Surge Arrester: Structure and Operating Principle

1. Fuction:

When lightning overvoltage invades a substation or other buildings along an overhead line, flashover may occur, and even the insulation of electrical equipment may be punctured. Therefore, if a protective device, namely a surge arrester, is connected in parallel at the power inlet end of the electrical equipment (as shown in Figure 1), when the overvoltage value reaches the specified operating voltage, the surge arrester will act immediately, conduct the charge, limit the amplitude of the overvoltage, and protect the equipment insulation. After the voltage returns to normal, the surge arrester will quickly return to its original state to ensure the normal power supply of the system.

1.1 The protective effect of a surge arrester is based on three prerequisites:

1.1.1 The volt-second characteristic has good coordination with that of the protected insulation.

1.1.2 Its residual voltage is guaranteed to be lower than the impulse dielectric strength of the protected insulation.

1.1.3 The protected insulation must be within the protective distance of the surge arrester.

1.2 The requirements for surge arresters:

1.2.1 No discharge during normal operation, and correct discharge action under overvoltage conditions.

1.2.2 Self-recovery function after discharge.

1.3 The relevant parameters of surge arresters:

1.3.1 Continuous operating voltage: The allowable long-term working voltage, which should be equal to or higher than the maximum phase voltage of the system.

1.3.2 Rated voltage (kV): The allowable short-term maximum power frequency voltage (arc-extinguishing voltage). The surge arrester can operate, discharge, and extinguish the arc under this power frequency voltage, but it cannot operate long-term under this voltage. It is a basic parameter for the characteristics and structure of the surge arrester, as well as the basis for design.

1.3.3 Power frequency withstand volt-second characteristic: Indicates the ability of a zinc oxide surge arrester to withstand overvoltage under specified conditions.

1.3.4 Nominal discharge current (kA): The peak value of the discharge current used to classify the surge arrester.

2. Classification and Structure

Common types of surge arresters include valve-type, tube-type, protective gap, and metal oxide surge arresters.

2.1 Valve-type surge arresters

Valve-type surge arresters are mainly divided into two categories: ordinary valve-type surge arresters (including FS and FZ series) and magnetically blown valve-type surge arresters (including FCD and FCZ series). The symbolic meanings in the model of valve-type surge arresters are as follows: F represents valve-type surge arrester; S denotes distribution (transformer) application; Z indicates substation use; Y signifies line use; D stands for rotating machine application; And C means with magnetically blown discharge gaps.

Valve-type surge arresters are primarily composed of parallel-plate spark gaps and silicon carbide resistive discs (valve discs) connected in series, enclosed within a sealed porcelain tube. The housing is equipped with terminal bolts for installation. The silicon carbide resistors in the arrester exhibit non-linear characteristics: their resistance is high under normal voltage but decreases significantly under overvoltage conditions.

Valve-type surge arresters feature spark gaps that remain unbroken under normal power frequency voltage, but break down when struck by lightning overvoltage waves. Concurrently, the resistance of silicon carbide discs drops sharply, allowing massive lightning currents to flow to the ground. As the lightning current subsides, the valve discs exhibit high resistance to the subsequent power frequency voltage, while the spark gaps block the power frequency current, restoring normal line operation. This coordinated action of valve discs and spark gaps resembles a "valve" that opens for lightning currents and closes for power frequency currents, hence the name "valve-type surge arrester".

The structure of FS series valve-type surge arresters is shown in Figure 2 (A). The arresters in this series have smaller valve disc diameters and lower current-carrying capacity, generally used to protect distribution equipment and lines. The structure of FZ series valve-type surge arresters is shown in Figure 2 (B). The arresters in this series have larger valve disc diameters, and the spark gaps are shunted by non-linear silicon carbide resistors, with larger current-carrying capacity. They are generally used to protect the electrical equipment of main step-down substations in medium and large factories at 35kV and above.

The magnetically blown valve-type surge arrester (FCD type) is internally equipped with a magnetic device to accelerate the extinction of arcs in the spark gaps, specifically designed to protect critical equipment or those with weak insulation, such as high-voltage motors.

