Zinc Oxide Surge Arresters play a crucial role in protecting distribution systems from the damaging effects of lightning. These devices are designed to divert excess voltage caused by lightning strikes, ensuring that the electrical equipment remains safe and operational. When lightning strikes the high-voltage side of a power system, it can cause a significant voltage spike that may damage transformers and other critical components.
In a three-phase four-wire distribution system, if lightning hits the high-voltage side, the resulting voltage drop across the grounding resistance, combined with the residual voltage from the zinc oxide arrester, can exceed the insulation limits of the transformer’s windings. To prevent this, the arrester’s ground lead is typically connected to the transformer’s housing and then grounded. This setup ensures that only the arrester’s residual voltage is applied to the high-voltage winding. However, this causes the transformer housing’s potential to rise, which could lead to a reverse flashover from the housing to the low-voltage winding.
To address this issue, the neutral point on the low-voltage side of the transformer is also grounded. This helps balance the potential difference and prevents reverse flashovers. This combined grounding method—connecting the zinc oxide arrester’s ground lead, the transformer’s neutral point, and the housing to a common ground—is known as the "trinity" grounding method. It significantly enhances the system’s ability to withstand lightning surges and protects against equipment damage.
When lightning strikes the high-voltage side, the zinc oxide arrester activates, allowing the lightning current to flow through the grounding resistor. This causes a voltage drop at the low-voltage neutral point. As a result, most of the voltage is applied to the low-voltage windings, while the inductive effect can cause high voltages to appear on the high-voltage windings. Although the arrester limits the terminal voltage, the voltage distribution along the high-voltage winding can still be dangerous, especially near the neutral point, where insulation failure might occur.
Additionally, the longitudinal voltage along the high-voltage winding can lead to breakdowns between layers or between turns, causing serious damage. To mitigate these risks, the grounding resistance should be minimized during installation, and the insulation level of the high-voltage winding’s neutral point should be checked and reinforced during inspections. On the low-voltage side, even though the insulation margin is generally higher, a lightning strike can still cause voltage to be stepped up to the high-voltage side, potentially leading to insulation failure on both sides. This can endanger people and livestock.
Therefore, it is essential to install low-voltage surge arresters on the secondary side of distribution transformers. This dual protection strategy ensures comprehensive lightning protection for the entire system, reducing the risk of equipment failure and enhancing overall safety.
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