Discussion on SF6 Ring Network Cabinet and Solid Insulated Ring Network Cabinet


Release time:

2021-01-12

All the live elements (load switch, earthing switch, busbar, etc.) used in SF6 ring network cabinet are sealed in SF6 stainless steel metal box filled with certain pressure and insulated with SF6 gas. The arc extinguishing medium, operating structure and fuse are still in the air for easy maintenance and replacement. The incoming and outgoing lines are connected to the cable through the cable terminal head, which is of a touchable type without exposed live parts. A pressure relief valve is installed on the back or lower part of the switch cabinet to ensure the safety of the operator in case of extreme failure.

 

  Introduction of SF6 ring network cabinet
All the live elements (load switch, earthing switch, busbar, etc.) used in SF6 ring network cabinet are sealed in SF6 stainless steel metal box filled with certain pressure and insulated with SF6 gas. The arc extinguishing medium, operating structure and fuse are still in the air for easy maintenance and replacement. The incoming and outgoing lines are connected to the cable through the cable terminal head, which is of a touchable type without exposed live parts. A pressure relief valve is installed on the back or lower part of the switch cabinet to ensure the safety of the operator in case 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 floods during operation, and can be used in most areas of our country. This type of switch cabinet now has two forms: ① according to the role of the switch to make an independent functional unit, when needed can be combined, connected. 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 engineering. 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 extended interfaces on the upper or side of the switch, which actually becomes a small ring network unit (RMU). According to different combinations, many models can be derived, such as 2-in-l-out, 2-in -2-out, 3-in -1-out, etc., and circuit breakers can be selected for incoming units. The biggest problem of the cabinet type is sealing, including the welding of the shell, the processing of the dynamic sealing, the cooperation of the expansion socket, the guarantee of safety and reliability, etc.


  Problems Existing in SF6 Ring Network Cabinet
The Problem of Trace Water Content in SF6 Gas
Whether imported or domestic SF6 ring network cabinet, rarely provide SF6 trace water content test report. As a unit of equipment operation, the power supply company cannot measure the trace water content. There is less gas in the gas bag of SF6 ring network cabinet. If the commonly used DP19 is used for micro-water test, the gas bag gas will almost be prevented and controlled. With the current equipment and technical conditions, micro-water measurement is not possible. However, the water content in SF6 gas directly affects the performance of SF6 arc extinguishing and the safe operation of the equipment. For the SF6 ring network cabinet that has been running for many years, how to judge its arc extinguishing performance will be very difficult.



  SF6 leakage problem
SF6 ring network cabinet may have gas leakage problem when sealing is not tight.
Most of the semi-inflatable ring network cabinets used in the power distribution network of the power supply company often do not set SF6 pressure indicator, and the actual operation shows that although the imported equipment has good sealing performance, there are also air leaks. Since there is no gas monitoring device, even if the gas leaks, the user does not know, and it is easy to become a hidden danger of accidents, especially the insulation performance and internal arc fault resistance of the ring network cabinet under zero gauge pressure. In addition, SF6 gas has a very high global warming potential (GWP) and was included in the list of greenhouse gases announced by the 1997 Kyoto Protocol. In the 100-year time frame, the GWP of SF6 is about 22500 times that of CO2. According to emissions, CO2 is the most important contributor to the greenhouse effect.



 Solid insulated ring network cabinet
Introduction of solid insulated ring network cabinet
Solid insulated ring network cabinet is a ring network cabinet that uses solid insulating material as the main insulating medium. It is a single or combined main electric circuit of vacuum interrupter and its conductive connection, isolating switch, grounding switch, main bus, branch bus and so on, which is covered and packaged with solid insulating medium into one or several modules with certain functions, which can be combined or expanded again and have full insulation and full sealing performance, A ring network cabinet where the surface of the module is coated with a conductive or semi-conductive shielding layer. The main components of the solid insulated ring network cabinet are: insulation module with switch function, insulated main bus, operating mechanism, cabinet body and control part.



  Advantages of solid insulated ring network cabinet
Environmental protection and no explosion hazard
The solid insulated ring network cabinet eliminates the pressure gauge, explosion-proof device, pressure sensor and pressure locking device, inflation valve, gas pressure alarm and other components. It adopts solid insulation technology to insulate and seal the vacuum interrupter and the main electric circuit separately. It adopts APG process to seal the switch components and the main electric circuit, thus solving the problems of low-temperature liquefaction of SF6 gas and high-temperature decomposition of toxic substances and leakage, can greatly improve the reliability of equipment operation. Through the connection technology of the insulated busbar, easy expansion, flexible combination and easy adaptation to changes in field conditions can be achieved. The phase-split structure and insulation technology will not cause phase-to-phase short-circuit fault. While ensuring the rated operating conditions of the ring network cabinet, maintenance-free is truly realized.
Adapt to smart grid construction
The structural design of the solid insulated ring network cabinet adopts the modular, primary and secondary fusion concept design, which can easily realize the automation of the power grid and ensure the reliability and flexibility of the system. At the same time, special PT can be connected on site according to user requirements, which can realize on-line temperature and insulation monitoring of some key positions in the cabinet, and can send the measured data to the control terminal through various communication modes, thus providing technical and equipment support for multi-level distribution network automation.





  Key Technologies to Promote the Development of Solid Insulated Ring Network Cabinet
High quality solid 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 technologies are after the preheated solid sealed parts are put into the preheated metal mold, the metal mold cavity is vacuumed and slowly injected with the heat-curing epoxy resin, and the heat-curing epoxy resin is taken out after curing. Such a solid sealing technology is not only low efficiency and high cost, the most important thing is that the bubbles are not treated thoroughly, which is easy to cause a large number of bubbles inside the solid insulation. After the equipment is put into operation, partial discharge is prone to occur, which leads to the breakdown of the solid insulation over time. Discharge affects the safe operation of the equipment. Therefore, the use of advanced, high-quality and high-efficiency epoxy resin sealing technology is extremely necessary.



Better electric field design
Corona discharge is a 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 occur around the tip due to electric field concentration. As a kind of partial discharge, corona discharge will lead to breakdown discharge with the operation of the equipment, which will affect the safe and reliable operation of the equipment. It can be seen that designing reasonable conductive parts to ensure that the electric field strength is weak and uniform is a key to solve the partial discharge of solid insulation. At present, the more effective design method is to use simulation software to calculate the electric field, to take a reasonable distribution of electric field, to optimize the insulation and electrode shape, if necessary, should also consider the addition of shielding cover and other measures to reduce the intensity of the electric field.


  Conclusion
The power supply mode of ring network is playing an increasingly important role in the distribution network of our country. As the main equipment of the power supply mode of ring network, the development of ring network cabinet is also increasingly rapid. SF6 ring network cabinet and solid insulation ring network cabinet have their own advantages and disadvantages, the development of solid insulation ring network cabinet also need to further improve the current solid sealing technology and insulation design level.