Distorted residential power, only at night? 3

[st50maint]

For many weeks I have had this problem with my power. I hear a slight buzz from my audio equipment, only at night. Hooking up an oscilloscope shows this waveform.

?
At dusk the waveform becomes dirty. At dawn it becomes clean. Everyday like clockwork.
Some more clues:
1. The problem is only on one of the 120 volt legs. The other leg is always relatively clean.
2. I thought that street lights might be a problem because of the timing. However I observed them on a few occasions and they did not come on or off exactly with the start or end of the distortion.
3. This is a residential only neighborhood with 14 houses and 3 streetlights on one transformer as far as I can tell.

Any thoughts on what is causing this and a possible solution?

 

[itsmoked]

magoo2; Around my neighborhood they change our voltage by switching in and out shunt capacitors. I call that voltage regulation. Granted it's crude but it's a form of regulation.

Anyway, that's what I meant.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[magoo2]

Keith,

Ok, I understand what you mean. It's a switched capacitor bank used to regulate voltage. I thought there might be some new gizmo out there.

Nonetheless, I wouldn't expect a cap bank to produce what was recorded.

 

[stevenal]

Gotta side with Davidbeach here. A normal linear capacitor switching in would cause a one or two cycle ringing, but this is continuous on the O-scope. If it is an abnormal cap, with a dielectric breakdown at the voltage peaks, it should quickly proceed to failure.

 

[st50maint]

I am starting to think that maybe it is the power company's problem as someone has suggested. I did not originally think so because only one leg from the transformer caused problems with my equipment. Also the scope waveform was much cleaner than on the problem leg.

I increased the gain of the scope to better see the distortion and there is similar distortion on the "good" leg but reduced in amplitude. I have taken pictures of the waveforms both at night and during the day. Any thoughts?

The "bad" leg at night.

The "good" leg at night.

The "good" leg during daytime.

The "bad" leg during daytime.

 

[PHovnanian]

I am starting to think that maybe it is the power company's problem as someone has suggested. I did not originally think so because only one leg from the transformer caused problems with my equipment. Also the scope waveform was much cleaner than on the problem leg.

Backwards logic. If you are seeing this glitch on only one leg of the service, it is almost certainly problem somewhere on the secondary. There is no way I can thing of that a glitch on the primary side of your transformer will only appear on one half of the transformer's secondary winding.

*For non US folks, our distribution transformers are usually single phase to neutral on the primary side and center tapped (120V from the center) to each side of a 240V winding.

 

[itsmoked]

Those daytime waveforms are hideous also. A problem.

Have you taken your scope somewhere else to make sure it's not your scope, or probe, or something else?

If you're sure your view is correct you should take this to the power company. Print those pictures out with dates and times and carry them in. Tell them it's causing buzzing in your audio equipment if it is.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[resqcapt19]

Scotty,

Don't forget that the US seem to use small pole-mount distribution transformers of a hundred kVA or so serving a small number of properties. ...

That would be a big transformer in my area. There are 7 houses on the 25 kVA transformer that serves my house.

 

[ScottyUK]

Thanks. I was guessing from what I see in the movies. In my patch there are several hundred houses hanging off a transformer that is rated perhaps 500kVA. I'm not scaling the transformer pen fence to find out the exact rating.


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If we learn from our mistakes I'm getting a great education!

 

[xfmrman]

Seven houses on one 25 KVA transformer ?
Where I live in the midwest your house has to have 100A service 100A*240V=24Kva.

 

[cranky108]

I've heard of 4 to 6 houses on a 25 kVA, but in many places that is streaching it. You see the larger number of houses in the older neiberhoods, but with compressor AC, and hot tubs, the transformer sizes are increasing as the need appears.

Many older homes have swamp coolers, or just fans for summer. The people moving here from somewhere else seem to want compressor AC.

It is possible this could be a lightning arrester failing, as the night time voltage goes up.

 

[davidbeach]

Nothing says that the utility has to have capacity to supply you 24kVA just because you have a service that could theoretically use 24kVA. Utilities are willing to overload transformers, within reason, and experience shows that there is lots of diversity in customer load. Seven on a 25 sounds a bit much, but seven on a 75 wouldn't attract notice here.

 

[LionelHutz]

That ringing after the peak almost looks like a capacitor switching on, but that makes little sense to be happening every cycle. It'd have to be some sort of thyristor switched capacitor bank and cap banks designed to switch on and off on a per cycle basis are typically designed to switch in a manner which avoids switching transients.

The utility doesn't follow the same rules as everyone else. For example, a building I worked in had #2 overhead feeding a 600A service. The #2 utility wire connected to something like dual 350mcm at the building.

 

[ghostbuster7]

As I said before,I would be looking for large 6 or 12 pulse loads on the primary side.It could be miles away.

 

[st50maint]

I went to the power company's office today. I asked to talk to a technical person but was denied. However, they took my information and pictures and said it will be looked into. We will see if anything happens

 

[Skogsgurra]

I wouldn't have much hope there. They probably don't have a 'technical person' that understands what you are saying.

Most utility guys think RMS and percent swell/sag. Some even think THD. But very few (none that I have met so far) will understand your waveforms.

You need to contact the power company's central office. There might be someone that understands.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[steelerfan28655]

As a utility engineer, I have seen a similar problem in one of our neighborhoods. It ended up being a loose neutral connection on some of our 120 volt street lights. I am not sure if it was the same harmonic that you are seeing.

We typically size our services by the projected load of the residence. Utility pole mounted transformers are ANSI rated to handle 140% of the nameplate rating for short durations. We use this extra capacity to reduce system losses.

Happiness is a way of travel, not a destination.

 

[st50maint]

The power company showed up at my house at 8 am today. The gentleman was not aware of the exact complaint. I explained that it only happened at night and showed him pictures of the waveforms. All he could do is check my service as he would normally do. Everything measured OK but he elected to redo the connections at the house and pole since they appeared to be quite old. I doubt if the problem is fixed but I will find out when I get home tonight.

 

[st50maint]

Power is still bad. At first I thought it was good but it turns out that they reversed the two legs which put the problem leg on different outlets.

 

[stevenal]

Tell them you have a power quality issue and suspect you have excessive harmonic distortion. Ask them to install a recorder.

 

[ghostbuster7]

In our area ,the cable tv power cabinets(2'x2')as well as the cable tv UPS systems,bus shelters and traffic lights are all connected to this same electrical feeder.These are some additional areas that may require investigation.