Voltage drop on grid power - Where does it come from? - Page 1
Forum section: electronics / general

author: SpookyGhost
author_rank: Contributor
timestamp: February 04, 2022, 05:39:36 pm
content: I live in Texas where our grid is made of mash potato. Since the big freeze and mass power outages last year, I've stepped up my game. I have solar (To stave off cost increases), a 27kw Generator, a Portable Generator and I got an IoTaWatt to monitor all my power usage/voltage/frequency etcSince its currently below freezing and everyone is scared about having issues, I've been keeping an eye on grid power just for my own interestI've noticed that the utility voltage that is usually pretty hot off the pole (253v at highest usually, steady around 250v) has dropped to as low as 233vThe obviously is no issue for me, but I'm just interested in where the drop comes from. The transformer on the pole serves 4 houses, and while I have natural gas, I think 1 or 2 may not. Is this just because of high load on the transformer? I assume it can't be further upstream since the transformer is fed with a high voltage line?And, at what point does it point to an issue with the utility?

author: SeanB
author_rank: Super Contributor
timestamp: February 04, 2022, 06:12:59 pm
content: Normal limits are 10% of nominal voltage, so for your nominal 230VAC supply 227VAC is the lower limit. Lower voltage is because all the houses fed from the local distribution transformer, not only your 4 fed from a single pole pig, but the hundreds also fed from the same 2 wire feeder are all drawing a heavy load to handle electric heating or heat pumps to keep warm. This causes a voltage drop on the long length of wire, and, as the local transformer for the area likely is a fixed tap type, and thus has manual voltage tap settings, which are effectively fixed, the only tap changers will be at larger substations, where they try to keep the output to all the intermediate power lines at a constant voltage. But all the extra load causes voltage drops on the cables, and this likely is higher percentage wise than the relatively thick wires from your local supply to the house, especially with you having a light load.So, not your connection, but the total of the entire neighbourhood drawing power, and this is showing by you, though if you turn on some heavy loads you might drop another volt in the feeder.

author: Faringdon
author_rank: Super Contributor
timestamp: February 04, 2022, 08:23:31 pm
content: Whereabouts in your hosue are you measuring the mains?.....beware of your house mains wiring voltage drops to the point where you are measuring it.

author: SpookyGhost
author_rank: Contributor
timestamp: February 04, 2022, 08:26:24 pm
content: Thanks guys, good info. I figured that the transformer would just output a set voltage no matter the input, I guess it makes sense that its relative to what's coming inI'm measuring right off the main panel. 2 Dedicated 15a circuits, one for each VT doing the measurements. Probably 2ft of 14/2 Romex in total, then about 2ft of 2/0 SER before it hits the meterMy generator ATS agrees with this measurements, which is the primary disconnect

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: February 04, 2022, 11:34:23 pm
content: Quote from: SeanB on February 04, 2022, 06:12:59 pmI think that should be 207VAC ( .9 * 230)

author: james_s
author_rank: Super Contributor
timestamp: February 04, 2022, 11:48:39 pm
content: Quote from: SpookyGhost on February 04, 2022, 08:26:24 pmThere exist constant voltage transformers, but they are special devices that bring various disadvantages. A conventional transformer will always put out a voltage proportional to the input voltage according to the turns ratio. Additionally it will have its own drop as load increases due to resistive losses in the windings and magnetic losses but this will typically be minimal in something like a distribution transformer. I suspect most of the variation in voltage will be due to the resistance of the miles and miles of wire between your distribution transformer and the power plant.

author: IanB
author_rank: Super Contributor
timestamp: February 04, 2022, 11:58:48 pm
content: In periods of heavy demand the operators might lower the supply voltage in order to reduce the load on the system. Many loads will draw less current when the voltage drops, so lowering the voltage can reduce total power consumption.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 05, 2022, 03:48:23 am
content: As you found out, sensing the frequency is a much better way to look at what's going on with the grid as a whole. Voltage tells you more about what's going on locally.Also of note is that Texas now has Ohmconnect, which rewards users for conserving energy at peak times. Unlike the California version, however, there's currently no way to sign up for it and still have time of use rates, so it's trading one peak demand reduction strategy for another.

author: SpookyGhost
author_rank: Contributor
timestamp: February 05, 2022, 04:13:59 am
content: Very interesting, I've never heard of Ohmconnect beforeIt's a shame, because I have 17kw of solar and get 1:1 net metering, so if I could get it on top of my current electrical plan, I could benefit greatly. With the exception of a very hot and cloudy December, I've never paid an electric bill since getting itAlso, my generator running at 25% load costs LESS than the current rate for electric on my plan, so I could even switch to generator when they want usage reduced, and get double benefit (My solar is on a line side tap, so I can switch to generator and still produce solar)

author: AlbertL
author_rank: Regular Contributor
timestamp: February 05, 2022, 03:51:02 pm
content: Utilities tend to keep the voltage closer to the high side of the range, because complaints from customers are usually about excessively low voltage. Most use automatically-controlled under-load tap-changer transformers ("ULTCs") at the substation, and/or tap-changer autotransformers ("regulators") at the substation or out on the line. Here's a typical regulator:https://www.eaton.com/content/dam/eaton/products/medium-voltage-power-distribution-control-systems/voltage-regulators/32-step-single-phase-voltage-regulator-catalog-CA225001EN.pdf.