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A current transformer is a device that is used for the transformation of current from a higher value into a proportionate current to a lower value. It transforms the high voltage current into the low voltage current due to which the heavy current flows through the transmission lines are safely monitored by the ammeter.
A lightning arrester (alternative spelling lightning arrestor) (also called lightning diverter) is a device used on electric power systems and telecommunication systems to protect the insulation and conductors of the system from the damaging effects of lightning. The typical lightning arrester has a high-voltage terminal and a ground terminal. When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester, the current from the surge is diverted through the arrester, in most cases to earth.
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.
There are various types of lightning arrester construction. They are
Rod gap LA
Expulsion type LA
Valve type LA
Horn gap LA
Pellet type LA
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.
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 length of the arrester lead should be as low as possible and should not exceed 10M.
The Arresters are installed both on the High Voltage and Low Voltage side of the transformers. The 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.
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 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
