Harmonics

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paulhalwe

New member
Location
Missouri
Assume a typical 75 KVA transformer feeding a branch circuit panel board is balanced between the three load side phase conductors. Even with this balanced condition, I am told that the neutral current can be as much as 1.73 times as high as the phase conductors if the transformer is feeding a great deal of harmonic producing load. My questions are:
1) Is this condition made worse if the phase conductors are not balanced, or even very much out of balance?
2) What is the effect of this condition on the line side of the transformer...that is, are conductors, panels, and switchgear upstream of the transformer effected?
 

Ed MacLaren

Senior Member
Re: Harmonics

1. Yes. the increased neutral current due to the harmonic loads will add to the current caused by the unbalanced loads.

The troublesome harmonics for single-phase loads are the 3rd and odd multiples of the 3rd (9th, 15th, etc.). These harmonics are called "triplens" and because the A-phase triplen harmonics, B-phase triplen harmonics and C-phase triplen harmonics are all in phase with each other, they will add rather than cancel on the neutral conductor of a 3-phase 4-wire system.



As you stated, this can overload the neutral if it is not sized to handle this type of load.


2. The problem is reduced by using the standard delta-wye connection.
Transformers connected delta primary to wye secondary have a natural phase shift of 30 degrees from the input to output voltage waveform. This 30 degree phase shift cancels or greatly reduces the 5th and 7th harmonics.

When connected with a delta primary, the triplen harmonics are captured. These harmonics produce heat, but if this is a K-rated transformer, it is not necessary to de-rate the transformer. With a standard transformer, de-rating is necessary when connected to harmonic generating loads.

Ed
 

Nick

Senior Member
Re: Harmonics

Ed,
Is it true in a delta/wye transformer that the 3rd order harmonic currents circulate in the delta thus creating the problem heating? Also the 5th and 7th order harmonics that are reduced are still transferred to the primary system and again add together when there are multiple transformers that have harmonic producing loads?
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Harmonics

Nick, triplen harmonics are trapped in the delta primary so they are not seen by the power source (utility). It is dissipated as heat. The most common method to deal with it a "K" rated transformer.

Paul, to answer your questions. Unbalanced loads do contribute to neutral current, as do harmonics. It is possible for neutral current to be as high as 1.73 times the highest phase current with high harmonic content.

Line side is pretty much unaffected as this is the purpose of the transformer. However 5th and 7th order harmonics can be reflected back to the primary. If 5th and 7th order harmonics are determined as a problem, a filter can be installed to be dissipated as heat.

[ March 30, 2003, 08:30 PM: Message edited by: dereckbc ]
 
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