A lightning arrester is an electrical appliance used to protect electrical equipment from the hazards of high transient overvoltage during lightning strikes, and to limit the freewheeling time and often the freewheeling amplitude. Lightning arresters are sometimes also called overvoltage protectors and overvoltage limiters.
The role of lightning arrester
When lightning overvoltage invades substations or other buildings along overhead lines, flashover will occur and even the insulation of electrical equipment will breakdown. Therefore, if a protective device, that is, a lightning arrester, is connected in parallel to the power incoming end of the electrical equipment. As shown in Figure 1, when the overvoltage value reaches the specified operating voltage, the arrester immediately starts to operate, flowing charge, limiting the overvoltage amplitude to protect the insulation of the equipment; after the voltage value becomes normal, the arrester quickly returns to its original state to ensure the normal power supply of the system.
The protective effect of lightning arresters is based on three premises:
1. Ensure that the volt-second characteristics match well with the volt-second characteristics of the protected insulation.
2. Ensure that the residual voltage is lower than the impact electrical strength of the protected insulation.
3. Ensure that the protected insulation must be within the protection distance of the arrester.
Lightning arrester requirements
1. Ensure that there is no discharge during normal operation and correct discharge action during over-voltage.
2. Ensure that it has a self-recovery function after discharge.
The scope of application of lightning arrester
AC gapless metal oxide arresters are used to protect the insulation of AC power transmission and transformation equipment from lightning overvoltage and operating overvoltage damage. Suitable for overvoltage protection of transformers, transmission lines, distribution panels, switch cabinets, power metering boxes, vacuum switches, parallel compensation capacitors, rotating electrical machines and semiconductor devices.
Characteristics and principles of lightning arresters
AC gapless metal oxide arresters have excellent nonlinear volt-ampere characteristics, good response characteristics, no freewheeling, large flow capacity, low residual voltage, strong overvoltage suppression ability, resistance to pollution, anti-aging, not subject to altitude restrictions, simple structure, no gaps, tight sealing, long life, etc.
Under normal system operating voltage, this arrester exhibits a high resistance state and only microampere current passes through it. Under the action of overvoltage and large current, it will show low resistance, thus limiting the residual voltage at both ends of the arrester.
Related parameters of lightning arrester
1. Continuous operating voltage: that is, the voltage allowed for long-term operation. It should be equal to or greater than the highest phase voltage of the system.
2. Rated voltage (kV): The maximum allowable short-term power frequency voltage (arc extinguishing voltage). The arrester can operate to discharge and extinguish the arc under this power frequency voltage, but it cannot operate under this voltage for a long time. It is the basic parameter of arrester characteristics and structure, and also the basis for design.
3. Power frequency withstand volt-second characteristics: indicates the zinc oxide arrester's ability to withstand overvoltage under specified conditions.
4. Nominal discharge current (kA): The peak discharge current used to classify arrester levels. Systems of 220kV and below should not exceed 5.
Classification and structure of lightning arresters
Commonly used arresters include valve type, tube type, protective gap and metal oxide.
(1) Valve arresters Valve arresters are mainly divided into two categories: ordinary valve arresters and magnetic blowing valve arresters.
Ordinary valve-type arresters are available in two series: FS and FZ; magnetic valve-type arresters are available in two series: FCD and FCZ.
The valve type arrester is composed of a spark gap and a valve resistor. The valve resistor is made of special silicon carbide. Valve resistors made of silicon carbide can effectively prevent lightning and high voltage and protect equipment. When there is high lightning voltage, the spark gap is broken down, the resistance value of the valve resistor decreases, and the lightning current is introduced into the earth, which protects cables or electrical equipment from the harm of lightning current. Under normal circumstances, the spark gap will not be broken down, and the resistance value of the valve resistor is relatively high, which will not affect the normal communication of the communication line.
The spark gap of the valve arrester is not broken down under normal power frequency voltage, but under lightning wave overvoltage, the spark gap of the arrester is broken down; the resistance of the silicon carbide resistor becomes very small, and the huge lightning current of the lightning wave smoothly flows into the earth through the resistor. The resistor valve plate shows a large resistance to the power frequency voltage that follows the lightning current, so the power frequency current is blocked by the spark gap, and the line returns to normal operation.
The structure of the FS series valve-type arrester is shown in Figure 2. The valve plate of this series of arresters has a small diameter and a low flow capacity. It is generally used to protect power transformation and distribution equipment and lines. The structure of the FZ series valve-type arrester is shown in Figure 2(b). The valve plate of this series of arresters has a larger diameter, and the spark gap is connected in parallel with a non-linear silicon carbide resistor. The flow capacity is large. It is generally used to protect the electrical equipment of the main step-down substation in large and medium-sized factories of 35kV and above.
The magnetic valve type arrester (FCD type) is equipped with a magnetic device inside to accelerate the extinguishing of the arc in the spark gap. It is specially used to protect important or weakly insulated equipment, such as high-voltage motors.
(2) Protection gap and tube ball arrester
The protection gap is the simplest lightning protection device, and its principle structure is shown in Figure 3. The protective gap is generally made of galvanized round steel and consists of a main gap and an auxiliary gap. The main gap is made into an angular shape and installed horizontally to facilitate arc extinguishing. In order to prevent the main gap from being short-circuited by foreign objects and causing malfunction, an auxiliary gap is connected in series below the main gap. Because the arc extinguishing ability of the protection gap is weak, it is generally required to be used in conjunction with an automatic reclosing device to improve the reliability of power supply.
The tubular arrester is actually a protective gap with high arc-extinguishing capability. It consists of two gaps in series. One gap is in the atmosphere, called the outer gap. Its task is to isolate the working voltage and prevent the gas production pipe from being burned by the power frequency leakage current flowing through the pipe; the other is installed in the gas pipe, called the inner gap or arc-extinguishing gap. The arc-extinguishing ability of the tubular arrester is related to the size of the power frequency freewheeling current. This is a protection gap type lightning arrester, which is mostly used for lightning protection on power supply lines.
Because tubular arresters rely on power frequency current to generate gas to extinguish arcs, if the short-circuit current is too large and the gas production exceeds the mechanical strength of the arc extinguishing tube, it will crack or explode. Therefore, tubular arresters are usually used outdoors.
(3) Metal oxide arrester
Gapless metal oxide arrester (also known as pressure-sensitive arrester) is a new type of arrester that began to appear in the 1970s. Compared with the traditional silicon carbide valve arrester, the gapless metal oxide arrester has no spark gap, uses zinc oxide (ZnO) instead of silicon carbide (SiC), and is structurally stacked with valve plates made of varistors. The valve plate has excellent nonlinear volt-ampere characteristics: under power frequency voltage, it presents a huge resistance, effectively suppressing power frequency current; and under lightning wave overvoltage, it presents extremely small resistance, which can well discharge lightning current.
Metal oxide arresters have the advantages of good protection characteristics, strong flow capacity, low residual voltage, small size, and easy installation. At present, metal oxide arresters have been widely used for the protection of high and low voltage electrical equipment.
