The working principle of surge protectors

Thread Content

For those working in the electrical field, it’s inevitable to come across surge protectors at work, but some people don’t know much about them. Principle: Under normal operating conditions, the lightning protection module is in a high-impedance state. When lightning strikes the power supply lines or when transient overvoltages occur during operations, the lightning protection module will switch on immediately with a response time of nanoseconds, limiting the lightning overvoltage or transient overvoltage to within the voltage range that the electrical equipment can tolerate, thereby ensuring the normal operation of the electronic devices. Once the lightning overvoltage or transient overvoltage subsides, the lightning protection module quickly returns to its high-impedance state, without affecting the normal power supply to the electrical grid. Working principle and structure of surge protectors (SPD)? A surge protection device is an essential component for protecting electronic equipment from lightning strikes; it was formerly referred to as a \"lightning arrester\" or \"overvoltage protector\", with the English abbreviation SPD. The function of a surge protector is to limit the instantaneous overvoltages that intrude into power lines and signal transmission lines to within the voltage range that the equipment or system can tolerate, or to discharge intense lightning currents into the ground, thereby protecting the protected equipment or system from damage caused by such surges. The types and structures of surge protectors vary depending on their intended use, but they must include at least one nonlinear voltage limiting element. The basic components used in surge protectors include: discharge gaps, gas-discharge tubes, varistors, suppression diodes, and chokes. I. Classification of SPDs: 1. By working principle: 1. Switching type: Its working principle is that it presents high impedance when there is no instantaneous overvoltage; however, once an instantaneous overvoltage caused by lightning occurs, its impedance drops sharply to a low value, allowing the lightning current to pass through. Devices used in such apparatus include discharge gaps, gas discharge tubes, thyristors, etc. 2. Voltage-limiting type: Its working principle is that it presents high impedance in the absence of instantaneous overvoltage, but its impedance decreases as the surge current and voltage increase; its current-voltage characteristics are highly nonlinear. Devices used in such devices include zinc oxide, varistors, suppression diodes, avalanche diodes, etc. 3. Shunt or choke type: Shunt type: It is connected in parallel with the equipment to be protected, presenting a low impedance to lightning pulses while exhibiting a high impedance at normal operating frequencies. Choke type: Connected in series with the equipment to be protected, it presents high impedance to lightning pulses and low impedance at normal operating frequencies. Devices used in such apparatus include: chokes, high-pass filters, low-pass filters, 1/4 wavelength short circuits, etc. By purpose: (1) Power protectors: AC power protectors, DC power protectors, switching power supply protectors, etc. (2) Signal protectors: low-frequency signal protectors, high-frequency signal protectors, antenna feed protectors, etc. II. Basic components of SPD and their working principles: 1. Discharge gap (also known as protection gap): It generally consists of two metal rods separated by a certain distance and exposed to the air. One of these rods is connected to the live wire L1 or the neutral wire (N) of the equipment that needs protection, while the other rod is connected to the ground wire (PE). When an instantaneous overvoltage occurs, the gap is broken down, allowing some of the excess voltage charge to be discharged into the ground, thereby preventing an increase in voltage across the protected equipment. The distance between the two metal rods in this discharge gap can be adjusted as needed; it has a simple structure, but its drawback is poor arc extinguishing performance. The improved discharge gap is of the angular type, and its arc-extinguishing capability is better than that of the previous type; the arc is extinguished due to the electrodynamic force F acting on the *circuit as well as the upward movement of hot air currents. 2. Gas discharge tube: It consists of a pair of cold cathodes separated from each other, enclosed within a glass or ceramic tube filled with an inert gas (Ar). To increase the triggering probability of the discharge tube, there is also a trigger aid inside it. Such gas discharge tubes come in two-terminal and three-terminal types. The main technical parameters of gas discharge tubes include: the DC discharge voltage Udc; the impulse discharge voltage Up (under normal conditions, Up≈(2～3)Udc); the rated current at power frequency In; and the rated current under impulse conditions Ip ； Insulation resistance R (>109Ω) ； The inter-electrode capacitance (1–5 PF) gas discharge tubes can be used in both DC and AC conditions. The selected DC discharge voltage Udc is as follows: for use in DC conditions, Udc ≥ 1.8U0 (where U0 is the DC voltage at which the circuit operates normally); for use in AC conditions, Udc ≥ 1.44Un (where Un is the effective value of the AC voltage at which the circuit operates normally). 