Power surge protector (SPD) is suitable for TT, TN-S, TN-C, IT and other power supply systems with AC 50/60Hz and rated working voltage of 220V/380V, as well as low-voltage power and control systems in factories. It protects against indirect lightning, direct lightning impact, or other transient overvoltage surges. It is mainly suitable for surge protection requirements in residential, tertiary industry, and industrial and mining enterprises.

In modern society, SPD has become an indispensable device in electronic device lightning protection. Its function is to limit the instantaneous overvoltage that enters power lines and signal transmission lines within the voltage range that the equipment or system can withstand, or to discharge strong lightning currents into the ground, protecting the protected equipment or system from impact and damage. Generally speaking, the basic structure of SPD includes: discharge gap, inflatable discharge tube, varistor, suppression diode, and choke coil, etc.

1. Basic components of SPD

(1) Discharge gap (also known as protection gap): It is generally composed of two metal rods exposed to the air with a certain gap between them. One metal rod is connected to the power phase line L1 or neutral line (N) of the required protection equipment, and the other metal rod is connected to the grounding wire (PE). When an instantaneous overvoltage strikes, the gap is broken down, introducing a portion of the overvoltage charge into the ground, avoiding an increase in voltage on the protected equipment. The distance between the two metal rods in this discharge gap can be adjusted as needed, and the structure is relatively simple. Its disadvantage is poor arc extinguishing performance. The improved discharge gap is an angular gap, which has better arc extinguishing function than the former. It relies on the electric force F of the circuit and the rising effect of the hot air flow to extinguish the arc.

(2) Gas discharge tube: composed of a pair of separated cold cathode plates enclosed in a glass or ceramic tube filled with a certain amount of inert gas (Ar). In order to increase the triggering probability of the discharge tube, there is also an auxiliary triggering agent inside the discharge tube. This type of inflatable discharge tube has two pole and three pole types. The technical parameters of the gas discharge tube mainly include: DC discharge voltage Udc; Impulse discharge voltage Up (usually Up ≈ (2-3) Udc; Power frequency and current in; Impulse induced current Ip; Insulation resistance R (>109 Ω); Interpolar capacitance (1-5PF).

Gas discharge tubes can be used under DC and AC conditions, and the selected DC discharge voltage Udc is as follows: used under DC conditions: Udc ≥ 1.8U0 (U0 is the DC voltage for normal operation of the circuit) used under AC conditions: Udc ≥ 1.44Un (Un is the effective value of AC voltage for normal operation of the circuit) (3) Varistor: It is a metal oxide semiconductor nonlinear resistor mainly composed of ZnO, When the voltage acting on both ends reaches a certain value, the resistance is very sensitive to the voltage. Its working principle is equivalent to the series parallel connection of multiple semiconductor P-N. The characteristic of varistors is their good nonlinear characteristics (I=CU α Nonlinear coefficients in α）， Large current capacity (~2KA/cm2), low normal leakage current (10-7-10-6A), low residual voltage (depending on the working voltage and current capacity of the varistor), fast response time to instantaneous overvoltage (~10-8s), no freewheeling.

The technical parameters of varistors mainly include: varistor voltage (i.e. switching voltage) UN and reference voltage Ulma; Residual pressure Ures; Residual pressure ratio K (K=Ures/UN); Maximum flow capacity Imax; Leakage current; Response time. The usage conditions of varistors include: Varistor voltage: UN ≥ [(√ 2 × 1.2)/0.7] U0 (U0 is the rated voltage of the power frequency power supply) Minimum reference voltage: Ulma ≥ (1.8-2) Uac (used under DC conditions) Ulma ≥ (2.2-2.5) Uac (used under AC conditions, Uac is the AC working voltage).

The maximum reference voltage of a varistor should be determined by the withstand voltage of the protected electronic device, and the residual voltage of the varistor should be lower than the loss voltage level of the protected electronic device, that is, (Ulma) max ≤ Ub/K. In the above equation, K is the residual voltage ratio, and Ub is the loss voltage of the protected device. (4) Suppressing diode: With clamping and voltage limiting function, it operates in the reverse breakdown region. Due to its advantages of low clamping voltage and fast action response, it is particularly suitable for use as the last few protection components in multi-stage protection circuits. The volt ampere characteristics of the suppression diode in the breakdown region can be expressed as follows: I=CU α， In the above equation α Is the nonlinear coefficient, for Zener diodes α= 7-9, in avalanche diodes α= 5-7.

