Transformer - Probability of Failure without Arresters

[NAZ55]

All,

I have been asked to look into the probability of Main Power Transformer (230/35kV) failure should one be implemented without high side arresters. The thought process behind this is to figure out the affect of energizing a transformer without an arrester due to lead times of the arresters, and install the arresters once they arrive.

Any leads will be greatly appreciated.

 

[waross]

How is your local weather? Any storm clouds on the horizon?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Marmite]

There are a lot of variables, one of which is weather as noted by Bill. It all boils down to risk and probability. Arresters are relatively cheap in comparison to the asset they protect (about $6-7k per phase). There is a potential for the transformer to be damaged by switching impulses or lightning resulting in total loss. What is your Company's appetite for risk? Personally I would cast the net further to try to get some arresters before you energise. Speak to the suppliers and see what they can do to shorten the lead time.
Regards
Marmite

 

[Yo121]

I think you need to run a transients study and get the isokeraunic data for your area. You can do a transients study via the ATP-EMTP free program (

http://www.emtp.org/),

set up a little model of your system and estimate what the voltage would reach; odds are good that lightning isn't going to strike the transformer directly or even near it on any other equipment/bus in the substation, assuming you have adequate station lightning protection (static wires/masts). Lightning should strike the lightning protection which is connected to the station ground grid and won't have any effect on the transformer. Lightning really is more probable to strike an overhead line conductor, first. Then the transient voltage wave will travel down the line toward the station and the incoming line surge arresters should clamp the voltage low enough not to do serious damage to your transformer, theoretically. If you run a simulation, you'd see that in many cases the transformer surge arresters aren't even necessary in this case (a line transient), due to the line surge arresters controlling the transient, enough. In my opinion, they're overused and it almost seems like they're on the transformer "just in case", given the significant investment a transformer is to a utility/plant and the fact the line surge arresters could fail or have bad ground connections, etc. Keep in mind that most overhead lines have grounded static wires for lightning protection, too. As for switching transients, you won't have much trouble unless you have some cap banks near the transformer...probably don't, and you don't need to worry about it. I'd risk it even without running an EMTP simulation, if you know you have adequate lightning protection in your substation and the incoming lines have properly rated surge arresters. However, you probably want to back your decision up with some hard facts you can present to the boss, if you risk it. You indicate this is a temporary thing...depends on how long you mean, a year, a few months, a few weeks....seems to me you'll probably only have to wait a few weeks which is not too big a risk.

 

[davidbeach]

Covering one mistake by making another? ;-)

 

[Marmite]

The practice of installing line entry surge arresters into AIS substations varies across the World. There is a potential for improved circuit availability, since tower arcing horn flashovers can be reduced. However a line entry arrester will not necessarily protect a transformer from fast front lightning (backflashover), since it will only reliably control the voltage locally (<20m) and not at the windings of a transformer 50m away. The protective margin drops off significantly as the distance from the arrester increases. As a rule of thumb, the arrester should be located within 15m of the plant item it is protecting.
Transformers are critical transmission assets and due to their complex design they are also very susceptible to over voltages. Surges entering the winding can cause part winding resonance, and a flashover between phases or to earth. The flashover of the winding dielectric insulation usually causes irreparable damage. In most cases the transformer will need to be replaced.

Regards
Marmite