Surge Arrester

[davva]

Where is the correct position to install a surge arrester.

The equipment to be protected is a 6.6kV motor. Currently there are surge arresters installed in the switchboard (vacuum contactor application). Searching the web it suggests that it is more appropriate to install at the motor but if you have surge suppressor then it is o.k at the switchboard.

I've spent some time reading about surge impedance and travelling pulse waves and then my eyes started to glaze over with all the equations!

Basically we have a number of surge arresters failing and I wondered if there location in the switchboard might be a factor.

Also I am not able to get the BIL insulation rating for the motor, what is a typical value for 6.6kV motor (500kW). I realise typical is not much used but I would like to get a ball park feeling of whether the correct surge arrester is being used.

The other factor is that the network is insulated neutral and the protection did not pick up the failed surge arrester. I wonder if an inappropriate rated voltage has been selected for the arrester and it was failing due to an undetected earthed fault, i.e. increased TOV (temporary over voltage)

 

[Skogsgurra]

Forget about the travelling wave. That is usually not the problem, its amplitude is seldom more than 1.8 times switched voltage.

The problem is the inductive kick-back and the fact that a vacuum bottle doesn't dissipate the stored energy in an arc (like normal contactors do) so voltage rises 'indefinitely' up to closest break-down level - which can be many times interrupted voltage.

You can put your surge arrester anywhere along the line between contactor and load. It will take care of the stored energy anyhow and reduce the overvoltage. Put it where it is safest and easiest to install. That is seldom at the load.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Mohsen1]

Dear Davva,

Skogsgurra is right, surge arrester use for vacuum contactor and if you have SF6 it is not there, the best position of Surge arrester at the load side at the motor or transformer, however the motor junction box space not enough to put it then they put it at the switchgear, the reason to put it at load side to be sure that no impulse voltage reaches to load, if you have long cable there is possibility to affect the load but it will not affect the surge arrester
Please check the voltage rated of surge arrester may be is not suitable