Magnetic parameters and equivalent circuit model of high temperature superconducting transformer

Author： GOZ Electric Time：2024-10-09 09:37:33 Read：10

1. Magnetic parameters

The high-voltage winding of the entire high-temperature superconducting transformer is divided into 4 parts, and its spatial structure is shown in Figure 1. The 4 windings of the high-voltage magnet are connected in parallel, and each high-voltage winding corresponds to a traction winding. The 4 traction windings work independently. Each traction winding is composed of 2 windings of the same structure connected in parallel. Each coil of the 2 identical windings is wound by two traction superconducting tapes. Its electrical principle is shown in the figure. The two windings of the high-voltage magnet on the same side are insulated by 20mm epoxy resin.

Figure 1

The transformer winding is made of multiple coils axially stacked in series. The inter-turn insulation of the high-voltage magnet and the low-voltage magnet is made of polyimide (PI) in a double 1/2 stacking manner. The thickness of the corona-resistant polyimide (with glue) is 25μm. The skeleton and interlayer baffle of the high-voltage winding are integrally formed with epoxy board G10. The space between the high-voltage winding and the low-voltage winding is filled with liquid nitrogen; the outside of the high-voltage winding is a stainless steel heat exchanger (the heat exchanger is grounded), and the space between the two is filled with liquid nitrogen, with a distance of 50mm.

Figure 2

2. Equivalent circuit model

When the transformer is subjected to lightning overvoltage, from the circuit point of view, the lightning full wave and lightning chopped wave have the characteristics of large steepness and high frequency, and the impact wavelength is inversely proportional to the frequency. It is necessary to establish a high-frequency equivalent model to study the voltage distribution of the magnet under lightning impact. Since the left and right magnets are symmetrical, the upper and lower windings of the magnet on the same side are connected in parallel. When the lightning impact wave invades, the upper and lower windings enter the wave at the same time, so one of the high-voltage windings is selected to establish an equivalent circuit model. In the model, each turn of the coil is regarded as a transmission line, and these transmission lines are connected end to end. The potential and current at the end of the i-th conductor are the potential and current at the beginning of the i+1th conductor. If there are N transmission lines in total, the boundary condition is 2N.

3. Inductance parameters

Taking into account the stray inductance between turns, the overall model of the high-voltage single-side magnet is composed of the self-inductance of all single-turn coils and the mutual inductance between single-turn coils. Schematic diagram of the physical connection of the high-voltage winding. The self-inductance of the coil can be calculated using the formula (omitted). When calculating the mutual inductance between coils, the Lyell method is used to calculate the mutual inductance.

4. Capacitance parameters

This paper uses the finite element software ANSYS to calculate the equivalent capacitance. The energy method is used to solve the equivalent capacitance inside the finite element software, as shown in the formula (omitted). In the two-dimensional axisymmetric electrostatic field model, regarding the material settings, the relative dielectric constant of the inter-turn insulating polyimide is set to 3.1, the relative dielectric constant of liquid nitrogen is set to 1.43, and the epoxy board G10 is set to 5.5. Since the number of turns of the entire high-voltage magnet is nearly 2000, it takes too long to apply excitation to the entire winding to solve the capacitance parameters, and the mutual capacitance value between the conductors far apart is too small to be completely ignored, so this model uses 4 double cakes (8 single cakes) to calculate the equivalent capacitance parameters.

Conclusion

This paper is the transient response of the high-voltage winding of the high-temperature superconducting transformer under lightning overvoltage. A high-frequency equivalent circuit model is established for the high-voltage winding, the state equation is derived, the distributed parameters are solved, and finally the transient response of each turn under lightning overvoltage is simulated. Overall, under the three types of lightning impulse waves, the turn-to-turn voltage at the head end of the winding is always relatively large. Whether the lightning impulse wave is truncated at the wave front or the wave tail, it can greatly reduce the maximum ground potential of each turn of the winding and the interlayer voltage at the end of the winding; truncation at the wave tail will raise the interlayer voltage between the first and second layers of the winding and the interturn voltage between the first three single wire cakes; and truncation at the wave front can significantly reduce the maximum ground potential of each turn, the maximum voltage between each layer, and the maximum voltage between each turn. Therefore, it is very important to select lightning arresters with fast response and quick action for insulation. According to relevant literature, when a full wave of lightning impulse is applied to a triangular plane electrode, 40mm liquid nitrogen can withstand a voltage of more than 130kV, and the liquid nitrogen between the high-voltage winding and the ground is 50mm. The insulation wrapped by the outermost turn can withstand a lightning impulse voltage of 128kV, while the maximum potential of the high-voltage winding to the ground under the full wave of lightning impulse in this article is 298kV. Therefore, regarding the main insulation, it is recommended to add a certain thickness of epoxy resin material between the high-voltage winding and the heat exchanger to strengthen the protection.