Analysis of Causes and Preventive Measures for Vacuum Circuit Breaker Burnout Accidents

1. Failure Mechanism Analysis of Vacuum Circuit Breakers

1.1 Arcing Process During Opening

Taking circuit breaker opening as an example, when the current triggers the operating mechanism to trip, the moving contact begins to separate from the fixed contact. As the distance between the moving and fixed contacts increases, the process proceeds through three stages: contact separation, arcing, and post-arc dielectric recovery. Once separation enters the arcing stage, the condition of the electric arc plays a decisive role in the health of the vacuum interrupter.

As the arc current increases, the vacuum arc evolves from the cathode spot region and arc column toward the anode region. With the continuous reduction of contact area, high current density generates high temperatures, causing evaporation of cathode metal material. Under the influence of the electric field, an initial gap plasma is formed. Cathode spots appear on the cathode surface, emitting electrons and forming field-emission current, continuously eroding metal material and sustaining metal vapor and plasma. At this stage, with relatively low arc current, only the cathode is active.

As the arc current increases further, the plasma injects energy into the anode, causing the anode arc mode to transition from a diffuse arc to a constricted arc. This transition is influenced by factors such as electrode material and current magnitude.

1.2 Contact Erosion Failure Analysis

Contact erosion is directly related to the interrupting current. Under rated power-frequency current, the degree of contact melting is almost negligible. Contact erosion occurs under high-current, high-temperature conditions. When the circuit breaker interrupts short-circuit currents exceeding its rated current, the degree of material erosion increases sharply, creating conditions for material loss.

Surface roughness of the contacts intensifies current concentration at surface protrusions, leading to more severe localized heating. Additionally, the duration of the arcing current is critical. Even if the current is a short-circuit current, if its duration is too short, the amount of material erosion remains small.

The root cause of contact failure is mass loss during the arcing process. Contact damage occurs in two stages:

• Material Erosion: Anode material erosion is powered by the plasma. The energy flux density on the anode surface is a key parameter measuring the plasma's effect on the anode. Research shows that anode energy flux density increases with higher arc current, larger contact gap, and smaller contact radius, promoting anode spot formation and material erosion.

• Material Loss: After arc extinction, molten metal droplets are expelled from the contact surface due to plasma pressure. This process is primarily influenced by material properties, with minimal further effect from the arc.

2. Causes of Vacuum Circuit Breaker Burnout Accidents

(1) Electrical Wear and Contact Gap Variation Leading to Increased Contact Resistance
Vacuum circuit breakers are sealed within a vacuum interrupter, with moving and fixed contacts in direct face-to-face contact. During interruption, contact erosion occurs, causing contact wear, reduced contact thickness, and changes in contact gap. As wear progresses, the contact surface deteriorates, increasing the contact resistance between moving and fixed contacts. Wear also alters the contact gap, reducing spring pressure between contacts, further increasing contact resistance.

(2) Out-of-Phase Operation Leading to Increased Resistance in Faulted Phase
If the mechanical performance of the vacuum circuit breaker is poor, repeated operations may result in out-of-phase operation due to mechanical issues. This prolongs opening and closing times, preventing effective arc extinction. Arcing can lead to welding (fusing) of contacts, significantly increasing contact resistance between moving and fixed contacts.

(3) Reduced Vacuum Integrity Leading to Contact Oxidation and Increased Resistance
The bellows in a vacuum interrupter are made of thin stainless steel and serve as a sealing element, maintaining vacuum integrity while allowing the conductive rod to move. The bellows' mechanical life is determined by the expansion and contraction forces during breaker operation. Heat transferred from the conductive rod to the bellows raises their temperature, affecting fatigue strength.

If the bellows material or manufacturing process is defective, or if the breaker experiences vibration, impact, or damage during transportation, installation, or maintenance, leaks or micro-cracks may develop. Over time, this leads to a decrease in vacuum level. Reduced vacuum allows contact oxidation, forming high-resistance copper oxide, which increases contact resistance.

Under load current, the contacts overheat continuously, further raising bellows temperature and potentially causing bellows failure. Additionally, with reduced vacuum, the circuit breaker loses its rated arc-quenching capability. When interrupting load or fault currents, insufficient arc extinction capability leads to sustained arcing, ultimately causing breaker burnout.

3. Preventive Measures for Vacuum Circuit Breaker Burnout Accidents

3.1 Technical Measures

The causes of reduced vacuum integrity are complex. Avoid vibration and impact during transportation, installation, and maintenance. However, manufacturing and assembly quality at the factory stage are critical factors affecting vacuum integrity.

(1) Improve Bellows Material and Assembly Quality
Vacuum interrupters use bellows for mechanical motion. After repeated opening and closing operations, micro-cracks may form, compromising vacuum integrity. Therefore, manufacturers must enhance bellows material strength and assembly quality to ensure sealing reliability.

(2) Regular Measurement of Mechanical Characteristics and Contact Resistance
During annual maintenance outages, regularly inspect contact electrical wear and gap variation. Perform tests on synchronism, over-travel, and other mechanical characteristics. Use the DC voltage drop method to measure loop resistance. Evaluate contact oxidation and wear based on resistance values, and address issues promptly.

(3) Regular Vacuum Integrity Testing
For plug-in type vacuum circuit breakers, operators often cannot visually detect external discharge on the interrupter during patrols. In practice, power-frequency withstand voltage tests are commonly used to periodically assess vacuum integrity. Although this is a destructive test, it effectively identifies vacuum defects. Alternatively, using a vacuum tester for qualitative vacuum measurement is the best method for assessing vacuum integrity. If vacuum degradation is detected, the vacuum interrupter must be replaced immediately.

(4) Install Online Vacuum Monitoring Devices
With the widespread use of wireless communication and SCADA systems in power networks, online vacuum monitoring has become feasible. Methods include pressure sensing, capacitive coupling, electro-optical conversion, ultrasonic detection, and non-contact microwave sensing.

• Pressure Sensing: Embed pressure sensors in the interrupter during manufacturing. As vacuum degrades, gas density and internal pressure increase. The pressure change is transmitted to the control system for real-time monitoring.

• Non-Contact Microwave Sensing: Uses passive sensing to detect microwave signals, capturing unique feedback signals when vacuum integrity is compromised, enabling real-time online monitoring.

3.2 Management Measures

In past incidents, operators failed to correctly identify circuit breaker faults, leading to burnout and accident escalation. This highlights insufficient familiarity with SCADA systems, on-site equipment, and operating procedures, as well as a lack of emergency response awareness. Therefore, operation management at main substations must be strengthened.

• Implement inspection systems rigorously to detect issues early.

• Enhance training for operators on SCADA systems, switchgear operation and maintenance, and emergency response procedures.

• Conduct regular drills for anti-accident and emergency response plans.

3.3 Improve "Five Prevention" Interlocking Functions in Mid-Mounted Switchgear

Technically upgrade the "Five Prevention" interlocking functions of mid-mounted switchgear to fully meet standard requirements. Complete high-voltage switchgear should have full "Five Prevention" functions with reliable performance.

• Install live-line indicators on the outgoing side of switchgear. These indicators should have self-test functionality and be interlocked with the line-side earthing switch.

• For installations with back-feed capability, the compartment door should be equipped with a mandatory lock controlled by a live-line indicator.

Through analysis of vacuum circuit breaker burnout accidents caused by reduced vacuum integrity—leading to contact oxidation, increased contact resistance, overheating, and eventual failure—this paper proposes targeted measures such as improving bellows material and assembly quality, and installing online vacuum monitoring devices. These measures help prevent and monitor vacuum degradation in real time, avoiding recurrence of similar accidents.