Why Test SF6 Gas?

  • Improve Safety
  • Minimize Environmental Impact
  • Save Maintenance Dollars
  • Improved Handling Practices

Sulfur Hexafluoride, SF6, is an excellent dielectric with unique arc interruption properties that have led to its successful and widespread use in circuit breakers as well as in gas insulated substations.

Under ideal circumstances, when a discharge occurs, each fluorine on the SF6 may capture an electron and dissociate from the sulfur. When the discharge has ended, each fluorine loses the captured electron and recombines with a sulfur to reform SF6. This is the “self-healing” or regenerative property of SF6. Regardless of circumstances, this is the predominate reaction occurring in SF6-filled high voltage electrical equipment. However, when other species such as oxygen and water from atmospheric contamination, carbon from Teflon interrupter components, copper and tungsten from contacts and aluminum are introduced into the discharge, they can react with the various species that have been created from the dissociation of SF6.

For all of its advantages, SF6-filled equipment is neither maintenance nor trouble free, and while SF6 may eliminate many of the issues of the oil-filled equipment, many new issues have arisen with its use. Safety and performance concerns about arc by-products, environmental concerns over the greenhouse effect of SF6 as well as the expense of new SF6 are among these issues. In addition to causing the utilities to change their basic handling practices, these concerns have created the incentives for utilities to assess, process and reuse their stocks of the gas.

Advantages:

  • Maintenance costs are reduced by identifying breakers in need of maintenance
  • Internal components are monitored reducing the need for internal inspections
  • Gas processing and handling costs are reduced
  • Reliability is improved
  • Safety is enhanced

Application of SF6 Analysis

Analysis of the gas for contaminants and arc decomposition products combined with an understanding of the reactions and reaction conditions provides the basis to assess the condition of the SF6-filled equipment.

Consider the following case history:

Circuit Breaker
 
Compound
Pole 1
Pole 2
Pole 3
 
SF6
Sulfur Hexafluoride
998191
995848
996190
N2
Nitrogen
964
649
2917
O2
Oxygen
341
136
638
CF4
Carbon Tetrafluoride
504
3365
255
SOF2
Thionyl Fluoride
ND
1
ND
CO2
Carbon Dioxde
ND
1
ND

This circuit breaker is a 161 k V, 2000 amp, SF6 single pressure, live tank unit with a true spring operating mechanism(2). It is operated 2 to 3 times per day to connect a 168 MVAR capacitor bank. On September 20, 1997 a remote dispatcher operated the breaker. Pole 2 failed to clear, resulting in operation of the overcurrent and breaker failure relay. The operation of these relays isolated the bus section that feeds the circuit breaker. After the clearance, the dispatcher was able to remotely energize the bus section. This failure did not result in a rupture of the interrupter.

The breaker was inspected for damage. All gas pressures were normal. The spring assembly was recharged after the last close operation. All tests, insulation, timing, function, and resistance were made with satisfactory results. Nothing indicated a problem with the breaker.

These gas samples were collected from all three poles. The high content of CF4 in pole 2 indicated erosion of Teflon. It was decided to remove the gas from the breaker and visually inspect the interrupter. Pole 2 showed arcing between the shields of the moving and stationary contacts. The arcing was not contained within the arcing contact area. Pole 2 interrupter was replaced.

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