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SF6 ring main unit and solid insulation ring main unit

Introduction of SF6 Ring Main Unit

   The SF6 ring network cabinet seals all the live components (load switch, grounding switch, bus bar, etc.) used in a SF6 stainless steel metal box filled with a certain pressure, and is insulated with SF6 gas. The arc extinguishing medium, operating structure, and fuse are still in the air, which is easy to maintain and replace. The incoming and outgoing wires are connected to the cable through the cable terminal head. The cable head is of a touchable type, and there is no exposed live part. A pressure relief valve is installed on the back or lower part of the switch cabinet to protect the safety of the operator in the event of extreme failure. Its structure is more compact, the size is smaller, the performance is more safe and reliable, and because it is completely sealed, it is not affected by dust, condensation or even flood during operation, and can be used in most areas of our country. This type of switch cabinet now has two forms: ① It is made into an independent functional unit according to the function of the switch, which can be combined and connected when needed. The cabinet is small in size and flexible in combination, but the disadvantage is that it is difficult to assemble the cabinet and requires high civil construction. It is very suitable for use in some cable branch boxes with switches and small outdoor switch stations. ② Combine several functional units and put them in a switch box, share SF6, and reserve expansion ports on the upper or side of the switch, which actually becomes a small ring network unit (RMU), which can be used according to different combinations Many models are derived, such as 2 in 1 out, 2 in 2 out, 3 in 1 out, etc., and the incoming unit can be a circuit breaker. The biggest problem of this cabinet type is sealing, including the welding of the shell, the handling of dynamic sealing, the coordination of the expansion socket, and the guarantee of safety and reliability.

   Problems in SF6 ring main unit

   SF6 gas trace moisture problem

   Regardless of whether imported or domestic SF6 ring network cabinets, SF6 trace moisture content test reports are rarely provided. As the equipment operation unit, the power supply company cannot measure the trace water content. There is less gas in the air bag of the SF6 ring network cabinet. If the commonly used DP19 is used for the micro-water test, the air bag gas will almost be prevented and controlled. With the current equipment and technical conditions, it is impossible to perform micro-water measurement. However, the moisture content in SF6 gas directly affects the SF6 arc extinguishing performance and the safe operation performance of the equipment. For the SF6 ring main unit that has been in operation for many years, it will be very difficult to judge its arc extinguishing performance.

  SF6 leakage problem

  SF6 ring main unit may have gas leakage problems when the seal is not tight.

   Most of the semi-inflated ring main units used in the power distribution network of the power supply company often do not set the SF6 pressure indicator, but the actual operation shows that although the imported equipment has good sealing performance, there are also air leakage. Because there is no gas monitoring device, even if the gas leaks, the user will not know it, which is likely to become a hidden danger of accidents, especially the insulation performance of the ring network cabinet under zero gauge pressure and the resistance to internal arc faults. In addition, SF6 gas has a very high global warming potential (GWP) and was included in the list of greenhouse gases announced in the 1997 Kyoto Protocol. In the time frame of 100 years, the GWP of SF6 is approximately 22500 times that of CO2, and CO2 is the main contributor to the greenhouse effect according to the amount of emissions.

  Solid insulated ring main unit

  Introduction of solid insulated ring network cabinet

The solid insulation ring network cabinet is a ring network cabinet that uses solid insulating materials as the main insulation medium. It is a single or combined main conductive circuit such as the vacuum interrupter and its conductive connection, isolation switch, grounding switch, main bus, and branch bus Encapsulated with a solid insulating medium into one or several fully insulated and fully sealed modules with certain functions that can be recombined or expanded. People can touch the ring network cabinet with conductive or semi-conductive shielding layer coated on the surface of the module . The main components of the solid insulated ring network cabinet are: insulation module with switch function, insulated main bus, operating mechanism, cabinet and control part.

  The advantages of solid insulated ring network cabinet

  Environmental protection and no explosion hazard

The solid-insulated ring main unit eliminates pressure gauges, explosion-proof devices, pressure sensors and pressure locking devices, inflation valves, gas pressure alarms and other components. It uses solid insulation technology to separate the vacuum interrupter and the main conductive circuit. Sealing, using the APG process to seal the switch components and the main conductive circuit, which solves the problem of SF6 gas low-temperature liquefaction and high-temperature decomposition of toxic substances and leakage, which can greatly improve the reliability of equipment operation. The connection technology of the insulated busbar can realize easy expansion, flexible combination and convenient adaptation to changes in site conditions. The phase-separated structure and insulation technology will prevent phase-to-phase short-circuit faults. While ensuring the rated operating conditions of the ring main unit, it is truly maintenance-free.

   Adapt to smart grid construction

   The structure design of the solid insulated ring main unit adopts modularization, one-two integration concept design, which can easily realize power grid automation and ensure the reliability and flexibility of the system. At the same time, special PT can be connected on site according to user needs, which can realize online temperature and insulation monitoring of some key positions in the cabinet, and can send the measured data to the control terminal through a variety of communication methods, providing technology and technology for multi-level distribution network automation Device support.

  The key technology to promote the development of solid insulation ring mains

  High-quality sealing technology

  According to the mechanism of partial discharge, it can be concluded that the internal discharge of solid insulation is mainly due to the existence of bubbles inside the solid insulation. Most of the existing solid-sealing technology is to put the pre-heated solid-sealed parts in the pre-heated metal mold, vacuum the metal mold cavity and slowly inject the heat-curing epoxy resin, and take it out after curing. This kind of solid sealing technology is not only low in efficiency and high in cost, but the most important thing is that the air bubbles are not completely handled, which easily causes a large number of bubbles in the solid insulation parts. After the equipment is put into operation, partial discharges are prone to occur, which will cause the solid insulation parts to strike over time. Wear and discharge will affect the safe operation of the equipment. Therefore, it is extremely necessary to adopt advanced, high-quality and high-efficiency epoxy resin sealing technology.

   Better electric field design

   Corona discharge is the discharge that occurs when the electric field strength near the conductor surface reaches the gas breakdown field strength when the electric field is extremely uneven. At the edge of the high-voltage electrode, corona discharge may be caused by electric field concentration around the tip. As a kind of partial discharge, corona discharge will cause breakdown discharge along with equipment operation, which will affect the safe and reliable operation of equipment. It can be seen that designing reasonable conductive parts to ensure that the electric field strength is sufficiently weak and uniform is a key to solving the partial discharge of solid insulating parts. At present, a more effective design method is to use simulation software to calculate the electric field, adopt reasonable electric field distribution, optimize the insulation and electrode shape, and consider adding shielding cover to reduce the electric field intensity when necessary.

  in conclusion

  The ring network power supply method is playing an increasingly important role in our country's distribution network. As the main equipment of the ring network power supply method, the ring network cabinet is developing rapidly. SF6 ring main unit and solid insulated ring main unit each have their own advantages and disadvantages. The development of solid insulated ring main unit still needs to further improve the current solid sealing technology and insulation design level.

Date:2020-09-04 16:45

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