Lightning discharge may occur between or inside the clouds, or between the clouds and the ground; in addition, the internal surge brought by the use of many large-capacity electrical equipment will affect the power supply system (China Low Voltage Power Supply System Standard: AC 50Hz 220 / 380V) The impact of electrical equipment and the maintenance of lightning protection and surge protection have become the focus of attention.
The lightning discharge between the cloud layer and the ground is composed of one or several independent lightnings, each of which carries several currents with high amplitude and short duration. A typical lightning discharge will consist of two or three lightnings, each separated by approximately one-twentieth of a second. Most lightning currents fall between 10,000 and 100,000 amps, and their duration is generally less than 100 microseconds. Due to the use of large-capacity equipment and frequency conversion equipment inside the power supply system, it has caused increasingly serious internal surge problems. We summarize it as the effect of transient overvoltage (TVS). There is an acceptable scale of power supply voltage for any electrical equipment. Sometimes even a very narrow overvoltage surge can cause damage to the power supply or all of the equipment. This is the effect of transient overvoltage (TVS) damage. Especially for some sensitive microelectronic devices, sometimes a small surge may cause fatal damage.
Impact of power supply system surge
The origin of the power supply system surge is divided into external (cause of lightning) and internal (start and stop of electrical equipment and faults, etc.).
Lightning strikes to the ground may have two effects on the low-voltage power supply system:
(1) Direct lightning strike: Lightning discharge directly hits the components of the power system, injecting a large pulse current. The probability of an attack is relatively low.
(2) Direct lightning strike: Lightning discharge hits the ground around the equipment, and induces a moderate current and voltage on the power line.
The cause of the internal surge is related to the problems of starting and stopping the equipment inside the power supply system and the operation of the power supply network:
The internal power supply system will cause internal surges due to the start and stop of high-power equipment, line faults, switching operations, and the operation of frequency conversion equipment, which will have an unfavorable effect on the electrical equipment. In particular, microelectronic devices such as computers and communications have brought fatal impacts. Even if there is no permanent equipment damage, the abnormal operation and halt of the system will bring very serious results. For example, nuclear power plants, medical systems, large-scale factory automation systems, securities trading systems, telecommunications exchanges, network key, etc.
Direct lightning strikes are the most serious thing, especially if the lightning strikes close to the user's line entrance to squeeze the power line. When these things happen, the voltage of the squeezed transmission line will rise to hundreds of thousands of volts, generally causing flashovers. The interval of lightning current transmission on the power line is one kilometer or more, and the peak current around the lightning point can reach 100kA or more. The current of the low-voltage line at the user inlet can reach 5kA to 10kA per phase. In areas with frequent lightning activity, power facilities may experience severe lightning currents caused by direct lightning strikes several times a year. With regard to the use of underground power cables for power supply or in areas where thunder and lightning activity is infrequent, these things rarely happen.
The probability of direct lightning strikes and internal surges is high, and most of the damage to electrical equipment is related to it. Therefore, the key point of power surge prevention is the absorption and suppression of this part of surge energy.
Regarding the low-voltage power supply system, the maintenance of transient overvoltage (TVS) caused by surges is best completed by means of hierarchical maintenance. Starting from the entrance of the power supply system (for example, the total distribution room of the building), the surge energy is gradually absorbed, and the transient overvoltage is suppressed in stages.
[The first line of defense] should be a large-capacity power surge protection device that connects between each phase of the user's power supply system inlet line and the ground. It is generally required that this level of power supply maintenance has a maximum impact capacity of 100KA / phase or more, and the required constrained voltage should be less than 2.8kv. We call it CLASS I power supply surge protection device (referred to as SPD). These power supply anti-surge maintenance devices are specially designed to withstand the high current and high energy surge energy absorption of lightning and induction lightning strikes, and can shunt a large amount of surge current to the ground. They are only for constrained voltage (the maximum voltage that appears on the line when the inrush current flows through the SPD becomes the constrained voltage) for medium-level maintenance, because CLASS I-level maintainers mainly absorb large surge currents. They alone cannot thoroughly maintain sensitive electrical equipment within the power supply system.
[Second line of defense] It should be a power surge protector installed at the branch distribution equipment that supplies important or sensitive electrical equipment. These SPDs have a better absorption of the remaining surge energy that has passed through the consumer-supplied surge arrester, and have an excellent suppression effect on transient overvoltage. The maximum surge capacity required by the power surge protector used here is above 40KA / phase, and the required restraint voltage should be less than 2.0kv. We are called CLASS II power supply surge protector. The general user power supply system can meet the requirements for the operation of electrical equipment after the second level of maintenance.
[The last line of defense] A built-in power surge protector can be used in the internal power supply of the electrical equipment to completely eliminate the small transient transient overvoltage. The maximum surge capacity required by the power surge protector used here is 20KA / phase or lower, and the required constrained voltage should be less than 1.8kv. With regard to some particularly important or sensitive electronic equipment, a third level of maintenance is necessary. At the same time, it can also maintain the electrical equipment from the transient overvoltage that occurs inside the system.