Measuring harmonic via CT - PT
Measuring harmonic via CT - PT
(OP)
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
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






RE: Measuring harmonic via CT - PT
There are some classic IEEE papers on CT and PT frequency response, but I can't lay my hands on my hard copies right now. If you search the IEEE database, you should find them.
RE: Measuring harmonic via CT - PT
* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *
RE: Measuring harmonic via CT - PT
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Measuring harmonic via CT - PT
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Measuring harmonic via CT - PT
Scotty, Gunnar you are right.
Like to Edison123, I have this Q a long time, but never ask:
why power meter's companies never write about it and EE installed power qulity meters hungreds meters from CT/VT.
Regards.
Slava
RE: Measuring harmonic via CT - PT
Current Transformer Accuracy with Asymmetric and High Frequency Fault Currents
Douglass, D.A.
IEEE Transactions on Power Apparatus and Systems
Volume PAS-100, Issue 3, March 1981 Page(s):1006 - 1012
Potential Transformer Accuracy at 60HZ Voltages Above and Below Rating and at Frequencies Above 60 HZ
Douglass, D.A.;
IEEE Transactions on Power Apparatus and Systems
Volume PAS-100, Issue 3, March 1981 Page(s):1370 - 1375
RE: Measuring harmonic via CT - PT
I agree with your comments in regards to CTs. However, I don't agree completely with VTs being good to the 50th harmonic.
The frequency response of VTs vary greatly by design and voltage class.
In many cases, VTs become quite unreliable above 1 kHz, especially units rated 15 kV and above, which is where I have the most experience with frequency response testing. I've not personally been privy to frequency testing on 600V VTs.
Of course, capacitor voltage transformers (CCVTs) should never be used for harmonic analysis, as they are only designed to be accurate at the fundamental.
RE: Measuring harmonic via CT - PT
dpc, I'll try to find these references, seems interresting.
And what about the connection of the PT-CT?
They must be wye-wye to not lose all the neutral harmonic i think!
RE: Measuring harmonic via CT - PT
15kHz may be possible for a good CT but that doesn't universally apply to all.
RE: Measuring harmonic via CT - PT
That's interesting. What kind of CT was it. I can't imagine any "normal" CT design that would have problems below 36 kHz, much less 1 kHz.
RE: Measuring harmonic via CT - PT
Thanks for the clarification - I agree that the VT are more of a concern than the CT. My experience has been mostly with 15 kV class PTs in metal-clad switchgear and 600 V PTs. I've never seen a major issue with ratio error, at least that was apparent at the time.
The Douglass paper I cited above indicates ratio error is highly sensitive to burden as frequency increases.
For a 7200 V VT with a 1000 ohm burden, the measured RCF at 10 kHz was 1.01. For a 4800 V VT, the error was 1.02. With a 100 ohm burden, the 4800 V VT RCF was off the chart at 10 kHz.
Sounds like your experience indicates the error could be much higher above 1 kHz??
Dave
RE: Measuring harmonic via CT - PT
I think it's important to remember that VT designs vary greatly from one manufacturer to another. In the case of frequency response, the winding design and the insulation grading plays a big part, in that they effect the capacitance and stray capacitance of the primary. Of course the capacitance seen by the primary, both across the primary and to ground, forms a very complex RLC circuit with implications on frequency response
A common mistake is to take measurements only at high and low frequency points and assume it's the same in between.
For instance, the response may look good at 1kHz and 10 kHz, but the response in between could be quite erratic.
This brochure:
http://www.pqs-consulting.com/ritzbrochure.pdf
Shows a good example of the frequency response of a 230 kV VT. We had other similar curves for MV VTs when I wrote this brochure, but the topic was in reference to HV applications in this case.
RE: Measuring harmonic via CT - PT
RE: Measuring harmonic via CT - PT
(From an older {early 1990s} power-measurement seminar) CTs are better than PTs are at passing harmonic quantities given their lower inductance [fewer turns].
RE: Measuring harmonic via CT - PT
I think you have that wrong, in the sense that the inductance of the winding of a CT or VT is not what causes error at higher frequencies. Rather, it's the capacitance of the winding/insulation reacting with the inductance of the winding.
Because VTs typically have many more turns on the primary and secondary, the inter-winding capacitances tend to be much higher, not to mention line-to-ground capacitances of insulation (i.e. across the same voltage being measured).
In theory, a transformer that is perfectly inductive in nature would perform linearly with frequency up to a certain point. After all, accuracy performance tends to be better (up to a point) with increasing frequency.
RE: Measuring harmonic via CT - PT
Scottf, I stand corrected. Thanks for the update.
RE: Measuring harmonic via CT - PT
Thanks for the additional info. I have a question.
I am often required to measure THD (up to 50th) of a new / rewound ac generator and always have used the generator PT's for such measurements. I could never be sure whether these PT's faithfully reproduce the acutual generator harmonics.
In such a case, what would be your suggestion to measure the harmonics ?
* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *
RE: Measuring harmonic via CT - PT
I would recommend resistive dividers if you need something remotely accurate. You could also try a capacitive divider if you can find one.
Or you could ask your vt supplier to confirm the frequency response, but I'm pretty sure it won't be good for what you're trying to measure.
I believe it is quite common to take THD measurements with normal VTs...most folks don't realize that they're probably looking at garbage :)
RE: Measuring harmonic via CT - PT
Resistive / Capacitive dividers ?
I am kinda dumb on them. Could you educate me more ?
Thanks again.
* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *
RE: Measuring harmonic via CT - PT
RE: Measuring harmonic via CT - PT
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.
RE: Measuring harmonic via CT - PT
Also http://www.rossengineeringcorp.com/hv_dividers.htm
RE: Measuring harmonic via CT - PT
* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *
RE: Measuring harmonic via CT - PT
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.
RE: Measuring harmonic via CT - PT
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".
RE: Measuring harmonic via CT - PT