Analysis of Function of Surge Protector in Power Supply System

Impact of power system surge

Sources of surges in the power supply system are divided into external (lightning causes) and internal (electrical equipment start-stops and faults, etc.). Lightning strikes against lightning may act on the low voltage supply system in two ways:

(1) Direct Lightning Strike: Lightning discharge directly hits parts of the power system and injects large pulse currents. The probability of occurrence is relatively low.

(2) Indirect Lightning Strike: Lightning discharge hits the ground near the equipment and induces moderate currents and voltages on the power line.

The reason for the internal surge is related to the internal equipment start-up and shutdown of the power supply system and the failure of the power supply network operation. The internal power supply system will bring about due to the start and stop of high-power equipment, line faults, switching operations, and operation of inverter equipment. Internal surges have an adverse effect on electrical equipment. In particular, microelectronic devices such as computers and telecommunications have caused fatal shocks. Even if it does not cause permanent equipment damage, system operation abnormalities and stalls can have serious consequences. Such as nuclear power plants, medical systems, large-scale factory automation systems, securities trading systems, telecommunications bureau switches, network hubs. Direct lightning strikes are the most serious incidents, especially if a lightning strike hits an overhead transmission line near the user's entrance. In the event of these events, the overhead power line voltage will rise to hundreds of thousands of volts, usually causing insulation flashovers. The lightning current is transmitted over the power line at a distance of one kilometer or more, and the peak current near the lightning strike point can reach 100 kA or more. The current of the low-voltage line at the user's inlet can reach 5kA to 10kA per phase. In regions where lightning activity is frequent, power facilities may experience severe lightning currents caused by lightning strikes on several occasions each year. However, these incidents rarely occur in areas that use underground power cables or where lightning activity is infrequent. The probability of indirect lightning strikes and internal surges is high, and most of the electrical equipment damage is related to it. Therefore, the focus of power surge protection is to absorb and suppress this part of surge energy.

Surge protection of power supply system

For low-voltage power supply systems, surge-induced transient over-voltage (TVS) protection is best done in a graded protection manner. From the entrance of the power supply system (such as the building's main power distribution room), the absorption of surge energy is gradually carried out, and the transient overvoltage is suppressed in stages.

The [first line of defense] should be a large-capacity power surge protection device connected between the incoming phase of the user's power supply system and the ground. It is generally required that the power protection device of this stage has a maximum impact capacity of 100 kA/phase or more, and the required limiting voltage should be less than 2800 V. We call it CLASSI power surge protector (SPD). These power surge protection devices are designed to withstand high currents and high energy surge energy absorption due to lightning and induced lightning strikes. They can divert a large amount of inrush current to the earth. They only provide limited voltage (the maximum voltage appearing on the line when the inrush current flows through the SPD becomes the limiting voltage) for medium-level protection, because the CLASSI-class protector is mainly absorbing large inrush currents. They alone cannot completely protect the sensitive electrical equipment inside the power supply system.

The [second line of defense] should be a power surge protector installed at a branch distribution facility that supplies power to important or sensitive electrical equipment. These SPDs provide better absorption of residual surge energy that has passed through the user-powered inlet surge arrester and have excellent suppression of transient overvoltages. The power surge protection device used here requires a maximum impact capacity of 40 kA/phase or higher, and the required limit voltage should be less than 2000V. We call it CLASSII power surge protector. The general user power supply system can reach the second level of protection to meet the requirements for the operation of electrical equipment.

[Last line of defense] A built-in power surge protector can be used in the internal power supply part of the electrical equipment to achieve the goal of completely eliminating the transient transient over voltage transients. The power surge protection device used here requires a maximum impact capacity of 20 kA/phase or lower, and the required limit voltage should be less than 1800V. For some particularly important or particularly sensitive electronic devices, it is necessary to have third-level protection. At the same time, it can also protect the power equipment from the transient overvoltage generated inside the system.