Voltage dips and frequency fluctuations at a residence.

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Sleken

Member
Location
Oklahoma
I've got kind of a puzzling problem.

I was called to a residence of a complaint of power dips. Customer stated that they usually came in batches of 5 to 6 dips at a time and seemingly at random times of the day.

I was lucky enough to experience it first hand. Fans plugged into outlets would very noticeably slow down and then recover.

This was independent of any major loads kicking on. My first thought was that a bad connection existed somewhere. The customer didn't want me to turn the power off that day so I'm going back later. I did notice that the problem seemed to be isolated to one leg of the single phase service.

Here's the puzzling part:

I flipped my meter over to hertz just out of curiosity. It was rock steady at 60 hz except during those dips where it would drop to the 57-59 hz range but then would bounce up to 120 - 155 hz. Some of the digital clocks on that leg had been speeding up since the problem started.

The more I've thought about this I just can't imagine any situation other than a utility problem that could cause that frequency fluctuation. However, I've never really monitored frequency during a voltage dip before.

Does anyone have any thoughts?
 

iwire

Moderator
Staff member
Location
Massachusetts
I flipped my meter over to hertz just out of curiosity. It was rock steady at 60 hz except during those dips where it would drop to the 57-59 hz range but then would bounce up to 120 - 155 hz.

I would really be questioning my meter, I don't see how the utility supply can hz could vary that much. :huh:
 

kwired

Electron manager
Location
NE Nebraska
Utility frequency will be pretty rock solid, some other source is injecting any varying frequency you are measuring.


I'd start with placing heavy loads on the service and watch what voltage does, a drop in voltage is expected, you need to determine what is reasonable for the conditions there. If there is a problem upstream from service entrance - it could be a neighbor's load that is causing the seemingly random change in this house - especially if they are both powered from same source.
 

Tony S

Senior Member
Does your meter have a recording facility?

Just as an experiment I set mine up to record my supply voltage over a 24Hr period. Being friends with a DNO (POCO) engineer we compared the fluctuations on my meter to the times the on load tap changes operated.

As for frequency varying, I doubt that very much.

Dynamic Demand
 

HEYDOG

Senior Member
I would be checking for a loose neutral either on customer or utility side. That would not effect the frequency but it seems more logical from your description.
 

big john

Senior Member
Location
Portland, ME
As for frequency varying, I doubt that very much...
Agreed. A frequency change like that would be a catastrophic utility event. This is measurement error.

That said: I'm thinkng interference from a large SMPS similar to how a multimeter will read the carrier frequency on a drive instead of the fundamental.

What are the chances the homeowner has an electric tankless water heater controlled by SCRs? That would definitely explain the VD and I know it's a sretch but maybe it causes frequency noise?

Also possible this is a local grid problem. See if neighbors also experience it. Is there an arc furnace right down the road? :D
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
Ditto about the frequency not really changing. Can't happen. Your meter is measuring some kind of interference. You could ask the POCO to check for a service problem. They will likely use a "Beast of Burden" to apply a load to each leg and measure for voltage drop. They can also install a digital power recorder if they have one available. Records voltage, current, frequency, harmonics....all kinds of stuff. I'd say, ignore the frequency readings and focus on voltage sag. But...you have to determine if it's a service side problem or a load side problem. You can measure at the main breaker and get a pretty good idea, though. If you're seeing it at the main, it's probably a utility problem. The service neutral and phase connections are behind a sealed cover, so leave that to the POCO. Just the way we would handle it as a utility, but we're NICE!:lol:
 

kwired

Electron manager
Location
NE Nebraska
I would be checking for a loose neutral either on customer or utility side. That would not effect the frequency but it seems more logical from your description.
But he should have an equal voltage rise on the oppposing line to the compromised neutral if that is the trouble.

Voltage change on one line only is due to resistance in that line - whether it be just conductor resistance or if it includes resistance from bad connection(s).
 

keith gigabyte

Senior Member
No way freq change. the voltage dip on one leg is most likely a loose connection somewhere Check meter base..main breaker..if all good. It's a poco problem. Visually look at service lateral maybe you can see some crappy splices or something.
 

Tony S

Senior Member
There’s only one real answer to this. Beg, borrow, steal or even hire a power analyser. They cost about £15 ($24) per day to hire over here.

Record the current on both lines and neutral along with the three voltages. They record frequency, I’ve used that once in the last 15 years so not even worth bothering about.

It depends on what the customer is willing to pay to find the root cause of the problem.
 
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