Sulphur hexafluoride (SF6) is an inert, colorless, odorless, non-flammable, and chemically stable heavy gas having good dielectric and arc extinguishing properties. SF6 is used as an insulating gas in substations, as an insulating and cooling medium in transformers and as an insulating and arc quenching medium in switchgear for high and medium voltage applications. Innumerable benefits such as less maintenance costs, less environmental effect and personnel’s safety can be achieved by application of sulfur hexafluoride (SF6) to substation equipment like gas-insulated circuit breaker (GCB), and gas insulated substation (GIS).
First, circuit breakers are classified in many ways such as by types of voltage, installation, external design, and interrupting medium. SF6 gas circuit breaker falls in the last category of circuit breakers. SF6 gas circuit breakers are used at EHV and UHV level. Measuring instruments are designed to monitor air, trace leaks and to quantify the SF6 quality of electrical operating equipment. The gas quality has to be monitored in accordance with the EC 842/2006 directive on fluorinated gases. While IEC 60480 directive which stipulates values for the re-utilization of SF6 gas, the main criteria for SF6 gas in circuit breakers is humidity, decomposition, and gas purity.
Second, GIS utilizes a superior dielectric gas, SF6, at moderate pressure for phase-tophase and phase-to-ground insulation. In a gas insulated substation, the high-voltage conductors, circuit breaker interrupters, switches, current transformers, and voltage transformers are encapsulated with SF6 gas inside a metal enclosure. The atmospheric air insulation utilized in a conventional, air-insulated substation (AIS) requires meters of air insulation, whereas, SF6 requires only a few centimeters. GIS can therefore be miniscule than AIS by up to a factor of 10.
Having said that, problems may arise in a GIS, for example, Very Fast Transient Overvoltages (VFTOs) are engendered due to switching operations of a circuit breaker or a disconnector switch. These transient overvoltages and the associated Very Fast Transient Currents (VFTC) could have elevate times as low as 3-20 ns. The peak magnitude of the transient current could be a few kA depending on the location of the switching apparatus under consideration. Principally, gas insulated systems are predicated on the concept of consummate encapsulation of all the HV energized components in a grounded enclosure and compressed SF6 gas is utilized as the insulating medium between the two. The transient voltages and currents radiate transient Electromagnetic (EM) fields during their propagation along with the gas insulated bus as the associated frequencies are in the range of a few MHz to about 300 MHz. The transient fields due to these transient currents may leak into the external environment through non-metallic flanges, gas-to-air bushing, gas-to-cable termination, non-metallic viewing ports etc. Malfunctioning of primary or secondary equipment have been reported during switching operations in substations due to induced voltages on the control circuits. Hence, characterization, quantification, and monitoring of radiated transient EMI in GCB as well as GIS during the switching events is essential.
Today‘s SF6 apparatus is very gas-tight and generally only a little of the gas can leak out. The gas is monitored perpetually, and any leakage could be discovered at an early stage. Therefore, insulating gas analysis can be most effective when applied to a number of routine testing and maintenance programs, especially verifying the quality and condition of SF6 content.
At Amperis, we understand your goals and always remain at the forefront of electrical technology, giving us a leading edge in the industry. Our latest generation instrumentation for testing, transformation diagnosis, thermographic inspection and electrical distribution is truly innovative. One of our latest innovation is the Amperis portable Pumback SF6 Gas Analyzer Transdox 6101 SF6, specifically designed for controlling and monitoring the quality of SF6 in gas insulated sub-stations and circuit breakers with zero waste and emissions. Once connected, the Transdox automatically samples the gas from the GIS or GCB by taking five periodic measurements before the gas is pumped back automatically. A vacuum purge cycle and gas storage system ascertains that no air is allowed to contaminate the gas sample and no SF6 is allowed to elude during the testing period.
This portable analyzer then simultaneously measures SF6, SO2 and H2O content by utilizing a high-precision infra-red SF6 detector, and offers exceptional accuracy and stability when measuring the purity of SF6. The specially designed modular structure can hold up to six sensors fitted within a single analyzer in order to suit a variety of customer requirements. With a 7” (18cm) full color touch screen interface with soft menu keys and a thermal printer for permanent record keeping, the Amperis fully automatic gas analyzer is not only easy to operate and configure, but fully complies with all current IEC and CIGRE test configurations, and permanently logging data. For additional features, specification, and applications, please see further details about this truly innovative product.