Power quality analysis

[Truemanator]

I'm studying the power quality for a client of ours. Over a 106 day period, our Fluke 1750 data recorders observed 40 voltage-triggered "impulses". An example of one of the transient event graphs is attached.

From my understanding, impulsive transients are typically caused by lightning strikes. The client's system is in a relatively likely spot for lightning, however I have yet to been able to obtain meteological data to confirm.

Does anyone have any ideas if the voltage forms are typical of a lightning strike?

 

[stevenal]

Is this typical of the 40? If you invert each of the voltages during the strange period, you can find another phase that it will align with. Not sure what would cause voltages to suddenly invert and swap. I suspect a faulty meter. Transients are narrow spikes that are not sinusoidal like these are. Send the recording to Fluke to get their take.

 

[Skogsgurra]

This is not lightning. That's a sure thing. You have milliseconds - lightning is microseconds. And does not look like this at all.

Looks more like a bad connection somewhere. Probably between grid and the 1750. Look for loose fuses, loose terminal blocks, loose wires, bad clips or just about anything.

Did you have any malfunctioning of other equipment when these "transients" occurred?

Gunnar Englund

http://www.gke.org

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

 

[Truemanator]

This is very typical of 39 of the 40. There was one anomaly transient, but I'm going to disregard it for the time being.

If it's not lightning, is there a possibility it is a result of switching off of large inductive loads (as I have read in some places)? We have been collecting data from two different circuits on site, but the other site did not have any transients occur, which may lend to the idea this is not lightning. I noticed how the voltage waveforms align perfectly when rearranged. Some of the transients even had the waveforms completed by the other phases (literally, the transient portion block of phase C was fitted perfectly into the missing transient portion of phase A and so on).

Unfortunately, I was simply handed the equipment with data on it and asked to analyze. I know little about the physical configuration of the site being tested, with the exception that this site is a 600V distribution circuit and the other is 120V at a regional prison. The data was recorded prior to my arrival with the company.

Thanks for the input.

 

[Truemanator]

Hmm I just read some information, is it possible this is an example of Voltage Notching?

 

[jghrist]

Voltage notching comes from phases being momentarily shorted when thyristors in an ac-dc converter fire at the same time. This will occur on a regular basis each cycle, not 40 times in 106 days. Notching is caused by the voltage drop across the system impedance during a high current. It wouldn't explain a voltage increase.

You seem to be getting some kind of a phase reversal that has to be generated by some power electronics. Notice that during the transient, VA = -VC (or what would have been -VC if the voltage were normal), VB = -VA, and VC = -VB, assuming VA is black, VB is blue, and VC is red.

The oscillations at the beginning of the transient period indicates that there really is some switching going on, and it isn't just an instrument error. I'd say this is an intermittent failure of some converter, variable speed drive, static var compensator, or some other power electronic device.

 

[slavag]

I think Jghrist is right.
It's seems as switching transient, maybe are some very bad contacts.
Best Regards.
Slava

 

[stevenal]

Truemanator,

There appears to be no load, just noise. Were the current probes connected? Was there any load during the recording period?

I have found Fluke to be responsive to my inquiries.

jghrist,

Shouldn't those oscillations occur at recovery also? That's why I suggested an instrument problem.

 

[jghrist]

stevenal,

I assumed that the current probes were not connected to anything and that the current traces were included just to confuse things.

You're right, I would expect oscillations at recovery also, but wouldn't this also be the case if the switching were within the instrument? Maybe, on second thought, this could be an intermittent instrument failure.

 

[Skogsgurra]

I maintain that this is nothing but a bad connection between instrument and grid. Thyristor notching does not look like that at all.

Also the high-frequency ringing that can be seen is typical when a wire comes loose so that the impedance in the circuit goes from milliohms (grid impedance) to megohms (open instrument input with wire attached to it). As you can see, the ringing does not happen close to the beginning of the sharp drop, but a bit into it. So, it can be concluded that ringing and sharp drop are not connected. It is just some noise being picked up as the wire is in high impedance state.

I attach a case of bad notching from an overloaded transformer with a thyristor rectifier.

Gunnar Englund

http://www.gke.org

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

 

[Truemanator]

Thanks for the help, guys.

There definitely would have been load during the contact period. Some of the data collected did not have the current probes connected for some reason. I have attached a similar graph while the current probes were connected.

I am leaning toward the intermittent equipment failure concept.

 

[stevenal]

I don't see the same invert and swap pattern in the voltages here. The current transitions smoothly across the sudden voltage changes, therefore current is not being affected by them. Also, current changes are not causing the voltage changes. If connections were bad, I would expect the voltages to move toward zero and not be coincident on all phases.

jghrist,

I don't believe the instrument is switching, but somehow miss-sampling or misplacing the data.

 

[Skogsgurra]

Depends on connection. Wye or delta?

Gunnar Englund

http://www.gke.org

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

 

[jghrist]

stevenal,

If the instrument is mis-sampling or misplacing data only, where does the ringing come from?

 

[stevenal]

The ringing is real, just in the wrong place.