Acceptable low voltage limit for residential appliances

Thread Starter

Joacchim

Joined Apr 11, 2020
8
Hello - I'm a retired EE who spent a career working in and around coal-fired electric generating plants. Nowadays I make my own electricity from the sun, while selling any excess back to the grid.

I came across this forum while googling for what the acceptable standards are for US household voltage. I found a couple of old threads and most seemed to indicate plus or minus 5% of 120 volts, but in my experience here in NE Ohio, my house sees over 130 volts quite often in the springtime, before my utility changes their xfmr taps for summer loads (at least, I think that's the explanation...). The reason for my search is that my home power system has grid-tied inverters that don't like it when the voltage on either 120 volt leg rises above 130.

I first observed this about 12 years ago after my first inverter install. At that time I enquired with the Ohio Consumers Council (a public consumer advocacy group) about it, and they told me the standard for us here was plus or minus 10%, which would allow voltages up to 132 volts and as low as 108.

Long story short, I recently installed an isolation transformer between the grid and one of my GTI's and set it up for 1:1 turns ratio, but now I'm considering changing that to 95% at the secondary, which would allow voltages as low as 103 volts.

I wonder if any of your forum readers would care to comment on acceptability of this low voltage? Not likely I'd see it much if at all, but...

Thanks in advance. - Mike

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Last edited:

Hymie

Joined Mar 30, 2018
976
Most utility mains supplies around the world can experience voltages ranging from +10% of the nominal value.

In my experience the regulation of 1:1 isolation transformers will be such that the output voltage under no load will be around 110% of the input voltage, and the input voltage equal to the output voltage at rated load.

Your problem may be that the electrical equipment will experience a brown-out (due to voltage dips), possibly resulting in the equipment switching off or entering standby.

Of course, under no load conditions, the output of the isolation transformer could be close to 140Vac, which could damage something.
 

Thread Starter

Joacchim

Joined Apr 11, 2020
8
Most utility mains supplies around the world can experience voltages ranging from +10% of the nominal value.

In my experience the regulation of 1:1 isolation transformers will be such that the output voltage under no load will be around 110% of the input voltage, and the input voltage equal to the output voltage at rated load.

Your problem may be that the electrical equipment will experience a brown-out (due to voltage dips), possibly resulting in the equipment switching off or entering standby.

Of course, under no load conditions, the output of the isolation transformer could be close to 140Vac, which could damage something.
Thanks - I watch the system pretty closely, it gives grid voltage readings (line to line) in real time and I've never seen anything approaching 280 volts, even with minimal loads.
I was always under the impression that most jurisdictions in N.A. the acceptable voltage tolerance was ±10%.
Max.
It is, but I found an old post on this forum where user (s) claimed 5%. No biggie.
 

Thread Starter

Joacchim

Joined Apr 11, 2020
8
The crooks at the management of your electricity utility raise the voltage so that your appliances use more power and you PAY MORE.
Not sure about that, this condition usually occurs (for me, anyway) in the springtime. Once the weather warms up, seems to go away. I live in an all electric housing development from the mid 50s and most homes heat with electric baseboard heating. We're also at the border of our electric distribution company so maybe that's part of the explanation for high voltage...
 

nsaspook

Joined Aug 27, 2009
8,380
Not sure about that, this condition usually occurs (for me, anyway) in the springtime. Once the weather warms up, seems to go away. I live in an all electric housing development from the mid 50s and most homes heat with electric baseboard heating. We're also at the border of our electric distribution company so maybe that's part of the explanation for high voltage...
High utility voltage at end-nodes is a known problem with RE on older grids.

https://link.springer.com/article/10.1007/s40565-015-0132-0
down.png
Voltage characteristics at LV networks
Voltage control is a critical issue for large scale PV integration in the LV grids. The resistivity of the residential LV grids makes the voltage control differs from high voltage (HV) transmission systems.
 

