Well, we may have fix and a possible cause!
Just to recap: we are on a long rural 22kV distribution line, with our own 15kW transformers. I began looking into our local over voltage issues, after Tesla support notified me of them when I called in to adjust the off-grid PV shutdown frequency down to 62.5Hz. Our meter was pretty routinely showing voltage at 254VAC or above during the day, and a UPS in the house with a voltage regulator subsystem was kicking in to bring the line voltage down to 120VAC for the components it supports.
Our utility tested the line and found no faults, or issues, and then installed a line voltage logging meter. The first logger failed to show any data, and the second did not capture any issues. At the same time as the first logger, the utility troubleman removed a capacitor bank from the line. (Don't you love that job title?)
The bottom line is that removing a capacitor bank seems to have lowered the voltage by about 3-4VAC, just enough to bring the maximum line voltage under the Rule 2 limit (252VAC).
The slightly longer story is that the issue seems to have been caused by adding residential solar (at least three installations since 2022), and the added power raises the voltage enough to occasionally have pushed the line voltage over 252VAC, triggering errors at the Tesla Gateway. Whatever the capacitor bank was installed to fix seems no longer to be an issue and its removal drops the line voltage back into nominal levels.
The following plot is our meter voltage since mid-February, courtesy of a helpful engineer at the utility. I think it is a rather informative plot when you dig into it. I have called out the Rule 2 voltage level, as that is the voltage level the utility is focused on.
While not directly related to this issue, here is the plot since 2018; I called out the wildfire, as the most of the distribution line and neighborhood were evacuated, with a resulting solid rise in line voltage.
The effect of the additional solar capacity on the distribution line can be seen in the general upward trend of the voltage, as the extra capacity comes on line.
The most recent troubleman to visit had decades of experience working on similar long rural lines, and he opined that for the long, sparsely populated, distribution lines, the same utility typically installs many, many automatic voltage regulators in other areas to keep the line voltages in spec for most of the time. For those who aren't familiar with the voltage regulators, they automatically switch boosters/buck coils in or out as the line voltage moves to bring the voltage back into compliance. For large customers, it is not uncommon to have two line voltage regulators, one upstream and one downstream of the customer to keep the customer demand/generation swings from having untoward impacts on the other customers on the circuit. 1,500lbs and up.
What I take away from this episode is that residential solar does bring maintenance, management, and voltage complexities to the utilities in ways that a grid scale PV installation doesn't. I have to admit that I came away from all of this with some changed views on grid sustainability and management challenges.
For an interesting viewpoint, here is what I think is a great podcast from a former CAISO employee who has an idea for reducing grid management complexity by making it much more like the hierarchy of the internet.
Show Volts, Ep Envisioning a more democratic, bottom-up energy system - May 15, 2024
podcasts.apple.com
All the best,
BG
