A lightning arrester is connected to protect a piece of equipment from lightning and switching surges.

Overvoltages may cause the burning of insulation of substation equipment if not well protected. Lightning is one of the most serious causes of overvoltages. 

When lightning protection is considered necessary, lightning arresters may be installed on or near the equipment being protected.

A Lightning arrester limits the high transient voltages generated during a system disturbance by diverting the excessive part of it to the ground. It also reduces the amplitude of the transient voltage wave across the equipment to a permissible safe value less than the impulse withstand level of the equipment

There are various types of lightning arrester construction. They are 

1. Rod gap LA
2. Expulsion type LA
3. Valve type LA
4. Horn gap LA
5. Pellet type LA
6. Thyrite type LA, etc.

An ideal Lightning Arrester should possess the following characteristics.

• It  must not take any current at normal system voltage
• Any transient wave with a voltage peak exceeding the spark over voltage must cause it to break down.
• After the breakdown, it must be capable of carrying the resulting discharge current without any damage to itself and without voltage across it exceeding the breakdown voltage.
• The power frequency current following the breakdown must be interrupted as soon as the transient voltage has fallen below the breakdown value.

Location of Lightning Arrester

Lightning arrester should be located close to the equipment that it is expected to protect.

In large substations, arrestors should be installed at take-off points of the lines and of the terminal apparatus.

Many factors like system voltages, basic impulse insulation level, arrestor rating, station layout, number and arrangement of lines, the position of isolators, the distance between equipment, etc. have to be taken into account in fixing the location of the arrestors. 

The Arresters are installed both on the High Voltage and Low Voltage side of the transformers. Junction of an OH line and the cable should be protected by LA. Separate earth should be provided for each LAs. LA ground leads should not be connected to the station earth bus.

Lightning Arrester Ratings

The rating of a lightning arrester is given below,

Normal or rated voltage:

It is designated by the maximum permissible value of power frequency voltage which it can support across its line and earth terminal while still carrying effectively and without the automatic extinction of the follow-up current.

The voltage rating of the arresters should be greater than the maximum sound phase to ground voltage.

Normal Discharge current:

It is the surge current that flows through the LA after the spark over, expressed in crest value (peak value) for a specified wave shape. Example 10, 5, 2.5, 1.5, 1 kA rating.

Power frequency spark over voltage:

It is the RMS value of the power frequency voltage applied between the line and earth terminals of the arrester and earth which causes spark over of the series gap. As per IS 3070, the recommended spark overvoltage is 1.5 times the rated voltage.

There are also other ratings like maximum impulse spark over-voltage, residual or discharge voltage, maximum discharge current, etc.

Selection of LA

Here we are selecting an appropriate rating of lightning arresters for the substation.

For the protection of substation above 66kV, an arrester of 10kA rating is used.

Voltage rating of LA = Line to line voltage × 1.1 × coefficient of earthing.

Power frequency spark over voltage = 1.5 ×Voltage rating of LA (Assuming the coefficient of earthing equals 0.8 for the effectively earthed system)

For 220KV side:

• Voltage rating = 1.1 × 220 × 0.8 = 193.6KV
• Power frequency spark over voltage = 1.5 ×193.6 = 290.4KV
• Rated discharge current = 10 kA

For 110KV side:

• Voltage rating = 1.1 × 110×0.8 = 96.8KV
• Power frequency spark over voltage = 1.5 × 96.8 = 145.2KV
• Rated discharge current  = 10kA

For 66kV Side 

• Voltage rating = 1.1 × 66×0.8 = 58.08kV
• Power frequency spark over voltage = 1.5 × 58.08 = 87.12kV
• Rated discharge current = 10kA

For 11 KV side:

• Voltage rating = 1.1× 11×0.8 = 9.68KV
• Power frequency spark over voltage = 1.5×9.68 = 14.52KV
• Nominal discharge current = 5kA