SafeRing Air - an innovative ring main unit

ABB has further extended the SafeRing/SafePlus ring main unit (RMU) portfolio with SafeRing Air that uses dry air as an alternative, environmentally friendly insulation gas.
Traditional SafeRing/SafePlus ring main units are well suited to the needs of customers looking for closed SF6 handling and a very low leakage rate. Now, the innovative SafeRing Air offers the perfect solution for customers who seek to lower their carbon footprint even further.

Designed specifically for the 12 kV RMU market, SafeRing Air has the same physical dimensions, operation sequence, technical performance and quality as the traditional SafeRing with SF6. But it also comes with a new feature – the ability to upgrade a load-break switch panel to a circuit breaker panel on-site. This is an important difference, not currently available elsewhere, made possible by the use of vacuum technology for current interruption on both fault currents as well as load currents.
A unique gas
SF6 is used as an electrical insulator, as a thermal conductor and to interrupt current flow to enable the design of gas insulated switchgear (GIS) that provides the desired compactness and technical performance.
No alternative gas has been identified so far that exhibits the excellent properties of SF6 for the electrical distribution industry. However, it is also classified as a greenhouse gas.
Challenges
The main technical challenge in developing an RMU with an alternative environmentally friendly insulation gas has been to maintain the same physical size. This was vital since the dimensions place strict conditions on the dielectric and thermal performance.
Dielectric design targeted the distribution of electrical fields within the unit, aiming to reduce the field strength of weak points to compensate for the reduced dielectric strength of alternative insulating gases. Key parameters for optimization include choice of insulating materials, geometrical shape of conducting surfaces and definition of conductor/insulator interfaces. Advanced simulation tools were used for this purpose, and the results validated by dielectric tests in full-scale units.
Thermal design was also critical due to the lower thermal properties of alternative insulating gases, compared with SF6, and had two main objectives. The first was to reduce ohmic losses to a minimum (hence increasing energy efficiency) by utilizing a sophisticated design of select materials, surface treatment, contact forces, etc.
The second objective was to distribute the heat-generated losses through a combination of conduction, convection and radiation. Computational fluid dynamics (CFD) analysis was used to understand and optimize the temperature distributions inside the switchgear. The results of these simulations were validated by temperature rise tests in full-scale units.
Besides these key challenges, other factors were considered in developing the SafeRing Air RMU. ABB is committed to providing customers with the safest products and SafeRing Air is no different. Technical parameters and tests set by the IEC and other relevant standards were utilised to ensure safety when operating the RMU.
Another important aspect was to keep the simple, easy and logical operation of the RMU. To take this fully into account, SafeRing Air was designed using the same logic as traditional RMUs, leading to a familiar operating sequence. And of course environmental measures such as life-cycle assessment (LCA), environmental product declaration (EPD), and environmental product information (EPI) have been met.
Initial customer feedback during the launch of SafeRing Air has confirmed an increasing demand for this type of solution in low-end applications (12 kV).