Application of Ring Main Units in Distribution Systems
With continuous economic development and the increasing impact of electricity on people's lives, particularly in urban areas with high load density, the reliability of power supply is especially crucial. Establishing a distribution network primarily based on a ring main structure can effectively improve power supply reliability, ensure supply continuity, and minimize the impact of distribution equipment failures and maintenance outages. As a key device in the ring main operation mode, the Ring Main Unit (RMU) is widely used in distribution substations and compact substations at load centers such as urban residential quarters, high-rise buildings, large public structures, and industrial plants, thanks to its advantages like simple structure, compact size, low cost, and ability to improve power supply parameters, performance, and safety.
1. Types of Ring Main Units
In distribution systems, equipment capable of performing ring main functions mainly includes ring main type cable branch boxes and RMUs. Ring main type cable branch boxes are lower in cost and offer more flexible installation locations. They are particularly advantageous in built-up urban areas where obtaining space for a distribution room (which is necessary for RMUs) is difficult, demonstrating their flexibility. However, compared to RMUs, the biggest disadvantages of ring main branch boxes are inferior safety performance (especially regarding measures against misoperation), unsatisfactory operating environments, and certain risks. If conditions permit, the author recommends prioritizing the use of RMUs for ring main configurations. RMUs can be classified into several types based on the type of load switch used: air-blast RMUs, piston-type RMUs, vacuum RMUs, and SF6 RMUs. Among these, air-blast and piston-type RMUs have been largely phased out due to fatal flaws and low reliability in their load switches. Vacuum RMUs and SF6 RMUs are widely used in distribution systems because of their high performance, reliable operation and maintenance.
1.1 Vacuum Ring Main Units
Years of domestic production and use of vacuum circuit breakers and vacuum switchgear have made vacuum technology relatively mature in China. Domestically developed vacuum RMUs have demonstrated high performance in type tests but have not seen widespread use. The main reason is the underperformance of the operating mechanism. The design of operating mechanisms for vacuum switches is relatively complex, and due to the level of domestic raw materials, processing technology, and quality control, the quality of operating mechanisms produced by domestic manufacturers has not yet truly met the mark. Operation and maintenance are difficult, and how to measure the vacuum degree in vacuum RMUs is a major challenge in maintenance.
1.2 Advantages and Disadvantages of SF6 Ring Main Units
The application of SF6 RMUs in distribution systems is mainly dominated by imported products. They are widely welcomed by power supply departments due to their excellent performance, reliable operation, fully insulated sealed design, and maintenance-free advantages. Typical SF6 RMUs include Schneider's RM6, ABB's SafeRing, and Siemens' 8DJ20. However, there are also some shortcomings during operation.
1.2.1 Advantages of SF6 RMUs:
(1) High Performance Specifications: SF6 RMUs have a high operating frequency, capable of making and breaking rated active loads up to 100 times. They also possess good breaking capacity and can withstand high currents.
(2) Convenient Maintenance: The cabinet surface design is user-friendly. Clear wiring diagram prompts on the panel provide guidance for operation. Some products even note precautions on the cabinet surface, further reducing the incidence of operator error. Most RMU products are equipped with devices that can detect the live status of the main circuit, providing indication of the live condition and, when combined with electromagnetic locks, preventing the handle door from being closed when live, reducing misoperations. Additionally, a transparent acrylic observation window on the front door allows direct viewing of the switch's open/close status, which is very convenient.
(3) Strong Flexibility: Modern RMUs can very flexibly meet the requirements of various distribution network designs and can be combined arbitrarily according to actual situations. Furthermore, cable connection methods are also very flexible, allowing for adaptable connections even on uneven ground surfaces without causing partial discharges.
1.2.2 Disadvantages of SF6 RMUs:
(1) Inflexible Configuration: They can only be selected from a limited number of schemes provided by the manufacturer, making it difficult to meet various user-specific needs.
(2) Inability to Expand: After the switchgear is commissioned, expansion is generally not possible.
(3) Requirement for Specialized Accessories: They require specialized accessories like specific cable terminations, which can be costly.
(4) Stringent Installation Requirements: If installation requirements are not met, the units may not achieve their intended performance.
