Medium Voltage GIS

It depends on what is in the maintenance information you received when it arrived, Murphy.

In an ideal gas, density and pressure are proportional. Therefore, either measures the other. Which do you think is easier?

It would depend on the quality of the switchgear I guess. My experience with high quality SF6 insulated 33kV switchgear (selling, designing with, testing, training and commissioning) would be that the pressure monitoring device is enough.

If the switchgear is of low quality where leakage, moisture ingress, etc is a potential problem due to poor containment design or SF6 gas degradation due to harsh usage, environmental conditions, end of life (etc) then pressure monitoring alone may not be enough.

A pressure monitor usually is just an absolute value device - it tells you the pressure no matter the conditions. The typical gas density switches used in GIS (and other SF6 insulated equipment) are temperature compensated, so that they measure accurately across the range of acceptable operating temps.

So if you expect the temp in which your GIS is operating will stay pretty consistent (such as indoors in a climate-controlled building), you can go with the manometer pressure monitor. If the temp is expected to vary, pay the extra for a temp-compensated gas density switch.

If I were paying the big bucks for a GIS installation, I'd also pay the little extra for option #2.

Couldn't he just take P and T readings and use simple math in a PLC to find density? That's assuming he needs density explicitly, maybe P and T OK for monitoring. Though it would have to be transducers, not switches.

The purpose is not specifically knowing what the pressure is, but rather that it is or is not adequate for insulation. The magnitude is not so important.

Usually there are contacts operated on specific pressure values, to indicate low gas pressure alarm point (Warning!) and low gas pressure trouble point (Danger!). The first should cause maintenance personnel to investigate and make repairs, while the second should initiate emergency action such as tripping the breaker, blocking operation, etc.

So the temperature compensated density switch is important, because at a high temp the manometer may indicate enough pressure to appear adequate and not cause an alarm, but have low enough density not to provide proper insulation. And at low temps, the manometer-based switch may indicate too low pressure and alarm unnecessarily for adequate gas density / insulation.

Thanks Mr. Peter T for the good explanation.