Current Transformer Ratio and Excitation Testing

[RedBrick]

I have a few questions for anyone who does a lot of current transformer testing. We use Vanguard's EZCT-2000C Plus for testing CT's.

NETA ATS-2017 Current Transformers 7.10.1

4. Ratio errors shall be in accordance with IEEE C57.13
- From what I have been able to find in that standard (2008) :
- Relay Class C Classification= 3% error at rated current and 10% at 20 times rated current.
- Metering Class will be given an accuracy rating for each standard burden, i.e. 0.3 B-0.1 (0.3% accuracy)
* What do most of you use for % error?

5. Excitation results shall match the curve supplied by the manufacturer or be in accordance with IEEE C57.13.1
- From that standard:
"Any substantial deviation of the excitation curve for the CT under test from curves of similar CT's or
manufacturer's data shall be investigated."
* There isn't really a definitive percentage or other quantity to describe "substantial". Anyone have any
information on this?
* How many of you actually find the excitation curves?

Any input on this subject would be greatly appreciated.

[anonymous]

I'm not an expert but we do use EZCT-2000 and I'll answer as best I can. Hopefully someone can correct me if I'm wrong on any of this.

4. Ratio errors shall be in accordance with IEEE C57.13
- From what I have been able to find in that standard (2008) :
- Relay Class C Classification= 3% error at rated current and 10% at 20 times rated current.
- Metering Class will be given an accuracy rating for each standard burden, i.e. 0.3 B-0.1 (0.3% accuracy)
* What do most of you use for % error?

The EZCT software will show you the % error (NP-RATIO is nameplate ratio, M-RATIO is measured ratio, % ERROR right under that is the percentage difference between these). As you said, the pass/fail criteria depends on the CT class, 3% or less for class C or T, 1% or less for class X. For field testing, "the purpose of the ratio test is to verify the stated ratio, not prove the accuracy of the ratio" (IEEE C57.13.1-2017 Clause 6.1). Basically the idea here is to identify damage to the CT such as shorted turns, or to identify incorrect tap connections for multi-ratio CTs. For metering CTs, the number you compare to will depend on the test setup. If you combine the ratio test with a burden test (EZCT can do this automatically), you can check the burden test results to figure out which accuracy rating to compare to.

5. Excitation results shall match the curve supplied by the manufacturer or be in accordance with IEEE C57.13.1
- From that standard:
"Any substantial deviation of the excitation curve for the CT under test from curves of similar CT's or
manufacturer's data shall be investigated."
* There isn't really a definitive percentage or other quantity to describe "substantial". Anyone have any
information on this?
* How many of you actually find the excitation curves?

Any input on this subject would be greatly appreciated.

This one is more complicated and to answer it you have to understand the operation and purpose of a CT. Relaying CTs are used as sensors for protective relays. These relays need to operate circuit breakers based on the relationship between the CT primary and secondary. As long as the CT is accurate, the relay will operate correctly for the amount of primary current actually present. Importantly, they need to operate correctly during fault conditions where currents are much higher than normal. Relaying CTs have a standard secondary current of 5A, and are rated to handle 20 times that (100A) with no more than a 10% ratio error. This means a 1200:5 CT should handle 24000A fault current on the primary side with less than 10% error, allowing relays to correctly respond to the fault.

The excitation test is meant to deliberately saturate the CT, meaning that the core cannot handle any more magnetizing flux ("saturated" is a fancy word for "full" or "soaked". Think of a soaked, dripping towel: you're not going to be able to pick up any more water with it – it's saturated). Because it's not practical to inject large amounts of current into the primary bus, the test is done by energizing the CT from the secondary side. Voltage is ramped up and current is measured as the voltage increases. The excitation curve shows secondary current vs secondary voltage as the CT is excited. The curve has a "knee point" where it starts to level off as voltage is increased. Down and left of the knee point the curve is steep and "linear" (C57.13.1 says "linear," but it's actually exponential and just looks like a straight line because it's plotted on a logarithmic scale. This is not important to your question, it just bothers me that the standard is written that way). Right around the knee point the curve's slope turns and then it becomes "linear" again but less steep to the right of the knee point. This area to the right is called the saturation region, where the CT is no longer as accurate due to increasing ratio errors (because the core has taken nearly all the flux it can take). In this area small increases in voltage cause a high increase in current.

So, to actually answer your question, one way to determine if a CT is bad without a manufacturer's curve to compare to (per C57.13.1-2017 12.3 (c)) is to look at the excitation curve on the EZCT test report, and draw a straight line along the saturation region of the curve out to 10 A. The voltage (Y-axis) at 10 A should be greater than the CT rated secondary voltage (the number after the "C" in the CT rating). If it's less than that number, the CT is bad. You also want to look at the shape of the curve. The linear parts should be straight, without any dips or spikes, and the voltage should not decrease in the saturation region. C57.13.1 figure 13 & 14 shows some examples of bad CT curves. Figure 12 shows a good curve (and also shows that a multi-ratio CT should have similarly shaped curves for each tap if you test multiple taps).

At some point, a power engineer should be reviewing relaying CT test results. They will know what sorts of fault currents their system is meant to handle, and they should be able to read or calculate everything they need to know from the EZCT test report. And of course they should have the published curve on record somewhere. The position of the knee point is important, and you can't tell if it's right without seeing the manufacturer's curve.

I hope this helps! Please let me know if I got anything wrong. Also the testguy article on this is pretty good: https://testguy.net/content/264-6-el...mers-explained