Electric utility, transmission and distribution companies are committed to strict worldwide greenhouse gas emission targets and regulations for sulfur hexafluoride (SF6) filled high voltage (HV) and medium voltage (MV) electrical installations.
High-quality SF6 gas is extensively used as an arc-extinguishing and insulation medium in the power transmission and distribution industry because of its electrical insulation properties.
In gas insulated switchgear (GIS), for example, the arc interruption and insulation is performed by SF6. In all SF6 applications, the presence of moisture in the SF6 gas will reduce its insulation properties, and in switchgear the moisture may react with SF6 arcing products to produce aggressive compounds. In switchgear, the arcing that occurs has considerable energy and can cause decomposition of the SF6 into other gaseous compounds of sulfur and fluorine. In the presence of water vapor, these fluorides will form highly corrosive compounds, such as hydrofluoric acid. These can cause the degradation of the insulator surfaces, which can then lead to their electrical failure and the potential release of SF6 to the environment. In this application, it is critical to minimize the moisture present in the insulation gas to avoid high repair and maintenance costs, and regulatory fines.
Since measurements are often necessary from many individual equipment items that might be in remote locations, the use of portable instruments is common. The main requirements are that the unit is compact and self-contained, uses a minimal sample volume, and that it gives quick and accurate responses with none of the SF6 exhausted to atmosphere.
To achieve this, Alpha Moisture Systems' portable Model SADPmini dewpoint hygrometer, Model GCTU-B Gas Collection and Transfer Unit, and the new Model GCTU-M Gas Collection and Transfer Unit contain all the necessary pipe work to allow sampling from a gas supply up to approximately 175 psi, including sample flow control, indication and collection of all sampled SF6. To provide quick response, the Model SADPmini dewpoint hygrometer also includes a desiccant dry down chamber. The sampled SF6 collected can be transferred back to the installation or an auxiliary vessel for recycling.
In January 2011, Utility Products featured an article on Alpha Moisture Systems' SF6 solution, the Model GCTU-B Gas Collection and Transfer Unit.
Since then, Alpha Moisture Systems has developed a mains/ac powered version of the Model GCTU-B (battery/cell version)-the Model GCTU-M Gas Collection and Transfer Unit.
Looking at this in more detail, the GCTU's components have undergone rigorous testing to ensure reliability. The unit's protective case is IP67 rated. When on site, the handle can be retracted; using the fixed anti-slip side handles provide more flexible positioning for sample taking.
The GCTU range has been developed with the user in mind. The GCTU-M offers added features and benefits that include a double self-sealing, single-gas collection and transfer point; double self-sealing inlet and outlet instrument points; process pressure indicator; bypass purge facility; sample flow regulator; and approximately 10 minutes of test time.
The key features of the new GCTU-M include:
• Variable mains (ac) input of 100 - 250V 50/60 Hz;
• Unlimited fill and discharge cycles compared with the battery/cell model;
• 175 psig maximum back fill pressure;
• User-friendly operation with built-in, fail-safe features;
• Extra strong IP67 (National Electrical Manufacturers Association [NEMA] 4X6) rugged transportation case with retractable handle and tough, in-line wheels;
• Process pressure indicator;
• By-pass purge facility and large gas collection vessel;
• Sample flow regulator and valve controllable between 0.2 and 4.2 standard cubic feet per hour (SCFH);
• QC4 female self-sealing analyzer connections;
• Approximately 10 minutes individual test time-assuming a sample flow rate of 4 SCFH;
• Quality, heavy-duty discharge pump-maximum discharge pressure 175 psig; and
• Quick, easy to operate with a start-to-finish, on-site sample undertaken in approximately 10 minutes, on average.
In addition to these key features, the GCTU-M accepts many instrument types for several species analysis. For moisture and dewpoint measurement, Alpha Moisture Systems recommends the use of its Model SADPmini hand held dewpoint meter, Model P35 SF6 analyzer, Model DSP-FCI and the SADPµ intrinsically safe dewpoint meters.
How the GCTU-M Works
1. Ensure the sample flow control valve is closed.
2. Attach a sample line from the SF6 source-normally polytetrafluoroethylene (PTFE) with female connector to the gas source male point on the GCTU-M panel.
3. Connect to the mains/ac point and power up.
4. Switch to the test position.
5. Press purge while opening the flow control. This is then collected into the GCTU-M's internal collection vessel of the GCTU-M.
6. Connect to a dew point meter, such as the SADPmini.
7. On analysis completion, press the discharge pump (transfer) function and recycle back to source, or, using an auxiliary line, return to an alternative safe, secure storage vessel.
8. When completed, close sample control valve and power down the GCTU-M.
9. Isolate gas source before removing instrument.
All the SF6 sample has now been safely transferred back to the switch, circuit breaker or auxiliary vessel, and the dewpoint measurement sample taken and logged. Periodically repeating this analysis will indicate early problems or leakages to the HV and MV switchgear or circuit breakers-reducing costs and down time.
The GCTU range is simple to use, inexpensive and an efficient solution to early problem detection with no SF6 release to the atmosphere.
For further details, please contact Alpha Moisture Systems Ltd, UK. Alternatively, the GCTU range is distributed in the US and Canada by Delta Instrument LLC in New Jersey.