Measuring harmonic via CT - PT 7

[tem1234]

Hi,

If i measure harmonic, the best is to measure it directly to the drive (on the power cable or bus), but if i can't, do i'll lost a lot of data if the power analyser is connected to CT and PT?

I was thinking, if the PT is connected in Delta, i'll probably lost a lot of 3rd harmonic?

So i want to know if it's acceptable to measure it that way or not, and if there's a standard to refer at. Do the standard IEC 61000-4-7 is very usefull and do i must buy it absolutely or not? I must measure untill the 50th harmonic and it's the harmonic current which are important for this case.

Thanks

 

[LionelHutz]

The CT's I saw were 600V class donuts. I believe they were something like a 400:5 C10 CT. The core losses were terrible over about 600 or 700Hz. The original poster asked about direct or VT/CT connections to a drive so I'd have to believe he's talking about a fairly low voltage.

 

[scottf]

LionelHutz-

You use the term "core losses". Do you mean accuracy? Core losses should actually improve as the frequency rises, since the flux density is lowered in the core.

Edison-

http://www.sandc.com/edocs_pdfs/EDOC_001668.pdf

Here is a link to a cut sheet on resistive voltage dividers.

 

[busbar]

Also

http://www.rossengineeringcorp.com/hv_dividers.htm

 

[edison123]

Thanks scottf and busbar (long time no see?).

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

 

[LionelHutz]

scottf - The core of the CT got hot during the testing. They measured completely wrong too.

I actually got curious about it and cut one apart and the core was a piece of steel rolled up (like most cores for this type of CT). However, there was nothing insulating the layers from each other and I believe it was eddy currents causing the problem. The core was the equivalent of a solid steel donut. I never did anything else to confirm this though.

 

[scottf]

Lionel-

Unless there was a winding error, about the only way the core would have become hot during testing is if the burden applied was much higher than rated or the secondary was open circuit, both of which cause saturation. Since flux density is inversly proportional to frequency, generally, as the frequency rises, the chance for saturation reduces as the flux lowers.

CT cores are almost always rolled cores as you describe. Eddy currents are generally not high in CTs and normally wrapped up into the term "excitation current".

 

[unclebob]

Regarding the harmonics, are there any reasons why someone needs them measured up to the 50th?