Understanding Transformer Tap Changer Operation

I. Transformer Operating Tap Positions

Is the number of tap positions on a transformer equal to its number of operating tap positions?

In China, on-load tap-changing (OLTC) transformers typically have 17 taps, while off-load tap-changing transformers usually have 5 taps—some may have 3 or 2.

Theoretically, the number of tap positions equals the number of available operating positions. During operation, the tap position of an OLTC transformer can be adjusted under load to regulate voltage. However, for off-load tap changers, adjustment must be performed only after the transformer is de-energized.

Transformer tap positions refer to physical taps on the winding coils—commonly 4 or 6 taps (yielding 3 or 5 positions, respectively), though configurations may vary. Each tap corresponds to a different number of turns, resulting in different voltage ratios. Thus, tap positions are used to adjust output voltage.

II. Determining the Operating Tap Position from the Nameplate

The nameplate indicates the voltage level for each tap position. To determine the current operating tap, measure the low-voltage side voltage, multiply it by the turns ratio, and compare the result with the primary-side grid voltage. This allows identification of the actual tap in use.

III. Checking and Adjusting the Tap Position After Power-Off

"High to high" adjustment: If the low-voltage side voltage is too high, move the tap connection toward a higher tap position.

"Low to low" adjustment: If the low-voltage side voltage is too low, shift the connection toward a lower tap position.

For exact voltage change per step, refer to the transformer nameplate.

A typical off-load tap changer has three positions, adjusting the neutral end connection of the high-voltage winding.

• "High" means the secondary voltage is too high; "to high" means switching to a tap marked for higher input voltage. A higher setting increases primary winding turns.

• "Low" means the secondary voltage is too low (needs increase); "to low" means moving to a tap indicating lower input voltage, which reduces primary turns.

Summary: With the secondary winding unchanged (constant turns), during "high to high" adjustment, the number of primary turns increases. Since supply voltage remains constant but the turns ratio increases, the secondary voltage decreases.

During "low to low" adjustment, primary turns decrease, reducing the turns ratio. With constant supply voltage, this results in a higher secondary voltage.

IV. How to Adjust the Transformer Tap Changer?

A transformer may have three standard tap positions:

• Position I: 10,500 V

• Position II: 10,000 V

• Position III: 9,500 V

Setting the tap to Position I means: when the high-voltage side is 10,500 V, the low-voltage output is 400 V.
Similarly, at Position II, 10,000 V input yields 400 V output; at Position III, 9,500 V input gives 400 V output.

Thus, Position I produces the lowest secondary voltage, while Position III yields the highest.

Adjust the tap based on the secondary bus voltage:

• If the secondary voltage is too low and needs to be raised, increase the tap position (e.g., from II to III).

• If too high, decrease the tap position.

For off-load tap changers, adjustment must be done with power off. After changing the tap, measure the DC resistance using a multimeter to verify good contact and balanced phase resistance (deviation ≤ 2%) before re-energizing.

Ordinary transformers can only change taps when de-energized. Therefore, a suitable tap must be selected in advance to ensure voltage remains within acceptable limits under both maximum and minimum load conditions.

On-load tap-changing transformers come in two types: one with an integrated regulating winding and OLTC mechanism, and another using an external booster regulator. OLTC-equipped transformers use a tap selector that enables adjustments under load.

Power transformer tap changers are classified as either on-load or off-load.

• On-load types allow live adjustment, typically motor-driven via up/down controls.

• Most small power transformers use off-load changers requiring manual access: open the tank cover, rotate the handle to the desired position, measure DC resistance of all three phases, confirm balance, then close and re-energize.