Network voltage is never constant: it rises during light loads and drops during peak demand. The tap changer is the transformer's tool for keeping up with this variation, based on a simple idea: changing the number of active turns in a winding.
The basic idea
The turns ratio equals the ratio of windings. By adding taps to one of the windings — usually the high-voltage winding — the number of turns brought into the circuit can be selected, thereby adjusting the output voltage without any change to the network or the load.
A distribution transformer with five taps in ±2.5% steps: the positions give ratios equivalent to 95%, 97.5%, 100%, 102.5%, and 105% of the base voltage. If the incoming network voltage drops, we move to a tap that reduces the active primary turns, raising the secondary voltage back to the required value.
Why is it usually on the high-voltage winding?
- Its current is lower, so the changeover switches are smaller, cheaper, and longer-lasting.
- It has a larger number of turns, so each tap represents a finer percentage of change.
Field handling guidelines
- First find out the type of tap changer: on-load (OLTC) or off-circuit (OCTC) — the handling differs fundamentally.
- Do not change the tap position except after clear voltage measurements and knowledge of the approved safe procedure.
- Record the tap position with every TTR test; the measured ratio depends on the current position.
Sample answer: Its function is to adjust the transformer's output voltage to counter fluctuations in network voltage and load, achieved by changing the number of active turns through taps on a winding — usually the high-voltage winding, due to its lower current and the finer percentage of each tap. This changes the turns ratio and brings the output voltage back to the required value. The change can be made off-circuit (OCTC) or on-load (OLTC) depending on the type of tap changer.
Operating an off-circuit tap changer while the transformer is in service. This interrupts the load current through switches not designed for it, causing an arc that damages the tap changer and possibly the transformer. Verify the type before touching it.
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