Hal wrote:
What sort of interference do you see?
There is a general "grass" on the entire waveform. At our location, the tops of the sine wave are clipped off (as the power is delivered). See attached image (the orange trace is the AC we receive from the grid; cyan is the distortion residual from a distortion analyzer -- there are rich harmonics out to the 20th or so within -50dB of the fundamental). The image is not properly scaled to show the high-frequency "grass."
What does an interesting transient event look like?
It all depends. They are not all that fast (unless your house feed gets struck by lightning, in which case "what does it look like in the data collection" is the least of your worries), and usually comparable in amplitude to the power signal +/- 10dB (again, unless there is a very close lightning strike), so they generate extra zero crossings spaced anywhere from low mS to tens of mS.
If you are going to post-process the data anyway, why not collect raw data and let the post-processing take care of the local interference? That lets you defer decisions about the appropriate filtering.
There is enough high-frequency "grass" to reduce the precision of your zero crossing determinations. Since there is no useful information on grid behavior at these frequencies, it is better to remove it to improve your zero-cross precision. You can do a lot with post-processing, but you can't fix EVERYTHING in the mix. You have to start with the best data collection you can get, which in this case means filtering out the low-amplitude stuff above 1kHz or so.
Is there any database of events that I can check when I see something interesting? Or turn things around and pick an event and see what it looks like when it gets here?
Not that I'm aware of. (But as I noted previously, I'm not personally a grid-nut).
Best regards, Charles
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