TapChanger

A tap changer is a device fitted to power transformers that adjusts the transformer's turns ratio in order to control the secondary voltage as input voltage or load current changes. By selecting different winding taps, it compensates for voltage variations along the distribution or transmission system, helping to maintain a stable output.

There are two main categories: on-load tap changers (OLTC) and off-load or no-load tap changers (NLTC). OLTCs operate while the transformer is energized and are common in distribution substations and large transformers; NLTCs require the transformer to be de-energized before a tap change can be made. OLTCs are often integrated in-tank and use diverter switches to connect to adjacent taps with small step changes, controlled by an automatic tap changer (ATC) or a local control. NLTCs are typically manual or motor-operated and are selected via a switch when the unit is de-energized.

Operation generally involves monitoring the output voltage and, when it drifts outside a preset band, the control system commands a step to a higher or lower tap. Each step produces a small change in the turns ratio, commonly a few percent per step, depending on design. Modern installations may also employ solid-state or electronic tap changers that regulate voltage without moving contacts, with the control logic supplied by the same ATC.

Applications include power distribution transformers, feeder substations, and other systems where voltage regulation is required under varying load. Advantages of tap changers include improved voltage regulation and reduced distribution losses, while drawbacks include added complexity, maintenance requirements, and potential arcing or wear on mechanical components in OLTCs.