Gas Monitor System:
The insulating and interrupting capability of the SF6 gas depends on the density of the SF6 gas
GIS is filled with SF to a density far enough above the minimum density for full dielectric and interrupting capability so that from 10 to 20% of the SF6 gas can be lost before the performance of the GIS deteriorates. The density alarms provide a warning of gas being lost, and can be used to operate the circuit breakers and switches to put a GIS that is losing gas into a condition selected by the user.
Because it is much easier to measure pressure than density, the gas monitor system usually has a pressure gage. A chart is provided to convert pressure and temperature measurements into density. Microprocessor-based measurement systems are available that provide pressure, temperature, density, and even percentage of proper SF6 content.
These can also calculate the rate at which SF6 is being lost. However, they are significantly more expensive than the mechanical temperature-compensated pressure switches, so they are supplied only when requested by the user.
Gas Compartments and Zones:
A GIS is divided by gas barrier insulators into gas compartments for gas handling purposes. In some cases, the use of a higher gas pressure in the circuit breaker than is needed for the other devices, requires that the circuit breaker be a separate gas compartment.
Gas handling systems are available to easily process and store about 1000 kg of SF6 at one time, but the length of time needed to do this is longer than most GIS users will accept. GIS is therefore divided into relatively small gas compartments of less than several hundred kg. These small compartments may be connected with external bypass piping to create a larger gas zone for density monitoring.
The electrical functions of the GIS are all on a three-phase basis, so there is no electrical reason not to connect the parallel phases of a single-phase enclosure type of GIS into one gas zone for monitoring.
Reasons for not connecting together many gas compartments into large gas zones include a concern with a fault in one gas compartment causing contamination in adjacent compartments and the greater amount of SF6 lost before a gas loss alarm. It is also easier to locate a leak if the alarms correspond to small gas zones, but a larger gas zone will, for the same size leak, give more time to add SF6 between the first alarm and second alarm.
Each GIS manufacturer has a standard approach to gas compartments and gas zones, but will, of course, modify the approach to satisfy the concerns of individual GIS users.
Source: ‘Electric Power Substations Engineering’