In domestic transformers, the high-voltage winding is generally connected in a Y-connection. The connection method for the medium-voltage and low-voltage windings depends on the system conditions. The system conditions refer to the relationship between the voltage phasors of the high-voltage transmission system and the voltage phasors of the medium-voltage or low-voltage transmission system. For low-voltage distribution systems, the connection method can be determined according to standard regulations.
The high-voltage winding is 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.
In domestic 500kV, 330kV, 220kV, and 110kV transmission systems, the voltage phasors are in phase. Therefore, for three-phase three-winding or three-phase autotransformers with these voltage ratios, both the high-voltage and medium-voltage windings should be connected in a star configuration. When using a three-phase three-core structure, the low-voltage winding can also be connected in a star or delta configuration, depending on whether the voltage phasors of the low-voltage transmission system are in phase with or lag the voltage phasors of the medium-voltage and high-voltage transmission systems by 30° electrical angle. In China, 60kV and 35kV transmission systems have two different phase angles.
For example, a 220kV transformer using the YNd11 connection and a 220kV transformer using the YN,yn0,d11 connection result in a 30° electrical angle difference between these two 60kV transmission systems.
When a 220kV/110kV transformer uses the YN,yn0,d11 connection and a 110kV/35kV transformer uses the YN,yn0,d11 connection, the voltage phasors of these two 35kV transmission systems also differ by a 30° electrical angle.
Therefore, the connection method for the 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 the 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; if the connection method is incorrect, the transformer cannot be connected to the transmission system. In China, 10kV, 6kV, 3kV, and 0.4kV transmission and distribution systems also have two phases in their phasors. In the Shanghai area, there is a 10kV and 110kV transmission system with a 60° electrical angle difference in voltage phasors. In this case, a three-phase, three-winding power transformer with a 110/35/10kV voltage ratio and a YN,yn0,y10 connection can be used, but it is limited to a three-phase, three-core column-type core.
However, it should be noted that single-phase transformers cannot use a YNy0 connection when connected in a three-phase group. Three-phase shell-type transformers also cannot use a YNy0 connection.
When a three-phase, five-limb core transformer must use a YN,yn0,yn0 connection, there must be a fourth winding connected in an angle configuration inside the transformer. Its output should not be brought out (except for outputs brought out for structural electrical testing).
When transformers with different connection groups are connected in parallel, the general rule is that the connection group designation must be the same.
