The basic principle of three-phase transformers

Three-Phase Transformers are the core equipment in the power system used to transform three-phase alternating voltage and current. Their core working principle is based on the law of electromagnetic induction, combined with the symmetrical characteristics of three-phase circuits. They are widely used in power generation, transmission, distribution, and industrial power supply scenarios.
I. Core physical basis: Law of electromagnetic induction
The working principle of three-phase transformers is the same as that of single-phase transformers, both relying on Faraday's law of electromagnetic induction. It consists of two key processes: mutual induction and self-induction:
Mutual induction phenomenon (core of energy transfer)
The three-phase transformer has two independent windings: the primary winding (primary coil) and the secondary winding (secondary coil). They are wound on the same closed iron core.
When the primary winding is connected to a three-phase alternating current power supply, an alternating current is generated, and a changing magnetic flux is formed in the iron core.
This changing magnetic flux passes through the secondary winding simultaneously. According to the law of electromagnetic induction, an induced electromotive force is generated in the secondary winding; if the secondary winding is connected to a load, an induced current will be generated, achieving the transfer of electrical energy from the primary winding to the secondary winding.
The medium for energy transfer is the changing magnetic flux in the iron core. There is no direct electrical connection between the primary and secondary windings.

The core and winding design of three-phase transformers fully utilize the symmetry of the three-phase circuit. It mainly consists of two types: core-type (the mainstream structure) and shell-type. The core structure includes three parts: 
The core is of a three-phase three-column or three-phase five-column structure, constructed by stacking silicon steel sheets (to reduce core loss). One phase of the primary and secondary windings is wound on each of the three core columns, and the three-phase magnetic flux is symmetrical, forming a closed circuit in the core. 
Each phase of the winding has an independent primary winding and secondary winding, which are wound concentrically or overlappingly on the iron core column. The connection method of the three-phase windings determines the connection group of The Transformer. Common ones include: 
Star connection (Y): One end of the three-phase winding is connected to form the neutral point, and the other end is the outgoing line end; a neutral wire can be drawn out, suitable for power distribution systems that require a neutral line.
Delta connection (△): The three-phase windings are connected in sequence at the beginning and end to form a closed loop, without a neutral point; suitable for high-voltage power transmission systems, can eliminate the third harmonic magnetic flux in the iron core.
Common wiring combinations in industry: Y/△ (step-down transformer), △/Y (step-up transformer), Y0/Y0 (distribution transformer, can provide three-phase four-wire power supply). 

The iron core and windings of the oil tank and its accessories are sealed within the oil tank. The oil tank is filled with transformer oil, which serves the functions of insulation and heat dissipation. The accessories include tap changers (for adjusting the ratio of turns and stabilizing the output voltage), radiators, gas relays (for fault protection), etc.

III. Working Characteristics of Three-Phase Transformers 
Symmetrical operation In an ideal state, the three-phase power supply is symmetrical, the winding parameters are consistent, and the voltages, currents and fluxes of the three phases of the transformer are all symmetrical. This is the basis for the efficient operation of a three-phase transformer. 
There are two types of losses in the operation of a transformer:

Iron loss: This is the hysteresis loss and eddy current loss caused by the alternating magnetic flux in the core. It is independent of the load size and only depends on the power supply voltage and frequency.
Copper loss: This is the loss caused by the resistance of the primary and secondary windings. It is proportional to the square of the load current. The efficiency of a transformer: iron loss + copper loss. The efficiency of industrial three-phase transformers is usually above 95%. 

Idle operation and load operation 
No-load operation: The secondary winding is open-circuited, and the primary winding is connected to the power supply. At this time, the current is very small (no-load current, approximately 2% to 10% of the rated current), and it is mainly used to establish the magnetic flux in the core.
Load operation: The secondary winding is connected to the load. As the load increases, the current in the secondary winding increases, and the current in the primary winding also increases to maintain the basic stability of the magnetic flux in the core.

IV. Core Value of Industrial Applications
Three-phase transformers are key equipment in the power system for voltage transformation, power transmission, and distribution:
On the power generation side: The step-up transformer raises the low-voltage electricity output by the generator to a high voltage, reducing line losses during transmission.
On the distribution side: The step-down transformer reduces the high voltage to the voltage required by industrial equipment (such as motors, switch cabinets) or for civilian use (such as 380V three-phase industrial electricity, 220V single-phase residential electricity).