Working Principle Of Power Transformer
In domestic transformers, the high-voltage windings are 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.
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.
1) 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 the following 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.
500/220/LVkV─YN,yn0,yn0 or YN,yn0,d11
220/110/LVkV─YN,yn0,yn0 or YN,yn0,d11
330/220/LVkV─YN,yn0,yn0 or YN,yn0,d11
330/110/LVkV─YN,yn0,yn0 or YN,yn0,d11
2). Domestic 60kV and 35kV transmission systems have two different phase angles.
For example, a 220/60kV transformer using the YNd11 connection and a 220/69/10kV transformer using the YN,yn0,d11 connection result in a 30° electrical phase angle difference between these two 60kV transmission systems.
When 220/110/35kV transformers use the YN, yn0, d11 connection method, and 110/35/10kV transformers use the YN, yn0, d11 connection method, the voltage phasors of these two 35kV transmission systems also differ by an electrical angle of 30°.
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.
3) Domestic 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 voltage phasor difference of 60°. 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.
4) However, it should be noted that: when a single-phase transformer is connected in a three-phase group, a YNy0 connection cannot be used. Three-phase shell-type transformers also cannot use the YNy0 connection.
When a three-phase, five-limb core transformer must use the YN, yn0, yn0 connection, there must be a fourth winding connected in an angle configuration inside the transformer. Its lead-out terminal is not brought out (lead-out terminals for structural electrical testing are an exception).
5) When transformers with different connection groups are connected in parallel, the general rule is that the connection group designation must be the same.
6) When distribution transformers are used in areas prone to lightning strikes, the Yzn11 connection method can be used. When using the z-connection method, the impedance voltage calculation differs from the Yyn0 connection method, and the copper consumption of the windings in the z-connection method is higher. Distribution transformers with the Yzn11 connection method have better lightning protection performance.
7) The YNy0 connection method cannot be used when a three-phase transformer uses four wound core frames.
8) The above are connection methods used for domestic transformers. For export, appropriate connection methods and connection group designations should be supplied according to requirements.
9) Generally, there are taps connected to tap changers within the high-voltage windings. Therefore, when selecting tap changers (including on-load tap changers and off-load tap changers), it is essential to ensure compatibility between the transformer connection method and the tap changer connection method (including connection method, test voltage, rated current, voltage per stage, voltage regulation range, etc.). For on-load tap changers used in YN-connected on-load tap changers, it is also important to ensure that the neutral point can be led out.