Due to the inflexible configuration of fully sealed SF6 RMUs, the usage of expandable semi-sealed SF6 RMUs in distribution networks has increased. Semi-sealed RMUs have independent gas compartments for each unit, making them easy to expand, install, and replace. Currently, widely used RMUs include Schneider's SM6, ABB's Uniswitch, and Siemens' 8DH10. As domestic manufacturers gradually master SF6 load switch technology, the quantity and quality of domestically produced SF6 RMUs are steadily improving. However, currently, the market for domestic 10kV and 20kV SF6 RMUs is still primarily dominated by foreign companies (like Schneider or ABB).
2. Issues with SF6 Ring Main Units
2.1 Moisture Content in SF6 Gas
SF6 RMUs seldom come with moisture content test reports. As the equipment operator, power supply companies often cannot measure the moisture content themselves. The moisture level in the SF6 gas directly affects its arc-extinguishing performance and the safe operating performance of the equipment. For SF6 RMUs that have been in operation for years, assessing the state of their arc-extinguishing capability is a challenge.
2.2 SF6 Gas Leakage Issues
SF6 RMUs may have sealing issues leading to gas leaks. Practical experience shows that although imported equipment generally has good sealing performance, leakage incidents still occur. Since most units lack gas monitoring devices, users may be unaware of leaks, potentially creating hidden dangers. This is particularly concerning regarding the performance (insulation, switching, etc.) of the RMU at zero gauge pressure and its ability to withstand internal arc faults. Many of these products use manual operating mechanisms, and operators work in close proximity. An accident could have serious consequences. Currently, including a pressure indicator has become a mandatory requirement, included as a necessary accessory for semi-sealed RMUs.
2.3 Mechanism Issues
In distribution transformer protection, combination units using load switches plus fuses are common. The load switch interrupts load current, and the fuse interrupts short-circuit and overload currents. In Hebei's distribution network, incidents occurred with fully sealed RMUs where the fuse blew, but the load switch failed to open reliably, preventing the faulty transformer from being de-energized and causing severe damage. The cause was an excessive travel in the trip wire of the operating mechanism controlled for tripping, which prevented the impact force from the fuse striker pin from successfully activating the load switch's tripping mechanism. This defect can be resolved by adjusting the trip wire and the tightness of the nuts. Additionally, simulating the fuse operation for transformer feeder units has been included as a mandatory pre-commissioning test.
2.4 Material Issue of the Equalizing Shield
Semi-sealed RMUs typically cannot use touchable cable terminations. Equalizing shields are often used to address insufficient phase-to-phase distance at the cable termination connection points. However, aluminum equalizing shields are highly susceptible to damp environments. Even when used with anti-condensation heaters, their effectiveness in humid conditions is limited. In 20kV distribution systems, severe corrosion of these shields has been observed. Surface roughness and white powdery corrosion products disrupt the electric field uniformity on the shield surface, negating the equalizing effect. Due to the small phase-to-phase distances around the shields, combined with daily temperature variations, condensation forms at the bottom of the gas compartment and can flow back to the shield area, creating a discharge path. The epoxy material of the insulating barriers at the bottom of the gas compartment can suffer severe electrical corrosion, eventually leading to interphase discharge paths and ultimately surface insulation breakdown. This entire discharge process is gradual. To address condensation, power supply companies can modify the RMU's equalizing shield, switching to silicone rubber insulating cable end covers. These covers internally use a semiconductor layer, which can still provide an equalizing effect. The improved RMU design has passed condensation and withstand voltage tests and is slated for trial operation in the distribution network.
3. Recommendations for SF6 RMU Selection
(1) Choose expandable RMUs: Their flexible configuration, easy installation, and ease of expansion represent the future direction for SF6 RMU use.
(2) Consider maintenance: Ideally, the SF6 load switch should be equipped with a device to monitor SF6 pressure. Otherwise, it should have passed a zero-gauge pressure switching test.
(3) Consider climate and location: Select products that have passed condensation tests. For small ring main units, like terminal units, where future expansion is not considered, using fully sealed RMUs can significantly reduce the impact of condensation on the equipment.
Summary
Years of operational practice show that among various RMU types, SF6 RMUs offer high performance, reliability, compact size, low space requirements, and minimal maintenance, leading to their widest application. Considering various factors such as maintenance costs, secondary investments, and reliability, it is recommended, where conditions allow, to prioritize the use of SF6 RMUs in distribution network renovation and construction projects. During planning and construction, sufficient consideration should be given to incorporating automation devices and adopting safe, reliable, and advanced equipment to enhance distribution levels, making the distribution network more reliable and secure.
