Thunder and Lightning Protection Equipment

Thunder and Lightning Protection Equipment

Created by: Glen Zhu | Updated Date: May 29th, 2024

Thunder and Lightning Protective Equipment

1. Thunder and lightning

1.1 Concept

  • Lightning discharge caused by charged thunderclouds
  • Most discharges occur between thunderclouds – not dangerous
  • A few discharges occur between thunderclouds and the ground – dangerous
  • Most thunderclouds that discharge to the ground carry negative charges, measured at 75% to 90%

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.

2. Lightning protection device

  • Lightning rod, lightning conductor
  • Surge arrester
  • Grounding device

3. Lightning rod

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.

4. Lightning conductor

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.

5. Lightning arrester

1) Principle

  • The lightning arrester is an electrical device used to limit the overvoltage caused by lightning or internal overvoltage caused by the operation of the circuit.
  • The protection principle of the lightning arrester is different from that of a lightning rod. It is essentially a discharge device connected in parallel near the protected equipment. When the applied voltage exceeds the discharge voltage of the lightning arrester, it will discharge first, limiting the development of overvoltage and protecting other electrical equipment from breakdown damage.

2) Development and Classification

  • Protection gap
  • Tubular arrester
  • Valve-type arrester
  • Zinc oxide arrester

3) Illustration of the protective effect of various surge arresters

4) Protection clearance

  • Air gaps are connected in parallel with equipment, and discharge occurs when overvoltage causes air gap breakdown.
  • The upper limit of the time-voltage characteristic of the gap should be lower than the lower limit of the impulse voltage-time characteristic of the protected equipment, with a certain margin.
  • There is no special arc extinguishing device for angular gaps. After overvoltage disappears, power frequency arc current (continuation current) will occur under working voltage, which often cannot self-extinguish and will cause circuit tripping. It can be used in conjunction with automatic reclosing.
  • When activated, it produces steep waveforms that cause serious harm to longitudinal insulation in high-voltage equipment such as motors and transformers.
  • It is mainly used to protect distribution systems, lines, and incoming sections of substations.

5) Pipe-type lightning arrester

  • Two gaps in series, when overvoltage arrives, the two gaps break down and discharge;
  • After the overvoltage disappears, under working voltage, it generates a power frequency continuous current due to heating which causes gas production in the gas-producing tube and blows out arcs. It can extinguish larger power frequency arcs.
  • The action produces very steep transient waves, causing serious harm to the longitudinal insulation of high-voltage equipment such as motors and transformers;
  • The volt-second characteristics of gap discharge are very steep and difficult to use for protecting insulation of devices with relatively flat volt-second characteristics;
  • Discharge characteristics are greatly affected by atmospheric conditions;
  • Mainly used for protection of incoming sections in substations.

(6) Valve-type surge arrester

Composition: Series connection of air gap and nonlinear resistor (valve plate)

  • Spark gap:

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.

  • Valve plates:

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.

(7) Zinc oxide surge arrester

  • Using zinc oxide resistors as valve sheets, zinc oxide has excellent nonlinearity.

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.

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