Due to global climate change, thunderstorms are becoming increasingly frequent. According to statistics from the World Meteorological Organization (WMO), the global average number of thunderstorm days in 2023 increased by 18% compared to a decade ago, with particularly significant growth in Southeast Asia, Africa, and North America. Lightning strikes and surge voltages cause over $5 billion in direct losses to global power systems annually, with transformers—the core equipment of power grids—being the most vulnerable due to their insulation systems.

To address this challenge, the International Electrotechnical Commission (IEC) and the Institute of Electrical and Electronics Engineers (IEEE) have released multiple standards (such as IEC 60099-4 and IEEE C62.11), promoting the development of collaborative protection technologies for lightning arresters and transformers. This article will analyze how to ensure the safety of power equipment through a multi-level protection strategy, combining international standards and global engineering practices.

Content

1. Lightning Strike Mechanisms and Direct Hazards

Lightning currents are characterized by high amplitude (10–200 kA) and rapid rise time (1–10 μs). Their energy can instantly break down insulation materials. For example, a typical negative lightning strike with a 10/350 μs waveform (as defined by IEC 62305) and a peak current of 100 kA releases energy equivalent to 1/10 of a ton of TNT explosion. When lightning currents invade substations through transmission lines, they create the following effects in transformer windings:

(1)Electromagnetic induction:The transient magnetic field induces voltages of several kilovolts in the windings.

(2)Ground potential rise (GPR):If the grounding system has high resistance, lightning currents cause a sudden rise in local ground potential, creating a "backflashover voltage."

●Without proper protection, lightning strikes can lead to:

(1)Insulation breakdown:High voltage gradients carbonize transformer oil-paper insulation, causing internal short circuits.

(2)Winding burnout:Joule heating (Q=I²Rt) from lightning currents can raise local temperatures above 1000°C.

(3)Equipment explosion:Insulating oil decomposes into flammable gases (e.g., hydrogen) under high temperatures, leading to explosions when exposed to arcs.

Case Study:In 2022, a lightning strike caused a transformer bushing breakdown at a Mumbai substation, resulting in an oil fire, explosion, a 12-hour blackout, and $2 million in direct losses.

2. Core Technologies of Lightning Arresters: From "Passive Discharge" to "Smart Voltage Limiting"

● Nonlinear Characteristics of Metal-Oxide Arresters (MOA)

Traditional silicon carbide (SiC) arresters are being replaced by metal-oxide arresters (MOA) due to their slow response time (~100 ns) and high residual voltage. MOAs use zinc oxide (ZnO) doped with trace metals (e.g., Bi₂O₃, CoO), exhibiting nonlinear volt-ampere characteristics:

(1)Low-voltage zone (<1 kV/mm):Resistivity reaches 10⁸ Ω·m, allowing almost no current flow.

(2)High-voltage zone (>3 kV/mm):Resistivity drops to 1 Ω·m, forming a low-resistance discharge path.

Effect:MOAs respond within 25 ns and reduce residual voltage by 40% compared to traditional arresters. For instance, under a 100 kA strike, MOAs limit line voltage to below 300 kV (meeting IEC 60099-4 standards), preventing transformer insulation failure.

Case Study:Thailand’s EGAT grid deployed MOAs in 230 kV substations, reducing transformer failures from 1.5 to 0.2 incidents per year—an 86% improvement in protection efficiency.

● Insulation Coordination Between Arresters and Transformers

The effectiveness of arresters depends on match transformer insulation levels. Per IEC 60071-1:

Uprotect ≤ 0.85 × Uwithstand
where:

(1)Uprotect: Arrester residual voltage

(2)Uwithstand: Transformer basic lightning impulse insulation level (BIL)

Engineering Practices:

(1)Distance optimization:Install arresters ≤50 meters from transformers (IEEE C62.22 recommendation) to minimize line inductance effects.

(2)Multi-stage protection:Use three-stage MOAs at line entrances, busbars, and transformer terminals to progressively reduce surge magnitude.

3. Reinforced Transformer Insulation Design Against Lightning Strikes

● Gradient Insulation and Electric Field Equalization

Modern transformers use alternating oil-paper gradient insulation, where dielectric constants (εᵣ) increase exponentially with distance from windings:

εᵣ(x) = εr₀ · eᵏˣ

This design reduces peak electric fields from 8 kV/mm to below 3 kV/mm.

Application:Ideal for substations in high-thunderstorm regions (e.g., Southeast Asia, Africa), meeting IEC 60076-15’s enhanced BIL requirements (20–30% improvement).

● Lightning-Resistant Winding Optimization

Key designs to mitigate current imbalance and overheating:

(1)Continuous transposition winding:Each turn is transposed 3–4 times, reducing eddy losses by 60%.

(2)Electrostatic shielding:Copper shields between windings and cores equalize electric fields via capacitive coupling, cutting local field strength by 50%.

Effect:ABB’s optimized transformers reduce winding temperature rise from 120°C to 65°C under 10 kA strikes, tripling insulation lifespan.

4. Grounding Systems and Smart Monitoring Synergy

● Low-Impedance Grounding Grids

Per IEEE 80, grounding resistance must satisfy:

Rground ≤ (voltage limits) / (lightning current)
For 50 kA strikes, Rground ≤ 0.001 Ω. Solutions           include:

(1)Deep-well electrodes:Buried 30–100 meters in low-resistivity soil (<50 Ω·m).

(2)Copper-mesh grids:Cross-sectional area ≥120 mm², spacing ≤5 meters, reducing step voltage to <40 V.

Application:Effective in high-resistivity areas (e.g., Middle Eastern deserts).

● Online Monitoring Systems

Real-time sensors provide early fault warnings:

(1)Arrester leakage current:Replace MOA valves when resistive current (IR) exceeds 15% of total current (IEC 60099-5).

(2)Transformer partial discharge:UHF sensors (300 MHz–3 GHz) locate defects with≤10 cm error using time-difference algorithms (TDOA).

Case Study:Germany’s E.ON grid extended arrester replacement cycles from 5 to 8 years, cut maintenance costs by 40%, and achieved 95% fault accuracy.

Table 1: Lightning Protection Measures and Applications

In Summary

Lightning protection relies on energy control and system synergy. Arresters divert lightning currents, transformers withstand surges via optimized insulation, and grounding/monitoring systems ensure reliability. For global users, selecting IEC/IEEE-compliant solutions tailored to regional climates is essential. Future advancements like wide-bandgap (SiC/GaN) arresters and self-healing insulation may achieve "zero lightning damage."

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