Zinc Oxide Surge Arresters play a crucial role in protecting distribution systems from lightning strikes. These devices are designed to limit voltage surges and prevent damage to electrical equipment. When lightning strikes the high-voltage side of a transformer, the surge can cause dangerous voltage spikes that may lead to insulation failure. By installing zinc oxide arresters, these voltage spikes are effectively suppressed, ensuring safer operation of the system.
In a three-phase four-wire distribution system, when a lightning strike occurs on the high-voltage side, the resulting voltage drop across the grounding resistance, combined with the residual voltage from the zinc oxide arrester, can cause significant stress on the transformer windings. This can lead to insulation breakdown, especially if the arrester’s ground connection and the transformer casing are not properly managed. To avoid reverse flashover—where the high voltage from the casing could affect the low-voltage winding—the neutral point on the low-voltage side is also grounded. This ensures that both the casing and the low-voltage side share the same potential, reducing the risk of damage.
This method of connecting the zinc oxide arrester's ground lead, the transformer's neutral point, and the casing together is known as the "trinity" grounding system. It provides an effective way to protect electrical equipment from lightning-induced surges and minimize the chances of equipment failure or damage.
However, even with this protection, when lightning strikes the high-voltage side, the surge arrester activates, and the voltage drop caused by the lightning current through the grounding resistor affects the low-voltage neutral point. This can result in a large portion of the voltage being applied to the low-voltage windings, potentially causing damage. The inductance of the windings can also lead to high voltages on the high-voltage side, which may exceed the insulation limits, especially at the neutral point. This can lead to insulation breakdown between turns or layers of the winding.
To mitigate this risk, it is essential to keep the grounding resistance as low as possible during installation. Additionally, during transformer inspections, the insulation level at the high-voltage winding neutral point should be reinforced. On the low-voltage side, even though the insulation margin is typically higher than on the high-voltage side, lightning surges can still cause damage. This is because the voltage from the low-voltage side, when stepped up through the transformer, can exceed the insulation capacity on the high-voltage side, leading to failures and even posing risks to people and livestock.
Therefore, it is highly recommended to install low-voltage surge arresters on the secondary side of distribution transformers. This additional layer of protection ensures comprehensive lightning protection for the entire system, safeguarding both equipment and personnel from the harmful effects of electrical surges.
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