Created by: Glen Zhu | Updated Date: May 29th, 2024
1.1 Concept
Understand the following points:
1) The essence of lightning discharge from a thundercloud to the ground is the sudden release of charge from the thundercloud to the ground.
2) The potential of a struck object depends on the product of lightning current and the impedance of the struck object.
3) From an electrical perspective, it is equivalent to the action process of a current source.
1.2 Lightning discharge process
1) Pilot;
2) Main discharge;
3) Residual light;
Maximum current and maximum rate of current increase: overvoltage, electromotive force, explosive force.
Long-term current during the afterglow stage: lightning thermal effect.
1.3 Equivalent circuit of lightning and amplitude of lightning current
The charge line density in the leader channel is σ, and the main discharge speed is VL. When the lightning resistance of the ground is zero, the current flowing through the channel is σVL.
1) Lightning current iL: When Zj=0, the current flows through the struck object. Therefore: iL = σVL.
2) The process of lightning striking an object can be seen as a process where a lightning current with a value of iL/2 propagates along a channel with wave impedance Z0 towards the struck object.
1.4 Statistical Analysis of Thunderstorms
1) Lightning current amplitude: The probability distribution is as follows, where IL represents the lightning current amplitude (kA) and P represents the probability of exceeding IL.
2) Lightning current waveform: the average tail of the wave is 40 μs, the head of the wave is 1-4 μs, and the average rise steepness is as shown on the right.
3) Thunderstorm days T (thunderstorm hours): The number of days (hours) with thunderstorms in a year.
4) Ground lightning density γ: The number of times the ground is struck by lightning per square kilometer on each thunderstorm day.
5) Number of lightning strikes on transmission lines N: The number of lightning strikes on a line with a height of h meters and a length of 100 km per year.
Principle: To attract lightning and safely guide the lightning current into the ground, thus protecting the equipment.
Protection range: Refers to a spatial range with a 0.1% probability of being struck by lightning, in a conical shape.
Application: Protecting power plants and substations.
Principle: To attract lightning and safely guide the lightning current into the ground, thereby protecting equipment.
Protection angle: The α angle in the figure. A range of 20~30 degrees can be considered within the protection scope of the lightning rod. For 500kV lines, it is required to be less than 15 degrees.
Application: Protecting transmission lines and 500kV substations.
1) Principle
2) Development and Classification
3) Illustration of the protective effect of various surge arresters
4) Protection clearance
5) Pipe-type lightning arrester
Composition: Series connection of air gap and nonlinear resistor (valve plate)
1) Composed of many small gaps connected in series;
2) Isolate the valve plate from the bus during normal operation to prevent burning of the valve plate under power frequency current;
3) Power frequency arc is extinguished by the natural arc extinction ability of the gap; magnetic blow type gap (connected with inductance) has stronger arc extinction ability than ordinary gap;
4) To ensure uniform and stable recovery voltage on each gap, resistors are connected in parallel next to the gap.
5) When overvoltage arrives, auxiliary gaps break down to prevent inductive current blocking.
1) Composed of many silicon carbide (SiC) valve plates in series;
2) When lightning strikes, it should be able to divert the lightning current into the ground, requiring low resistance at this time;
3) When the lightning strike disappears, it should be able to limit the power frequency continuous flow, extinguishing the arc when the air gap crosses zero for the first time at power frequency, requiring high resistance at this time;
Parameters:
1) Residual voltage: The pressure drop generated on the valve plate when the surge current passes through it; 35-220kV power grids are designed for 5kA lightning current, and 330kV power grids are designed for 10kA; it is required to be less than the equipment’s impulse withstand voltage.
2) Current capacity: The ability to pass current; for example: For a 330kV power grid, ordinary valve plates should be able to withstand 5kA surge current and 100A half-wave frequency of 20 times each.
3) Arc extinction voltage: Refers to the highest frequency voltage applied to the arrester that can ensure arc extinction at the first zero crossing point of the working frequency continuous flow; it is required to be higher than the highest possible working frequency voltage that may appear on the busbar.
4) Protection ratio: The ratio of residual voltage to arc extinction voltage; a smaller protection ratio indicates lower residual voltage or higher arc extinction voltage, which means better protective performance of the arrester.
Parameter:
1) Starting action voltage: transition voltage, usually the voltage under 1mA; approximately 105% to 115% of the peak value of the maximum allowable operating voltage.
2) Voltage ratio: refers to the ratio of residual voltage during high current flow to the starting action voltage; a smaller voltage ratio indicates lower residual voltage and better protective performance.
3) Charging rate: the ratio of maximum continuous operating peak voltage to starting action voltage; a higher charging rate indicates better stability and stronger aging resistance for lightning arresters. Its limit value is 1.
4) Protection ratio: the ratio of residual voltage at nominal discharge current to maximum continuous operating peak voltage (ratio of voltage ratio to charging rate). The smaller, the better.
Feature:
1) No gap: Under power frequency voltage, the resistance is high and the current is small, so there is no need for a gap, avoiding problems caused by gaps. The response characteristics are good under steep waves.
2) No follow current: Under power frequency voltage, the resistance is high and the current is small, so there is no need for a large heat capacity.
3) Low overvoltage stress on equipment: Due to no gap, zinc oxide varistors discharge throughout the entire overvoltage process, while valve-type surge arresters only start to discharge when the breakdown occurs in the gap.
4) High current-carrying capacity: Zinc oxide has a large current-carrying capacity which can limit internal over-voltages.
5) Due to no gap and high current-carrying capacity, it has a small volume, lightweight, and simple structure; due to no follow current, it can be used in DC systems.
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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