High-voltage arrester
Core Role and Working Principle
'Voltage limiting' rather than 'lightning protection':
The name of lightning arrester can easily be misunderstood as it can "avoid" lightning. In fact, its core function is to limit the amplitude of overvoltage applied to the device ("voltage limiting") and safely discharge the large current generated by overvoltage to the ground.
It is connected in parallel between the protected equipment (such as a transformer) and the ground.
Nonlinear characteristics:
The core component is a resistor with nonlinear volt ampere characteristics (modern high-voltage lightning arresters mainly use zinc oxide resistors).
During normal operation: The operating voltage of the system is much lower than the operating voltage of the lightning arrester (reference voltage or transition voltage), and the lightning arrester exhibits extremely high resistance (approximately open circuit state), with only a very small leakage current (usually microampere level) flowing, which hardly affects the operation of the system.
When overvoltage occurs:
- When an overvoltage wave invades and its amplitude exceeds the operating voltage threshold of the lightning arrester.
- The resistance of zinc oxide resistors will instantly decrease sharply (becoming a low resistance state).
- The lightning arrester quickly conducts, releasing the energy (high current) generated by overvoltage into the ground.
- At the same time, clamp the voltage at both ends of the lightning arrester (i.e. the voltage applied to the protected equipment) below a safe level that the equipment can withstand (this voltage is called residual voltage).
After the overvoltage disappears: When the system voltage returns to the normal operating level, the resistance of the lightning arrester automatically returns to a very high state, restoring the "open circuit" state and automatically cutting off the power frequency continuous current (different from the old silicon carbide lightning arrester that needs to cut off the power frequency continuous current), waiting for the next action.
Key components
Zinc oxide resistor:
The core of modern high-voltage lightning arresters. It is mainly composed of zinc oxide (ZnO) and sintered with various metal oxide additives.
Has excellent nonlinear volt ampere characteristics and outstanding protective performance.
No freewheeling: When the system voltage is restored, there is almost no power frequency freewheeling, and there is no need for a series gap to cut off the power frequency current, greatly improving reliability and protection characteristics. Therefore, it is also known as gapless metal oxide surge arrester.
Insulated jacket:
Provide external insulation to prevent internal components from discharging the casing and withstand the effects of system operating voltage and external environment (such as rain and pollution). The material is usually ceramic (porcelain sleeve) or organic composite insulation material (silicone rubber).
Voltage equalizing ring: (common on high voltage level lightning arresters)
Installed at or near the top of the lightning arrester.
The function is to improve the voltage distribution along the insulation jacket at the high voltage end, and prevent partial discharge or flashover caused by electric field concentration.
Pressure relief device: (in ceramic bushing lightning arrester)
It is a weak link (such as a thin metal sheet).
If there is a malfunction inside the lightning arrester causing a sharp increase in pressure, the device will open before the porcelain bushing explodes, releasing pressure and preventing the porcelain bushing from exploding and causing more serious harm.
Online monitor:
Usually installed on the grounding wire of the lightning arrester.
Used for real-time monitoring of small leakage currents (total current, resistive current component) of lightning arresters during normal operation.
The abnormal increase in leakage current is an important warning signal for moisture, aging, or damage inside the lightning arrester.
Main application scenarios (location in the power system)
Substation entrance/exit: Protect the entire substation equipment from lightning wave damage caused by line intrusion.
At the outlet of the high and low voltage sides of the transformer: directly protect the most important main transformer.
Generator outlet: Protect the generator from lightning or overvoltage damage during operation.
Near GIS equipment: protect gas insulated switchgear.
Key locations of transmission lines, such as important crossings and large towers, are used to limit the overvoltage level of the lines.
Near the capacitor bank: limit operation overvoltage and protect the capacitors.
The main advantages of high-voltage lightning arresters (compared to old-fashioned lightning arresters with series gaps)
Excellent nonlinear characteristics: steep volt ampere characteristics and low protection level (residual voltage).
No continuous current: automatically restores high impedance state after action, no power frequency continuous current problem, high reliability.
Large current capacity: Zinc oxide valve plates can absorb very large amounts of overvoltage energy.
Fast response speed: responds quickly to rapidly rising steep waves.
Simple structure: no gaps, compact structure.
Easy to monitor: By monitoring the leakage current under operating voltage, the operating status can be effectively evaluated.
Important technical parameters
Rated voltage: The maximum continuous operating voltage that the design can withstand. Requires a maximum system continuous operating voltage greater than the installation point.
Nominal discharge current: used to classify lightning arrester levels (such as 5kA, 10kA, 20kA, etc.), indicating its ability to discharge typical lightning overcurrent.
Protection level:
Lightning impulse residual voltage: The residual voltage measured at the nominal discharge current. It is the foundation of insulation coordination for protected equipment.
Operating impulse residual voltage: The residual voltage measured under a specific waveform (such as 250/2500 µ s) of operating impulse current.
Steep wave impulse residual voltage: The residual voltage measured under an impulse current with an extremely steep rising edge (such as 1 µ s).
Continuous operating voltage: The effective value of the power frequency voltage (less than or equal to the rated voltage) that is allowed to be applied to both ends of the lightning arrester for a long time.
DC reference voltage: The terminal voltage of the lightning arrester measured at a DC current of 1mA is an important parameter for checking the aging status of the valve plate.
Leakage current at 0.75 times DC reference voltage: an important indicator for measuring the performance of lightning arresters under aging conditions.
summary
High voltage lightning arrester is the guardian of the safe operation of the power system. It utilizes the nonlinear characteristics of zinc oxide resistors to not interfere with system operation during normal times. Once a dangerous overvoltage (such as lightning strikes or switch operations) strikes, it quickly "acts" to release the huge energy generated by the overvoltage into the ground, while limiting the voltage applied to important equipment (such as transformers) to a safe level (residual voltage), thereby protecting equipment insulation from breakdown and damage, ensuring the reliability and continuity of power supply.