ring main unit (RMU for short) of high-voltage switchgear
The ring main unit (RMU for short) of high-voltage switchgear is the core equipment for realizing ring network power
supply in the power distribution system, featuring compact structure, convenient operation, and high reliability.
The following are its common faults and corresponding countermeasures:
I. Faults of Core Components
1. Faults of the Load Break Switch
- **Manifestations**:
- Difficulty in opening and closing, contact ablation, and the contact resistance exceeding the standard (＞50μΩ)
- The arc cannot be extinguished due to insufficient gas generation in the arc extinguishing chamber
- **Causes**:
- Oxidation of contacts, spring fatigue, and leakage of the arc extinguishing medium (such as SF6 gas)
- **Solutions**:
- Clean the contacts and apply conductive paste, and replace the spring components
- Supplement SF6 gas to the rated pressure (usually 0.4~0.6MPa)
2. Abnormal Fuse Blowing
- **Phenomena**:
- The fuse blows frequently or fails to operate
- Cracks or carbonization marks appear on the bushing after the fuse blows
- **Causes**:
- Improper selection of the rated current of the fuse element (not selected according to 1.5 to 2 times the load current)
- Poor contact between the fuse and the base leads to local overheating
- **Countermeasures**:
- Reset the parameters of the fuse element according to the IEC 60269 standard
- Use a torque wrench to tighten the fuse interface (refer to the manufacturer's manual for the torque value)
II. Faults Caused by the Characteristics of the Ring Network
1. Exceeding the Short-circuit Current Limit of the Ring Network
- **Risks**:
- The short-circuit current during the operation of the ring network may exceed the rated breaking capacity
of the equipment (such as 25kA)
- **Case**:
- In 2024, due to the inter-phase short-circuit of the cable in an industrial park, the ring main unit exploded
due to insufficient breaking capacity
- **Prevention**:
- Adopt the combination of a current-limiting fuse and a load break switch (such as the FLN36-12D type)
- Regularly calculate the short-circuit current and update the equipment selection
2. Failure of the Automatic Recovery of the Ring Network
- **Problems**:
- The automatic reclosing function does not start or malfunctions after a fault
- **Troubleshooting**:
- Check whether the power supply of the reclosing controller (usually DC220V) is normal
- Verify the matching between the transformation ratio of the zero-sequence current transformer (CT) and the
protection setting value
III. Mechanical and Operational Faults
1. Failure of the Five-prevention Function
- **Manifestations**:
- The cabinet door can be opened without power outage (violating the principle of "preventing misoperation")
- The interlock between the grounding switch and the circuit breaker fails
- **Maintenance**:
- Check the wear of the mechanical interlock device and replace the copper lock bolt if necessary
- Simulate the operation to verify the five-prevention logic (such as "the grounding switch cannot be operated
in the closed state")
2. Sticking of the Operating Mechanism
- **Inducing Factors**:
- Lack of oil in the transmission gear (it is recommended to use molybdenum disulfide grease)
- Thermal expansion and contraction of components caused by changes in the ambient temperature
- **Solutions**:
- Lubricate and maintain the operating mechanism every two years
- Install a temperature control device to maintain the temperature inside the cabinet between -5℃ and 40℃
IV. Environmental and Insulation Faults
1. Condensation and Insulation Degradation
- **Hazards**:
- Creepage on the surface of the insulation parts (the creepage discharge voltage is lower than 30kV)
- Corrosion of metal components leads to an increase in the grounding resistance (＞4Ω)
- **Treatment**:
- Install a semiconductor heating tape (power ≥ 50W/m)
- Use moisture-proof sealing compound to seal the cable inlets and outlets
2. External Insulation Pollution Flashover
- **Scenarios**:
- In coastal areas or industrial zones, salt spray/dust accumulates on the surface of insulators
- **Measures**:
- Select insulators with an anti-pollution level above level III (creepage distance ≥ 31mm/kV)
- Use RTV anti-pollution flashover coating for surface treatment every year
V. Typical Fault Handling Process
1. **Fault Location**:
- Determine the fault section through the fault indicator (such as the TBP-800 type)
- Locate the discharge point using a partial discharge detector (such as the PD-8000)
2. **Emergency Operation**:
- Confirm that the circuit breaker has been opened before manually operating the grounding switch
- It is prohibited to force the power supply when the fault has not been eliminated
3. **Power Restoration**:
- Transfer the load by the "loop closing and power transfer" method (the phase angle difference needs to be < 15°)
- Conduct a power frequency withstand voltage test (42kV/1min) after replacing the faulty equipment
VI. Suggestions for Operation and Maintenance Optimization
1. **Condition Monitoring**:
- Install a contact temperature sensor (accuracy ±1℃)
- Configure an intelligent terminal to achieve remote opening and closing (supporting the IEC 61850 protocol)
2. **Spare Parts Management**:
- Stock commonly used consumables such as fuses (such as NT00-63A) and sealing rings
- Key spare parts need to be certified through type tests
3. **Personnel Training**:
- Carry out simulation drills for the switching operations of the ring main unit
- Be familiar with the standard of "DL/T 404-2017 3.6kV~40.5kV AC Metal-enclosed Switchgear"
**Conclusion**: Most of the faults of the ring main unit are caused by the load break switch, fuse, and
environmental factors. It is necessary to formulate targeted maintenance strategies in combination with the
characteristics of ring network power supply. Through condition monitoring technology and standardized
operation processes, the reliability of the equipment can be significantly improved, and the continuous power
supply of the power distribution system can be ensured.
