GIS - Loss of Gas pressure & Dielectric strength

[RRaghunath]

33kv GIS data sheet shows the filling level for SF6 gas is 1.3bar gauge, Alarm level is 1.25bar gauge and lockout level is 1.2bar gauge.
Under a specific scenario involving rapid loss of gas pressure reaching the lockout level within a short time, is it safe to leave the switchgear in service as the operations team make preparation for planned shutdown!
In other words, What would be the dielectric strength available when the SF6 gas pressure in GIS becomes zero bar gauge due to gas leak!
Thanks in anticipation.

 

[isquaredr]

It depends, is the sf6 their for arc extinction or just to increase the bil rating of the swgr. Some low pressure sf6 swgr have vacuum interupters and then surrounded with sf6 to increase bil rating.

 

[RRaghunath]

isquaredr,
Thanks for the mail.
Yes, the SF6 is only for increasing the BIL and keeping the dust & moisture out. The interruption is in vacuum (VCBs).

 

[rasevskii]

raghun:

Well, you had better contact the OEM immediately. If the gas pressure actually reaches zero, you are risking a flashover with damage, irrespective of whether the interrupters are vacuum or whatever. There should be a busbar differential protection in service in any case which will trip the bus upon a fault, which will at least limit the damage. Otherwise there is a risk of explosion, depending on SC levels and settings of upstream protections.

Th OEM does not put in pressure alarms without reason.

regards, rasevskii

 

[RRaghunath]

rasevskii,
Thanks for your mail.
The issue is only at discussion stage in our office now, some of the colleagues arguing that the SF6 gas pressure low alarm should have been wired for upstream trip.
I think not wiring it to upstream trip is prudent one as the alarm allows the operator to switchover the power supplies and shutdown the panel with minimal impact on production.
I would appreciate your views.

 

[rasevskii]

raghun:

I assume that you are talking about GIS (gas insulated switchgear) where the HV components are totally enclosed in pressurized pipes, rather than SF6 gas breakers which are part of switchgear which is air or compound insulated.

In GIS, the spacing between phases or phase to earth is only some centimeters and the gas density provides the insulating property compared to air. At 33kv the spacing in air between phases or phase to earth is something like one meter ( others can give a more accurate figure). You can imagine what will happen if gas pressure is lost in GIS.

Usually there are 2 or 3 stages of alarm as you say. The lowest stage must be a trip in principle.

If it is SF6 breakers only, the lowest stage must be a lockout, in other words the breaker cannot be allowed to trip. If it did, the arc would not extinguish, and destruction of the gear would be the result. In that case an upstream trip would have to occur, and the concerned SF6
breaker would be locked out of operation.

The OEM has to be consulted to get a recommendation, that would cover any discussions in your office.

regards, rasevskii