current transformer testing

[guido52]

Due to NERC's focus on Reliability Standards I find myself testing CT's as part of Generation and Transmission protection system testing and maintenance. My question deals with the saturation curve. We used an AVO CTER-91 CT test set and applied voltage to the secondary leads (X1 and X2)of the CT in question and measured amperage induced in the primary side (H1 and H2). Tested CT's 13.8 kV, 69 kV and 161 kV. CT's with ratios of 600:5 up to 3000:5. We produced a variety of curves that resemble portions of the standard saturation curve. Some we did not saturate, nice ascending line, some we did, line parallel X axis.
How do I use the data we collected testing, the nameplate data of the CT's and formulas to verify the CT is adequate, in size, accuracy, etc...? We gathered a lot of data but I don't know how to interpret it. Thanks.

 

[DiscoP]

We test CTs to see if they comply with the nameplate, using tests specified in the relevant standard.

We don't normally measure the primary side current, normally we raise the voltage on the secondary side and measure the secondary current.

The knee-point in our part of the world is defined as the point where a 10% rise in volts produces a 50% increase in current. We would then check that this point is above what the nameplate guarantees.

Getting a copy of the relevant standard would be the 1st step in my opinion - it may have the answers you need.

 

[oldfieldguy]

If you're applying a voltage to the secondary terminals of the CT under test, you should be measuring the voltage at the primary to obtain the ratio. For example, for your 600:5 CT, apply 120 volts to the secondary, and read one volt on the primary. That verifies the ratio. Other secondary voltages can be used, but the idea is the same.

For saturation, with NO connection to the primary, apply voltage to the secondary. Step the voltage up in discrete steps, recording the current flowing in the secondary winding at each voltage step. In the unsaturated region of the curve, a small current change will take place with each increase in voltage. As you get into saturation, a small change in voltage will cause a large change in current.

If I remember correctly, the CTER-91 also has a circuit to check polarity. However, in a maintenance test, I have NEVER seen polarity change on a CT. Still, it's an easy check.

If your CT is multi-ratio and the saturation voltage is too high for your test equipment at full ratio, the saturation voltage will be proportionally lower at lower ratios and the test will still be valid.

One last check to make while you're at the CT secondary terminals is to use the CTER-91 test set and inject one amp of current into the current circuit and measure the voltage needed to push that current. This is a measure of circuit burden. It also should not have changed in a maintenance test situation, but occasionally connections get loose or shorting screws get left in from other work.

Evaluation of the results should be made by comparing saturation voltages and currents with manufacturer curves. If these curves are not available, compare like items. In other words, A, B, & C-phase CT's should saturate within a couple of volts of one another.

Ratio checks should be compared with nameplate.

Circuit burden checks should be compared with previous test results.

Testing good CT's is not very difficult work. The problem comes in when things don't seem to be right. Then it can get interesting.

old field guy

 

[scottf]

guido-

Performing an excitation test is useful in 2 ways:

1) To confirm a protection class rating.

For instance, if the CT has a protection class rating of C800, then the excitation curve should show that a voltage of 800V can be developed without the excitation current exceeding 10A.

2) To compare versus the factory measured or factory supplied typical excitation curve

Knee-point voltage should be comparable between your measured results and factory supplied results. This can be used to detect turn-to-turn shorts, turn-to-core shorts, etc....


As an aside, typically excitation curves are generated by applying voltage to the secondary winding and measuring the current flowing through it with the primary winding open circuited.