×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

CT Error Calculation
3

CT Error Calculation

CT Error Calculation

(OP)
I am trying to find a reference which explains the process for calculating the ratio and phase angle error of a metering CT, based on:
- Calibration certificate data at two burdens (VA1, VA2) both at the same power factor
- The actual burden and power factor of the load

I have several metering and protection references which describe the Farber or Circle method applied to CT's.  This involves plotting the ratio error against the phase error in centiradians.

However, the reference are unclear if this method can also be applied to current transformers.

Thanks

Chris


RE: CT Error Calculation

The GE Website used to have .pdf files scanned in from old publications.  They have an Instrument Transformers Buyers Guide that covers some aspects of CT error calculations.  Maybe that will be a good starting point.  Most of my info is regarding relaying applications.  

RE: CT Error Calculation

cdhorne-

Ignoring the phase angle of the connected burden for a moment, the accuracy of CTs change linearly with burden at a given current level.

The accuracy measurements listed on your test reports should be at 2 burden points (at least) for each current point measured.

You can plot out the 2 accuracy points and draw a line between them. If you then know the connected burden, you can plot that point on the line and that will give you the accuracy at your burden, at that current level.

Depending on the standard, your metering accuracy measurements were probably taken at burden pf's of 0.9 (IEEE) or 0.8 (IEC) lagging. In practice, with modern electronic meters and relays your connected burden should be very close to purely resistive. It's tough to convert the accuracy measurements at a particular burden power factor to that with a different power factor by calculation. In actuallity, it doesn't really make that much of a difference, assuming the burden's pf is between 0.5 lagging and unity. The effect of pf varies a bit with the design of the CT's core/coil, but that's a topic probably too deep for this format.

Bottom line, in my opinion, I wouldn't worry so much about the burden's pf.

RE: CT Error Calculation

(OP)
Thanks to "busbar", dpc and scottf.  I now have all the information I need (and also a much deeper understanding of CT and CT error compensation).

Perhaps it's time we clubbed together and wrote a short definitive paper on the issue?  Most of the texts appear to be a billion years old and with current trends in power engineering (in the UK at least) there's a danger that the current knowledge will be lost...

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources