Events affecting negative of sine wave only

[buzzp]

For the power supplied by a utility, are there any events/problems that would affect the negative part of the sine wave and not the positive part?

 

[peebee]

Sure -- as a very simple example, a half-wave rectifier (which might be inadvertently created by the partial failure of a full-wave rectifier) will only affect half of the sine wave.

Any single diode will affect only half the wave. I think SCRs & triacs may also fall into that boat (someone else please confirm, I'm certainly no expert on silicon power devices). That's why you'll usually see those things installed at a minimum in groups of two (single-phase) or six (three-phase). Fail half of a pair, though, and you're back to half-wave rectification which would draw power only during half the cycle and would notch only the top (or bottom) of the voltage waveform.

Transient events (switching, lightning, etc) could also be of short-enough duration to affect less than one-half wave (although that would not be a periodic disturbance).

 

[buzzp]

I understand about rectification and problems that would arise when diodes, scr's, IGBT's, etc fail. But this would not typically be a concern for power provided by the utility (utilities don't use recitifiers, scrs, IGBTs except at the DC ties on the US's grid as far as I know).

Would faults on xmission/distribution lines, faults within xformers, capacitor bank switching, etc cause only the negative part of a sine wave to be affected? (I cant imagine only the negative being affected but want others input). Can you think of any events (caused by the utility either directly or indirectly) that would cause only the negative part of the wave to be affected?

Or for a better illustration, say I was going to monitor the voltage (from the utility) to some critical (or not so critical) piece of equipment. Say I was only going to monitor the positive part (half wave rectified) of the waveform. What events could occur that would be missed because we are not monitoring the negative part of the wave? I hope this clarifies my question.

 

[peebee]

Ahh -- you are only interested in utility-owned equipment that could cause such problems?

I do know that S&C sells a big-mama static switch intended for use by utilities on MV systems. I have no idea if they've ever sold one. But that could cause the rectifier problems I mentioned above.

It's not clear to me how long the transient from switching a cap bank would last -- possibly sub-cycle? But you'd have a 50/50 chance of catching it even if it was. . .

I can't think of anything other than transients & solid-state devices that would affect only half a wave.

 

[busbar]

Aside — Characteristics of DC ‘riding’ on AC waveforms readily generate even-order harmonic content. Three-phase {three-diode} halfwave rectification is great for producing textbook even-order harmonics and nice iron-saturating AC-supply-side dissymmetry/offset.

AC-revenue metering and instrument transformers go nuts over even-order harmonic content,

 

[jghrist]

CT saturation is a good example of the negative portion of the sinewave being affected. Under heavy saturation, the negative portion of the CT current is very small compared to the positive portion.

 

[alehman]

Transformer inrush current. Residual flux in the core causes inrush current to be asymmetrical.

Also, fault current typically has some amount of asymmetry which decays over a few cycles. The amount of asymmetry depends on the point in the cycle where the fault occurs. The rate of decay depends on X/R.

 

[ScottyUK]

As busbar has already suggested, the presence of DC in the winding of a transformer will bias the magnetic flux within the core and may well create assymetry in the secondary waveform because the core will approach or enter saturation on one or other half-cycle. Core saturation will affect the magnetising current on the primary side too, with the mag current being higher on the half-cyle entering saturation.




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I'm getting a great education!

 

[buzzp]

So we have transformer saturation, residual flux, and cap bank switching on the negative part of the waveform. This is easy enough to understand.

The presence of DC on the waveform being monitored would not necessarily be bad, its easily filtered. However, I understand the effects of DC on transformers (saturation).

 

[mc5w]

Metal oxides in wiring joints and fuse clips can also produce more voltage drop in 1 direction than in the other. If somebody is only using 1/4 maybe 1/2 of their capacity a wiring joint can have a significant amount of voltage drop without burning up.

Mike Cole, mc5w@earthlink.net