What is a third winding typically called on a three-winding transformer?

A third winding on a three-winding transformer is typically referred to as a tertiary winding. This winding is added to provide additional functionality or support, such as stabilizing the voltage, providing a path for circulating currents, or supplying power to auxiliary systems. The tertiary winding can assist the transformer in managing unbalanced loads and can be used for a variety of purposes, including supplying power to a delta load or providing voltage support to other systems connected to the transformer.

In the context of a transformer’s operation, each winding serves a distinct purpose, and the tertiary winding is essential for certain applications where enhanced performance or additional load handling is required. Its role is significant in maintaining system balance and enabling greater versatility in power distribution solutions. Other options like 'spare winding', 'LV winding', and 'unused winding' do not accurately capture the specific designation and function associated with a third winding in a three-winding transformer setup.