What causes the "hissing sound" in SF 6 Gas insulated HV Switch Gear?

SHUT IT DOWN! This could be internal corona discharge. The switchgear should be de-energized and closely examined. That means pump out the SF6 and take it apart. Examine all insulating components.

I agree with Robert's recommendation, especially if the sound can be localized to portions of the switchgear which do not have bushings for connection to overhead lines. Even if the sound is in the area of air bushings, deenergizing will allow more in-depth inspection and addressing any sharp edges or cracked insulators, etc.

Robert and Allen are of course absolutely on point! Take this pieces of equipment out of service immediately, perform a "hi-pot" or high potential test on the various elements of the switchgear and attempt to locate the area that is "leaking" to ground (or between phases). Inspect closely for indications of tracking on insulators from corona discharge and replace any compromised components. After component replacement, installation of new SF6 gas and other repairs, re-run the Hi-Pot test to confirm that the switchgear is able to withstand voltages at least 50% greater than the name-plate rating. Of course all of this advice is worthless if the unit has already failed. Good Luck Benard!

Thanks for your comments and advise, Let me plan for prescribed "ACTIONS". Keep that spirit.

Remember that a Hi-pot is actually a destructive test. It challenges the insulation to the point of breakdown. Check the vendor recommendations before you Hi-pot equipment that has been in-service.

I agree with Marie, but i don't think this will effect on SF6 SWG's . SF6 can fill easy and gauges will show us the reading values.

But Your point is more effective on XLPE/PVC Cables, Metal-Clad SWG's.