Views: 0 Author: Site Editor Publish Time: 2023-09-11 Origin: Site
As the name suggests, it is an instantaneous overvoltage that exceeds the normal operating voltage. It is called transient pulse voltage, transient overcurrent, surge or surge, etc. It is a short-term current and voltage fluctuation that occurs in the circuit，a violent pulse in an electrical circuit that usually lasts about a millionth of a second. Surge also refers to a voltage change in which the effective value of the power grid output voltage is greater than 110% of the rated value, and its duration ranges from one cycle (20ms) to several cycles.
1. Generation of surge
There are two categories: external surges and internal surges.
External surge: the main source is lightning;
1. Lightning surge overvoltage
Surges caused by lightning strikes are the most harmful. During lightning discharges, dangerous overvoltages may occur within a range of 1.5 to 2km centered on the lightning strike. The (external) surge caused by lightning strikes is characterized by a single-phase pulse type with huge energy. The voltage of an external surge can quickly rise from a few hundred volts to 20KV in a few microseconds, and can be transmitted over a considerable distance. According to statistics, power surges outside the system mainly come from lightning and other system impacts, accounting for about 20%.
(1) Induced lightning surge overvoltage: The high-speed changing electromagnetic field generated by lightning strikes and the electric field radiated by lightning act on the conductor, inducing a very high overvoltage. This type of overvoltage has a steep front and rapidly decays.
(2) Direct lightning surge overvoltage: Direct lightning strikes on the power grid. Due to the huge instantaneous energy and extremely destructive power, there is no equipment that can protect direct lightning strikes.
(3) Lightning strike conducted surge overvoltage: conducted from distant overhead lines. Since the equipment connected to the power grid has different suppression capabilities for overvoltage, the conducted overvoltage energy weakens with the extension of the line.
(4) Oscillating surge overvoltage: The power line is equivalent to an inductor, and there is distributed capacitance between the earth and nearby metal objects, forming a parallel resonant circuit. In TT and TN power supply systems,
When a single-phase ground fault occurs, the high-frequency component resonates and generates a high overvoltage on the line, which mainly damages the secondary instrument.
The impact of lightning strikes and surges is an important risk factor that is often ignored. Lack of preventive measures will lead to serious consequences, such as fires, important equipment shutdowns, and misoperation. Therefore, lightning and surge protection measures are an important part of the safety plan, because modern society is a coordinated and coordinated whole.
Lightning and surge protection should be taken into account in the early stages of project planning, making early implementation much easier. Subsequent transformation will be difficult to achieve and involve very high costs of talent, capital and time.
IEC 62305-2 stipulates risk analysis methods. Predictive and advanced risk assessment reports will help investors make more accurate and reasonable decisions. Efficiently control predictable risks.
National standard GB 50057 "Design specifications for lightning protection of buildings" / GB 50343 "Technical specifications for lightning protection of building electronic information systems"
IEC 62305 / GB/T 21714 The selection of lightning and surge protection solutions and shielding measures in compliance with the standard. In terms of system planning, the problem of preventing property damage and endangering human life is to take into account the human orientation, and there are many more factors, For example: It is also important to avoid failures in electrical and electronic systems,
Therefore, large-scale enterprise or government project evaluations attach great importance to Part 4 of IEC 62305/GB/T 21714!
To sum up, it can be seen from the standard requirements that lightning protection design is crucial, and risk assessment must be carried out in the early stage of project implementation.
