Voltage response of high-temperature superconducting transformer under lightning impulse

Author： GOZ Electric Time：2024-10-07 09:25:53 Read：13

In order to study the transient voltage response of the high-voltage magnet of a 6.6MW high-temperature superconducting transformer under lightning impulse overvoltage, a high-frequency equivalent circuit model with a single turn as a unit was established based on the multi-conductor transmission line theory. The distributed parameters were obtained by using finite element software and programming. By deriving the state equation and solving the state variables, the transient response of each turn under the lightning impulse full-wave voltage and lightning impulse chopped wave voltage was obtained in the time domain. Under the three lightning impulse waves, the insulation of the high-voltage magnet meets the requirements of safe operation. The insulation between the first end turns is the weakest. The truncation of the lightning wave can greatly reduce the maximum ground potential of each turn of the winding and the interlayer voltage at the end of the winding. When the wave front is truncated, the inter-turn voltage and inter-layer voltage of the magnet can be greatly reduced, indicating that the selection of fast-acting arresters is crucial.

Keywords: multi-conductor transmission line; lightning overvoltage; high-temperature superconducting magnet; main insulation; longitudinal insulation

When superconducting materials are used in transformers, they can greatly improve operating efficiency, reduce energy consumption, and reduce the weight and volume of equipment. However, its application faces great difficulties. The current superconducting material manufacturing technology can make the thickness of the conductive tape only 0.1mm. Due to its small size, it is easy to produce tip discharge, so it is very difficult to achieve good insulation. Conventional conductors use continuous, inserted shielding and entangled technical processes to improve lightning resistance and wave distribution. It is difficult to achieve these measures to improve wave distribution for high-temperature superconducting transformers. Although lightning protection devices such as lightning conductors and lightning rods are arranged around the substation, lightning shock waves may still bypass these lightning protection facilities and invade the transformer. According to statistics, more than half of the transformer insulation damage is caused by lightning strikes. Once a high-temperature superconducting transformer is struck by lightning, it may cause irreversible damage to the transformer insulation. When struck by lightning, the traction transformer, as a core device, has a very complex electromagnetic coupling relationship between the turns at any position in its winding and the other turns and grounding bodies, which may induce very high voltages. Since the magnets of high-temperature superconducting transformers and high-temperature superconducting current limiters, high-temperature superconducting magnetic energy storage and other superconducting power equipment have similar structures, studying the voltage response of high-temperature superconducting transformer magnets when subjected to lightning impulses has certain academic value in the field of high-temperature superconducting power equipment. In 2017, Zhou Aobo, Shi Jing and others from Huazhong University of Science and Technology established a high-frequency equivalent circuit model for the 1MJ high-temperature superconducting magnetic energy storage device, studied the transient overvoltage it suffered, gave the design process of electromagnetic design and electrical design, and obtained the optimal design scheme. In 2021, Zhang Jing, Dai Shaotao, Ma Tao and others from Beijing Jiaotong University used the method of solving the state equation to study the insulation problem of 160kV superconducting fault current limiters under lightning impulse overvoltage. Based on the longitudinal insulation analysis under lightning impulse withstand voltage test, operating impulse withstand voltage test and power switching test, the turn-to-turn distance of the double-wire coil was optimized. According to the simulation and experimental results, the maximum voltage drop occurs in the first turn of the double-wire coil.

At present, there are few studies on the transient response of high-temperature superconducting transformers subjected to lightning impulses. This paper focuses on the double-pancake structure high-voltage winding of a 6.6MW high-temperature superconducting transformer. Based on the multi-conductor transmission line theory, an equivalent circuit model under transient overvoltage is established with a single turn of superconducting tape as a unit. The response of each turn of the high-voltage winding under different lightning impulse voltages is obtained by solving the state equation. The simulation results are analyzed and suggestions for insulation protection are put forward.