How to Adjust Transformer Tap Positions Correctly？

I. Transformer Operating Tap Positions

How many tap positions a transformer has, that's how many operating tap positions it has?

In China, on-load tap-changing transformers usually have 17 taps, while off-load tap-changing transformers generally have 5 taps, though some have 3 or 2.

Theoretically, the number of transformer tap positions equals the number of its operating tap positions. When voltage fluctuates during operation, the tap position of an on-load tap-changing transformer can be adjusted, but the tap position of an off-load tap-changing transformer cannot be changed while energized—it can only be adjusted after power is shut down.

The number of tap positions on a transformer refers to taps on the winding coils—these taps are coil taps, which come in various numbers, generally 4 or 6, sometimes more. For 4 taps there are 3 positions; for 6 taps there are 5. Each tap corresponds to a different number of winding turns, resulting in different voltages at each tap position. Therefore, transformer tap positions are used to adjust voltage.

II. How to Determine the Operating Tap Position from the Nameplate

The nameplate shows the voltage level of the tap positions. To determine which tap position the transformer is operating at, measure the low-voltage side voltage multiplied by the turns ratio and compare it with the primary-side grid voltage to identify the current tap.

III. Checking the Transformer Tap Position After Power-Off

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

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

As for how many volts one step adjustment corresponds to, refer to the transformer nameplate.

A non on-load tap changer usually has three positions, adjusting the neutral point connection of the high-voltage winding. "High" means the low-voltage side voltage is too high; "to high" means moving the tap changer to the position indicating higher voltage. A higher voltage setting means more turns in the primary winding.

Similarly, in "low to low," "low" means the low-voltage side voltage is too low (needs to be increased), and "to low" means adjusting the tap changer to the lower voltage indication position. A lower primary voltage indicates fewer turns in the primary winding.

Summary: With the secondary winding unchanged (number of turns constant), during "high to high adjustment," the number of primary winding turns increases. Since the supply voltage remains unchanged but the primary turns increase, the transformation ratio increases, thus lowering the low-voltage side output voltage.

During "low to low adjustment," the number of primary winding turns decreases, reducing the transformation ratio. With the supply voltage unchanged, the secondary voltage increases.

IV. How to Adjust the Transformer Tap Changer?

Three tap positions of the transformer:

• Position I: 10,500 V

• Position II: 10,000 V

• Position III: 9,500 V

• Setting the switch to Position I means: when high-voltage side is 10,500 V, the low-voltage output is 400 V.

• Setting the switch to Position II means: when high-voltage side is 10,000 V, the low-voltage output is 400 V.

• Setting the switch to Position III means: when high-voltage side is 9,500 V, the low-voltage output is 400 V.

That is, Position I gives the lowest output voltage, and Position III gives the highest output voltage.

Adjust the tap changer according to the secondary bus voltage. When the secondary voltage is too low and needs to be raised, increase the tap position by one (e.g., if originally at Position II, adjust to Position III). Conversely, do the opposite.

For off-load tap changers, voltage regulation must be performed with power off. After adjustment, use a multimeter to check the DC resistance to ensure good contact at the new tap position before re-energizing.

Ordinary transformers can only change tap positions when de-energized, not under load. For such transformers, a suitable tap must be selected in advance so that voltage deviation remains within allowable limits under both maximum and minimum load conditions.

On-load tap-changing transformers come in two types: one type has its own regulating winding with an on-load tap changer; the other uses an external boosting regulator. On-load tap-changing transformers with regulating windings are equipped with a tap selector that allows tap changes under load.

Power transformer tap positions (more accurately called "tap changers") are either "on-load" or "off-load." On-load tap changers can be adjusted while energized and under load, and are typically motor-driven—adjustment is done simply by pressing up or down buttons. Most small power transformers use off-load tap changers, which require shutdown. The cover of the tap changer on the transformer tank must be opened, and the handle rotated to the desired position. Then, the DC resistance of the three-phase windings must be measured to confirm balanced values (generally no more than 2% difference) before replacing the cover and re-energizing.