SF6 (sulfur hexafluoride) circuit breakers are widely used in modern power systems, particularly in medium and high voltage applications, due to their excellent arc extinguishing and insulation properties. However, their complex structure, coupled with stringent requirements for sealing and operating mechanisms, also presents some common faults and potential hazards.
The following are common SF6 circuit breaker faults and potential hazards, their causes, and their corresponding preventative measures.
Ⅰ. SF6 Gas-Related Failures (Most Common Problems)
This is the most unique and critical type of SF6 circuit breaker failure.
1. Failure Symptom: SF6 Gas Leakage
◆Hazards and Consequences:
Degraded Insulation Performance: Reduced gas pressure leads to insufficient insulation strength, potentially causing serious accidents such as internal breakdown and flashover to ground.
Degraded Arc Extinguishing Capacity: When interrupting fault currents, insufficient gas pressure can cause arc extinguishing failure, potentially causing circuit breaker explosion.
Equipment Shutdown: The low-voltage lockout function prohibits the circuit breaker from opening or closing, causing a power outage.
Environmental Pollution: SF6 is a potent greenhouse gas, and leakage is harmful to the environment.
◆Main causes:
Aging or cracking of seals (such as O-rings and gaskets).
Improper sealing surface treatment during installation or maintenance, resulting in scratches or impurities.
Loose flange bolts.
Porosity or cracks in the porcelain sleeve or tank body.
Leakage from the density relay or pressure gauge interface.
Abnormal opening of the safety valve or explosion-proof disk.
◆Preventive Measures:
Regular Leak Testing: Use an SF6 gas leak detector (such as a laser leak detector or infrared leak detector) to perform annual precision leak testing on key areas such as valves, pipe joints, and sealing surfaces. Qualitative leak testing (using a leak detector probe) can be performed during routine inspections.
Monitoring Gas Pressure and Density: Regularly record and compare the readings of the SF6 density relay (pressure gauge) to observe whether the curve changes normally with ambient temperature. The density relay has built-in temperature compensation and can accurately reflect the gas density (i.e., the amount of gas stored).
Intensified Inspections: Observe whether the pressure gauge pointer is within the normal range (green zone) and whether there are any alarm signals.
Strict Maintenance Procedures: When replacing seals, always use qualified new ones, clean the sealing surfaces, and tighten bolts to the specified torque.
SF6 Gas Recovery Management: When recharging and performing maintenance, professional SF6 gas recovery equipment must be used. Direct discharge is strictly prohibited.
2. Fault Symptom: SF6 gas moisture content (humidity) exceeds the specified limit.
◆Hazards and Consequences:
Degraded insulation performance: Moisture condenses on the insulation surface, reducing the surface flashover voltage.
Generation of Corrosive Substances: Under high arc temperatures, moisture reacts with SF6 gas and its decomposition products to produce highly corrosive substances such as hydrogen fluoride (HF) and sulfur dioxide (SO2). These substances can corrode the circuit breaker's internal metal parts and insulation, leading to mechanical jamming or insulation breakdown.
Liquidation Risk: High humidity can cause liquefaction at low temperatures, affecting gas pressure.
◆Main causes:
Insufficient fresh air purity and inherent moisture.
Excessive exposure of the internal components to humid air during installation or maintenance.
Saturation failure of the adsorbent (molecular sieve).
Moisture release from internal insulation materials or components.
Moisture intrusion through leaks.
◆Preventive Measures:
Strict Gas Management: Fresh gas must be rigorously tested for moisture content and purity before storage and charging to ensure compliance.
Controlled Exposure Time: During maintenance, internal exposure time must be strictly limited and performed on sunny days with dry air.
Regular Moisture Testing: SF6 gas moisture content should be tested at least annually and compared with historical data. If the moisture content increases rapidly, the cause should be investigated.
Timely Replacement of Adsorbent: The circuit breaker is equipped with adsorbent. After opening the gas chamber for maintenance, the adsorbent must be replaced.
Charging and Discharging Dry Nitrogen: During maintenance, dry nitrogen can be used to flush and remove moisture.
Ⅱ. Mechanical Characteristics and Operating Mechanism Failure
This is the primary cause of circuit breaker failure or malfunction, directly impacting power supply reliability.
1. Fault Symptom: Operating Mechanism Failure (Refusal to Open, Refusal to Close, Malfunction)
◆Hazards and Consequences: This is one of the most dangerous faults. When a circuit breaker fails to operate due to a line fault, it can expand the scope of the accident, causing upstream circuit breakers to trip and resulting in widespread power outages.
