The winding of a dry transformer is a set of turns that form an electrical circuit corresponding to a voltage value indicated by a dry transformer. The detailed application of winding is the dry transformer, the dry transformer has a transformer with a primary winding and several secondary winding wound on the iron core. The number of turns of each secondary winding is different, and the end voltage is different, so the dry-type transformer multi-winding transformer can supply power to several electrical equipment with different voltages. Three-winding transformers are most commonly used in dry transformer power systems. Using a three-winding transformer to connect the transmission system with three different voltages is more economical, less occupied and more convenient to maintain and manage than using two general transformers. Three-phase three-winding transformers generally use Y-Y-4 connection, that is, the primary and secondary windings are Y connection third winding into 4. The 4 connection itself is a closed loop, which is allowed to pass through the third harmonic current in the same phase, so that the third harmonic voltage is not present in the primary and secondary windings of the Y connection. This provides a neutral point for both the primary and secondary sides. In a long-distance transmission system, the third winding can also be connected to a synchronous modulation camera to improve the power factor of the dry-type transformer line.