I need help determining cause of wave form distortion

[MrJam]

The below wave forms where captured with a hioki 3196 attached to the load side of the service disconnect of a 300 amp service. The service feeds a lot of computer equipment 3 liebert units and a large Mitsubishi U.P. S system. The distortion occurs at irregular patterns 2 to 4 times an hour. Is the magnitude of the distortion something to be concerned about? Any help opinion or lead to a probable cause is greatly appreciated.

 

[burnt2x]

MrJam,
Hello.
It seems that some equipment was switched on phases 1 and 2 (red and green) making their respective phase currents almost in-phase with their respective phase voltages. The "blue" (phase 3) remained lagging its phase voltage.
Maybe power factor correction caps switching with the phase 3 cap not functioning well, or it' just my eyes!
Please try compare the peaks of the voltage waves with the respective current waves.
Others might have different takes on this, so just wait for a while.

 

[DanDel]

This isn't a 'distortion' per se, it is a switching in of a load. The voltage waveforms show a typical switching transient at two points for CH2 and one point each for CH1 & CH2, corresponding to the two points on CH2.

The current waveforms show what looks like a >100A spike for each of these transients. The dI/dt looks quite dramatic, and the load tapers off from the peaks over the next few cycles. I would imagine it is one of your loads switching on. There appears to be a moderate amount of current THD both before and after the events.

burnt2x has quite admirably noticed the change of phasing alignments from before to after the events.

Of most concern is the actual amplitude of the load imposed, but an overall effect on the voltage waveform past the transient itself is not evident.

 

[MrJam]

If it is a single piece of equipment causing the event, why would the load be distributed in such a way that it first spikes on C phase and B phases and then on A phase and B phase. Why wouldn’t it spike on all three at the same time? Is this because of rotor position at start up (assuming it’s a motor)? As far as power factor correction capacitors, I don’t think there are any in the building; they would have to be located down stream from the metering location because voltage and current are moving in opposite directions in the wave form captured the spikes are a little over 250 amps in some occurrences. The customer called me in to monitor because two circuit boards on two separate liebert units got fried. I’m trying to figure out if this is connected to the problem. I see this as a motor related load but, I’m not so sure of my self. This is the most frequently occurring event in the monitoring data as I said before 2 to 4 times per hour. Thank you guys and keep the suggestions coming. Heres another Picture

 

[MrJam]

Heres the Picture

 

[Bronzeado]

MrJam,

It seems to be a monitor problem.
Note that the voltages don't change when the currents increase. Is it a strong system?

Regards,

H. Bronzeado

 

[MrJam]

I would appreciate if you could explain what you mean when you say it’s a monitor problem. Also I'm not sure what you mean by a strong system.

 

[DanDel]

The multiple spikes could be electronic switching, or even contact bounce on a electromechanical contactor.

Can you isolate the problem? If it occurs 2-4 times an hour, you could check each individual circuit being fed from the original monitoring point, and so on down the line until you find the problem.

 

[Bronzeado]

MrJam,

For monitor problem I mean "instrument problem".

The suddenly change in the currents without a "response" in the voltages seems it may not be real, unless the short-circuit level at that point of measurement is very high in coparison with the load size (strong system).

H. Bronzeado

 

[slavag]

Hello.
I totally agree with DanDel. Try isolate problem, on the first step don't think about reason.
Regards.
Slava

 

[jghrist]

If it is a single piece of equipment causing the event, why would the load be distributed in such a way that it first spikes on C phase and B phases and then on A phase and B phase. Why wouldn’t it spike on all three at the same time?

The contactors on the three phases energizing the load do not all close at exactly the same time.

The customer called me in to monitor because two circuit boards on two separate liebert units got fried.

Where are the circuit boards in relation to the switched load? There is not much of a voltage spike at the point where you monitored the overall load, but if the circuit boards are at the end of the circuit where the switched load is, there might be a much larger voltage spike at that point because of the impedance of the circuit to the load.

 

[MrJam]

This is really great, I appreciate everyone’s input. I would love to track down the source of this anomaly, but unless it has anything to do with the burnt circuit boards I can’t really justify doing so. Is there anyway for these anomalies to effect the units if there not on the same branch circuit? The voltage response during the event looks like a sag to me, would it be likely a sag in voltage could fry a circuit board?

 

[burnt2x]

MrJam,

The events are not voltage sags but occurrences of voltage distortions! The distortion value is approximately 20V or so. As far as I can remember, the possible causes of these phenomena are switching on of the following equipment:
[ol A]
[li]Operation of battery chargers[/li]
[li]Large computers[/li]
[li]Variable Speed Drives (VSD)[/li]
[li]Arc welders[/li]
[li]Fluorescent lighting with electronic ballasts[/li]
[/ol]
You may want to survey the area for these electrical equipment or initiate anti-distortion solutions (use of special circuit and grounding practices or install harmonic filters).

 

[unclebob]

The disturbances appear between phase A and B (reb and green) and phase B and C (green and blue). Why do no disturbance appears between phase A and C (red and blue)?
This is odd.

 

[MrJam]

Thank you Burnt2x.

 

[jghrist]

The disturbances appear between phase A and B (reb and green) and phase B and C (green and blue). Why do no disturbance appears between phase A and C (red and blue)?
This is odd.

Good observation. The load must be delta-connected, not wye-connected. If this were the case, there would be no current spike when the first phase closed, say ØC. When the second phase closed, say ØB, V[sub]A[/sub] was zero, and V[sub]BC[/sub] was high, causing a spike in the ØB to ØC connected leg. When ØA closed, V[sub]AB[/sub] was high, but V[sub]CA[/sub] was zero, resulting in a surge in the ØA to ØB connected load, but not in the ØA to ØC connected load.

 

[jghrist]

I just opened the new issue of EC&M. It has an article Tracking Down Transient Voltages that looks directly related to your problem. Basically, the method used is to monitor the current on feeders, and then downstream panelboards until you find the source of the current surges.

The January issue is not on the EC&M website yet.