Function of Surge Arresters

When lightning-induced overvoltages travel along overhead lines and invade substations or other buildings, they can cause flashovers and even break down the insulation of electrical equipment. Therefore, if a protective device such as a surge arrester is installed in parallel at the incoming power supply of electrical equipment, as shown in P1, the surge arrester will immediately operate when the overvoltage reaches the specified action voltage. It will discharge the charge, limit the overvoltage amplitude, and protect the equipment's insulation. Once the voltage returns to normal, the surge arrester quickly resets to ensure the system's normal power supply.

Surge Arrester Protection Conditions

The protection provided by a surge arrester is based on three conditions:

The volt-second characteristic of the arrester should match well with the volt-second characteristic of the protected insulation.

Ensure that the residual voltage is lower than the impulse withstand voltage of the protected insulation.

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

Requirements for Surge Arresters

Do not discharge during normal operation, discharge correctly during overvoltage.

Have a self-recovery function after discharge.

Parameters of Surge Arresters

Continuous Operating Voltage: The allowable long-term working voltage, which should be equal to or greater than the highest phase voltage of the system.

Rated Voltage (kV): The allowable short-term maximum power frequency voltage (arc-extinguishing voltage). The arrester can operate and discharge under this power frequency voltage and extinguish the arc but cannot run continuously at this voltage. It is a fundamental parameter of the arrester's characteristics and structure.

Power Frequency Withstand Volt-Second Characteristic: Indicates the ability of a zinc oxide arrester to withstand overvoltages under specified conditions.

Nominal Discharge Current (kA): The peak value of the discharge current used to classify the arrester. It should not exceed 5 kA for systems of 220 kV and below.

Residual Voltage: The voltage generated across the arrester terminals under impulse current, which can also be understood as the highest voltage the arrester can withstand.

Types and Structure of Surge Arresters

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

1.Valve Type Arresters

Valve-type arresters are divided into ordinary valve arresters and magnetic blow valve arresters. The ordinary valve arresters include FS and FZ series, while magnetic blow valve arresters include FCD and FCZ series.

The symbols in the model numbers of valve-type arresters have specific meanings: F - valve arrester; S - for distribution; Z - for substations; Y - for lines; D - for rotating machinery; C - with magnetic blow discharge gap.

The valve-type arrester is mainly composed of a flat spark gap and silicon carbide resistor (valve piece) in series, housed in a sealed porcelain tube, with connecting bolts for installation. The silicon carbide resistor has a nonlinear characteristic, with high resistance under normal voltage and low resistance during overvoltage.

2.Magnetic Blow Valve Arresters (FCD Type)

These arresters have an internal magnetic device to accelerate arc extinction in the spark gap, specially designed to protect critical or weakly insulated equipment, such as high-voltage motors.

3.Protective Gaps and Tube Arresters

The protective gap is the simplest lightning protection device, consisting of a main gap and an auxiliary gap, typically made of galvanized round steel. The tube arrester has a spark gap installed inside a gas-producing tube, with gaps formed by rod and ring electrodes.

4.Metal Oxide Surge Arresters (MOSA)

MOSA without gaps, also known as varistor surge arresters, emerged in the 1970s. They use zinc oxide (ZnO) instead of silicon carbide (SiC) and have superior nonlinear volt-ampere characteristics. Under power frequency voltage, they exhibit high resistance, effectively suppressing power frequency currents, while under lightning overvoltage, they exhibit low resistance, effectively discharging lightning currents.

Test Items and Standards for Surge Arresters

Insulation Resistance Measurement: Using a 2500V or higher megohmmeter, values should be no less than 2500MΩ for systems 35kV and above, and no less than 1000MΩ for systems below 35kV.

Measurement of DC 1mA Voltage and Leakage Current: Apply DC voltage, record the voltage at 1mA current, then reduce to 75% of this voltage and measure the leakage current, which should not exceed 50μA.

AC Leakage Current Under Operating Voltage: Measure the total current, resistive current, or power loss under operating voltage, compare with initial values, and if the resistive current doubles, the arrester should be inspected. If the resistive current increases to 150% of the initial value, the monitoring period should be shortened.

These tests can identify moisture, aging, surface cracks, and insulation aging defects in the arrester valve pieces.