked fine, but I noticed some anomalies:

  • My roof solar panels were not working.

  • The clicking "igniter" on the gas range clicked faster than usual.

  • The microwave, when cooking, made a louder higher noise than usual.

So my question is: what was "wrong" with the power coming through the lines that distinguished it from what comes in normally? How did these appliances "know" something was odd? Was the Hz too fast?

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  • \$\begingroup\$ Voltage and frequency were dependent on varying diesel. Voltage would dip as load increased. Diesel would speed up. Frequency would vary. \$\endgroup\$ – StainlessSteelRat Jul 21 at 2:11
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    \$\begingroup\$ Long ago (when electric typewriters were still common,) I worked in a place that had backup generators. They tested the generators periodically by running the whole place on generator power for a few hours. The tests were announced ahead of time, but you could tell when the generators were on by listening to the motor whine of the electric typewriters. The whine would wander up and down noticeably on generator power because the frequency control was a lot laxer on the generator than the frequency control from the utility company. Whooooooo= utility. Whuoouooou = generator. \$\endgroup\$ – JRE Jul 21 at 15:42
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It sounds like the voltage and frequency of the temporary generator power were both overly high.

  • Grid-tied solar power systems measure the frequency and voltage of incoming power and do not supply their own if it is not within expected ranges. This is for a number of reasons that can be generally described as "if there is a problem, don't make it worse", including but not limited to;
    • If there is a power failure, it is not desirable for home solar power to try to run all the neighborhood loads, because it will almost certainly be insufficient capacity and unable to provide stable power — and even if they could, there would be a phase synchronization problem upon attempting to restore power.
    • High frequency is a sign of momentarily under-loaded generators (just like a motor runs faster without load, so does an AC generator), as is high voltage, and adding more power to the grid in that situation would be bad. (Note that moderate frequency shifts are actually normal, as they effectively act as a way to signal between multiple power plants to help them share the load; the way this works is partly simple physics and partly deliberate regulation and is a bit more than I'm equipped to correctly explain in a brief paragraph.)
  • The hums from the magnetron, fan motor, and turntable motor in your microwave will all be the line frequency and multiples (harmonics) of it.
  • The igniter has to create an even higher voltage from line voltage, and such a circuit might fire faster if it gets more input voltage.

There is no inherent reason why a portable generator should have consistently high (rather than low or just right) voltage and frequency — they're just not necessarily as rigorously controlled as the many interconnected grid power plants.

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  • \$\begingroup\$ Yes, the effect on the microwave and igniter might have been just at that moment. — I was amazed that the generators were prepared to cope with the fluctuations in demand. At 3 in the afternoon on a blistering hot day everyone turned on their air conditioning and the power kept on chugging. \$\endgroup\$ – matt Jul 21 at 4:37
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    \$\begingroup\$ @matt A single rental 20ft container is typically around 1 MVA (800 kW), that's plenty to supply a few houses. \$\endgroup\$ – Jeroen3 Jul 21 at 5:50
  • \$\begingroup\$ A grid-tied solar inverter will drop out on low frequency as well. \$\endgroup\$ – Simon B Jul 21 at 7:54
  • \$\begingroup\$ @SimonB Edited; I was trying to to specify only things I was 100% certain of but I think this edit is a better overview if I got it right. \$\endgroup\$ – Kevin Reid Jul 21 at 15:32
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The standby mains voltage and frequency would not be as stable as the grid.Where would exported solar energy go if the neighbours were not drawing much power ?.The voltage waveform would probably have more harmonics present .This is why the power is not as good .Also the power is expensive to generate .

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