Autotransformer Selection Tips
An autotransformer is a special type of transformer in which the secondary and primary windings share a common winding, also known as a single-winding transformer.
The following characteristics of autotransformers are noteworthy:
- Due to the shared winding, an autotransformer of the same rated capacity is more compact in size. This reduces the amount of active materials (such as silicon steel sheets and windings) and structural materials (such as steel), thereby decreasing overall cost.
- Copper and iron losses are also lower as a result of the reduced material usage, leading to improved efficiency.
- The per-unit short-circuit impedance of an autotransformer is smaller than that of a two-winding transformer. This results in a smaller voltage regulation but a higher short-circuit current.
- There is a direct electrical connection between the primary and secondary sides. Overvoltage on the high-voltage side can cause severe overvoltage on the low-voltage side, so surge arresters must be installed on both sides.
- On-load tap changing in autotransformers can only be achieved through line-end voltage regulation.
However, due to its structural features, autotransformers also have the following drawbacks that cannot be overlooked:
- There is an electrical connection between the high- and medium-voltage windings, and their short-circuit impedance is only (1-1/K)² times that of a conventional two-winding transformer of the same capacity.
- The direct electrical link between high- and medium-voltage windings complicates voltage regulation. Common solutions include built-in on-load tap changers or additional transformers installed on the high- and medium-voltage lines, which are costly and can affect operation.
- Overvoltage protection and relay protection systems become more complex.
When selecting an autotransformer, in addition to the above advantages and disadvantages, the following factors should also be considered: transformer ratio, operating mode, short-circuit current level, relay protection, electrical layout, and transportation.
Autotransformers are more economical when the transformation ratio is close to 1. Under the same transportation conditions, an autotransformer of the same physical size can have a larger capacity compared to a two-winding transformer. However, they are not suitable for applications requiring electrical isolation or where equipment and operational safety are a priority.
Shenzhen Shentebian Electric Equipment Co., Ltd., a wholly-owned subsidiary of the Jishengchang Group, was founded in 2004. The company operates its own industrial park and is an advanced, well-established manufacturer specializing in transformers. It is one of the earliest enterprises in the region to enter the power transformer manufacturing sector. STB is the transformer brand under Shenzhen Shentebian Electric Equipment Co., Ltd.
