What is a tap changer and how does it regulate voltage?

Tap changers adjust transformer turns ratio by selecting different winding taps, typically plus/minus 10% in 1.25-2.5% steps. No-load tap changers (NLTC) are adjusted when transformer is de-energized for seasonal or permanent adjustments. On-load tap changers (OLTC) change taps under load using a mechanism with transition resistors or reactors to prevent current interruption. OLTCs operate automatically based on voltage regulator signals, maintaining secondary voltage within limits despite load or primary voltage variations.