Auto-transformer refers to a type of transformer where the primary and secondary windings are part of the same winding. Unlike conventional transformers where primary and secondary windings are separate, in an auto-transformer, they are directly connected in series. This configuration allows for self-coupling of the transformer. Depending on the structure, auto-transformers can be classified into adjustable and fixed types.

The coupling in an auto-transformer refers to electromagnetic coupling, similar to conventional transformers. However, in auto-transformers, there is a direct electrical connection between the primary and secondary sides. This means that a portion of the low-voltage coil is part of the high-voltage coil, leading to more efficient energy transfer.

Auto-transformers find applications in various areas of power systems where voltage transformation is required. One of the key reasons for using auto-transformers is their ability to provide voltage regulation and step-up or step-down voltage transformation more efficiently compared to standard two-winding transformers. They are commonly used in power transmission and distribution systems, as well as in industrial applications.

Auto-transformers differ from standard transformers primarily in their configuration and mode of operation. In standard transformers, the primary and secondary windings are electrically isolated, whereas in auto-transformers, they are directly connected. This difference leads to differences in performance and applications.

Standards, such as those set by the International Electrotechnical Commission (IEC) and the Institute of Electrical and Electronics Engineers (IEEE), define auto-transformers as transformers where at least two windings have a common section or part. These standards provide guidelines for the design, testing, and usage of auto-transformers in power systems to ensure safety and reliability.

Auto-transformers play a crucial role in power systems for voltage transformation and regulation. Their unique configuration and mode of operation make them suitable for various applications where efficient voltage conversion is required. Standards set by organizations such as IEC and IEEE help ensure the proper design and implementation of auto-transformers in power systems.