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Harmonic Neutral Current - Phase Shifting Transformers

Do you have any info on the use of phase shifting transformers to eliminate neutral current caused by high levels of harmonic distortion? Mirus makes a product called a Combined Neutral Current Eliminator (CNCE) that apparently is effective. It is an autotransformer that shifts the harmonic current 180 degrees canceling the fifth and seventh harmonic. Any info on reliability, effectiveness, popularity of this technique would be greatly appreciated. Thanks.
David Glover, P. Eng. dglover@bgei.com

Response No. 1

The autotransformer for the application in question will work. I am not familiar with the brand you sited. I would caution you to the following concerning this application:
1. What type load is applied? For example a drive that is misfiring could show some even harmonics that could be the problem. It is not always, though most often, the odd harmonics.
2. What kind of problems are the harmonics causing? It may not be a harmonic problem. The same problem that masts as harmonic problems can be something else, such as improper function data etc.
3. Are your harmonics within the signal to noise ratio of allowed by affected equipment?
4. Are you using an analyzer such as a Power Platform or Rustrack to determine the waveform?

These are some questions you may want to consider when trying to troubleshoot. My company does harmonic
studies for several companies and we find out that the problem is often not caused by the harmonics, but by something else. All electrical equipment can handle a certain amount of harmonics. I hope this helps, if you have more info I may give you more detailed direction or send you to a specialist.

David R. Carpenter, integrityco@mindspring.com>

Response No. 2

These transformers are good but you have to also design for the neutral that is a current carrying conductor and size it per Section 520-53(o) "at twice the ampacity for the largest ungrounded supply terminal". If the neutral is not properly sized the terminal and conductorís insulation will be overheated and it could fail.
John D. Rowland Sr. P.E. John.Rowland@GDSeng.com

Mike Holtís Comment

I donít know anything about phase shifting transformers to eliminate neutral current, but I donít see how Section 520-53(o) applies, not that Iím questioning the method (size the neutral 2 times the load), just why would Section 520-53(o) apply.

If you would like more information on harmonic currents go to mikeholt.com/studies/harmonic.htm

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