What type of transformer is used to increase voltage on a feeder by 5 or 10 percent?

A booster transformer is specifically designed to increase voltage and is commonly employed in electrical systems to adjust voltage levels on feeders. When the voltage on a feeder line is slightly lower than the required level for optimal operation, a booster can elevate that voltage by increments such as 5 or 10 percent, ensuring that the system continues to function efficiently without the need for a complete redesign or major equipment changes.

This type of transformer works by taking the input voltage and adding a specific voltage boost, which is particularly useful in scenarios where voltage drops might occur due to long transmission lines or heavy loads. It's designed for voltage regulation rather than isolation or stepping down voltage, making it the correct choice for applications focused on increasing voltage levels in distribution systems.

In contrast, other types of transformers serve different functions. An autotransformer primarily adjusts voltage but does not isolate the primary side from the secondary side. A step-down transformer is designed to reduce voltage, which does not align with the requirement to increase voltage. An isolation transformer provides electrical separation between the primary and secondary circuits without altering voltage levels, which again does not meet the need to boost voltage. Therefore, the booster transformer is the ideal solution for the specified requirement in this context.