Thread Starter

Joacchim

Joined Apr 11, 2020
8
High utility voltage at end-nodes is a known problem with RE on older grids.

https://link.springer.com/article/10.1007/s40565-015-0132-0
View attachment 204260
Voltage characteristics at LV networks
Voltage control is a critical issue for large scale PV integration in the LV grids. The resistivity of the residential LV grids makes the voltage control differs from high voltage (HV) transmission systems.
Thanks - Where I live, there's not a lot of grid-tied PV/RE connected, but your point about end of line voltage rise is surely relevant. I have several different GTI's, ranging from a pair of 2007 Outback Power GVFX3524's (hybrid inverters with battery backup) to 44, Enphase M215's (30 in 2012, 14 more in 2019) to an Outback Skybox in 2019, added along with the 14 additional M215's. Only the Skybox is capable of the advanced grid support functions your article talks about, but AFAIK none are available to users yet.

Still wondering whether it's advisable to reduce the output voltage on my isolation transformer. It is only connected to the Skybox, which has a grid low voltage cutoff/trip set for 88% of nominal grid voltage, about 105 volts. The loads on the Skybox are not extensive - Just a mini-split heat pump and a small fridge that runs without a compressor. I guess I could try it to see what happens...of course, could wait for voltages to drop. But in the meantime, when there's a spike in voltage the Skybox disconnects from the grid and causes my battery to overcharge from an AC coupled array that won't respond quickly enough to the SB dithering its output frequency.
 

Reloadron

Joined Jan 15, 2015
5,934
As far as any supporting documentation of published specifications you can look at ANSI C84.1a or any subsequent revisions. I am also located in the Greater Cleveland, Ohio suburbs so in general NE Ohio. Served by First Energy. There was a time when I was curious about my mains voltage delivered so I started charting it. I have to give them credit as at my service entry it is 246 VAC split phase and a remarkable split into 123 VAC each side. I data logged this for a month and it was always within +/- 1.0 volt. We also seldom have outages but when we do I rely on a natural gas fired auto transfer generator. I hate dark and cold. :)

Ron
 

Thread Starter

Joacchim

Joined Apr 11, 2020
8
As far as any supporting documentation of published specifications you can look at ANSI C84.1a or any subsequent revisions. I am also located in the Greater Cleveland, Ohio suburbs so in general NE Ohio. Served by First Energy. There was a time when I was curious about my mains voltage delivered so I started charting it. I have to give them credit as at my service entry it is 246 VAC split phase and a remarkable split into 123 VAC each side. I data logged this for a month and it was always within +/- 1.0 volt. We also seldom have outages but when we do I rely on a natural gas fired auto transfer generator. I hate dark and cold. :)

Ron
Thanks. I looked at your ANSI link and my experience here (FirstEnergy also, right on the border between the Illuminating Co. and Ohio Edison service territories) definitely is outside their standard ranges. Like I said before, I was told by an OCC rep awhile back that +/- 10% was what FE was allowed to sell...
PS I hate dark and cold too!
 

MisterBill2

Joined Jan 23, 2018
8,721
While a wandering voltage level is inconvenient I would be more concerned about being able to sell your excess power. To do that you need a higher output voltage and that means that you may need to make adjustments, possibly even a few times a day. So your end may become more complicated.
I don't think that I have ever seen 130 volts at my location, I am far more concerned about the times when I see ZERO volts because something failed. But a 10% variation does sometimes happen, always in the downward direction.
 

Thread Starter

Joacchim

Joined Apr 11, 2020
8
Thanks Bill - Unfortunately my inverters have little to no ability to adjust their AC output voltage. In fact it's the oldest, 2007 vintage inverters that actually offer two levels for output: one labeled IEEE, the standard output level, and a higher voltage (I'm assuming, no numerical values are given) output labeled "user", which the manufacturer at that time warned in their hard copy manual not to use without consulting with them first. I did but it seemed more a formality than any concern for unforeseen bad consequences or rule-breaking. Those inverters did improve their sell capability a bit as a result of this setting change but there are still quite a few instances when selling is prevented by high grid voltage.

The newer inverter doesn't have any user adjustments for output voltage, and that's the one I'm concerned about. It has an AC-coupled PV array connected to its output side, so when it rejects grid voltage it disconnects, and so that AC array can no longer sell to the grid. The main inverter is supposed to dither its output frequency to cause the AC microinverters to disconnect, but they don't respond quickly enough, battery gets overcharged, and battery voltage rises past its HV trip point. This basically knocks my whole system off until I physically go out to reset it. High voltage is not good for LiFePO4 batteries...

130 volts is pretty common here in the springtime, maybe because loads are dropping off. I've never observed low voltage and that's partly why I'm considering changing the output tap on the isolation transformer to reduce the grid tie voltage the inverters see.
 

