Precision of Measurements

General questions and discussions focused around the EKM Metering System. Discuss your meters, installations, and meter communications here.
Post Reply
romemeek
Posts: 5
Joined: Tue Jul 25, 2017 7:50 am

Precision of Measurements

Post by romemeek » Tue Aug 01, 2017 11:24 am

In reference to the spec sheet: Split Core, 13mm, 200A CT, under Testing.
Please define the terms: “Input”, “Accuracy f (%)”, and “Phase Angle”
I’m trying to determine the precision of my measurements and I don’t want to misinterpret the spec sheet.

Thank You!!!

Jameson
Posts: 774
Joined: Fri Nov 04, 2011 7:42 pm
Location: Santa Cruz, CA
Contact:

Re: Precision of Measurements

Post by Jameson » Tue Aug 01, 2017 9:03 pm

Here is the spec sheet for the SCT-013-200 that you are referring to: http://documents.ekmmetering.com/EKM-SC ... -sheet.pdf

You are looking at this part of the spec sheet:
Screenshot 2017-08-01 20.51.15.png
Screenshot 2017-08-01 20.51.15.png (61.39 KiB) Viewed 3528 times
Input is the percentage of the CT Rating, so for example when the 200 amp CT is measuring 100 amp (50% of its rating) it is 0.159% accurate

Accuracy is what the accuracy is as described above

Phase Angle is.... well in the context of what our chart is trying to tell you, you got me. I will have to get back to you. But essentially phase angle measurement is helpful when using the CTs to look for power factor. Our CTs are really pretty good at measuring phase angle because of how they are built. A Power Factor of other than 1.00 is when the Current sine wave, and the Voltage sine wave do not exactly line up, which is important when calculating kWh. Let me know if you would like more information about Power Factor.
Jameson
EKM METERING
http://www.ekmmetering.com
831.425.7371

BlueHornet
Posts: 56
Joined: Fri Dec 16, 2011 10:42 pm
Location: Victoria, Australia

Re: Precision of Measurements

Post by BlueHornet » Thu Aug 24, 2017 1:36 am

You are looking at this part of the spec sheet:


Image


Input is the percentage of the CT Rating,
Correct so when a 200A CT is measuring 50% the current passing through it is 100A
so for example when the 200 amp CT is measuring 100 amp (50% of its rating) it is 0.159% accurate
NO! - Well I hope not anyway!!!

As per your explanation that would be an extremely poor quality and inaccurate CT. If it were 0.159% accurate then the CT & metering wouldn't be worth a cracker and we could well make a better measurement using a wet string and a darning needle.

What it most likely refers to is what is known as the "Accuracy Error", although I've never seen the symbol "f" used for this equation in the past. The symbol "f" is used for frequency in electrical terms.

You see CT's always exhibit two basic or primary errors (amongst a variety of others); the two in question being:
1. Accuracy Errors
2. Phase Angle Errors

Accuracy Errors relate to gain and/or linearity and are fairly easy to understand, so if a CT has an accuracy error of 0.159% at 50% input (lets assume it's a 200A CT) therefore the input would be 100A then the CT would report a current of 100.159 amps, similarly at 5% input (accuracy error of 0.142%) which would be 10 Amps the CT will report 10.0142 Amps
Phase Angle is.... well in the context of what our chart is trying to tell you, you got me. I will have to get back to you. But essentially phase angle measurement is helpful when using the CTs to look for power factor. Our CTs are really pretty good at measuring phase angle because of how they are built. A Power Factor of other than 1.00 is when the Current sine wave, and the Voltage sine wave do not exactly line up, which is important when calculating kWh. Let me know if you would like more information about Power Factor.
What was all that?

Phase Angle Errors however are much more complicated and best explained/understood through the study of phasor diagram for a CT, however that is not the scope of this forum, so I'll keep it as simple as possible.

CT's will introduce a phase shift (or time delay) in the AC current signal, which is relative to the actual current. This is commonly measured in degrees and varies from say 0.1 degrees or 6 arc minutes (or possibly even better for some CT's) for highly accurate CTs to as high as 6 degrees or 360 arc minutes. When the power factor is at or near unity the CT phase angle errors have little effect on the measured power and energy. However, at lower supply power factors, such as 0.7 or below and especially below 0.5, even a small phase angle error can cause large errors in the measured power and energy as we start to see a compounding effect of poor power factor and phase angle errors.

If we had the ideal CT the angle between the primary and reversed secondary current vector would be zero. But in practical real world terms a CT will always pose a difference in phase between two due to the fact that primary current has to supply the exciting current component. The angle between the above two phases is what is referred to as the Phase Angle Error in current transformer or CT.
Phase Angle Errors are normally expressed in Arc Minutes

Post Reply