Rating of a Lightning Arrestor
Posted by Anup Mohan on Thursday, November 17, 2011 Under: Design
When you look into a substation layout, say for instance a 132kV Switchyard you would notice that all the equipment installed will be same as the nominal system voltage except for the lightning arrestor. A few days back I found that many of them still have this as unanswered question so as to why is the LA rating chosen lower than the system voltage. So I thought I could share my idea on this with everyone through this post.
When you consider a three phase system, the LA is placed between each Line and Ground and that is the reason why you have three individual LAs in the circuit where as to one three phase circuit breaker or isolator.
The basic need of having a Lightning Arrestor or Surge Arrestor installed is to protect our equipment from surges or simply saying protect it from damage caused by any rise in voltage. I came to know that a there are quite a few who are of the impression that the rating of LA / SA is chosen depending on surge voltage. That it is absolutely incorrect. The reason being that the surge voltage cannot be predetermined.
The nominal system voltage is one of the important factor to choose the LA/SA rating. As I mentioned earlier, since the LA is placed in between the Line and ground, hence we have to consider the line voltage and then the maximum voltage. The rating of the LA is chosen which is the nearest to this value.
For instance, consider a 132kV substation. The line to ground voltage will be 132/sqrt(3); i.e 76.21kV
The peak value for this line voltage will be 76.21 X sqrt(2); i.e 108kV
Consider a margin of 10%, we can say the voltage will be 119.37kV
The standard rating of the LA that is nearest to this value is 120kV and hence at a 132kV substation, we choose an LA of 120kV rating.
In case of a Traction Substation ( for information on Traction Substation, refer the previous post "Traction Power System), the input is a three phase supply, yet a double transmission line. In this case the secondary side bus voltage will be 25kV AC single phase.
The peak value will be 25 X sqrt(2); i.e 35.35kV and considering a margin of 10% to the peak value we have 38.885kV
The nearest standard rating available is that of 42kV and the same is chosen.
This is why the primary side LA rating in a TSS is less than the nominal voltage and that at the secondary side is higher than the system voltage.
When you consider a three phase system, the LA is placed between each Line and Ground and that is the reason why you have three individual LAs in the circuit where as to one three phase circuit breaker or isolator.
The basic need of having a Lightning Arrestor or Surge Arrestor installed is to protect our equipment from surges or simply saying protect it from damage caused by any rise in voltage. I came to know that a there are quite a few who are of the impression that the rating of LA / SA is chosen depending on surge voltage. That it is absolutely incorrect. The reason being that the surge voltage cannot be predetermined.
The nominal system voltage is one of the important factor to choose the LA/SA rating. As I mentioned earlier, since the LA is placed in between the Line and ground, hence we have to consider the line voltage and then the maximum voltage. The rating of the LA is chosen which is the nearest to this value.
For instance, consider a 132kV substation. The line to ground voltage will be 132/sqrt(3); i.e 76.21kV
The peak value for this line voltage will be 76.21 X sqrt(2); i.e 108kV
Consider a margin of 10%, we can say the voltage will be 119.37kV
The standard rating of the LA that is nearest to this value is 120kV and hence at a 132kV substation, we choose an LA of 120kV rating.
In case of a Traction Substation ( for information on Traction Substation, refer the previous post "Traction Power System), the input is a three phase supply, yet a double transmission line. In this case the secondary side bus voltage will be 25kV AC single phase.
The peak value will be 25 X sqrt(2); i.e 35.35kV and considering a margin of 10% to the peak value we have 38.885kV
The nearest standard rating available is that of 42kV and the same is chosen.
This is why the primary side LA rating in a TSS is less than the nominal voltage and that at the secondary side is higher than the system voltage.
In : Design
blog comments powered by Disqus
