Efficient Voltage Control: Guide to Transformer Tap Changer Adjustments

Why Voltage Adjustment is Important?

Voltage is a pretty big deal when it comes to how well an electrical system works. It's one of the main things that shows if the system is handling reactive power properly. If the voltage is too high or low, it can mess up how devices work, and might even cause some damage. According to the rules, for 10kV transformers, the voltage on the low-voltage side (0.4kV) can only vary by +7%.

What's the Tap Changer in Oil-Immersed Transformers?

A tap changer is the part inside a transformer that lets you switch between different voltage settings. Its main job is to change the number of turns in the transformer's high-voltage winding. This can be adjusted from +5% to -5% from the rated value when the transformer is off. By doing this, it helps adjust the voltage on the low-voltage side to match what the user needs.

Tap Changer Positions on a 10kV Transformer

1. Three-Position Tap Changer

· Position 1: 10kV +5%, transformation ratio = 10.5kV/0.4kV

· Position 2: 10kV, transformation ratio = 10kV/0.4kV

· Position 3: 10kV -5%, transformation ratio = 9.5kV/0.4kV

2. Five-Position Tap Changer

· Position 1: 10kV +5%, transformation ratio = 10.5kV/0.4kV

· Position 2: 10kV +2.5%, transformation ratio = 10.25kV/0.4kV

· Position 3: 10kV, transformation ratio = 10kV/0.4kV

· Position 4: 10kV -2.5%, transformation ratio = 9.75kV/0.4kV

· Position 5: 10kV -5%, transformation ratio = 9.5kV/0.4kV

Adjusting the Tap Changer When the Power is Off

1. Measuring the Resistance
So, before you make any adjustments, grab a multimeter and measure the resistance between the lines (AB, BC, and CA) on the high-voltage side. This is just a rough check to help you figure out the right range for more accurate readings later when you use a bridge meter.

2. Check the Tap Changer
Take a look at the tap changer and see if there's any dirt or oil on it. If so, just clean it off with a cloth.

3. Adjust the Tap Setting

· If you want to raise the output voltage, rotate the switch from position 2 to position 3.

· If you want to lower it, rotate the switch from position 2 to position 1.

· The five-position tap changer works the same way.

4. Double-Check the Position
After you've adjusted it, you should measure the resistance between the high-voltage and low-voltage sides again. The difference should meet the required standards:

· For transformers up to 1600kVA, the high-voltage side resistance difference should be ≤2%, and the low-voltage side should be ≤4%.

· For transformers over 1600kVA, the high-voltage side difference should be ≤1%, and the low-voltage side should be ≤2%.

5. Compare the Readings
Always compare the resistance values before and after the adjustment. You want to make sure the tap changer is in the right position.

Remove Safety Measures and Turn the Power On

· Take off the grounding wire

· Remove any barriers or warning signs

· Turn the power back on

· Measure the voltage on the low-voltage side

Common Mistakes

· Wrong Order of Power-Off Switches: Always turn off the low-voltage main switch first, then turn off the high-voltage drop-out switch.

· Not Disconnecting High-Voltage Leads: Be sure to disconnect and secure the high-voltage leads before measuring.

· Not Inspecting the Tap Changer: Always check for dirt or oil buildup on the tap changer before making adjustments.

· Not Measuring Resistance Before Adjustment: Record the high-voltage side resistance before making changes to the tap changer.

· Not Comparing Values Before and After Adjustment: Always compare the resistance before and after adjustments to make sure the tap changer is set correctly.

Important Things to Keep in Mind

· Weather Conditions: Don't install or remove grounding wires if it's storming or if the wind is too strong (level 5 or higher). Everyone should move to the ground.

· Tap Changer Shaft Sticking: If the rotating shaft gets stuck, don't force it. Try rotating it back and forth to free it up.

· Non-Compliant Resistance Readings: If the resistance is off, readjust the tap changer until it matches the standard.