When are surge arrestors needed on medium voltage switchgear?

[bdn2004]

Attached is a simplified sketch of the installation we are planning: a medium voltage (15kV) double ended switchgear that will connect to a single transformer. The new switchgear will be arc resistant and sit inside an existing electrical building.

All of the existing outside conductors are open bus. This open bus actually feeds several transformers, each one with it's own set of surge arrestors. All of the new conductors will be shielded armored 3/C cable.

My question is around surge arrestors at the new medium voltage switchgear on the incoming feeders. Are they required? Good idea to protect the switchgear?

 

[odlanor]

if you have only CB in switchgear it is not necessary.
If you have TP and TC it will be necessary.
If you don´t know ,its better to install;- surge arrester is not so expensive.
It is less expensive then the studies necessary to proof you need surge-arrester.

 

[bdn2004]

odlandor,

TP?
TC?

 

[odlanor]

potential transformer, current transformer , any equipment with winding.

 

[jghrist]

If lighting hits the open bus, the voltage will be limited at the bus by the transformer arresters. There will be a surge through the cable and reflections at the switchgear end, increasing the voltage.

You could review IEEE Std C62.22, IEEE Guide for the Application of Metal-Oxide Surge Arresters for Alternating-Current Systems, for calculation of how far you can go with the cable before you need arresters at the end while maintaining adequate protective margins.

More than likely, at 15 kV, you won't need arresters at the switchgear. But, as odlanor points out, the arresters may be less expensive that the study required to prove that they aren't needed.

 

[GEORGE2311]

Don't you need anyway an insulation coordination study for the dimensioning of the arrester, taking in mind all possible cases like system symmetrical or asymmetrical faults, switching over voltages, energizing of lines ,load rejections etc?

G.F.

http://www.engineering-services.gr

 

[jghrist]

Don't you need anyway an insulation coordination study for the dimensioning of the arrester, taking in mind all possible cases like system symmetrical or asymmetrical faults, switching over voltages, energizing of lines ,load rejections etc?

You do if you like to keep consultants like us in business. Or, you could pick out the recommended arrester for the voltage from the manufacturer's catalog.