What produces Second Harmonic Current? 2

[axg002]

Does anyone know what could produce 120 hz current on two legs of a 3 phase system? (120 hz is being produced on a and b phase, not c) We know that it's not a drive misfiring and we know it isn't a transformer saturating. Any input would be so much appreciated!!
Thanks, Ashley

 

[dpc]

How are you measuring this? Any chance it could be a measurement artifact? Are the loads balanced on all three phases?

 

[marks1080]

What is the configuration of the transformer? What type of equipment is installed on the load side?

Mark

 

[axg002]

Hey Dpc - The service entrance transformer is a Wye – Grounded Wye winding configuration. The load bus is commercial facility, nothing out of the ordinary. One hint is that we only see elevated levels of the 2nd harmonic current on two of the three phases; with an average of 8% Distortion level as expressed as percentage of the fundamental, with fundamental current measuring an average of 100 amps. This is measured at 480V; at the clients main switchboard. Thank you for your reply.

Mark1080- I am not sure what you mean by measurement artifact. This was measured over a two day period; and I am beginning to question whether or not a recent client lighting retrofit with high efficiency ballast might be the source; whereas they do have some drives, these are not energized during the predominant period of measurement; but when AC Drive is online and working, approximately 15 minutes out of two days profile, it provided slightly higher 2nd order current as well. I scoped the drives, there is no problem with pulse firing sequences; as that was the first place I looked given historically having seen 2nd order current produced by a misfiring SCR on a six pulse drive. But that is not the case here. Thank you for your reply!

Thanks guys, any thoughts will help! - Ashley

 

[axg002]

And Dpc, The loads are somewhat balance across all three phases

 

[axg002]

I apologize dpc and mark1080, I answered your questions backwards. The first reply should be to mark and the second to dpc. Sorry about that. Thanks again.

 

[PHovnanian]

Look for devices across these two phases with half wave rectifiers. Or perhaps a phase to phase load with a failed leg in a bridge rectifier. In general, things that produce an asymmetry between the positive and negative halves of a sine wave produce even harmonics.

 

[axg002]

Thank you Phovanian for your feedback. I will look into that. I appreciate the help!

 

[wroggent]

Could the isolated neutral on the primary cause problems? The presence of zero sequence current would cause that neutral point to shift. Perhaps this would cause the transformer to become saturated during half of a cycle for one or more of the phases. An asymmetrical voltage may then be imposed on the secondary thus resulting in even harmonics.

 

[Skogsgurra]

A thyristor rectifier can produce even harmonics without misfiring. Older analogue gating units are not always symmetrical and slight deviations from 60 degrees cause even harmonics.

I have also had "nervous" armature current controllers that locks to mains frequency. That also produces even harmonics.

Use a current clamp plus scope and inspect outgoing current in every cable. Then, if you find bad waveshapes, follow that cable and see what is connected to it. You could also use a current clamp with individual harmonics display like a C&A F607.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[axg002]

Thank you all so much for your questions and assistance in the Power Quality problem; We have identified the source of the 120 hertz current.....when reviewing current spectrums as compared from one phase to another, we noticed that the 2nd order current was only in elevated levels on two of the three phases in the system. This pointed us in the direction that the source of the current is single phase loads. When we went through system panel schedules, the only harmonic producing load that remotely met the profile of what we were looking for was a high efficiency fluorescent lighting; the client had just recently had a lighting retrofit and replaced HID lighting with fluorescent style ballasts. We asked the client to monitor the second harmonic current on all three phases, and then cycle the lighting panel on and off. When they did so, the two phases that expressed elevated harmonic current at 120 hertz dropped off significantly, while the third phase never moved. When lighting panels were turned back on, both phases harmonic content jumped back up to recorded levels.

These are 277 volt lights; and apparently the client’s electrical installer only used two phases of the system when powering them; no engineering support was used for project install and design. In this case, it might have been for the best; as had all three phases had lighting evenly distributed across them, we might still be wondering as to what the source of this unusual negative sequence component could be. Now that the ballasts have been identified, and approximately 6 amps of 2nd harmonic current are recorded in the system, what would your comments be in next step? Client is soon to add approximately 16 motors and AC Drives to the system; and I am concerned that the negative sequence current poses counterclockwise torque and heating in motor shafts. Would this concern warrant another lighting change out in your opinion? At what levels of 2nd order / negative sequence current would one feel action was needed to protect 480V motor loads in parallel to system lighting circuits?

Thanks,
Ashley

 

[PHovnanian]

Balancing the lighting loads would be a good idea. Also, specifying ballasts for new work with tighter harmonics would be a benefit. There is a cost tradeoff between swapping out the existing ballasts and installing a filter to mitigate their effect on the rest of the system.

The primary issue with other loads is harmonic voltage, not current. Any loads connected in parallel with this lighting will see the system voltages caused by the harmonic currents and the source impedance at the harmonic frequency. If a filter (low impedance path for these currents) is installed, the motors won't see harmonic voltages and will be OK. If there is no low impedance path, then all loads will see the resulting voltages and will be a part of the harmonic current return path. This is the situation you want to prevent.

 

[LiteYear]

Hopefully someone will chime in and explain why this is a bad idea: could you not, other than balancing the loads across three phases, balance the polarity of the loads? Two half-wave rectified loads in opposite polarity will look to the source like a full-wave rectified load. This will shift your harmonic current from 2nd to higher odd harmonics, depending on the particulars. If the only significant harmonic you're seeing is in fact the 2nd, balancing the polarity would in theory result in no significant harmonics.

 

[PHovnanian]

LiteYear, half wave rectified loads are an anomaly on most power systems and are generally not used. They do result from component failures, where one side of a bridge rectifier fails. The DC component resulting from half wave rectification (and a few other causes) is undesirable on AC systems as well. It can result in problems like saturation of magnetics.

 

[LiteYear]

PHovnanian, I understand that but in the OP's case half-wave rectifiers (or something similar like an unsymmetrical full-wave) are being used, as evidenced by the 2nd harmonic. A full wave rectifier creates no 2nd harmonic. I'm only suggesting he take his existing loads and swap the polarity on half of them. Two unsymmetrical loads in opposite polarity equals a symmetric load equals no 2nd harmonic.

 

[axg002]

Liteyear, How do you go about swapping polarity as you've suggested? We have a 3 phase Y power system and we don't understand how to implement your suggestions. Are you saying to go into the lighting ballast and swapping the DC leads? We dont even know if we can do that. Thank you for your response and help.

 

[waross]

Go to the ballast and swap the hot and neutral leads.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

If it is an emergency fitting be careful with the above suggestion as many use a common neutral for the maintained and switched supplies.