Fault Analysis of the Neutral Busbar Circuit Breaker During Blocking of Ultra-High-Voltage Converter Valves

1.Blocking Principle of Ultra-High-Voltage Converter Valves
1.1 Working Principle of Converter Valves
Ultra-high-voltage (UHV) converter valves typically employ thyristor valves or insulated-gate bipolar transistor (IGBT) valves to convert alternating current (AC) to direct current (DC) and vice versa. Taking the thyristor valve as an example, it consists of multiple thyristors connected in series and parallel. By controlling the triggering (turn-on) and turn-off of thyristors, the valve regulates and converts electrical current. During normal operation, the converter valve converts AC to DC or DC to AC according to a predefined firing sequence and timing [1].

1.2 Causes and Process of Converter Valve Blocking
Converter valve blocking can be triggered by various factors, including overvoltage, overcurrent, internal component failures, and abnormalities in the control and protection system. When such anomalies are detected, the control and protection system rapidly issues a blocking command, ceasing the triggering of all thyristors or IGBT valves, thereby blocking the converter valve.

During the blocking process, significant changes occur in the system’s electrical parameters. For instance, on the rectifier side, after the converter valve is blocked, the AC-side current drops rapidly. However, due to line inductance, the DC-side current does not immediately fall to zero and instead continues to flow through paths such as the neutral busbar, forming a freewheeling current. At this moment, the neutral busbar circuit breaker must operate quickly to interrupt the DC current and protect system equipment from damage caused by excessive current [2].

2.Operating Conditions of the Neutral Busbar Circuit Breaker During Converter Valve Blocking
2.1 Changes in Electrical Parameters
When the converter valve is blocked, the voltage and current across the neutral busbar circuit breaker undergo drastic changes. On the DC side, since the blocked converter valve prevents normal current flow, overcurrent occurs in the neutral busbar and associated equipment. Meanwhile, due to electromagnetic transient processes in the system, overvoltage may appear across the neutral busbar circuit breaker.

For example, in a certain UHV DC transmission project, after converter valve blocking, the neutral busbar current instantly surged to 2–3 times the rated current, and the voltage across the neutral busbar circuit breaker exhibited significant fluctuations, peaking at 1.5 times the normal operating voltage. Table 1 visually illustrates the changes in electrical parameters during converter valve blocking.

Table 1: Changes in Electrical Parameters During Converter Valve Blocking in a Certain UHV DC Transmission Project

2.2 Stress Variations
When the converter valve is blocked, the neutral busbar circuit breaker must withstand not only electrical stress but also mechanical stress. Electrical stress mainly arises from overvoltage and overcurrent, which intensify electrical erosion of the breaker contacts and shorten their service life. Mechanical stress primarily results from impact forces generated by the operating mechanism during rapid opening and closing operations, as well as electromagnetic forces caused by rapid current changes. For example, in frequent converter valve blocking events, components of the neutral busbar circuit breaker’s operating mechanism may become loose or worn, adversely affecting its normal opening and closing performance [3].

3.Common Fault Types and Cause Analysis of Neutral Busbar Circuit Breakers During UHV Converter Valve Blocking
3.1 Insulation Failure
3.1.1 Fault Manifestations
Insulation failure is one of the more common fault types for neutral busbar circuit breakers during converter valve blocking. It mainly manifests as aging or damage of internal insulation materials, leading to degraded insulation performance and resulting in flashover or breakdown. For instance, in some long-operating UHV DC transmission projects, surface contamination and cracks have appeared on the insulating porcelain bushings inside the neutral busbar circuit breaker, severely compromising insulation performance.

3.1.2 Cause Analysis
The causes of insulation failure include several aspects. First, prolonged operation under high voltage and large current gradually ages insulation materials, reducing their insulation capability over time. Second, the overvoltage and overcurrent generated during converter valve blocking impose severe stress on insulation materials, accelerating the aging process. Additionally, harsh operating environments—such as high humidity and heavy pollution—cause insulation surfaces to accumulate contaminants, further degrading insulation performance. For example, in a coastal UHV DC transmission project with high humidity and salt-laden air, a conductive film readily forms on the surface of the neutral busbar circuit breaker’s insulating porcelain, significantly reducing insulation strength and causing frequent flashover faults.

3.2 Operating Mechanism Failure
3.2.1 Fault Manifestations
Operating mechanism failures mainly manifest as abnormal opening/closing times or failure to open/close (refusal to operate). For example, during converter valve blocking, the neutral busbar circuit breaker may exhibit excessively long opening times, failing to interrupt DC current promptly, or may fail to close properly, resulting in poor contact.