The technical parameters of the suppression diode mainly include: · rated breakdown voltage, which refers to the breakdown voltage at a specified reverse breakdown current (usually lma). This is similar to the rated breakdown voltage of Zener diodes, which is generally in the range of 2.9V to 4.7V, while the rated breakdown voltage of avalanche diodes is usually in the range of 5.6V to 200V· Maximum clamping voltage: It refers to the highest voltage that occurs at both ends of a tube when it passes through a specified waveform of high current· Pulse power: It refers to the current waveform specified (such as 10/1000) μ s) Below, the product of the maximum clamping voltage at both ends of the tube and the equivalent current in the tube.

·applied at both ends of a tube in the reverse leakage zone, at which the tube should not break down. This reverse displacement voltage should be significantly higher than the peak operating voltage of the protected electronic system, which means it cannot be in a weak conduction state during normal system operation· Maximum leakage current: It refers to the maximum reverse current flowing through a tube under the action of reverse displacement voltage· Response time: 10-11s. (5) Choke coil: Choke coil is a common mode interference suppression device with ferrite as the magnetic core. It consists of two coils of the same size and number of turns symmetrically wound on the same ferrite ring magnetic core, forming a four terminal device. It has a suppressive effect on the large inductance of common mode signals, but has little effect on the small leakage inductance of differential mode signals. The use of choke coils in balanced transmission lines can effectively suppress common mode interference signals (such as lightning interference), without affecting the normal transmission of differential mode signals on the line.

This type of choke coil should meet the following requirements during production: the wires wound on the coil core should be insulated from each other to ensure that there is no breakdown or short circuit between the turns of the coil under instantaneous overvoltage· Do not saturate the magnetic core when a large instantaneous current flows through the coil· The magnetic core in the coil should be insulated from the coil to prevent breakdown between the two under instantaneous overvoltage· The coil should be wound in a single layer as much as possible, which can reduce the parasitic capacitance of the coil and enhance its ability to withstand instantaneous overvoltage.

2. Working principle of SPD

Power surge protectors are divided into two types: explosion-proof box type and modular type. We all use a varistor with excellent nonlinear characteristics. Under normal circumstances, the surge protector is in an extremely high resistance state, with almost zero leakage current, ensuring the normal power supply of the power system. When the power system experiences surge overvoltage, the power surge protector immediately conducts within nanoseconds, limiting the amplitude of the overvoltage within the safe operating range of the equipment, and releasing the surge energy into the ground. Subsequently, the surge protector quickly changed to a high resistance state, thus not affecting normal power supply.

In terms of the classification of power surge protectors, there are three levels: the first level can discharge direct lightning current, or discharge the huge energy conducted when the power transmission line is directly struck by lightning. The first level of protection should be a three-phase voltage switch type power surge protector, and its lightning current flow should not be less than 60KA. Generally used for general distribution.

The purpose of the second level is to further limit the residual surge voltage through the first level lightning arrester to 1500-2000V and implement equipotential connection between LPZ1-LPZ2. When the power lightning arrester for the output of the distribution cabinet line is used as the second level protection, it should be a voltage limiting power lightning arrester, and its lightning current capacity should not be less than 20KA. The third level objective is to ultimately protect the equipment by reducing the residual surge voltage to below 1000V. As the third level protection, it should be a series voltage limiting power lightning arrester, and its lightning current capacity should not be less than 10KA. Generally used for terminal distribution equipment.

Different distribution systems should choose corresponding surge protectors, which can be divided into TN (TN-S, TN-C, TN-C-S), IT, TT. In short, it is necessary to connect surge protectors to electronic devices. The internal components of modern electrical equipment such as computers, electronic instruments, and experimental instruments are very precise and complex, and they are also particularly sensitive to surges. Surge is an inherent phenomenon in the power system, therefore, surge protectors are essential in daily life and work.