Page 155 - Practical Power System and Protective Relays Commissioning

154 Practical Power System and Protective Relays Commissioning

14.12.2 Gas Service Truck
All SF 6 equipment must be filled initially and after a time may need refill-
ing. To perform overhaul or maintenance on this equipment the gas must be
removed. Releasing the gas into atmosphere would be very expensive and
possibly dangerous if done indoors. Most SF 6 gas service trucks are equipped
to provide storage, purification, and transfer of gas to and from equipment
(Fig. 14.14). The primary functions of an SF 6 gas service truck are to remove
the SF 6 from a switchgear or CB, store it, clean and dry it, and return it to
the switchgear or CB. The major components required to achieve this are:
Vacuum pump, for pulling SF 6 out of the switchgear, and for creating a
vacuum prior to refilling the switchgear with SF 6 .
An SF 6 storage tank.
A compressor for pumping SF 6 into the storage tank or back into the
switchgear.
A filtering system for removing moisture and other contaminants from
the gas prior to its return to the switchgear.
These major components are interconnected by a series of valves and
pipe work in such a way that the gas truck can be operated in several differ-
ent modes. Several important functions of the truck are monitored. Before
filling the equipment, the ambient temperature must be measured and the
pressure, temperature, and density chart consulted to determine the correct
operating pressure for the temperature.
Every effort must be made to prevent air from entering and mixing with
the gas, by means of purging or evacuating portable hoses, and by correct
valve selection.
When equipment has been open to the atmosphere, it must be evacuated
to remove air and any absorbed moisture before refilling. After filling the

FIGURE 14.14 SF 6 gas filling truck. HMI, human machine interface.