What are the most important factors of quality criteria for SF₆ gas in gas insulated switchgear (GIS)?

The following is of great importance: gas density, moisture concentration, decomposition products, gas purity (air concentration) which are determined by means of special SF₆ measuring instruments as well as gas density controllers. DILO supplies the measuring devices required for the determination of the SF₆ gas quality and for the verification of the switching points of density controllers.

Why is the SF₆ quality in the GIS circuit breaker of great importance?

The gas quality is the decisive factor for the correct and reliable function of the switchgear. Moisture or toxic and corrosive decomposition products in circuit breakers might have a negative impact on the insulation properties and switching capacity.

When can SF₆ be reused?

Reuse of SF₆ is allowed if the previous measuring results of the switchgear comply with the limit values stipulated in the IEC 60480 regulation (air, moisture and SO₂ concentration). But essentially the indications of the switchgear manufacturers regarding the minimum quality of SF₆ gas are to be followed. 

What is to be observed when working with SF₆ gas?

In accordance with the EC 517/2014 directive and by implementing the 2015/2066 regulation, only certified personnel is allowed to carry out SF₆ gas handling in Europe. The 2015/2066 regulation stipulates the minimum requirements for the certification of personnel.

DILO is an accredited company and is therefore allowed to hold training courses for the acquisition of the permission to carry out the recovery of SF₆ gas.

What is the function of a service cart?

A service cart enables the recovery of the SF₆ gas from a gas compartment to a final vacuum of < 1 mbar. Thereby the gas flows automatically via dry and particle filters which effectively adsorb decomposition products and moisture from the gas. During the gas recovery process the SF₆ is stored either in gaseous or in liquid form. Furthermore, the air is evacuated from gas compartments after maintenance works. Then, the gas compartments are refilled with SF₆ to the desired operating pressure. The gas is led via filters and is purified once again while filling. 

What are the most important criteria when selecting a service cart?

The most important criteria are the volume of the gas compartment, filling pressure, gas volume to be stored, the desired time for the gas recovery, evacuation and filling processes as well as the final vacuum desired for the gas recovery process (< 5 or < 1 mbar). Our service cart selection tool allows for (pre-)selection of the appropriate service carts, taking your particular requirements into account.

How can SF₆ gas be stored?

SF₆ can be stored in gaseous form, and pressure liquefied depending on the temperature. Pressure liquefaction is achieved by means of compressors installed in the SF₆ maintenance equipment. Thus, it is possible to store considerably higher gas quantities into pressure vessels.

What is the final vacuum that SF₆ should be recovered to?

The IEC 62271-4 guideline stipulates a limit value of < 20 mbar. However DILO recommends a recovery down to < 1 mbar and therefore offers SF₆ maintenance equipment allowing a recovery of < 1 mbar.

When should the SF₆ gas quality be measured?

The SF₆ gas quality must be checked prior to the recovery of SF₆ from gas compartments, in particular with regard to possible reuse in accordance with the IEC 60480 directive. Also check whether a pre-filer unit should be used to protect the service cart or whether personal protective measure should be taken.

The key parameters here are the SF₆ purity (%), the moisture content and the concentration of decomposition products. In general, the gas quality must always be checked if a decision regarding the reuse of the SF₆ gas must be made or in the context of condition-based maintenance to prepare a trend analysis.

What must be done after having found gas contaminated with decomposition products in the circuit breaker?

In this case a pre-filter unit should be installed between the gas compartment and the service cart which adsorbs most of the decomposition products and moisture. This is to avoid contamination of the SF₆ service cart and purifies the gas again during the recovery. In addition, personal protective measures should be taken before opening the gas compartment.

How can the gas quantity in the SF6-filled production equipment be determined?

Since the introduction of the first (EU) F-gas regulations, the SF₆ gas quantity filled in the components must be indicated on the type plate. The SF₆ gas quantity can be recorded during the gas recovery by means of a mass flow meter or cylinder scales at liquid storage. In case of old SF₆ filled components please get in contact with the manufacturer.

Why must electrical production equipment be evacuated prior to SF₆ filling?

Before filling with SF₆ gas the air and moisture must be evacuated from the gas compartment to a final vacuum of < 1 mbar in order to prevent mixing with air and / or humidification. 

When are N₂/SF₆ gas mixtures or CF₄/SF₆ gas mixtures used?

These mixtures are used at low temperatures to avoid liquefaction of the gas at low temperatures. The same puncture strength as pure SF₆ is achieved by higher system pressures.

Is it possible to separate SF6 by means of a SF₆ maintenance device?

No it isn't possible. For this purpose special SF₆/N₂ separating units are necessary. After the separating process the gas complies with the quality criteria for reuse stipulated in the IEC 60480 directive. However, DILO separating units allow the SF₆ gas mixture to be purified to the extent that it even meets the standards of IEC 60376 (directive for new SF₆ gas technical quality) with regard to moisture, SF₆ percentage and decomposition products (SO₂).

However, the SF₆ service cart can improve the quality of the gas regarding the decomposition products and humidity. You will find more detailed information on the preparation of used SF₆ gas in the DILO Certified Gas section.

Which decomposition products may occur in case of failure in a GIS unit?

Toxic and corrosive decomposition products may occur. Corresponding measures for occupational health and safety in accordance with the local regulations DGUV Information 213-013 (= German social accident insurance concerning SF₆ machinery and equipment) which are in effect in your country are to be taken. For this purpose a special working protection kit has been compiled. 

Which safety measures are to be taken if decomposition products are found in circuit breakers?

Please follow the local regulations for occupational health and safety DGUV Information 213-013 (= German social accident insurance concerning SF₆ machinery and equipment) which are in effect in your country. 

What must be observed for the transportation of SF₆ maintenance devices containing SF₆ gas?

In accordance with the ADR guidelines applicable in Germany, SF₆ may only be transported on roads under an exemption if the pressure of the gas in the maintenance device or storage tank is a maximum of 2 bar at a temperature of 20 °C. Further information on the transport regulations applicable in Germany can be found in the DILO Certified Gas/Transport Regulations section.

How can I reduce the pressure of a SF₆ service cart to transport pressure?

In order to reach a transport pressure of max. 2 bar p_e, a transport pressure reduction with pressure equalisation generally needs to be carried out in the device. Most DILO devices are equipped with a special function implemented in the control which automatically establishes this pressure equalisation.

What is meant by SF₆ monitoring?

This means the stock management of SF₆ gas. The aim is to record all incoming and outgoing SF₆ gas in order to create a mass balance.

How can I detect an SF₆ leak?

A drop in pressure or density, which can be read on the pressure/density monitors installed at the SF₆ gas compartment, indicates a leakage. Room monitoring devices also trigger an alarm in the event of an SF₆ leak.

What can I do with used SF₆ gas? Can I reuse it?

If the limit values stipulated in the IEC 60480 regulation for SF₆ purity (%), moisture content and concentration of decomposition products are met, the gas can be reused. Otherwise DILO offers the preparation of used SF₆ gas. Further information can be found in the "DILO Certified Gas" section.