Optimization method of lightning impulse withstand voltage of epoxy dry-type transformer

Author： GOZ Electric Time：2024-09-17 09:40:57 Read：10

The interlayer capacitance of the segmented layer winding itself is large, that is, the longitudinal capacitance is large, while the capacitance to the ground on the inner and outer surfaces of the winding is much smaller, so its a value is small, which has the inherent advantage of good lightning impulse characteristics. Our optimization and improvement are also limited to the local position of the winding.

(1) From the simulation analysis, it can be seen that the shock wave first destroys the wire turns, outer surface and outer airway of the incoming line end (head end), followed by the neutral point side (end), so the first thing is to improve the wire turns of the head and end. Insert the capacitor screen wire at the head and end, because the capacitor screen wire is disconnected and suspended, and no current passes through the transformer during operation, but it acts in the form of capacitance under the high-frequency characteristics of the shock wave. The capacitor screen wire at the head end can have 3 to 6 layers to increase the equivalent capacitance; the end can be smaller than the head end. When the capacitor screen wire is not used, other methods are also needed to increase the inlet capacitance, such as using wide electromagnetic wire, increasing the number of turns and layers of the first section wire turns; using double parallel wire segments at the head end is also an effective measure.

(2) Regarding the breakdown of air in the airway, the relevant literature adopts the method of setting a shield around the airway. Through simulation, it is found that after adding capacitor screen wires to the head and the end, the maximum field strength value in the outer airway has decreased; it is only necessary to increase the mutual capacitance between the corresponding wire turns on both sides of the airway. Because the existence of the airway reduces this mutual capacitance, it plays a role in equalizing the pressure and field, and basically suppresses the breakdown of the air in the airway.

(3) The outer surface around the inlet end of the high-voltage winding may cause the interface between the resin and the air to produce a higher electric field strength due to the interface polarization effect. The improvement method is mainly to increase the radius of curvature to improve the field strength distribution at the end. First, the thicker the wire turns of this section of the inlet end, the larger the edge arc, and the head end bears the highest voltage, and the electromagnetic wire is strengthened. The second is to ensure that the end of the entire wire segment is flat during winding; the third is to set a voltage equalizing ring (cover) on the exposed outlet terminal to improve the electric field at the inlet end.

(4) Partition compensation: Divide the winding into four areas along the axial direction, combine the capacitance parameter calculation of the finite element method, and reasonably configure the capacitance of each area. In fact, it is to adjust the number of turns of each layer to make the capacitance optimally distributed and the impulse voltage more evenly distributed.

(5) The high-voltage winding is wired in the middle, and the winding is arranged symmetrically in the axial direction. This structure has better electric field distribution, which can reduce the insulation size of the iron yoke and help improve the lightning impulse level. However, considering the transportation height limit, when the winding structure with end wires is used for a capacity of more than 30MVA, the iron yoke insulation distance must be increased, and an isolation angle ring must be added between the winding end and the iron yoke.

Through the above optimization method, the air electric field around the high-voltage winding is significantly improved. It can be seen from the simulation-assisted analysis that, except for the slightly higher level in the first 15μs (maximum 3.4kV/mm), the maximum field strength in the airway at other times is reduced to below 2.8 kV/mm, and the maximum field strength in the main airway is below 2.5kV/mm, as shown in the figure. The windings manufactured by the above optimization method passed the 432kV and 477kV lightning impulse tests at one time, achieving the set goal.

Figure 1

Conclusion

Through the test and simulation-assisted analysis of the actual winding, it is found that the reason why the lightning impulse withstand voltage of epoxy cast dry-type transformer cannot be further broken through is not the performance limitation of epoxy resin itself, but the pain point is that the air in the airway, the outer surface of the head end and the main airway of the commonly used segmented layer high-voltage winding is broken down due to excessive electric field strength. This paper proposes the main optimization methods such as setting capacitor screen wires at the head and end of the high-voltage winding, improving the electric field distribution of the winding head end segment, and increasing the mutual capacitance between the corresponding wire turn layers on both sides of the airway, so that the lightning impulse withstand voltage of epoxy cast dry-type transformer reaches the standard value of 480kV oil-immersed transformer. In fact, since dry-type transformers are installed and used indoors, their lightning impulse voltage standards should be lower than 480kV. At the same time, the author also believes that 110 kV is the voltage level limit for dry-type transformer manufacturing.