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Distorted residential power, only at night?
3

Distorted residential power, only at night?

Distorted residential power, only at night?

(OP)
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?

 

RE: Distorted residential power, only at night?

2
The flat top voltage looks like there are a lot of switched-mode power supplies on line.  Switched-mode power supplies power computers and similar equipment and conduct current only at voltage peaks.  This causes voltage drop at voltage peaks, flattening the waveform.  Squiggley CFL bulbs also use switched-mode power supplies.  Maybe your neighbors are environmentally conscious and have quit using the old nasty resistive type incandescent lamps that don't cause powerline distortion.
 

RE: Distorted residential power, only at night?

st50maint,

That appears to be harmonic distortion. Have you tried using your osilloscope to check for which odd harmonic it being applied into your local line?

RE: Distorted residential power, only at night?

(OP)
The flattening of the waveform is not what comes and goes. It is the spikes just after the peaks that I am concerned with.
 

RE: Distorted residential power, only at night?

The first step would be to start switching off circuit breakers in your house to determine how much of this distortion is being created by your loads.

14 houses on one transformer is a lot of houses.  Are you sure about this?

Also, where are you measuring this voltage?

One of your neighbors have an indoor plant growing operation?  

David Castor
www.cvoes.com

RE: Distorted residential power, only at night?

Did any of your neighbors get an electric car recently?  

RE: Distorted residential power, only at night?

That's a very intelligent observation, Compositepro.

This is not a combination of several SMPS-es, but looks more like one hefty rectifier taking a lot of current over a conduction angle that is close to 45 degrees, which is much more than one can see in a typical SMPS. The fact that current snaps off distintly and causes significant ringing at the end of the conduction angle also tells that this is one consumer and not the typical 'co-operation' of many small loads.

The ringing is probably the origin of the buzz you hear.

Charging an electric car could very well be such a load.

Interesting.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Distorted residential power, only at night?

(OP)
I have switched off all breakers except the one powering the scope. Then I put the scope on a different line and shut off the original breaker. This was done at the outlets closest to the circuit breakers.

I am pretty sure that there are 14 houses on the transformer.

I am not aware of any electric cars or indoor plant growing but I will ask around.



 

RE: Distorted residential power, only at night?

Quote:

... indoor plant growing...
Careful, those guys tend to not appreciate being found!

I would start by finding a friendly neighbor and ask to connect your scope to his system too and see if the problem exists outside of your home. If they don't have it, the problem is closer to home. If they do, and if you can get to enough other friendly neighbors, you may find a variation in the level of attenuation going on by distance and that may allow some gross triangulation toward the culprit.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Distorted residential power, only at night?

You might be able to take a small portable radio tuned to the hi end or low end of the AM dial and walk down the street if front of the 14 houses and see if the noise lever increases in front of one house.  Even better is to check at the utility meters. Sounds like a nearby neighbor may have a light dimmer, old electric blanket, a lamp that goes on/off by touching it.  May not even be on the same utility transformer.

RE: Distorted residential power, only at night?

Not much HF in that. Will probably not be picked up by a radio. Not a low wattage load like a lamp or blanket either. This is substantial.  

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Distorted residential power, only at night?

If you 'scope the neighbors services, you'll have to take a look at their current waveforms as well as voltage. Everyone will be seeing more or less the same voltage. Particularly if this waveform is caused by the interaction of a non-linear load with the common utility transformer.

You'll be looking for a step in the load current that coincides with the notch in the voltage waveform.

Searching for radiated RF can fool you. At times, the signal can be conducted a ways along a service line and then radiated from a point some distance away from the culprit.

RE: Distorted residential power, only at night?

skogs,

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 certainly contrasts with British practice where a suburban distribution transformer is more typically in the 0.5 - 1.5 MVA range and serving a much larger number of properties. The 'single big load' might not have to be all that big to cause a problem: maybe someone with a home workshop running a lathe or milling machine on an inverter? I have never looked at my domestic mains when my lathe is running, but it certainly upsets the ADSL broadband modem connection. surprise

Another thought: how would a high primary voltage pushing the transformer core in to moderate saturation manifest itself in terms of secondary voltage waveform? Light loads on the system in the evening would tally with a high primary voltage.
  

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

RE: Distorted residential power, only at night?

st50maint;  The flat top is what you should be interested in not the little squiggle after it.   It's the flat top that's causing the squiggle.

Is the time this starts in the evening and ends in the morning accurate and repeatable?  I ask because if it is then it's probably not a consumer but rather a power company caused function like a switched capacitor voltage regulator.  If it were, say, an electric car being charged it would end at different times and start with a multi minute variation.  

Keith Cress
kcress - http://www.flaminsystems.com

RE: Distorted residential power, only at night?

Yes, I have had one of them just outside my window in Elizabeth, NJ. Not exactly beautiful. And that arrangement with two transformers in one pole to get that open delta does not make it any more beautiful. 'My' transformer kept buzzing all day and all night. I couldn't sleep - waiting for it to explode any minute.

Yes, but even 100 kVA is big compared normal switch-mode power supplies. A conduction angle close to 45 degrees is not usual. I am very excited to learn what it actually turns out to be.

