10kV SF₆ Ring Main Units (RMUs) Common Fault Prevention Measures

10kV SF₆ Ring Main Units (RMUs) Common Fault Prevention Measures​

During the development of urban distribution network cabling, 10kV SF₆ Ring Main Units (RMUs) (European-style), serving as ring power supply nodes, have been widely adopted due to their features such as full insulation, complete enclosure, maintenance-free operation, compact size, and flexible, convenient installation. However, as the number of RMUs in use increases, the occurrence of faults within RMUs has also gradually risen.

​1 Common Faults​

• ​RMU Busbar Connection Faults:​​ RMU expansion busbars mostly utilize plug-in silicone rubber connectors, fully insulated and shielded to ensure electrical conductivity reliability and resistance to surrounding environmental influences. They allow for arbitrary connection and combination based on actual needs. However, leakage of SF₆ gas, caused by various reasons, reduces the RMU's insulation level and arc-extinguishing capability, making busbar connection faults and insulation breakdowns still likely to occur.
• ​Faults at the Junction between RMU and Three-Core Cable:​​ During the installation of three-core cables, phase sequence verification is often necessary, requiring the application of external torsional force before fixing. After installation, the internal stress generated by this twisting gradually releases, creating a restoring torque acting on the bushings. This easily leads to bushing cracks, causing high-voltage short circuits.
• ​Faults at the RMU Cable Termination:​​ The cable compartment space in RMUs is relatively small, placing high demands on the cable termination manufacturing process. Inadequate handling of the conductor, semiconductor layer, or shielding layer can easily lead to cable breakdown due to insufficient creepage distance at the termination.

​2 Prevention Measures​

• ​Securing Large Cross-Sectional Cables Entering the RMU:​​
  Three-core cables entering the RMU ​must be secured​ using cable clamps directly below the high-voltage bushings. Otherwise, the cable will exert twisting or pulling forces on the bushings. Sustained stress can damage the seal between the bushings and the cabinet, leading to SF₆ gas leakage, bushing cracks, and ultimately high-voltage short circuits.
  Ensure the vertical symmetry of the cable cores without twisting. The branch glove should be installed as low as possible, and the cable clamp position should also be as low as possible, with a ​minimum vertical distance of 750mm​ from the bushing.
  During construction, when feeding the cable from under the RMU foundation into the cable compartment, ​cut off​ the end of the cable damaged during pulling. Then, verify the phase sequence, correct the entry angle of the cable into the RMU to align the three cores with their respective bushings. If the cable entry angle is excessive, withdraw the cable back into the cable trench, adjust the angle, re-feed it into the RMU, and secure it with a cable clamp.
• ​Cable Phase Separation and Termination:​​
  When performing phase separation, ​first secure the lower end of the cable branch glove​ with a cable clamp, and then trim the cable core lengths.
  Align the L2 core with the L2 bushing. Slightly bend the L1 and L3 cores outward from the root first, then align them vertically upwards with their bushings. Screw on the double-ended fixing bolt, temporarily hang the cable lug on the bushing, compare the cable length, and saw off any excess core. ​Ensure the three cable cores are of correct, equal length and flush​ to avoid stress on the bushings and poor contact between the cable lug and the bushing face.
  ​Failing to secure the cable before trimming the core lengths means there is no reference point, leading to errors. Therefore, securing the cable first is crucial.​​
  ​Pay attention to the following points during cable stripping:​​
• Stripping dimensions ​must strictly follow​ the specifications provided by the cable T-body connector manufacturer and their accompanying process dimensions.
• ​Extreme care must be taken​ when removing outer layers to avoid damaging inner layers.
• ​Absolutely avoid​ longitudinal scratches on the core insulation to prevent internal creepage.
• ​Always use​ the manufacturer's designated special cleaning wipes; avoid using alternatives like industrial alcohol.
• For installation lubricant, ​it is recommended to use polyfluoropolyether (PFPE)​​ grease products. These are non-reactive with silicone rubber, ensuring long-term sealing and insulation performance. ​Avoid using silicone-based greases, as mutual dissolution and drying with silicone rubber can create a risk of interfacial creepage.
• ​Ensuring Proper Fit Between Stress Cone and Cable Cross-Section:​​
  The interference fit (overlap) must be appropriate. Excessive interference makes installation difficult and risks cracking the components. Insufficient interference compromises sealing and can lead to severe surface discharge.
  For cable T-body connectors, the stress cone, insulating outer sheath, and the cable itself have specific relative positioning requirements, offering less flexibility. Installation ​must be performed strictly according to the requirements​ (standards vary between manufacturers) to meet stress control and insulation sealing demands.
  Furthermore, during installation, ensure the stress cone body is positioned within the ​vertical section​ of the cable where possible, to guarantee the best sealing effect. ​Take special care to prevent sharp objects​ from scratching the inner or outer surfaces of silicone rubber stress cone components. Apply the designated installation lubricant ​evenly and separately​ to the contact surfaces forming the interference fit.
• ​Installing Elbow Connectors:​​
  The conductor connection within cable elbow connectors is completed inside the insulated outer housing, making the contact condition difficult to observe and inconvenient to test. Therefore, ​it must be ensured​ that the lug face is ​parallel and in flush contact​ with the conductive face of the RMU bushing. This minimizes the stress exerted by the lug on the bushing while ensuring full, good contact to prevent heating during operation.
  The crimping of cable lugs onto wire cores ​must follow​ the installation procedure. ​Pay strict attention​ to the orientation of the lug face; it should be parallel to the copper face of the busbar bushing to ensure flush contact. When using a crimping tool, ​hold the dies closed for 10-15 seconds​ after reaching the full crimp position to allow the metal at the crimp to stabilize plastically. After crimping, use a file to smooth any burrs or sharp edges on the lug surface, then clean both the core insulation and the lug. Slide the cable lug onto the fixing stud, push the cable elbow connector into the bushing, and install it ensuring the lug face is ​in tight, flush contact​ with the copper face of the bushing.