Fatal Harmonics

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danilo

Member
Hi,
i just tested our 225kva dry-type xformer,480-230v,k-4 rated that serve mostly non-linear loads..the test result shows that our system is positive to a k factor of 9 which exceeds the limit of 4..im worried coz the xformer is only 50% loaded!..any suggestion on what action should i make about this problem?

thanks as usual,

Danilo
 

charlie

Senior Member
Location
Indianapolis
Re: Fatal Harmonics

A K rated transformer is not required to handle high harmonics but it is desirable. A K rated transformer will handle more harmonics because of the way it has been beefed up to withstand the additional currents. If you keep an eye on the true loading with a true RMS meter, you can use a normal transformer to handle loads in an area that has a lot of harmonic loads. You may only be able to use the transformer to serve ? of the load that you wold be able to serve with a K13 though. :D
 

charlie tuna

Senior Member
Location
Florida
Re: Fatal Harmonics

what were the actual readings from your data loggers? how unbalanced was the load. i have come across k rated transformers that run so hot i expect them to fail at any moment! they've been that way for 15 years and are still fine. and expected them to be near full load and find them less than 50 per cent loaded???????
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: Fatal Harmonics

The harmonic currents from non-linear loads can be nearly double the "normal" load. If you think the transformer is 50% loaded, then it is really almost 100% loaded.

(ASIDE: I have heard that the theoretical limit for linear plus non-linear is 173.2% of the linear ? it's that square root of three thing again. But I don't know how to do the math to prove it.)

One of your other choices is to replace the transformer with one of the kind that blocks harmonics, instead of just being able to tolerate them. Another is to add an occasional 1:1 delta-delta transformer, which would block the triplens harmonics (i.e., 3rd, 9th, 15th, etc.) from reaching the transformer that you are discussing.
 

Dnkldorf

Senior Member
Re: Fatal Harmonics

Anyone had any luck with HSS.

(Harmonic Supression systems)

I've been reading up on them, and they sound like they would do the job removing the 3rd from the transformers, but the potential for noise bothers me.

Any input?


Dnk....
 

danilo

Member
Re: Fatal Harmonics

Originally posted by charlie tuna:
what were the actual readings from your data loggers? how unbalanced was the load. i have come across k rated transformers that run so hot i expect them to fail at any moment! they've been that way for 15 years and are still fine. and expected them to be near full load and find them less than 50 per cent loaded???????
hi,

the actual total kva reading was 101kva.no significant unbalance load among phase currents..Maximum voltage THD for Phase A=4%,Phase B=3%,Phase C=4%..Maximum Current,THD for Phase A=53%,Phase B=57%,Phase C=67%.

i suspect it exceeds the 20% limit per IEEE standard.

regards,

Danilo
 

wirenut1980

Senior Member
Location
Plainfield, IN
Re: Fatal Harmonics

the actual total kva reading was 101kva.no significant unbalance load among phase currents..Maximum voltage THD for Phase A=4%,Phase B=3%,Phase C=4%..Maximum Current,THD for Phase A=53%,Phase B=57%,Phase C=67%.
What does this mean? In other words, what are these percentages of? Is it the percentage of nominal voltage and rated current of the transformer that is being utilized by harmonics? :confused:
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: Fatal Harmonics

This is for the benefit of the non-engineers, for I know you will have seen this before.

Any signal that has a repetitive wave shape can be modeled with a set of pure and simple sine waves. The set will include the cosine of 60 hertz, and the cosine of twice 60 hertz, and 3 times, and 4 and 5 and 6 and so on, until you've had enough. All you have to do is to you pick the right amplitudes for each of these cosine waves, and you can get as close to the original signal as you wish. It turns out that for a power system of our interest, the amplitude for all the "even harmonics" (i.e., 120 hertz, 240 hertz, 360 hertz, etc.) is zero. So we don't worry about "even harmonics."

They percentages that danilo is citing are essentially a mathematical combination of the amplitude of all harmonics, as a percentage of the amplitude of the fundamental (i.e., 60 hertz) wave. I don't remember how the harmonics are combined. My guess is that it is a "square root of the sum of the squares" sort of thing.
 

wirenut1980

Senior Member
Location
Plainfield, IN
Re: Fatal Harmonics

Ah, yes Charlie, thanks for shaking up my memory bank. I missed in the post (yes, the one I quoted :eek: ) that danilo mentioned THD, which is a percentage of the harmonics to the fundamental frequency. It is calculated by taking the square root of the sum of all odd harmonics divided by the fundamental. I must be getting old ;) .
 
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