Working principle of power transformer

In domestic transformers, high-voltage windings are generally connected in a Y-connection. The connection methods for medium-voltage and low-voltage windings depend on the system conditions. System conditions refer to the relationship between the voltage phasors of the high-voltage transmission system and the voltage phasors of the medium- or low-voltage transmission system. For low-voltage distribution systems, the connection method can be determined according to standard regulations.

High-voltage windings are often connected in a Y-connection because the phase voltage can be equal to 57.7% of the line voltage, allowing for a lower voltage per turn.

When a 220/110/35kV transformer uses a YN, yn0, d11 connection, and a 110/35/10kV transformer uses a YN, yn0, d11 connection, the voltage phasors of these two 35kV transmission systems differ by an electrical angle of 30°.

Therefore, the connection method for 60kV and 35kV windings must be carefully determined, and the connection method must meet the requirements of the transmission system's voltage phasors. The connection method for 60kV and 35kV windings is determined based on the relative relationship of the voltage phasors. Otherwise, even if the capacity and voltage ratio are correct, the transformer cannot be used because the connection method is incorrect, and the transformer cannot be connected to the power transmission system.