MisterBill2

Joined Jan 23, 2018
8,721
IT may be that you should consider an automated tap changing system. three levels should cover all possibilities, while just two may work well enough. I have read about the solar power systems and how they work but I do not claim any expertise. But I am familiar with power systems a bit.
 

Thread Starter

Joacchim

Joined Apr 11, 2020
8
My son in law lived for a while in west Texas. That’s sunshine country and many people had PV systems.

He experienced the same over voltage conditions you are experiencing during the spring time.
He would be replacing light bulbs constantly, and in one instance a stereo system blew its large power supply caps.

So I fully understand what you are going thru.
 

MisterBill2

Joined Jan 23, 2018
8,721
Heres the transformer I'm using: https://documentcloud.adobe.com/link/review?uri=urn:aaid:scds:US:9e883f01-5513-47a3-a96b-3342b5fd5649. It changes the turns ratio by using various combinations of winding tap connections, not a mechanical device so not easily automated without spending more time, effort and $$$ than I'm willing to...Also, the secondary only has a 5% reduction option. If I had low voltage there'd be more options to boost it, in 2.5% increments.
OK, I see the product and I see that they do not provide a diagram of what it is. In fact there is really not a whole lot of useful information provided at all. That is unfortunate. The power companies have a "wonderful" thing called a tap changer, and I have watched it, on a hot summer day, changing the voltage up and down in 2 volt steps. That system alternated between attempting to hold the voltage versus trying to avoid an overload. Eventually avoiding an overload and shutdown won, and the voltage stayed a bit on the low side.
What I think that I see is that the intended primary is 240/480, and the intended secondary is 120/240 volts. Using it as simply a 1:1 correction device does limit your options. An auto-transformer with a set of taps at one end would be what my suggestion required. And I understand about the $$$$ concern, that is an expensive item indeed.
 

Reloadron

Joined Jan 15, 2015
5,934
OK, I see the product and I see that they do not provide a diagram of what it is. In fact there is really not a whole lot of useful information provided at all. That is unfortunate. The power companies have a "wonderful" thing called a tap changer, and I have watched it, on a hot summer day, changing the voltage up and down in 2 volt steps. That system alternated between attempting to hold the voltage versus trying to avoid an overload. Eventually avoiding an overload and shutdown won, and the voltage stayed a bit on the low side.
What I think that I see is that the intended primary is 240/480, and the intended secondary is 120/240 volts. Using it as simply a 1:1 correction device does limit your options. An auto-transformer with a set of taps at one end would be what my suggestion required. And I understand about the $$$$ concern, that is an expensive item indeed.
Had my guys hand plenty of them. Here is the data sheet including the tap configurations. They are pretty common. Taps are all done on the primary side and while switching can be added it is an expensive proposition.

Ron
 

MisterBill2

Joined Jan 23, 2018
8,721
Had my guys hand plenty of them. Here is the data sheet including the tap configurations. They are pretty common. Taps are all done on the primary side and while switching can be added it is an expensive proposition.

Ron
Certainly tap switching at even low power levels is complicated, no question about that. I am trying to visualize a way to do it mechanically with some sort of cam and switch arrangement. Even as a "mechanical Marvel" it would be very complicated.
So is there some way to have just a "high" and "low" connection and then a single change over switch? Could that arrangement be useful?
 

Reloadron

Joined Jan 15, 2015
5,934
Certainly tap switching at even low power levels is complicated, no question about that. I am trying to visualize a way to do it mechanically with some sort of cam and switch arrangement. Even as a "mechanical Marvel" it would be very complicated.
So is there some way to have just a "high" and "low" connection and then a single change over switch? Could that arrangement be useful?
No easy way I have seen. Normally mains supply voltage should be stable so when setup you choose the taps, use the taps and that's it. The large transformers in our old sub were dated 1953 and those taps never needed moved. :) Yes, it can be done with a large maze or array of contactors but it is one expensive proposition.

Ron
 

MisterBill2

Joined Jan 23, 2018
8,721
That is why I suggested a cam and switch creation. Some of the 1920's electrical equipment in the Smithsonian Institute museum of science and industry are examples of what I am thinking about. Some of that ancient technology was really quite brilliant.
 
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