OnLoadTapChanger

An On-Load Tap Changer (OLTC) is a device mounted on a power transformer that allows changing the transformer's turns ratio while the transformer remains energized and connected to the network. Its primary purpose is to regulate the secondary voltage in response to load fluctuations, line drops, and feeder voltage requirements without interrupting service.

OLTCs use a multi-tap transformer winding with several selectable taps and a switching assembly that connects a chosen tap to the circuit. The changeover is typically performed as a make-before-break operation to maintain continuous connection during the transition, reducing voltage transients. The switching action is driven by an actuator—commonly an electric motor with a gear train or a hydraulic/pneumatic system—and controlled by a tap changer controller that monitors voltage (and sometimes current or frequency) and issues stepwise commands to move to adjacent taps until the target voltage is reached.

In standard operation, the regulator follows a voltage setpoint and applies taps in discrete steps, with settling time and deadbands to damp oscillations. Some OLTCs support remote control, fault protection, and diagnostics, and they may include features to prevent excessive switching, such as limits on the number of operations per hour and voltage-swing restrictions.

OLTCs are widely used on distribution and transmission transformers to maintain grid voltage within acceptable limits. They require regular maintenance of the drive mechanism, contacts, insulation, and control circuitry to ensure reliable operation and to manage contact wear, arcing, and insulation aging during switching.