Created by: Glen Zhu | Updated Date: July 2nd, 2024
Lightning protection: Lightning – 200,000-300,000A, temperature of 20,000 degrees.
Classification and Hazards of Lightning
Formation mechanism of thunderclouds:
The widely accepted formation mechanism of thunderclouds is the theory of water droplet fragmentation electrification.
Clouds are composed of many tiny water droplets, with ions adsorbed on the droplets, forming spherical charges. Due to the large mass of water droplets, they move clumsily; even a few micrometer-sized water droplets can be a heavy burden for gas ions. Therefore, the movement of charges in clouds is slow and it is difficult to reach electrical equilibrium. Under the influence of atmospheric electric fields, positive and negative charges accumulate in different layers of the cloud. Positive charges often gather in the upper layer of the cloud, while negative charges gather in the lower layer.
When a charged cloud is close to the ground, it forms a huge capacitor with the ground. The cloud and ground are each poles of this capacitor, and the atmosphere between them acts as the dielectric material. During a thunderstorm, there is a significant voltage difference between these poles that can reach tens of thousands of volts per meter.
Cloud-to-ground lightning process:
When the thundercloud is low and there are no other charged thunderclouds nearby, the ground induces opposite charges. An induced electric field forms between the thundercloud and the ground, leading to discharge when it reaches 25-30 kV/cm.
In the initial stage of discharge, a very fine discharge channel (leader channel) is formed with speeds reaching 100-1000 km/s. When the leader channel is close to the ground, discharge occurs as the charges from both the ground and thundercloud neutralize within this channel. The discharge channel develops from the ground towards the thundercloud at speeds of 15000-150000 km/s.
The direction of leader discharges is random; when at a certain height above the ground, their direction changes towards areas where induced charges are concentrated.
Induced overvoltage – Overvoltage occurs in the circuit due to the effect of static electricity induction.
Formation of induced overvoltage:
During the pilot discharge process, negative charges accumulate in the pilot channel, inducing positive charges from the ground, and forming an electric field. Under the action of the electric field, negative charges in the line conductors are repelled to a distance and gradually leak into the ground, while positive charges gather on and bind to the conductors.
During the main discharge process, negative charges in the pilot channel quickly neutralize with positive charges from the ground. The electric field undergoes a sudden change, releasing positive charges on the conductor. The released charges move at electromagnetic speed along both sides of the conductor, causing voltage increases wherever they go (induced overvoltage).
Induced overvoltage generally does not exceed 300 kV and rarely reaches 500-600 kV. It poses reduced threats to power systems above 110 kV.
Lightning current steepness:
Lightning current steepness – the rate at which lightning current rises (changes) over time.
(Rate of change of lightning current with respect to time)
1) Electrical damage:
Lightning discharge – extremely high impulse voltage – insulation breakdown – short circuit – fire or explosion (electric shock).
A huge lightning current flowing into the ground can directly cause electric shock accidents due to contact voltage or step voltage.
During the thunderstorm season, there is a risk of electric shock from step voltages near the lightning strike point and the grounding point of lightning rods.
2) Thermal damage:
Current – Conductor – Resistance – Heat Energy – Scorching (Melting) – Spark (Explosion)
3) Mechanical damage:
Current – Wooden Conductor – Moisture Content – Expansion- Burst
4) Lightning induction:
Current- Moisture Content- Burst
Electrostatic repulsion, electromagnetic force, and shock waves increase the risk of bursting.
5) Electromagnetic induction:
Strong lightning current – strong alternating electromagnetic field – induced current in closed loop metal object – local heating – spark discharge (ignition explosion)
6) Lightning intrusion wave:
Lightning shock voltage – entering the building – breakdown of insulation – short circuit – discharge (ignition explosion)
Therefore, during thunderstorms, indoor electrical appliances should have their power plugs pulled out to prevent damage.
7) Counterattack effect:
Lightning strike lightning protection device – arrester lead grounding body high voltage – discharge to nearby pipelines, electrical equipment – insulation breakdown, pipeline burn-through (ignition explosion)
8) Person struck by lightning:
Struck by lightning – immediate respiratory paralysis – ventricular fibrillation – brain damage – death (internal burns)
The main hazards of lightning:
1) Building lightning protection levels:
Class 1: Buildings storing (producing) explosives;
Class 2: National key buildings;
Class 3: Provincial key buildings.
Most chemical companies are classified as Class 1 lightning protection buildings.
2) Lightning protection measures:
Lightning rod – tall buildings, flammable and explosive warehouses;
Lightning conductor – power transmission lines;
Surge arrester – near transformer electrical lines;
Lightning protection net – a network composed of various lightning protection facilities.
Chimneys, water towers, derricks, tall buildings, as well as houses containing flammable and explosive materials should be equipped with lightning rods.
The grounding of the lightning rod should be secure, and the grounding resistance generally should not exceed 10Ω.
Lightning rod protection:
Protection range – that is the protected space.
Overvoltage protection regulations stipulate: that the probability of lightning striking within the protected area does not exceed 0.1%)
The height of the lightning rod is h:
The ground protection radius is 1.5h;
The upper part is conical;
The downward protection range increases at h/2.
When multiple lightning rods work together, their external protection range is the respective boundary, and the protection range between two rods is related to the effective height of the lightning rod.
Lightning rod
High-voltage transformers and distribution equipment of 35kV and below should use independent lightning rods or lightning protection lines to prevent direct lightning strikes.
Protection of power transmission lines by lightning protection wires:
The actual lightning protection wire is suspended and offset by wind.
