Ratio Trial Verification Method for Current Transformers

Oliver Watts

07/05/2025

Fellow electric power calibration workers, you must have encountered this situation: The nameplate of an outdoor current transformer has been tormented by wind, sun, rain, and freezing, to the point where the transformation ratio is indistinguishable! Don't panic, we have a solution - use a current transformer calibrator and through the “transformation ratio trial calibration method”, we can figure out the actual transformation ratio and errors clearly. Here, taking the SHGQ - DC type calibrator as an example, I'll chat with you about the specific operation. To make it more accessible, it's convenient for us front - line workers to follow.

1. Start the Trial Calibration with a Small Transformation Ratio

First step, let's first try a smaller transformation ratio, such as calibrating at 150/5 first. When operating, pay attention to these points:

Load Box Matching: Switch the load box to the corresponding capacity, that is, the corresponding volt - ampere value. This step must be done correctly; if wrong, the subsequent data will be inaccurate.
Correct Wiring: The wiring of the current transformer is generally done according to the subtractive polarity wiring. Don't connect it in reverse; if connected wrong, the error will be out of control.
Thorough Demagnetization Test: The test voltage must start from zero, rise uniformly to 120% $U N .U N$ is the rated voltage of the transformer), and then drop back to zero at a constant pace. This set of operations is called a demagnetization test. What's it for? It's to clear the residual magnetism in the iron core of the current transformer, so as not to affect the subsequent measurement error.

At the same time, keep an eye on the polarity indicator light of the calibrator to see if it moves or turns red. If the light turns red, it means that this transformer either has an extremely large error or the transformation ratio is simply wrong - if the transformation ratio is wrong, the measurement error itself will be unacceptably large. In case of this situation, write it down and analyze it later.

2. Continue Calibration with a Larger Transformation Ratio

After testing the small transformation ratio just now, then use the same method to calibrate at the 200/5 transformation ratio. At this time, look at the polarity indicator light: if the light doesn't turn on, congratulations! It means that the error of this transformer is not too large, and the transformation ratio is probably correct (that is, the actual transformation ratio is 200/5).

Next, enter a more detailed calibration: slowly raise the test voltage from zero, successively to 5%

U N

, 10%

U N

, 20%

U N

, 100%

U N

, and finally to 120%

U N

. At each node, record the error. After recording the rising process, then lower the voltage from 120%

U N

, 100%

U N

, 20%

U N

, 10%

U N

, 5%

U N

to zero, and record the transformation ratio error and phase angle error at each measurement point.

3. Error Analysis to Determine the Result

Now it's time to analyze the error records and check whether the error at each test point exceeds the specified value. For example, when the current transformer is at 20%

U N

, the specified transformation ratio error is ±0.35%, and the actual measured value is - 0.25%, which means there is no over - error. Check each point like this. If the errors of all points are within the specified range, it means that the transformation ratio of this transformer is correct and the error is acceptable, so it can be used!

But if any point exceeds the limit, for example, at 100%

U N

, the specified transformation ratio error is ±0.2%, and the actual value is - 0.5%, it means that this measurement point has an over - error. At this time, it can be judged that: this transformer is unqualified, but the transformation ratio is correct (that is, it is indeed a 200/5 transformation ratio).

4. How to Deal with Special Situations
(1) Encountering Transformers with Altered Nameplates

Some unethical people deliberately damage or swap the nameplates of current transformers to muddle through. Don't be afraid; we can still measure the actual transformation ratio with our method. The principle is the same; just follow the previous steps.

(2) Transformers with Extremely Large Errors

If the transformer itself has an extremely large error and should be directly discarded, the above method may not work well at this time - because when the error is large, the polarity indicator light of the calibrator will also turn red, and you can't tell whether it's because the transformation ratio is wrong or the large error itself causes it. At this time, if you want to determine the actual transformation ratio, you have to change the method: apply a standard current value on the primary side of the transformer, then measure the actual current value on the secondary side, and finally calculate the transformation ratio.

In short, this “transformation ratio trial calibration method” is quite practical when the nameplate of an outdoor transformer is unclear. Let's front - line workers practice more, and we won't panic when encountering such tasks!

Topics

CT Inspection and testing

Oliver Watts

Hey! I'm Oliver Watts, an electrical engineer in Inspection and Testing. With years of hands - on experience, I ensure electrical systems meet top safety and performance standards. Using advanced gear, I conduct diverse tests, easily spotting issues in both large - scale industrial and small - scale commercial setups. I love teaming up, sharing knowledge, and keeping up with industry regs. Also, I'm skilled at data analysis with software. If you're into electrical inspection or just want to chat engineering, reach out. Let's connect and explore!