2.2 Protective Gaps and Tube-Type Surge Arresters

Protective gaps are the simplest lightning protection devices, with their principle shown in Figure 3. Typically made of galvanized round steel, they consist of two parts: a main gap and an auxiliary gap. The main gap is shaped like an angle and installed horizontally to facilitate arc extinction. To prevent the main gap from being short-circuited by foreign objects (causing misoperation), an auxiliary gap is connected in series below the main gap. Due to the weak arc-extinguishing capability of protective gaps, they generally need to be used in conjunction with automatic reclosing devices to improve power supply reliability.

The basic component of a tube-type surge arrester is a spark gap installed inside a gas-producing tube, which consists of rod-shaped and ring-shaped electrodes, as shown in Figure 4. It is composed of an internal gap within the arc-extinguishing tube and an external gap. The arc-extinguishing tube is typically made of materials like fiber bakelite that can generate gas at high temperatures.

When a lightning overvoltage wave arrives, both the internal and external gaps of the arrester break down, allowing the lightning current to discharge to the ground through the grounding wire. The subsequent power frequency current generates a strong arc, which burns the tube wall and produces a large amount of gas ejected from the tube opening, rapidly extinguishing the arc. Meanwhile, the external gap restores insulation, isolating the arc-extinguishing tube (or arrester) from the system and resuming normal operation.

Since tube-type surge arresters rely on power frequency current to generate gas for arc extinction, if the interrupted short-circuit current is too large, excessive gas production exceeding the mechanical strength of the arc-extinguishing tube may cause it to crack or explode. Therefore, tube-type surge arresters are typically used outdoors.

2.3 Metal Oxide Surge Arresters

Gapless metal oxide surge arresters (GMOAs), also known as varistor arresters, emerged as a new type of arrester in the 1970s. Compared with traditional silicon carbide (SiC) valve-type arresters, GMOAs feature no spark gaps and use zinc oxide (ZnO) instead of SiC. Structurally, they are composed of stacked varistor discs made of piezoresistive materials, which exhibit excellent non-linear volt-ampere characteristics: under power frequency voltage, the discs show extremely high resistance, effectively suppressing power frequency current; under lightning overvoltage, their resistance drops drastically, enabling efficient discharge of lightning current.

Metal Oxide Surge Arresters (MOAs) boast advantages such as excellent protective characteristics, strong current-carrying capacity, low residual voltage, compact size, and convenient installation. Currently, MOAs have been widely applied to protect high and low voltage electrical equipment.

2.4 Metal Oxide (Zinc Oxide) Surge Arresters with Series Gaps

Metal oxide (zinc oxide) surge arresters with series gaps are composed of zinc oxide varistor blocks connected in series with a gap component (featuring two disc-shaped electrodes in a porcelain ring) inside a composite jacket, suitable for neutral non-effectively grounded systems. When single-phase ground faults or arcing ground faults occur, severe transient overvoltages with long durations may arise, which gapless ZnO arresters can hardly withstand. Such series-gap arresters address this issue: under single-phase ground or low-amplitude arcing ground overvoltages, the series gaps remain inactive to isolate the arrester from the system; when overvoltages exceed the threshold, the gaps discharge, and the excellent volt-ampere characteristics of ZnO varistors limit the residual voltage across the arrester, while the minimal follow current is easily interrupted, providing reliable insulation protection for transformers.

3. Routine Test Items and Corresponding Standards

3.1 Insulation Resistance Measurement

Use a 2500V or higher megohmmeter: for 35kV and above systems, the insulation resistance should be no less than 2500MΩ; For systems below 35kV, it should be no less than 1000MΩ.

3.2 DC 1mA Reference Voltage and 75% Voltage Leakage Current Test

Apply DC voltage to the arrester: as the voltage increases, the leakage current gradually rises. Record the voltage value when the current reaches 1mA (denoted as U1mA), then reduce the voltage to 75% of U1mA and record the leakage current, which should not exceed 50μA.

3.3 AC Leakage Current Measurement Under Operating Voltage

Measure the total current, resistive current, or power loss under operating voltage. The measured values should not differ significantly from the initial values. If the resistive current doubles, an outage inspection is mandatory; If it increases to 150% of the initial value, the monitoring cycle should be shortened appropriately.