Module 3 · Condition monitoring

Watch SF6 density, leaks and partial discharge

Every compartment has its own temperature-compensated density monitor. Open a leak and follow the two-stage alarm into a breaker lockout, heat the hall and see why the reading does not move, then read the UHF partial-discharge pattern of four classic defects.

Gas density monitors · Line bay E-05 · bar(g) at 20 °C

Elapsed 0 yr

Alarm fascia

GZ-4 SF6 LOW · STAGE 1 REFILL

GZ-4 SF6 LOW · STAGE 2 LOCKOUT

52 TRIP / CLOSE BLOCKED

SF6 LOW · REFILL · OTHER ZONE

SF6 BELOW MIN. FUNCTIONAL

UHF PD ACTIVITY HIGH

Leak simulator

Compartment

Leak

Hall temperature

Gujarat GIS halls swing from winter mornings to summer afternoons. Pressure follows temperature; density does not.

Ambient: 25 °C

Pressure gauge: 6.10 bar
Density monitor: 6.00 bar

Both read the selected compartment. A plain pressure switch would false-alarm on a cold morning and miss a leak on a hot afternoon; the density monitor's temperature-compensated element refers every reading to 20 °C.

Density trend · GZ-4 · Breaker 52

Normal

Start a leak to plot the density of the selected compartment. The time axis is compressed so a slow seal weep and a cracked casting can be compared on one screen.

UHF partial discharge · sensor S-3 on GZ-3 · phase-resolved

0 pulses / cycle

No defect

Healthy

Only the sensor's background noise floor shows: a few random low-level points with no relation to the supply phase.

What to do

Nothing beyond routine trending. Keep the alarm threshold just above the noise floor.

Peak level: −70 dBm
Pulses: 0.1 / cycle

Why density, not pressure

In a sealed compartment the mass of gas — its density — sets the dielectric strength. Pressure rises and falls with temperature by roughly 0.02 bar per kelvin at 6 bar, so a pressure reading alone cannot tell a cold morning from a leak. A density monitor uses a temperature-compensated element (or a pressure and temperature sensor with the correction done electronically) and reports bar at 20 °C.

Leak rate limits and gas quality

IEC 62271-203 limits the relative leakage rate of each compartment to 0.5 % per year, and many specifications now ask for 0.1 %. Leaks are found with an SF6 sniffer or an infrared gas-imaging camera. During maintenance the gas is checked for moisture (dew point), purity and decomposition products such as SO2, against IEC 60480 for reuse.

SF6 and the climate

SF6 has a global warming potential of about 23 500 times CO2 over 100 years (IPCC AR5) and lasts in the atmosphere for millennia. Every kilogram is recovered with a gas cart, never vented, and inventories are reported. Alternatives based on fluoronitrile mixtures, and vacuum interrupters with clean air, are entering service at transmission voltages.