How to protect the power transformer from lightning?

During thunderstorms, distribution transformers are prone to lightning-induced damage—primarily driven by forward and inverse transformation overvoltages when the power distribution system is struck by lightning. Understanding these two overvoltage mechanisms is key to implementing effective protection. Below is a detailed breakdown of their causes and the importance of targeted lightning protection for transformers.

Positive transformation overvoltage originates from lightning strikes on the low-voltage side of the distribution system. When lightning hits low-voltage lines, the lightning current penetrates the low-voltage winding and discharges to the ground through the neutral point grounding device. As this grounding current passes through the grounding resistance, it creates a significant voltage drop, which sharply elevates the neutral point potential on the low-voltage side.

This elevated potential superimposes an overvoltage on the low-voltage winding, posing a direct threat to its insulation. Additionally, through electromagnetic induction between the high and low-voltage windings, this low-voltage side overvoltage is stepped up to the high-voltage side in proportion to the transformer’s transformation ratio. When combined with the existing phase voltage of the high-voltage winding, it results in a dangerous overvoltage that can damage the high-voltage winding.

In short, positive transformation overvoltage refers to the phenomenon where lightning-induced overvoltage on the low-voltage side is inductively transferred to the high-voltage side, causing hazardous overvoltage in the high-voltage winding.

Inverse transformation overvoltage is triggered by lightning strikes on the high-voltage side lines. When lightning strikes, the lightning current is diverted to the ground via the high-voltage side surge arrester. As this current flows through the grounding resistance, it generates a voltage drop that acts directly on the neutral point of the low-voltage side.

At this point, the low-voltage outgoing lines behave as if grounded through a resistor, meaning most of the voltage drop is applied to the low-voltage winding. Through electromagnetic induction, this voltage is then stepped up to the high-voltage side according to the transformation ratio. When superimposed with the phase voltage of the high-voltage winding, it creates an excessive overvoltage that can lead to insulation breakdown and serious accidents in the high-voltage winding.

This process—where a high-voltage side lightning strike indirectly induces overvoltage on the low-voltage side, which is then inductively transferred back to the high-voltage side—defines inverse transformation overvoltage.

Ensuring the safe and reliable operation of power transformers requires addressing both forward and inverse transformation overvoltages. These two mechanisms demonstrate that lightning damage can originate from either the high or low-voltage side, with risks propagating through electromagnetic induction between windings.

To protect transformers effectively, it is essential to implement comprehensive lightning protection measures tailored to these specific overvoltage risks. This not only safeguards the transformer’s windings and insulation but also minimizes downtime and maintenance costs associated with thunderstorm-related damage.