Lightening surge, travels on line, hence it should be first at receiving end  on top of line, and could protect all, substation equipments from, over voltage surges, applicaple for lines up to 66KV,switching surges of over voltages produced by above 66KV lines are higher than lightening. There should be earthing rods too, at tall structures, in sub station to protect its equipments, from direct strokes.  

Yes , The class of arrestor required depends on MVA rating of transformer. Intermediate class arrestors are used for  protection of dry-type transformers, for

use in switching and sectionalizing equipment and for the protection of

cables. Traditional applications include equipment protection in the

range of 1 to 20 MVA for substations and rotating machines.

Another class called Distribution class arresters are frequently used for smaller liquid-filled and dry-type transformers 1000 kVA and less.

If two identical transformers are to be used in two different location (one at a place which has less lightning effect and the other at lightning prone zone), can we have two different rating of lightning arrester?

Same rating of lightning arrestor would worked, for both transformer, as has to handle same power in a stroke. In prone zone you could used more lightening(ground) rods for tall structures to reduce lightening effect in substation.

Intensity of lightning activity i.e. lightning frequency may affect just on a frequency of arrester's switching. In that  lightning arresters have infinity switching resource location or altitude seem no required to be taken into consideration at option of arresters. 

The rating of a lightning arrestor depends on

1. The normal working voltage of the line / transformer

2. The discharge current due to the lightning (kA)

3. The normal frequency spark over voltage.

 1 and 3 depend on the type of line and you have complete information about them. But 2 is the discharge current when the lightning strikes. It depends on the severity of strike. I dont think that it can be calculated by any means or atleast I am not aware of any method to measure the lightning current. They go by the practice being observed in that locality or area. Location with more strikes need definitely higher rated (kA) LAs.

Note that the standard IEC60099-4 state the normal service conditons for surge arrester. The altitude shall not be exceed 1000m.  The use of surge arrester in abnormal service condition is subject to agreement between the manufacterer and the user. 

We should make a clear distinction between external insulation (bushings, insulators) and internal insulation (oil-impregnated paper for oil-immersed transformers, Mylar or similar turn-to-turn insulation for dry-type transformers).

Internal insulation is of course not affected by air density (altitude) or even contamination, precipitation, etc. 

External insulation - bushings - is affected by the environmental conditions.  What problems are you experiencing? External flashover of bushings through air?  Dielectric withstand of air is affected by atmospheric conditions, and this may require different rating of arresters for particular altitudes.  For dry-type transformers, contamination on the transformer surface may cause problems (tracking, flashover), but arresters would not be the proper protection for this condition.

Regards, GG

yes, the transformers have lightning arrester for different altitude and location. The rating of a lightning arrestor depends on the normal working voltage of the line-transformer and also the discharge current due to the lightning.

Very interesting question dear Mr. Phub Dorji,

Please ask the manufacturer directly, especially the ones installed in the regions that you are performing your studies. If the arresters do have air gaps or are filled with air please do observe caution in regards to isolation properties related to atmosferic pressure close and within the arresters. Good luck! Very interesting topic and especially for a PhD Thesis. Please do not forget to consider all types of arresters: gapped, MOVs, etc.

A LA comprises of Insulator housing (Polymer or Porcelain) and active element (Zn O2) inside the housing For quality check before assembly, the housing is checked for system dielectric and air gap which is effected by air density/ altitude but once assembly is completed and proper sealing is achieved, altitude has no effect. However incidence of lightnings are effected by altitudes. High altitude locations are more prone to lightning and life of LA gets reduced due to frequent discharges.

I am S/S and TL Designer and experience in execution of Projects  

The insulation level is affected by both the altitude and humidity.  The altidute  correction factor in the standards is for  2000 m above the sea level.  Check the altidute/humidity condition at the transofrmer location. It could be necessary to chance the LA type.

The transformer except bushing is not affected by altitude/ humidity. The Insulation level (outdoor insulation) is affected by altitude/ humidity and corrections are applied accordingly and as such bushing is tested independently for dielectric ( Power frequency, lightning and Switching) as also for pollution level (Cat light, medium, heavy and very heavy) and  correction factors are applied. Same methodology is adopted for LA and  LA housing is tested similarly as that of transformer bushing. However performance of bushing of transformer and that of LA is at different over voltages. The operation of LA takes place against over voltage(BIL , SIL, Dynamic) and these over voltages afford protection minimum margin of 15% over BIL and  25% over switching voltages of Transformer..   LA conducts at lower voltages than that of withstand voltages of transformer and protect the transformer. The procedure for selection of LA rating and its other characteristics and parameters of Transformer ( BIL, SIL, PF withstand, PD Discharge, Audible noise, RIV, etc) .