Flatened top sine wave, does it mean anything?

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This happens increasingly.
The most common reason is that a large part of the load on the utility supply consists of switched mode power supplies, electronic lighting ballasts and variable speed drives for motors.
Such loads normally consist of a bridge rectifier and reservior capacitor, followed by other electronics.
In normal use, the capacitor charges up to nearly the peak of the input voltage, therefore current can only flow into the capacitor when the line voltage exceeds the voltage across the capacitor.
The loads therefore draw relatively large currents but only at the peaks of the sinewave.
The impedance of cables, transformers etc, results in the peak voltage dropping, but only the peaks, not the rest of the sinwave since no/very little current is drawn for the rest of the sinewave.
The utility cant fully compensate for this by altering transformer taps, since the RMS voltage would become excessive.
 
That was a 208Y/120 service for an assisted living center. All most all the lighting is florescent or low voltage.

I am here because they are blowing capacitors on HVAC equipment and going through a lot of ballasts. (This place is new, all modern electronic ballasts.)

They have a 480 service that supplys the HVAC and now my meter is on that service, that shows lots of spikes on the current waves.
 
Besoeker said:
It could be that the facility is not causing the problem and that it is actually on the incoming supply regardless of the loading at the facility.
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Good point. But if the facility has its own dedicated step down transformer, harmonics tend to cancel out in the primary (if its a delta).
 
I was just in the boiler room, at a quick glance I saw a number of small VFDs and a couple of larger ones (25 HP or less)
 
What percentage of capacity is the transformer loaded at?

But I'm thinking it's coming from the primary side by the way that the peaks are low but not really jagged.

That isn't a result of the sampling rate of your scope is it? I've really only worked with analog, so I don't know. But If I saw that on an analog scope, I'd think it was coming from the other side of the iron.
 
steve66 said:
Good point. But if the facility has its own dedicated step down transformer, harmonics tend to cancel out in the primary (if its a delta).
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Third harmonic currents, if I remember back to university days, circulate in the delta winding.
What we're looking at here are third harmonic voltages.
The voltage appearing on the primary windings will appear on the secondary windings.
If the primary voltage is distorted, the secondary will be too.

This is from a report I did some while back for one of our installations:

(a) the supply impedance is low relative to the load being drawn.
(b) the supply voltage distortion is being imposed on the site and the cause of the distortion is external.
Clearly, in view of the above, compliance with spec. *** at the point of common coupling is not possible. The excessive level of supply voltage distortion being imposed should be brought to the attention of the supply authority.
The HV distortion will be reflected on the LV side of the transformer and this, together with the present 6-pulse load, means that the LV distortion will be close to the 6% limit which the customer wishes to meet. With the new loading which employs 12-pulse converters calculations indicate that the level of distortion will be similar.
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It was a water pumping station with a number VSDs and a dedicated 11kV delta to 400V to four wire star transformer.
 
I'm not sure if the waveforms have anything to do with ballasts and blown hvac capacitors.

It could just be a case of a higher mortality rate from everything being new.
 
steve66 said:
I'm not sure if the waveforms have anything to do with ballasts and blown hvac capacitors.

It could just be a case of a higher mortality rate from everything being new.
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Because of poor quality control by the manufacturer?
 
120113-1950 EST

iwire:

If the nominal steady-state RMS AC line voltage is 120 V +/-10 V, then good quality well designed equipment should not fail as a result of the waveform distortion you are seeing.

The cause of the flatten peak is a result of all the equipment with rectifiers followed by a capacitor input filter that are loads on the entire grid. I think you will see this in most parts of the country. I have scope photos from years ago that display very nicely formed sine waves from the line. Today my supply has flattened tops, and these are not primarily from my loads.

Disassemble some of the failed components and see if there is a common type of failure.

Are there sustained under or over voltage conditions? Are there large voltage transients? Are components subjected high temperature?

.

.
 
gar said:
120113-1950 EST

iwire:

If the nominal steady-state RMS AC line voltage is 120 V +/-10 V, then good quality well designed equipment should not fail as a result of the waveform distortion you are seeing.
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The flattened top results from typically third harmonic voltage. That means that there is a 180Hz component. Given that Xc = 1/ω the 180Hz component will increase the loading on the capacitors.
And, according to iwire, capacitors are failing.
And ballasts. Conventional magnetic ballasts, also with capacitors maybe?

Seen it happen. Mostly in industrial applications I have to say. But then again, I don't often get involved in commercial and almost never in residential. And the capacitors don't care where.
 
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