Single-Phase Transformer with High Lightning Impulse Withstand Capability

06/17/2025

1 Introduction

To ensure the safe operation of railways and reduce the risk of lightning damage to railway telecommunication control systems, the author has specially researched and designed a single - phase series transformer with a relatively high impulse voltage withstand level, with the model number D10 - 1.2 - 30/10. This transformer is equipped with an oil conservator and adopts a fully sealed structure (it can also be designed as a dry - type structure according to actual needs). This series of transformers is a special - purpose device for railway control signals and can also be applied in small - scale power distribution scenarios of industrial and agricultural power grids, having a certain degree of versatility.

2 Analysis of Lightning and Its Hazards
2.1 Physical Characteristics of Lightning

Lightning is essentially a non - periodic shock wave. The front part of its wave rises very rapidly and then decreases slowly. Due to the extremely large rising steepness of the lightning wave, it can cause very serious harm to electrical equipment.

2.2 Classification and Causes of Lightning

Lightning is mainly divided into two types: direct lightning and inductive lightning. Direct lightning is a form of lightning that directly acts on lines or equipment. Although the degree of harm it causes is extremely great, the actual probability of occurrence is relatively low; however, most of the lightning damage accidents are caused by inductive lightning. Inductive lightning is further subdivided into electrostatic inductive lightning and electromagnetic inductive lightning: Electrostatic inductive lightning is generated by the over - voltage induced by the thundercloud electric field between the overhead line and the earth; Electromagnetic inductive lightning is caused by the over - voltage appearing on the line due to the electromagnetic induction effect when the thundercloud nearby the line discharges. However, its impact degree is much smaller than that of electrostatic inductive lightning.

2.3 Hazard Manifestations of Lightning on Transformers

During the actual operation process, accidents of transformers being damaged by lightning strikes occur from time to time. Such accidents will not only cause damage to the transformer itself but also cause damage to the secondary equipment through the wave - impact effect, leading to a wider range of fault impacts.

2.4 Mechanism of Transformer Damage by Lightning Waves

The damage of transformers by lightning waves mainly comes from two factors: First, the impulse voltage value is quite high, reaching a maximum of 8 - 12 times the phase voltage; Second, the lightning wave will cause a high concentration of the electric field, thereby damaging the insulation performance of the transformer. Under the action of the shock wave, the main insulation of the transformer may be damaged. This is because the lightning wave has a high frequency and a steep wave front, which will make the potential gradient at the beginning of the winding reach the maximum value, making the longitudinal insulation extremely easy to be broken down.

2.5 Voltage Transmission of Lightning Shock Waves in Transformer Windings

When a lightning shock wave acts on the primary winding of a transformer, the voltage of the winding will rise rapidly, which is equivalent to applying a high - voltage with a very high frequency. In this case, an

over - voltage will also be generated on the secondary side accordingly. Due to the existence of electrostatic capacitance coupling and magnetic field coupling between the primary and secondary windings,

although the over - voltage generated on the secondary side is related to the transformation ratio, it is not a simple transformation ratio relationship.

In some specific situations, this over - voltage may greatly exceed the insulation level of the secondary winding and the electrical equipment it carries, eventually leading to damage to the electrical equipment connected to the secondary winding. The over - voltage acting on the secondary winding is composed of both an electrostatic component and an electromagnetic component. The electromagnetic component can be calculated by the formula me/n (in the formula,

n

is the transformation ratio,

e

is the voltage on the primary side,

m

is the coupling coefficient, and the approximate value is 1).

Stray capacitances exist between a transformer's primary - secondary windings and between windings and the ground. When an impulse voltage is applied between the primary winding and the ground, the electrostatic impulse voltage on the secondary side depends on the distributed capacitances between windings and the ground, not the turns ratio. The transfer voltage

t 2

between the secondary winding and

the ground is

t 2 =&t 1 (&

: transfer/voltage transfer coefficient;

t 1

: impulse voltage on the primary - ground).

3 Single - phase Transformers with High Impulse Voltage Withstand Level

A power transformer’s voltage transfer coefficient (

t 2 /t 1

) usually ranges 0.2–0.9; a tested transformer had 0.25.

Transformers undergo rated lightning impulse withstand voltage tests per voltage levels/national standards. This product (10 kV grid, tested at 15 kV) suffered no damage.Specially designed, the high - impulse - voltage - withstand transformer minimizes secondary over - voltage, resists lightning shocks, blocks interference currents, and boosts electrical performance. Tested by the Academy of Railway Sciences, its voltage transfer coefficient ≤ 1/200, reducing shock - wave transmission from primary to secondary below 1/200.

Effective for protecting low - voltage equipment from lightning, it requires reliable grounding (potential differences during lightning can damage equipment; grounding the shell balances potentials, reducing impulse voltage).Impulse voltage intrusion paths into low - voltage equipment are complex (primary/secondary/ground - side; single or simultaneous). Reliable grounding is key.

4 Conclusion

The single - phase series transformer (with oil conservator, high impulse voltage withstand) abandons traditional oil conservator structures, achieving material - saving, easy - processing, and attractive design.The single - phase oil - immersed series (with oil conservator/fully sealed) has high lightning impulse resistance, reduces over - voltage, protects secondary equipment, and cuts power - line noise for power - supply lightning protection.

Since the 1990s, many such transformers have operated across railway bureaus (hydropower/signal/power - supply sections, etc.), covering most stations, especially lightning - prone areas. Proven in thunderstorms, they offer low loss, material savings, energy efficiency, and reliability, ensuring electrical equipment safety.With railway modernization and technological progress, these transformers will see wider use.

Topics

Single phase distribution transformer Design and Simulation

Hello，I'm Wdwiin. A decade of hands-on experience in electrical engineering, specializing in high-voltage systems, smart grids, and renewable energy technologies. Passionate about technical exchange and knowledge sharing, committed to interpreting industry trends with professional insights to empower peers. Connection creates value—let’s explore the boundless possibilities of the electrical world together!