Harmonic- transformer

[SELEC]

Hi, Folks,

I am doing a harmonic study for a facility using ETAP software.On ETAP transformer editor, there is a harmonic page confusing me, which requires the inputs for 6,12,18 pulse data regarding harmonics. I consulted ETAP. They were saying lighty-loaded transformers generate harmonic, which doesn't make sense to me.To my knowledge, only when transformers are being enegized, they can generate harmonic. Who can guide me to learn more about this?

thank you in advance.

 

[mpparent]

The energization of transformers generates the 2nd harmonic. I haven't heard of the "lightly loaded" transformer generating harmonics, but I certainly could be wrong.

Mike

 

[CJCPE]

The magnetizing current of a transformer contains odd harmonics. That harmonic current is typically about 2% of the rated current. Since it doesn't vary with load, it becomes a higher percentage of the total current when the transformer is lightly loaded.

 

[amps21]

The exciting current in the transformer cannot be sinusoidal if you need a sin voltage output. This current (like u know) drives the flux unidirectional in all phases and hence it does not add up to zero (general 3 legged core form). The 3rd harmonic voltage either need to be given a path (hence harmonic current) thru the neutral of the wye winding to be grounded so as to keep your voltage waveform siney or should be provided a path like thru a delta winding where the 3rd Harm voltage , in effect, gets shortcircuited and hence 3rd harmonic current shall flow in the delta limited only by the winding impedence.
If This is not ensured (path is not provided for 3rd harmonic curretns), the flux will not vary cosinusoidally and hence the voltage will have 3rd harmonic component at the winding terminals. (you probably know this).

The flux density , core material (CRGO)and core design as also the winding connections effect magnitude of 3rd harmonics. But like someone said this current is very small and hence practically neglected in most cases. There are standards in transformer design where u can calculate these and very well measure these as well.

So 3rd harmonics are inherent to transformers but i woulld not call transformers active 3rd harmonic generators.

Thanks

 

[stevenal]

6, 12, 18, pulses refer to rectifiers. Looks like ETAP allows you to create a so called "rectifier transformer" or "DC transformer." These are transformers directly connected to rectifiers with no other loads.

 

[mpparent]

Possibly not what you are looking for, but here is a link to a paper which talks about the 2nd harmonic upon energization of a transformer:

http://www.selinc.com/techpprs/6025.pdf

Mike