3. Varistor: ??? It is a nonlinear semiconductor resistor made of metal oxides, with ZnO as its main component; when the voltage applied across it reaches a certain level, its resistance becomes highly sensitive to that voltage. Its working principle is equivalent to the series and parallel combination of multiple semiconductor P-N junctions. Varistors are characterized by good nonlinear behavior (the nonlinear coefficient α in I=CUα), a high current-carrying capacity (~2 KA/cm2), low leakage current under normal conditions (10-7 to 10-6 A), low residual voltage (depending on the operating voltage and current-carrying capacity of the varistor), a fast response time to instantaneous overvoltages (~10-8 s), and no backflow of current. The main technical parameters of varistors are: the voltage at which breakdown occurs (i.e., the switching voltage) UN, and the reference voltage Ulma ； Residual pressure Ures ； Residual pressure ratio K (K=Ures/UN) ； Maximum flow capacity Imax ； Leakage current ； Response time. The operating conditions for varistors are as follows: Voltage rating: UN ≥ U0 (where U0 is the rated voltage of the power supply at line frequency). Maximum voltage: Ulma ≥ (1.8–2)Uac when used in DC conditions; Ulma ≥ (2.2–2.5)Uac when used in AC conditions (where Uac is the AC operating voltage). The maximum voltage of the varistor should be determined based on the voltage tolerance of the electronic equipment to be protected, such that the residual voltage of the varistor remains below the damage threshold of that equipment, that is, (Ulma)max ≤ Ub/K, where K is the residual voltage ratio and Ub is the damage voltage of the protected equipment. 4. Clamping diode: ??? The clamping diode has a voltage-clamping function; it operates in the reverse breakdown region. Thanks to its advantages of low clamping voltage and fast response time, it is particularly suitable as the final protection element in multi-stage protection circuits. The volt-ampere characteristic of a suppression diode in its breakdown region can be expressed by the formula I=CUα, where α is the non-linearity coefficient; for Zener diodes, α ranges from 7 to 9, while for avalanche diodes, it ranges from 5 to 7. The key technical parameters of suppressor diodes include (1) the rated breakdown voltage, which refers to the breakdown voltage at a specified reverse breakdown current (usually 1 mA). The rated breakdown voltage of Zener diodes generally ranges from 2.9V to 4.7V, whereas that of avalanche diodes typically falls within the range of 5.6V to 200V. (2) Maximum clamping voltage: It refers to the highest voltage that appears across the tube when a large current of a specified waveform passes through it. (3) Pulse power: It refers to the product of the maximum clamping voltage across the tube under a specified current waveform (such as 10/1000 μs) and the current flowing through the tube. (4) Reverse displacement voltage: It refers to the maximum voltage that can be applied across the ends of a tube in its reverse leakage region, without the tube breaking down at this voltage. This reverse bias voltage should be significantly higher than the peak operating voltage of the protected electronic system; in other words, it must not be in a weakly conductive state during normal system operation. (5) Maximum leakage current: It refers to the maximum reverse current that flows through the tube under the action of a reverse bias voltage. (6) Response time: 10–11 s. 5. Choke coil: A choke coil is a device used to suppress common-mode interference, with a ferrite core as its magnetic core. It consists of two coils of the same size and with the same number of turns, which are wound symmetrically around the same ferrite ring core, forming a four-terminal device. This device exhibits high inductance to suppress common-mode signals, while showing very low leakage inductance and thus having little effect on differential-mode signals. Choke coils, when used in balanced circuits, can effectively suppress common-mode interference signals (such as lightning interference), without affecting the differential-mode signals that are transmitted normally along the circuit. When manufacturing such choke coils, the following requirements must be met: 1) The wires wound around the coil core must be insulated from one another to prevent short circuits between turns of the coil under the effect of transient overvoltages. 2) The core should not become saturated when a large instantaneous current flows through the coil. 3) The core in the coil should be insulated from the coil to prevent breakdown between them under the effect of transient overvoltages. 4) The coil should be wound in a single layer as much as possible; this reduces the parasitic capacitance of the coil and enhances its ability to withstand transient overvoltages. 6. 1/4 wavelength short circuiter: The 1/4 wavelength short circuiter is a microwave signal surge protector designed based on the spectral analysis of lightning waves and the theory of standing waves in feed lines. The length of the metal shorting rod in such a protector is determined according to 1/4 of the wavelength corresponding to the operating signal frequency (such as 900 MHz or 1800 MHz). For the frequency of the operating signal, the impedance of this parallel shorting rod is infinite, meaning it acts as an open circuit and does not affect the transmission of that signal. However, for lightning waves, since lightning energy is primarily distributed below n+KHZ, the impedance of this shorting rod for lightning waves is very low, meaning it acts as a short circuit, and the lightning energy is discharged into the ground. Since the diameter of 1/4 wavelength short-circuit rods is generally a few millimeters, they exhibit good performance in withstanding inrush currents, capable of withstanding levels above 30KA (8/20μs), and the residual voltage is very low; this residual voltage is mainly caused by the inductance of the short-circuit rod itself. The disadvantages are that the operating frequency range is narrow, at around 2% to 20%, and another drawback is that it is not possible to apply a DC bias to the antenna feed system, which limits certain applications.