even harmonics

[Pafi]

Trying to design a PFC bank for medium voltage application, 10kV system voltage and measuring the system harmonics, I found out that the even number harmonics are really big: 4,65% the second order current harmonic, 2% the 4-th and 1.25% 6-th harmonic. Also pretty big was the third and triplen harmonics (third harmonic level 3.7%)As the desired sollution was to implement a passive filter with three branches for 5-th, 7-th and 11-th harmonics - which are the biggest), the existence of the 4-th and 6-th harmonics rises difficult problems regarding the amplification of these harmonics in the presence of the filter. Can anyone explain which is the possible cause of these un-characteristic harmonics ? The plant is packed with power SCR converters(6 and 12 pulses) and VFD drives, supplied on low voltage.
Thank you!
Pafi

 

[electricuwe]

Even harmonics are caused by half-controlled three-phase rectifiers (three SCRs and three diodes) or by poor phase control of fully controlled rectififers resulting in differnet firing angles of upper and lower thyristors.

Some firing schemes for reducing apparant power during phase control also give rise to even harmonics.

 

[jbartos]

Suggestion: Generally, even and odd harmonics are caused by general (not even or not odd) nonlinear device characteristics. Even harmonics are caused by even characteristics and odd harmonics are caused by odd characteristics.
The half-wave rectifier is causing even harmonics. This is in agreement with the above posting.
It is good to refresh the Fourier Series or Fourier Transform Analysis for the elementary issues of harmonics.

 

[alehman]

A failed SCR can also produce even harmonics. As suggested by jbartos, even harmonics result from non-symmetrical waveforms.

 

[4654]

Unless there is an abnormality in eithr the source or the load, the even harmonics will not be generated. In general only 3rd harmonics will be generated with such type of rectifiers(not wiht half wave). The even harmonics will be cancelled with the load itself.

concentrate on nature of load alos.

 

[jbartos]

Suggestion: The even harmonics cause serious malfunctions of motors and transformers since they tend to shift the B-H hysteresis loops.

 

[jbartos]

Suggestion: Visit

http://ercap.org/licencing/connectioncode.htm

http://www.nrel.gov/docs/fy03osti/32864.pdf

for total even harmonic distortion TED