Protection angle – determined by the vertical line of the lightning protection wire and the angle between the lightning protection wire and the conductor.
The smaller the protection angle, the greater the shielding effect of the lightning protection wire on the conductor.
Generally, when the protection angle is less than 20°, there is a very small probability (less than 0.001) that thunder will strike around or hit directly on the conductors.
When it exceeds 30°, this probability significantly increases.
The general range for determining a suitable protective angle is between 20°-30°.
Transmission lines above 110KV need to be equipped with lightning protection lines along the entire route.
lightning arrester
Several commonly used lightning arresters.
Commonly used – gap type, valve type, tube type, zinc oxide; valve type is commonly used.
To prevent lightning and electromagnetic wave intrusion, it is generally installed on the high-voltage side of the distribution transformer.
Protection of arresters:
Working principle:
When the circuit is struck by lightning, under the action of atmospheric overvoltage, the external and internal gaps are successively punctured. The lightning current flows into the ground through the grounding wire, forming a short circuit between the circuit and the ground. At this time, the power frequency short-circuit current continues to flow, generating a strong electric arc that causes a large amount of gas to be generated on the inner wall of the tube and sprayed out from the pipe opening to extinguish the electric arc. The internal and external gaps of the tube restore insulation.
The internal diameter of the arrester tube will gradually expand after multiple uses, causing changes in the upper and lower limits of its interrupting current. Maintenance should be carried out when the internal diameter increases by 20-25%.
To prevent the misoperation of the arrester tube, it should have a minimum external clearance corresponding to the rated voltage of the transmission line.
Rated voltage (kilovolt) | 3 | 6 | 10 | 20 | 35 | 60 | 110 |
External spark gap (mm) | 8 | 10 | 15 | 60 | 100 | 200 | 350 |
Installed at: the incoming end of substation (requires release of a large amount of lightning current);
weak insulation point on the line.
Power capacitors do not need to use pipe-type surge arresters to prevent lightning intrusion waves, and it is also best not to use valve-type surge arresters.
Reason: When the overvoltage value of a common valve-type arrester is lower than the discharge voltage of the arrester, the impact overvoltage charges the capacitor. Until the overvoltage value reaches the discharge voltage of the arrester, the gap of the valve-type arrester is punctured, at which point the capacitor will discharge towards the arrester. Since there is very low impedance between capacitors and arresters, and both lightning current and capacitor discharge current are high in combination, it may damage both capacitors and arresters.
Currently, zinc oxide surge arresters with low residual voltage, large current capacity, fast response time, continuous operation capability, and long service life are widely used.
Lightning protection net
Constructed from reinforced concrete structure with steel mesh + external exposed lightning protection device
Lightning surge protection device – air terminal, down conductor, grounding device.
In order to prevent step voltage from harming people, the distance of lightning rod grounding devices from buildings, structures entrances and sidewalks should not be less than 3m.
The minimum distance between lightning rods and their down conductors and other conductors in the air generally should not be less than 3m.
Flash arrester: (device to receive direct lightning discharge)
Lightning rods, lightning belts, lightning nets, permanent metal objects on the roof and metal roof, and steel bars in concrete components.
When metal roofs are used as flashings, the metal sheets must not be insulated between them, and the overlap should be no less than 100mm. Class I buildings shall not be used as flashings.
The lower guide rail shall be made of round steel or flat steel, with no less than 2 pieces.
Grounding device: (grounding wire, grounding body)
1) Protection against direct lightning strikes:
Install independent lightning rods, overhead lightning conductors, and overhead lightning nets, all with independent grounding devices, with an impulse grounding resistance not greater than 10Ω;
For pipelines discharging explosive substances, their discharge outlets should be within the protection range of air terminals.
For discharge pipes that ignite emissions or do not reach explosive concentrations, the air terminal can only protect up to the pipe outlet.
2) Lightning Protection Induction:
The pipelines and metal objects inside the building should be connected to the base device of lightning protection induction;
The grounding device of lightning protection induction shall have a power frequency resistance not greater than 10Ω and shall be shared with the grounding device of electrical equipment.
There should be no less than 2 connections between the main ground wire of lightning protection induction and the grounding device, with a distance not exceeding 16-24 meters.
3) Prevention of lightning wave intrusion:
All important users use cable burial for installation, and connect the outer sheath of the metal cable to the grounding device for lightning protection at the household end;
For overhead pipelines entering and exiting buildings, they are connected to the grounding device for lightning protection.
1) The thickness of the top plate of the tank is >4mm and equipped with a breathing valve, so there is no need to install a lightning protection device;
2) If the thickness of the top plate of the tank is <4mm, even if it is equipped with a breathing valve, a lightning rod should be installed on the top of the tank;
3) Floating roof oil tanks do not need to be equipped with lightning protection devices;
4) Tanks storing non-metallic flammable liquids should have independent lightning rods and also take measures to prevent induction lightning;
5) Underground oil tanks with soil cover thickness >0.5m do not need to have lightning protection facilities, but exposed parts should have a good grounding.
6) Independent lightning rods should be installed for open storage tanks of flammable liquids, and their impulse grounding resistance should not exceed 5Ω.
7) Lightning protection measures for outdoor overhead pipelines:
For pipelines carrying flammable gases, grounding should be done at the beginning, end, branches, corners and every 100 meters along the straight line with a grounding resistance not greater than 30Ω. Grounding devices can use electrical equipment’s grounding devices.
LSP’s reliable surge protection devices (SPDs) are designed to meet the protection needs of installations against lightning and surges. Contact our Experts!
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