Transformer Tap Changer

Supply voltage variations throughout the day — caused by changing load patterns, renewable energy injection, and network switching — can push consumer voltage outside acceptable limits. Transformer tap changers compensate by adjusting the effective turns ratio. Off-circuit tap changers (OCTC) are simple mechanical switches on the HV winding that can only be adjusted when the transformer is de-energised — typically set during commissioning and rarely changed. They usually offer ±5% adjustment in 2.5% steps (five positions). On-load tap changers (OLTC) are complex mechanical devices that adjust turns ratio under load, typically with ±10% range in 1.25% or 1.67% steps. OLTCs use diverter switches that make-before-break to maintain continuous current flow during tap changes. Modern OLTCs are controlled by automatic voltage regulators (AVRs) that monitor the secondary voltage and adjust taps to maintain the target voltage within a deadband (typically ±1.5%). In distribution networks, tap changer settings directly affect the voltage available to consumer installations. When calculating maximum and minimum fault levels, engineers must consider the tap position that produces the worst-case scenario for each verification.