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September 8, 2020 / Jim Fenton

Line voltage fluctuations

Voltmeter showing 101.3V

This past July, we replaced our roof and at the same time updated our solar panels and inverter (I’ll write about the new solar equipment in the near future). I was monitoring the new equipment somewhat more closely than usual, and noticed on one warm August day that the inverter had shut down due to low line voltage. Having home solar generation shut down on a warm day with a high air conditioning load is the opposite of what the utility, Pacific Gas & Electric (PG&E), should want to happen. In addition to shutting down solar power inverters, low line voltage can be hard on power equipment, such as motors.

At a time when our voltage was particularly low, I opened a low line voltage case with PG&E. This resulted in a call from a field technician that told me several things:

  • PG&E has been aware of the voltage regulation problem in my neighborhood for some time
  • The problem is likely to be due to the older 4-kilovolt service in my part of town. Newer areas have 12-kilovolt service that would be expected to have about 1/9 the voltage drop with an equivalent load.
  • Another possible cause is the pole transformer that feeds our house and nearby neighbors that the technician told me is overloaded. [Other neighbors that aren’t as close are reporting these problems as well, so they would have to have similarly overloaded transformers.]
  • Line voltage at my home is supposed to be between 114 and 126 VAC.

Another technician from PG&E came out a couple of days later to install a voltage monitor on the line. But it occurred to me that I have been collecting data since 2007 from my solar inverter that includes voltage data. A total of about 3.2 million data points. So I thought I’d check to see what I can find out from that.

My data are in a MySQL database that I can query easily. So asked it how many days there have been where the line voltage went below 110 VAC (giving PG&E some margin here) and the solar inverter was fully operating. There were 37 such days, including very brief voltage dips (<10 minutes) up to over 5 hours undervoltage on September 2, 2017. The line voltage that day looked like this:

A more recent representative sample is this:

Part of my concern is that this problem seems to be getting worse. Here is a table of the number days where <110 VAC lasted for more than 10 minutes:

YearDays with
2020 (to June 30)2160

And as I mentioned above, the problem seems to occur on particularly hot days (which is when others run their air conditioners; we don’t have air conditioning). Fortunately, the NOAA National Centers for Environmental Information provide good historical data on high and low temperatures. I was able to download the data for Los Altos and relate it to the days with the outages. Indeed, the days with the most serious voltage problems are very warm (high of 110 on 9/2/2017 and 100 degrees on 6/3/2020 shown above).

Does that mean we’re seeing purely a temperature effect that is happening more often due to global warming? It doesn’t seem likely because there have been very warm days in past years with little voltage drop. Here’s a day with a recorded high temperature of 108 in 2009:

My street, and the City of Los Altos more generally, has seen a lot of extensive home renovations and tear-down/rebuilds the past few years. The section of the street I live on, which has about 50 homes, currently has three homes being completely rebuilt and currently unoccupied. So this is only going to get worse.

The ongoing renovations and rebuilds in Los Altos are all considerably larger than the homes (built in the 1950s) that they replace, and I expect nearly all have air conditioning while the original homes didn’t. This is resulting in a considerably higher electrical load on infrastructure that wasn’t designed for this. While this is mitigated somewhat by the prevalence of solar panels in our area, the City needs to require that PG&E upgrade its infrastructure before issuing new building permits that will exacerbate this problem.

SolarEdge inverter display

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