High voltage power transformer

The reason why high-voltage windings are often connected in a Y connection is that the phase voltage can be equal to 57.7% of the line voltage, and the voltage per turn can be lower.

1). The voltage phasors of domestic transmission systems of 500, 330, 220, and 110kV are all in phase. Therefore, for three-phase three winding or three-phase autotransformers with the following voltage ratios, star connection should be used for both high and medium voltage windings. When the three-phase three iron core column iron core structure is used, the low-voltage winding can also adopt star connection or angle connection, which depends on whether the voltage phasor of the low-voltage transmission system is in phase with or lagging behind the voltage phasor 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). There are two different phase angles for the voltage of the domestic 60 kV and 35 kV transmission systems.

If the 220/60kV transformer adopts YNd11 connection method, and the 220/69/10kV transformer adopts YN, yn0, d11 connection method, there is a 30 ° electrical angle difference between these two 60kV transmission systems.

When the 220/110/35kV transformer adopts YN, yn0, d11 connection method, and the 110/35/10kV transformer adopts YN, yn0, d11 connection method, the voltage phasor of the two 35kV transmission systems also differs by 30 ° electrical angle.

So, when deciding on the connection method for 60 and 35kV windings, caution must be exercised, and the connection method must meet the requirements of the voltage phasor of the transmission system. Determine the connection method of 60 and 35kV windings based on the relative relationship of voltage phasor. Otherwise, even if the capacity and voltage ratio are correct, the transformer cannot be used, the connection method is incorrect, and the transformer cannot be connected to the power transmission system.

3). There are also two types of phasor in domestic 10, 6, 3, and 0.4kV transmission and distribution systems. In the Shanghai area, there is a type of 10kV and 110kV transmission system with a voltage phasor difference of 60 ° electrical angle. At this time, a three-phase three winding power transformer with a voltage ratio of 110/35/10kV and YN, yn0, y10 connection can be used, but only with a three-phase three iron core column iron core.

4). However, it should be noted that single-phase transformers cannot be connected in three-phase groups using YNy0 connection method. The YNy0 connection method cannot be used for three-phase shell transformers.

When a three-phase five column iron core transformer must use YN, yn0, and yn0 connections, there must be a fourth winding connected in an angular manner inside the transformer, and its head should not be led out (the head that needs to be led out during electrical testing is not included in this example).

5). When transformers with different connection groups are operated in parallel, the general rule is that the connection group numbers must be the same.

6). When the distribution transformer is used in areas with multiple lightning strikes, the Yzn11 connection method can be used. When the z-connection method is used, the impedance voltage algorithm is different from the Yyn0 connection method, and the copper consumption of the z-connection winding is higher. The Yzn11 connection distribution transformer has good lightning protection performance.

7). YNy0 connection method cannot be used for three-phase transformers with four coiled iron core frames.

8). The above are all connection methods used for domestic transformers. If exported, suitable connection methods and connection group numbers should be supplied according to requirements.

9). Generally, there are taps and tap changers connected to high-voltage windings. Therefore, when choosing a tap changer (including on load tap changers and off excitation tap changers), it is necessary to pay attention to the coordination between the transformer connection method and the tap changer connection method (including connection method, test voltage, rated current, each level voltage, voltage regulation range, etc.). For the on load tap changer used in YN connected on load tap changers, it is also important to note that the midpoint must be able to be pulled out.