Tap Changer

A tap changer is the mechanism used to adjust the effective turns ratio of a transformer so that the voltage on one side can be regulated. By moving between winding taps, the transformer can compensate for changes in load, source voltage, or network operating conditions.

Tap changing is essential in both transmission and distribution systems because voltage cannot be kept within acceptable limits by fixed transformer ratio alone. Depending on the equipment design, the adjustment may require the transformer to be de-energized or may be performed while it remains in service.

Key Aspects of Tap Changers:

• Voltage-Regulation Function: The main purpose of a tap changer is to hold system voltage within an acceptable band as loading and upstream conditions change. It is one of the most widely used forms of practical voltage control in the grid.
• On-Load vs Off-Circuit: Off-circuit tap changers require the transformer to be disconnected before adjustment, while on-load tap changers can move taps during energized operation. OLTC designs are therefore critical for continuously regulated network points.
• Step Range and Resolution: Tap changers usually provide a finite number of discrete steps over a defined adjustment range, often around plus or minus 10 percent. The number of steps affects how finely the voltage can be controlled.
• Mechanical and Maintenance Demands: OLTC mechanisms operate under demanding switching duty and are among the most maintenance-sensitive parts of a power transformer. Contact wear, mechanism reliability, and oil condition are key asset-management concerns.
• Control Coordination: Tap changers must be coordinated with capacitor banks, SVC or STATCOM devices, and distributed-resource voltage controls. Poor coordination can create unnecessary tap operations, voltage hunting, or inefficient reactive-power behavior.