Thread: Mysterious vibration in long conduit between solar inverters and main service panel

1. Moderator
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Dec 2012
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Originally Posted by Redwood Infrared
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Still don't really understand relationship between PF and THD.
Displacement Power Factor corresponds to a phase shift in current relative to voltage (at the fundamental frequency, and assuming basically that the voltage waveform is a pure sine wave.) It happens as a result of linear reactive loads (motors or capacitor bank, for example) and does not involve any waveform distortion as measured by THD.
A non-linear load (whether resistive or reactive in nature) results in distortion of the current waveform (assuming again that the voltage is pure sine wave). It results in a discrepancy between the product of RMS voltage and RMS current which shows up as a reduced power factor. And that distorted waveform can be built up from a summation of the fundamental frequency and suitably phased harmonics. One measure of that array of harmonics is THD. THD does not tell you anything about relative amplitude or phase of individual harmonics. It cannot be corrected for by any combination of linear reactive loads, but can be compensated by a deliberately non-linear load that exactly cancels out the harmonic current. That can be done within the input stages of otherwise non-linear loads like the DC power supplies in an electronic ballast or switching power supply or VFD.

2. gar
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180617-2458 EDT

Redwood Infrared:

To get an idea of instrumentation problems I ran the following experiment.

1. Made a hairpin loop of #16 insulated wire. Lashed it together at a point with fiberglass tape. Then again about 6" away.

2. Powered this from a 16 V transformer with its input from a Variac.

3. Used a quartz accelerometer in this 6" region, and scope synced to the line to monitor vibration.

4. Did not measure current, but with probably several amps I saw a relative clean double frequency sine wave waveform well above the quartz amplifier noise level. No significant harmonics. Amplitude increases with current. Thus, checks theory.

5. Moving the transducer from contact with the wire, but just about as close and no signal was seen. If the two wires were at a potential difference of 240 V would I see capacitive coupling of an unwanted 60 Hz signal? I don't know.

6. I would like to see this experiment with a 3 phase balanced load, and a significant unbalanced load.

You need to do some of your own bench experiments, and with and without conduit, and at current levels seen in your problem site.

Can you simulate the noise in a lab situation?

.

3. gar
Senior Member
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Apr 2008
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Ann Arbor, Michigan
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180618-0925 EDT

To clarify my comment on no significant harmonics in the force (accelerometer) curve ---

1. Today and probably most places in the world there is clipping distortion at the peak of the AC supply voltage curve. This did not exist 60 years ago.

2. This clipping produces some harmonics in the voltage waveform.

3. Force between conductors is a function of the current in the conductors and the interaction.

4. In my experiment I had a transformer with series leakage inductance, and nearly a short circuit on the secondary. Thus, input voltage harmonics would be reduced in the output current by the low pass filtering action of the series LR circuit.

Theory checks out nicely.

.

4. Join Date
Jun 2018
Location
Arcata, CA, USA
Posts
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Gar,

Given the scale of our problem site - over a couple hundred amps along conductors spanning 250 feet - I have no idea how I could prepare a bench experiment that wouldn't lack verisimilitude.

Originally Posted by gar
180617-2458 EDT

Redwood Infrared:

To get an idea of instrumentation problems I ran the following experiment.

1. Made a hairpin loop of #16 insulated wire. Lashed it together at a point with fiberglass tape. Then again about 6" away.

2. Powered this from a 16 V transformer with its input from a Variac.

3. Used a quartz accelerometer in this 6" region, and scope synced to the line to monitor vibration.

4. Did not measure current, but with probably several amps I saw a relative clean double frequency sine wave waveform well above the quartz amplifier noise level. No significant harmonics. Amplitude increases with current. Thus, checks theory.

5. Moving the transducer from contact with the wire, but just about as close and no signal was seen. If the two wires were at a potential difference of 240 V would I see capacitive coupling of an unwanted 60 Hz signal? I don't know.

6. I would like to see this experiment with a 3 phase balanced load, and a significant unbalanced load.

You need to do some of your own bench experiments, and with and without conduit, and at current levels seen in your problem site.

Can you simulate the noise in a lab situation?

.

5. Originally Posted by cowboyjwc
Gotta throw it out there sometime, I once walked up on two electricians an and engineer trying to solve a problem on why a motor wasn't working, I asked if they checked to see if a mouse had gotten in and chewed on the windings and shorted it out, they all laughed and kept checking stuff, I asked again and they told me that if I wasn't going to be serious I could leave. I told them I was serious and they asked why? I said because I can see the mouse and the arc mark it left on the side of the motor. Sometimes the simplest answer is the one.

Occam's razor or a more modern expression: KISS="Keep It Simple Stupid"

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