Gas density monitoring with reference gas comparison

The reference gas comparison principle was invented by Trafag in mid-1980s and was continuously improved. Today it is the leading industry standard for temperature compensated insulating gas density monitoring in applications with high demand for reliability, accuracy, stability and longevity.

Necessity for temperature independent density monitoring

Density measurement in pressurised, gas-insulated compartments is all about physics. Pressure, density and temperature are in a certain relationship to each other. The relationship is defined by isochores (constant-volume process) for each specific insulation gas. The insulating performance of a gas-insulated compartment is achieved through  defined density which results into a certain pressure at a given temperature. In a closed and tight compartment, the overall density always remains constant, but temperature variations lead to a variation of the system pressure.

Lines exemplary representing constant SF6 gas density (isochores): Changes in pressure and temperature with constant volume.

Absolute monitoring principle (temperature compensated due to reference principle)

A density monitor is typically directly mounted to the pressure compartment of the high-voltage equipment (a) via a customizable process connection (b). Trafag density monitors are based on a reference chamber (c) incorporating a metal bellows system (d), which is pre-pressurized with the customer specific insulating gas. The metal bellows system allows a direct temperature coupling of pressure compartment gas and gas filling in the reference chamber. Ambient temperature changes affect the pressure (isochoric change) in the gas compartment to the same extent as they affect the pressure in the reference chamber.

Therefore, the effect of temperature on insulating gas pressure is inherently compensated and a very precise insulating gas pressure @ 20°C (equalling the density), at any temperature, is indicated on a dial face (i). No false alarm is triggered due to temperature-induced pressure changes. Reference gas chamber and pressure compartment are both hermetically sealed systems. Ambient pressure has no influence on the operating principle. Therefore, it is an absolute monitoring principle.

Bellows system actuates microswitches 

The pressure, more specifically the density of the insulating gas compartment is compared via the outer bellows volume (e) with the pre-defined density of the hermetically sealed inner bellows volume (f) of the reference chamber. If the density of the gas compartment alters, the bellows system actuates via a switch rod and a spring-loaded switch plate (g) up to four independent microswitches (h). Each microswitch can be factory-calibrated either to increase or decrease pressure alarm.

That means when the density drops below pre-defined switchpoint (SP) settings, the microswitch contacts gradually close or open. The switchpoint accuracy is factory tested
at -25°C, +20°C and 50°C.

Supporting measures for demanding outdoor applications

If the local, environmental effects hamper a direct temperature coupling of pressure compartment (a) and reference gas chamber (c), e.g. outdoor installation with diurnal solar radiation or rapidly changing or extreme weather conditions, specifically designed thermal covers maintain the necessary equality between pressure compartment and reference gas chamber.

Practical example:

  • Filling pressure (density) of insulating gas compartment: 6.1 bar abs. @ 20°C, pure SF6
  • SP1: 5.7 bar abs. @ 20°C, decreasing warning switchpoint for compartment re-filling
  • SP2: 5.5 bar abs. @ 20°C, decreasing lock-out alarm switchpoint
  • SP3: 5.5 bar abs. @ 20°C, redundant decreasing lock-out alarm switchpoint
  • SP4: 6.4 bar abs. @ 20°C, increasing high-alarm switchpoint for compartment overpressure
  • Factory pre-pressurised inner bellows volume of reference chamber: 5.7 bar abs. @ 20°C, SF6, hermetically sealed, according to SP1

If the insulating gas compartment pressure (a,e) drops due to leakage, the hermetically sealed inner bellows volume pressure (f) gains impact towards the dropping compartment pressure. Switch rod with switch plate (g) move down.

While the pressure drops below switchpoint 1 (SP1) at 5.7 bar abs. @ 20°C, the first microswitch changes over and induces first-alarm. Usually, the first-alarm indicates that the pressure compartment must be re-filled.

If the pressure drops further, in the example below 5.5 bar abs. @ 20°C, then usually two more, redundant microswitches change over (SP2 and SP3). By default, these switchpoints are used as emergency stop; the operational safety of the system is no longer guaranteed. A fourth microswitch (SP4) e.g. can be used to monitor undesired overpressure conditions during re-filling routines of the pressure compartment. If the pressure rises above 6.4 bar abs. @ 20°C, the microswitch changes over and induces high-alarm.

Reference gas comparison is deployed in the following Trafag devices: