Understanding the Impact of Indirect Lightning Induced Overvoltages

When we consider the phenomenon of lightning, it's crucial to differentiate between direct and indirect lightning strikes. Direct lightning strikes occur when the bolt strikes directly on a power line or electrical equipment, whereas indirect strikes involve the bolt striking a nearby object, which then induces overvoltages. This article aims to explain the mechanisms behind indirect lightning-induced overvoltages and their effects on electrical systems.

Indirect Lightning Strikes: What They Are and How They Occur

Indirect lightning strikes happen when a lightning bolt strikes a nearby object or the ground, creating an electromagnetic pulse. This pulse travels through the air and can induce overvoltages in nearby power lines and electrical equipment. Unlike a direct strike, which causes enormous and immediate damage, indirect strikes can be even more insidious because they can affect circuits and devices that are not directly hit.

When a lightning bolt strikes a building or the ground, it creates a massive current flow. This current flow is perpendicular to the path of the power lines, leading to the phenomenon known as induction. Similar to how a radio signal induces a voltage in a radio antenna, the electromagnetic pulse induced by the lightning current can also induce a voltage and current in the power lines. This induced voltage can be surprisingly high, sometimes reaching 150kV or more.

The Mechanism Behind Inductive Overvoltages

The mechanism behind inductive overvoltages involves the electromagnetic field created by the lightning current. When a lightning bolt strikes, it generates an intense electromagnetic field. This field can travel through the air and induce voltages in nearby conductors. The key principle here is Faraday's law of induction, which states that a changing magnetic field can produce an electric current or voltage.

The process can be visualized using Fleming's left-hand rule. According to this rule, the direction of the induced voltage is determined by the direction of the current and the direction of the magnetic field. In the case of lightning, the magnetic field is perpendicular to the power lines, causing the current to induce a very high voltage pulse into the conductors. This pulse travels along the conductors for several kilometers before dissipating.

Consequences of Indirect Lightning Strikes

Indirect lightning strikes can cause significant damage even when the lightning is far from the power lines. The induced overvoltages are capable of reaching the basic insulation level of electrical equipment, leading to insulation puncture. Once the insulation is punctured, the 50Hz current can follow through a short circuit, causing explosions, burning, and significant equipment damage.

According to statistics in the UK, the lightning density is about 2 strikes per square kilometer per year in the worst locations. Most of the UK experiences less than 0.2 strikes per year. For high-voltage overhead lines, the "collection corridor" is a key concept. For an 11kV line, this corridor is a strip about 20 meters wide either side of the line, extending 6 meters above the ground. Strikes outside this 20-meter corridor rarely hit the line or poles.

Protective Measures and Strategies

To mitigate the risks associated with indirect lightning strikes, several protective measures can be implemented. These include:

Earth electrodes and grounding systems to dissipate the induced currents and voltages. Surge protection devices (SPDs) to protect sensitive electrical equipment from overvoltages. Regular maintenance and inspections of electrical installations to ensure they are in good working condition. Installation of lightning rods or lightning arresters to divert the lightning strike away from the power lines.

Understanding the mechanisms of indirect lightning strikes and their consequences is crucial for the design and maintenance of electrical systems. By implementing proper protective measures, we can significantly reduce the risk of equipment damage and ensure safer, more reliable electrical systems.

Keywords: indirect lightning, overvoltages, lightning induction