Events affecting negative of sine wave only
Events affecting negative of sine wave only
(OP)
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?
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Events affecting negative of sine wave only
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RE: Events affecting negative of sine wave only
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).
RE: Events affecting negative of sine wave only
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.
RE: Events affecting negative of sine wave only
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.
RE: Events affecting negative of sine wave only
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,
RE: Events affecting negative of sine wave only
RE: Events affecting negative of sine wave only
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.
RE: Events affecting negative of sine wave only
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RE: Events affecting negative of sine wave only
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).
RE: Events affecting negative of sine wave only
Mike Cole, mc5w@earthlink.net