Lightning arrester or surge arrestors

What is a surge arrester?

Surge arresters are devices that help prevent damage to apparatus due to high voltages. The arrester provides a low-impedance path to ground for the current from a lightning strike or transient voltage and then restores to a normal operating conditions.

A surge arrester may be compared to a relief valve on a boiler or hot water heater. It will release high pressure until a normal operating condition is reached. When the pressure is returned to normal, the safety valve is ready for the next operation.

When a high voltage (greater than the normal line voltage) exists on the line, the arrester immediately furnishes a path to ground and thus limits and drains off the excess voltage. The arrester must provide this relief and then prevent any further flow of current to ground. The arrester has two functions; it must provide a point in the circuit at which an over-voltage pulse can pass to ground and second, to prevent any follow-up current from flowing to ground.
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Causes of over voltages

    Internal causes
    External causes

Internal causes
Switching surge

The overvoltages produced on the power system due to switching are known as switching surge.
Insulation failure

The most common case of insulation failure in a power system is the grounding of conductors (i.e. insulation failure between line and earth) which may cause overvoltage in the system.
Arcing ground

The phenomenon of intermittent arc taking place in line to ground fault of a 3phase system with consequent production of transients is known as arcing ground.
Resonance

It occurs in an electrical system when inductive reactance of the circuit becomes equal to capacitive reactance. under resonance , the impedance of the circuit is equal to resistance of the circuit and the p.f is unity.
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Types of lightning strokes

    Direct stroke
    Indirect stroke

(1) Direct stroke

In direct stroke, the lightning discharge is directly from the cloud to the subject equipment. From the line, the current path may be over the insulator down the pole to the ground.
(2) Indirect stroke

Indirect stroke results from the electro statically induced charges on the conductors due to the presence of charge clouds.
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Harmful effects of lightning

The traveling waves produced due to lightning will shatter the insulators. If the traveling waves hit the windings of a transformer or generator it may cause considerable damage.
Protection against lightning

Different types of protective devices are:

    Earthing screen
    Overhead ground wires
    Lightning arresters

(1) The Earthing screen

The power station & sub-station can be protected against direct lightning strokes by providing earthing screens. On occurrence of direct stroke on the station ,screen provides a low resistance path by which lightning surges are conducted to ground.

Limitations: It does not provide protection against the traveling waves which may reach the equipments in the station.
(2) Overhead ground wires

It is the most effective way of providing protection to transmission lines against direct lightning strokes. It provides damping effect on any disturbance traveling along the lines as it acts as a short-circuited secondary.

Limitations:

    It requires additional cost.
    There is a possibility of its breaking and falling across the line conductors, thereby causing a short-circuit fault.

(3) Lightning Arresters

It is a protective device which conducts the high voltage surge on the power system to ground. The earthing screen and ground wires fail to provide protection against traveling waves. The lightning arrester provides protection against surges.
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AC Power Surge Arrester
Type 1 Surge Protectors

Type 1 surge protectors are designed to be installed where a direct lightning strike risk is high, especially when the building is equipped with external lightning protection system (LPS or lightning rod).

In this situation IEC 61643-11 standards require the Class I test to be applied to surge protectors : this test is characterized by the injection of 10/350 μs impulse current in order to simulate the direct lightning strike consequence. Therefore these Type 1 surge protectors must be especially powerful to conduct this high energy impulse current.
Type 2 surge protectors

Type 2 surge protectors are designed to be installed at the beginning of the installation, in the main switchboard, or close to sensitive terminals, on installations without LPS (lightning rods). These protectors are tested following the Class II test from IEC61643-11 based on 8/20 μs impulse current injection.
Type 3 surge protectors

In case of very sensitive or remote equipment, secondary stage of surge protectors is required : these low energy SPDs could be Type 2 or Type 3. Type 3 SPDs are tested with a combination waveform (1,2/50 μs – 8/20 μs) following Class III test.