K-factor Transformers

[patpappas]

Can a K rated transformer carry more harmonics than a regular transformer or do they handle it better than a regular transformer. What would be the limit of harmonics on K rated vs regular?

 

[zog]

Yes and Yes, a K factor transformer has a larger core to disapate the additional heating caused by the additional core losses from the high frequency currents from harmonics

 

[patpappas]

When we had transformer testing done 6-7 year ago I seem to remember that the testing person mentioned that a regular tranformer limit was 5% on voltage and 3% on current for harmionics in the system, do these number sound reasonable? What about a K rated transformer.

 

[robbietan]

our utility limits harmonics on the system at 5% VTHD, it penalizes customers who emit harmonics to the grid.

current harmonics are more tricky, there are some electrical gadgets that must operate on 5% ITHD, else it wont work at all.

although several utility meters, transformers, etc that work well on environments above 5% VTHD

 

[beanland]

K-Factor

K-Factor

K-factor is a calculation of the heating that would be caused in a transformer by harmonics. The load has a K-factor based on the current drawn and harmonics in the supply voltage. A transformer has a K-factor rating based on its design (larger core, better ventilation, more copper, etc.) However, a standard 100kVA transformer can be loaded to 50kVA by a highly harmonic (high K-factor) load and still function fine. It is a matter of heating of the core and coils.

At a POCO, we smoked a 500kVA transformer because there was 500kVA of load and the harmonics were very high. We could not get a K-factor rated transformer but calculated that a 750kVA standard transformer could carry the 500kVA of high K-factor load.

 

[Besoeker]

A related document.
http://www.xitrontech.com/images/support/app_notes/AN102%20K-Factor%20Defined.pdf