Principle of power surge protection
Surge Protective Device (SPD), also known as Surge Protector, is a nonlinear protective device used in live systems to limit transient overvoltage and guide discharge surge current. It is used to protect electrical or electronic systems with low voltage withstand levels from lightning strikes, electromagnetic pulses or operational overvoltage damage. In recent years, electronic information systems (such as television, telephone, communication, computer networks, etc.) have developed rapidly, and a large number of electronic information devices have emerged and popularized. These types of systems and equipment are often expensive and important, with low operating voltage and withstand voltage levels, making them highly susceptible to lightning electromagnetic pulses. Therefore, SPD should be used for overvoltage protection.
Due to the different standards followed by different countries, the specifications of products are not unified, and the identification of parameters also has their own emphasis, which is far inferior to other electrical product specifications, which brings great inconvenience to design and selection. In engineering design, common brands can be mainly divided into domestic products, European products, and American products based on their place of origin. The parameter settings of domestic products are chaotic, with diverse specifications and high residual voltage. Some of the standardized product models are set to imitate European products, while others follow national standard parameters. Most products are labeled with In and Imax. Due to the low requirements of domestic products for application sites, low building levels, and high equipment withstand voltage values, some parameter requirements can be appropriately relaxed.
European products are generally labeled with the maximum discharge current, and the product model is also set based on this parameter. For example, a well-known European brand XXX65 and XXX40, where the values of 65 and 40 are Imax. However, Chinese standards clearly stipulate that the nominal discharge current In should be used for selection, which is an awkward situation encountered in engineering design at present. After checking the product information, the In value of XX65 does not exceed 20kA, and the In value of XX40 does not exceed 15kA. If the recommended values of GB50343 are followed, these two products can only be used for three-level protection at the end of the equipment. However, in actual design, they are installed on the first and second levels, which is clearly inconsistent with the selection parameters of national standards, and the residual voltage is high. Ordinary models generally exceed 1200V. Once the wiring environment is not good, it is easy to exceed the equipment's withstand voltage value. Generally, European products have a relatively small Uc value and are opportunistic in labeling line voltage, making it easier to mislead when selecting models.
The working principle of SPD
The surge protector is suitable for the protection of 220/380V low-voltage power supply and is a nonlinear component. According to IEC standards, the surge protector is a device mainly used to suppress the overvoltage and overcurrent of the transmission line. The basic requirement for a surge protector to play a protective role is to withstand the expected lightning current and effectively extinguish the power frequency continuous current generated after the lightning flow through the maximum clamping voltage of the surge. It limits the instantaneous overvoltage that enters the power line or signal transmission line to the voltage range that the equipment or system can withstand, or discharges strong lightning current into the ground to protect the protected equipment or system from damage caused by impact.
The types and structures of surge protectors vary according to their different uses, but they contain at least one nonlinear voltage limiting element. Commonly used surge protectors include MOVs (MetalOxideVaristors) and gas discharge tubes. Electric surges contain powerful energy and cannot be stopped. For this reason, the strategy to protect sensitive electrical equipment from surge damage is to divert the surge from the equipment and then flow into the ground.
The surge protector MOV consists of three parts: a metal oxide material in the middle, connected by two semiconductors to the power supply and ground wire. When a surge occurs, the MOV immediately acts with a response time of 1-3 nanoseconds. The "V" in MOV is a rheostat. At the moment of response, the resistance of the MOV drops from its maximum value to almost zero ohms, and overcurrent flows into the ground through the MOV. The protected electrical equipment continues to operate at normal operating voltage. Its semiconductor components have the property of changing resistance with voltage changes. When the voltage drops below a specific value, the movement of electrons in a semiconductor generates high resistance. On the contrary, when the voltage exceeds this specific value, the motion of electrons will change, and the semiconductor resistance will decrease to nearly zero ohms. The voltage is normal, and the surge protector MOV is idle on the side, without affecting the power line.
The indicators for the advantages and disadvantages of surge protective devices (MOVs) are: (1) Clamping voltage: represents the voltage value that will cause the MOVs to connect to the ground wire. The lower the clamping voltage, the better the protection performance. (2) Energy absorption/dissipation capacity: This nominal value represents how much energy the surge protector can absorb before being burned down, in joules. The higher the value, the better the protection performance. (3) Response time: Surge protectors do not immediately disconnect, and there will be a slight delay in their response to surges.
Another common surge protection device is a gas discharge tube. These gas discharge tubes have the same function as MOVs, moving excess current from the live wire to the ground wire, and achieving this function by using inert gas as a conductor between the two wires. When the voltage is within a specific range, the composition of the gas determines that it is a poor conductor. If the voltage surges and exceeds this range, the strength of the current will be sufficient to ionize the gas, making the gas discharge tube a very good conductor. It will conduct current to the ground wire until the voltage returns to normal levels, and then become a faulty conductor
