Overvoltages may cause burning of insulation of substation equipment if not well protected. Lightning is one of the most serious causes of overvoltages. Lightning arrestors/ surge arrestors are connected to protect the equipments from lightning and switching surges.
Various types of lightning arrestor construction are Rod gap, Expulsion type, Valve type, Horn gap, Pellet type, Thyrite type etc.
An ideal Lightning Arrester should posses the following characteristics.
- 1. It must not take any current at normal system voltage
- 2. Any transient wave with voltage peak exceeding the spark over voltage must cause it to break down.
- 3. After break down it must be capable of carrying the resulting discharge current without any damage to itself and without voltage across it exceeding the breakdown voltage.
- 4. 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 Arrestor
Lightning Arrestor should be located close to the equipment that it is expected to protect. In large sub stations arrestors should be installed at take off points of the lines and of terminal apparatus. Many factors like system voltages, basic impulse insulation level, arrestor rating, station lay out, number and arrangement of lines, position of isolators, distance between equipments etc. have to be taken into account in fixing the location of the arrestors.
The length of the arrestor lead should be as low as possible and should not exceed 10M. Arrestors are installed both on HV and LV 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 Arrestor Ratings
The Rating of lightning arrestor are 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 arrestors should be greater than the maximum sound phase to ground voltage.
- Normal Discharge current: It is the surge current which 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 arrestor and earth which causes spark over of the series gap. As per IS 3070, the recommended spark over voltage 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 Lightning Arrestor
For the protection of substation above 66KV an arrestor 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 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
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