BTW, many PWM inverters take out communication. But they do it with HF from the PWM switchiing. Not with low-frequency distortion from the rectifier.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Distorted residential power, only at night?

(OP)
It is always good when I get up about 8 AM. It is always bad when I get home after dark. The few times I have been able to observe the transition it has been fairly accurate and repeatable, within minutes of my street lights going on or off.

"Is the time this starts in the evening and ends in the morning accurate and repeatable?  I ask because if it is then it's probably not a consumer but rather a power company caused function like a switched capacitor voltage regulator.  If it were, say, an electric car being charged it would end at different times and start with a multi minute variation."

RE: Distorted residential power, only at night?

Quote (st50maint):

within minutes of my street lights going on or off
A bad street lighting ballast.  The problem may be thermally induced and therefore not present at turn on, but only shortly thereafter.

RE: Distorted residential power, only at night?

We had a case recently,in an industrial area where the voltage on the step down side of the dedicated factory transformer would increase by approx. 20% with lots of harmonic distortion.It would stay at this level for 10-15 minutes each time.It was later found down the street,the scrap metal yard(supplied from another step down tx.) was turning on their large  electromagnet to pick-up the cars.

The point is :harmonic/waveform distortion on the secondary/primary tx. of offending client can be transferred to the tx.secondary side of an unsuspecting client.

I would lodge a complaint with the utility and let them investigate.It could be a large load miles away that is turning on.  

RE: Distorted residential power, only at night?

a switched capacitor voltage regulator?  What is that?

A switched capacitor is a shunt capacitor with an on/off switch.

A voltage regulator is an autotransformer with a tap changer.

I'm just curious what you mean by a switched capacitor voltage regulator.

 

RE: Distorted residential power, only at night?

I agree with Gunnar that it looks like a single load.  A bunch of small loads might cause the flat spot, but they all would not fire at exactly the same angle and the transient ringing would cancel out.  It isn't switched utility capacitors.  These could cause a high frequency ringing, but it would be a single transient when the capacitor is switched, not something that happens every cycle and shows up on an oscillograph.

How big a load?  It looks like about 30 volts is chopped off the voltage waveform.  Say the transformer is 25 kVA with a 2% impedance.  This is about  0.012 ohms at 120 volts.  Say the load is 200 ft from the transformer and the secondary is #1/0 aluminum.  The two-way impedance would be 0.076 ohms.  These are at 60 Hz.  The impedance for the peak part of the current would be probably be 4 times this because of the increased frequency.  So, we have 30 volts divided by 0.088 ohms, or 340 A peak, but only lasting .  That's a very large load. If it was an electric car charging overnight, I would expect the load to be very small by morning when the batteries are fully charged.
 

RE: Distorted residential power, only at night?

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

RE: Distorted residential power, only at night?

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.

RE: Distorted residential power, only at night?

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.  

RE: Distorted residential power, only at night?

(OP)
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.
  

RE: Distorted residential power, only at night?

Quote:

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.  

RE: Distorted residential power, only at night?

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

RE: Distorted residential power, only at night?

Scotty,

Quote:

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.    

RE: Distorted residential power, only at night?

Thanks. I was guessing from what I see in the movies. blush 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. smile
  

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

RE: Distorted residential power, only at night?

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

RE: Distorted residential power, only at night?

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.

RE: Distorted residential power, only at night?

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.

RE: Distorted residential power, only at night?

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.

RE: Distorted residential power, only at night?

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

 

RE: Distorted residential power, only at night?

(OP)
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

RE: Distorted residential power, only at night?

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

RE: Distorted residential power, only at night?

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.

RE: Distorted residential power, only at night?

(OP)
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.  

RE: Distorted residential power, only at night?

(OP)
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.  

RE: Distorted residential power, only at night?

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

RE: Distorted residential power, only at night?

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.

RE: Distorted residential power, only at night?

Can your scope do a FFT?  You can prove harmonic distortion (or disprove it) yourself.

RE: Distorted residential power, only at night?

(OP)
No, my Tektronix RM504 can not do FFT. However, I did hook up an audio distortion analyzer to the AC. It reads about 3.5 percent THD all the time. I assume this means that the problem is not harmonically related to the 60 Hz.  

RE: Distorted residential power, only at night?

An audio distorsion analyzer isn't the best choice for AC power. But the number seems to be in the right community or even ballpark. The THD doesn't say much, I have had installations with 14 % THD and no problems. What makes your equipment buzz is that higher frequency transient at the end of the flat top.

You never told us if the buzz comes from the speakers or from your amplifier. Which one is it?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Distorted residential power, only at night?

(OP)
The buzz is not heard from the speakers. It is heard faintly from the electronics, noticeable only when the speakers are silent.

RE: Distorted residential power, only at night?

Does the electronics have a good old-fashioned power transformer in the PSU - or an SMPS?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Distorted residential power, only at night?

(OP)
Oddly enough the unit with the buzz is an AC power regenerator that feeds AC to the tube amplifiers. It is a PS Audio- Power Plant Premier. I suspect that it does have some sort of SMPS.

RE: Distorted residential power, only at night?

(OP)
Got home from work yesterday and found a tag on the door from the power company. At 1 PM they had checked my power and found no problems. I had called last week and again explained that the problem was only at night yet they still come during the day.
 

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