The magnitude of K allows us to directly judge the function of the transformer. For ease of understanding, I have organized the following table for you:
Table
Transformer Type Turns Ratio (K) Range Winding Relationship Voltage Relationship Typical Applications
Step-down Transformer K > 1
Maintenance power supply, safety isolation
On the engineering nameplate, the turns ratio is sometimes expressed in ratio form, for example, “100:1” usually indicates step-down, and “1:10” usually indicates step-up.
3. Why measure the turns ratio? (Testing and Inspection)
In the commissioning tests, preventive tests, or fault diagnosis of power equipment, measuring the turns ratio is a very important part. This is not just to see if the voltage has changed, but also for a “physical examination”.
Check if the number of winding turns is correct: Verify whether the actual turns ratio matches the nameplate design value.
Diagnose inter-turn short circuits: If a slight inter-turn short circuit occurs inside the transformer, the turns ratio will change significantly (usually the deviation will exceed ±0.5%).
Check the tap changer: Confirm whether the tap changer (the gear for adjusting voltage) has good contact and whether the position indicator is accurate.
Judge three-phase balance: For three-phase transformers, the turns ratio of each phase should be basically consistent. If the deviation is too large, it indicates that there may be an asymmetric fault inside.
4. Measurement Methods and Standards
Currently, fully automatic transformer turns ratio testers are mainly used for measurement, which can directly apply test voltage and calculate results, with high accuracy and safety.
Acceptance criteria: According to power industry standards (such as DL/T 596), at the rated tap position, the error between the measured turns ratio and the nameplate value should usually be within ±0.5%.
Test procedure:
Disconnect the external connections of the transformer to ensure safe grounding.
Connect the high-voltage terminal (H) of the tester to the high-voltage side of the transformer, and the low-voltage terminal (L) to the low-voltage side.
Input the transformer nameplate parameters (rated voltage, connection group, etc.).
Start the test, and the instrument will automatically measure the turns ratio, error, and phase angle of each phase.
In summary, the turns ratio is the “identity ID” of the transformer. It not only determines the voltage transformation ratio, but also serves as an important indicator for judging the internal health of the transformer in our daily operation and maintenance.
