Tap Changer — How Transformers Regulate Voltage

The tap changer is a mechanism on a transformer that allows the voltage ratio to be changed by adjusting the number of turns on the primary or secondary winding. It is the primary tool for voltage regulation in power systems.

Why Is Voltage Regulation Necessary?

In a power system, voltage at each node changes continuously due to load fluctuations and generation output. Operating standards require voltage to stay within ±5% to ±10% of the rated value. The tap changer helps meet this requirement.

Types of Tap Changers

1. Off-Circuit Tap Changer (OCTC / DETC)

Can only be adjusted when the transformer is completely de-energized. Typically has 5 taps with steps of ±2.5% or ±5%. Commonly found on distribution transformers rated 22/0.4 kV.

Tap Position	Primary Voltage
+5%	23.1 kV
+2.5%	22.55 kV
0 (nominal)	22.0 kV
-2.5%	21.45 kV
-5%	20.9 kV

2. On-Load Tap Changer (OLTC)

Changes taps while the transformer is energized under load — no need to disconnect. This is more complex equipment, typically installed on power transformers at 110 kV, 220 kV, and 500 kV substations.

The OLTC uses a transition resistor or transition reactor switching to prevent short-circuiting between adjacent taps during the changeover process.

How the OLTC Works

The tap-change operation happens very quickly (20–40 ms) and follows this sequence:

1. Current contact still conducting; new tap contact synchronized via transition resistor
2. Both taps briefly connected in series through the resistor (bridge state)
3. Old tap contact opens — full current transfers to new tap
4. Transition resistor disconnects — tap change complete

Automatic Voltage Regulation (AVR)

Modern substations are equipped with an Automatic Voltage Regulator (AVR) or Automatic Tap Changer Control (ATCC). This device continuously monitors secondary voltage and issues tap-change commands when voltage deviates beyond the set deadband.

Typical Tap Changer Specifications

Transformer	Number of Taps	Step Size	Regulation Range
110/22 kV (OLTC)	17 taps	±1.25%	±10%
220/110 kV (OLTC)	21 taps	±1.25%	±12.5%
500/220 kV (OLTC)	25 taps	±1%	±12%

Operation and Maintenance Notes

• OLTC must be serviced periodically based on operation count (typically every 50,000–100,000 operations or 1–2 years)
• Oil in the OLTC compartment must be tested and replaced independently from the main transformer oil
• When operating two transformers in parallel, tap positions should be coordinated to avoid circulating currents
• Never operate the OCTC while the transformer is energized — this can cause arcing and serious damage

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