◆Main causes (varies by mechanism type):
Spring mechanism: Closing spring not storing energy, pawl or cam wear, connecting rod mechanism stuck, opening and closing latch malfunction.
Hydraulic mechanism: Frequent oil pump startup (internal leakage), abnormally high/low oil pressure, oil line blockage, valve body sticking, damaged seal and oil leakage, abnormal nitrogen pre-pressure.
Pneumatic mechanism: Low compressed air pressure, air compressor failure, pipeline leak, valve body sticking.
Common causes: Auxiliary switch not in place or poor contact, opening and closing coil burned out, loose or broken secondary circuit wiring.
◆Preventive Measures:
Regular Mechanical Property Testing: Test opening and closing times, opening and closing speeds, synchronicity, travel, and overtravel annually or as required. Compare the data with factory settings or historical data to promptly identify potential mechanical wear and jamming.
Check the Operating Mechanism: Regularly check the spring charge status, hydraulic oil level and pressure, and air pressure for normal operation.
Regular Lubrication: Lubricate transmission connecting rods, bearings, and other parts with specialized grease to prevent jamming.
Tightening and Cleaning: Inspect and tighten all mechanical connections, and keep the interior of the mechanism clean and dry.
Secondary Circuit Maintenance: Check the opening and closing coil resistance, auxiliary switch contacts, and circuit insulation to ensure the integrity of the control circuit.
III. Electrical Performance Failure
1. Failure Symptom: Excessive Contact Resistance
◆Hazards and Consequences: Overheating of the main contacts during current flow can lead to insulation degradation and even contact burnout, potentially resulting in a short-circuit explosion.
◆Main Causes: Oxidation, wear, or burnout of the contact surface; decreased contact spring pressure; loose internal connections.
◆Preventative Measures:
Regularly measure loop resistance: At least once a year, use a high-current loop resistance tester and compare the results with the factory setting. If there is a significant increase, investigate the cause.
Monitor Operating Temperature: Use an infrared thermal imager to regularly inspect and measure the temperature at the circuit breaker's moving and static contact junctions to promptly detect overheating risks.
2. Fault Symptom: Insulation Failure
◆Hazards and Consequences: Internal or external insulation breakdown, resulting in a short circuit or grounding accident.
◆Main Causes:
Internal: Moisture, aging, or cracking of the insulation rod; dirt on the insulation surface; the presence of free conductive materials such as metal particles; excessive moisture in the gas.
External: The porcelain bushing is dirty, cracked, or damaged by external forces.
◆Preventative Measures:
Regular insulation testing: Conduct insulation resistance measurements at the fracture surface and to ground, and perform AC withstand voltage tests (or series resonant withstand voltage tests).
Maintain External Insulation Cleanliness: Regularly clean dirt from the porcelain bushing surface, especially in heavily polluted areas.
Inspect External Insulation: Inspect the porcelain bushing for cracks or damage.
IV. Other Hazards
1. Free Metal Particles
Hazard: Under the influence of an electric field, particles can migrate, potentially causing partial discharge or insulation flashover.
Prevention: Ensure internal cleanliness during installation; monitor operation using methods such as partial discharge detection or ultrasonic testing.
2. Shunt Capacitor or Closing Resistor Failure
Hazard: Failure of the shunt capacitor used for voltage balancing or the closing resistor used to suppress switching overvoltage can affect the circuit breaker's interrupting performance or cause overvoltage.
Prevention: Regularly measure the capacitance and dielectric loss of the capacitors and the resistance of the closing resistors.
Summary：Core Maintenance and Preventive Strategies
1.Condition-Based Maintenance: Shift from "periodic maintenance" to "condition-based maintenance," relying on various online and offline test data to assess equipment health.
2.Establish a comprehensive inspection plan:
Daily inspections: Check pressure, oil level, energy storage status, and abnormal noise.
Regular preventive testing: Strictly follow the "Procedure for Preventive Testing of Power Equipment," including:
SF6 gas moisture content and leak detection
Circuit resistance measurement
Mechanical property testing
Insulation testing
Infrared temperature measurement
3.Improve maintenance quality: Use qualified spare parts during maintenance and strictly adhere to process standards, especially for sealing and vacuum gas injection procedures.
4.Strengthen technical training: Ensure that operation and maintenance personnel have a deep understanding of the structure, principles, and key points of SF6 circuit breakers, enabling them to accurately analyze fault symptoms.
Through these systematic preventive measures, the failure rate of SF6 circuit breakers can be significantly reduced, ensuring the safe and stable operation of the power grid.
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