DC Offset at Substation 1

[bytebyte]

We have an RPM monitoring device installed at our substation and it is recording a positive and negative DC offset but only in the neutral current. What can this indicate?

We are trying to identify the source of the votlage sags on the circuit that may be caused by a customer that is violating our flicker limits, and I wasn't sure whether the DC offset may provide some clues.

 

[dpc]

I doubt it is has anything to do with flicker problems.

i'd start with the test equipment setup and work my way back through the PT and CT wiring. I suspect whatever you are seeing is an artifact of the test equipment.

Without more information, it is all speculation.

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[racobb]

Do you, or anyone else, have any switched cap banks on the bus or system? Bad vacuum bottles or mis-timed switches have been a problem in the past, but I would expect you would be seeing spikes, and real trashy waveforms in addition to the dips if this were the case. Just a thought.

 

[Skogsgurra]

dpc is right on!

I suspect that you are using a DC clamp-on ammeter?

If so, and there is a lot of harmonics adding in the neutral, you are probably overloading the DC range with positive and negative peaks.

That would not be a problem if the circuitry clipped symmetrically. The net result would still be zero.

The problem is that very few instruments are ideal. Most clip unsymmetrically. That means that the result is non-zero. So, the instrument reports a fictive DC. That does not exist IRL.

Try going up to next range. The DC should not be present there because the harmonics are now below the clipping levels.

Have had this not only in the neutral, but also in screens in motor cables fed from VFDs. It is very confusing until you understand what is going on,

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

Good point Skoggs!

Welcome to Eng-Tips racobb.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Bronzeado]

Hi folks,

Momentary DC offset in AC systems may occurs when a transformer is energized due to the DC component of the inrush current.

Regards,

H. Bronzeado

 

[danw2]

Gennar gets a star. Thanks for explanation on phantom DC.

 

[BSEE1980]

Did you say a Positive AND Negative DC offset? hmmm...that seems strange. I agree: check the test instruments.

Mr. Bronzeado is correct and I add any transient, like energizing a xmfr or a short circuit, may have a DC component that decays exponentially.

What is the magnitude of this DC offset current compared to the neutral current? I would suggest the grounding system be checked, maybe even an earth resistance test done. Certain soils or conditions (like bad CAD welds) can have a rectifying effect on current. Do you know the neutral-ground bond current? How much ground current is flowing?
You said neutral current: maybe you already ground current?

Good luck,
Steve