3.2.2 Cause Analysis
The causes of operating mechanism failures are complex. On one hand, mechanical components degrade over time due to frequent operations, suffering wear or deformation that impairs performance. For instance, springs in the mechanism may lose elasticity due to fatigue, leading to insufficient opening/closing force. On the other hand, faults in the control circuit—such as relay failure or broken control cables—can prevent the mechanism from receiving or executing commands correctly. Moreover, electromagnetic interference during converter valve blocking may disrupt control signals, causing malfunctions or refusal to operate. For example, in a certain UHV DC transmission project, control cables routed near high-current busbars experienced strong magnetic interference during valve blocking, leading to breaker refusal to open.

3.3 Contact Failure
3.3.1 Fault Manifestations
Contact failures primarily include contact erosion, increased contact resistance, and contact welding. During converter valve blocking, when the neutral busbar circuit breaker interrupts large currents, high-temperature arcs form, causing contact surface erosion. Prolonged erosion leads to uneven contact surfaces and higher resistance, impairing normal operation. In severe cases, contacts may weld together, preventing the breaker from opening.

3.3.2 Cause Analysis
The main cause of contact failure is the large current and high-temperature arc generated during converter valve blocking. Large current flow produces Joule heating, raising contact temperature, while the arc’s intense heat accelerates erosion. Additionally, contact material properties and manufacturing quality affect arc resistance. Contacts made of materials with poor high-temperature or arc resistance, or those produced with substandard processes, are more prone to erosion. For example, in a UHV DC project, the neutral busbar circuit breaker used contacts with inadequate arc resistance; after multiple blocking events, severe erosion occurred, significantly increasing contact resistance and disrupting normal operation.

3.4 Current Transformer Failure
3.4.1 Fault Manifestations
Current transformer failures mainly include secondary-side open circuits, winding insulation damage, and core saturation. During converter valve blocking, the abrupt change in DC current subjects the current transformer to significant stress, making it prone to failure. For instance, an open secondary circuit can generate dangerously high voltages, endangering equipment and personnel; winding insulation damage can cause internal short circuits, degrading measurement accuracy; and core saturation increases measurement errors, potentially triggering incorrect protective actions.

3.4.2 Cause Analysis
Causes of current transformer failure include the following: First, overcurrent during converter valve blocking subjects windings to high thermal and electromagnetic stress, possibly damaging insulation. Second, insulation performance naturally degrades over time, making transformers more vulnerable to failure under abnormal conditions like valve blocking. Additionally, improper design or selection—such as incorrect rated current or accuracy class—can lead to core saturation during blocking events. For example, in one UHV DC project, the current transformer’s rated current was too low; during valve blocking, the core quickly saturated, failing to measure current accurately and causing protective relays to malfunction.

To better understand the proportion of each fault type among neutral busbar circuit breaker failures during converter valve blocking, this paper conducted a statistical analysis of fault data from multiple UHV DC transmission projects, with results shown in Table 2.

Table 2: Proportion of Neutral Busbar Circuit Breaker Fault Types During UHV Converter Valve Blocking

4.Fault Prevention and Handling Measures for Neutral Busbar Circuit Breakers During UHV Converter Valve Blocking
4.1 Fault Prevention Measures
4.1.1 Optimization of Equipment Selection and Design
During the construction phase of UHV DC transmission projects, the impact of abnormal conditions such as converter valve blocking on neutral busbar circuit breakers should be fully considered, and equipment selection and design should be optimized accordingly. Key components—such as circuit breakers with high insulation performance, excellent arc-resistance contacts, reliable operating mechanisms, and appropriately rated current transformers—should be selected. For example, insulating porcelain bushings made from advanced insulation materials and manufacturing processes can enhance insulation reliability; contact materials with strong arc-resistance extend contact life; and a well-designed operating mechanism ensures accurate and reliable opening/closing under various operating conditions.

4.1.2 Enhanced Equipment Monitoring and Maintenance
A comprehensive equipment monitoring system should be established to continuously monitor operational parameters of the neutral busbar circuit breaker, including electrical parameters, temperature, pressure, vibration, and other status indicators. Through data analysis, potential fault risks can be identified early. For instance, infrared thermography can be used to monitor temperatures at contacts and connection points; abnormal temperature rises trigger timely inspections and corrective actions. Online monitoring of insulation resistance and partial discharge helps assess insulation condition. Additionally, routine maintenance—including cleaning, lubrication, and tightening—should be strengthened to ensure the equipment remains in optimal operating condition.

4.1.3 Improvement of Operating Environment Quality
The operating environment of the neutral busbar circuit breaker should be improved to mitigate adverse environmental impacts. For example, air purification systems can be installed in substations to reduce airborne contaminants and corrosive gases; effective moisture control measures—such as dehumidifiers—can maintain dry conditions around the equipment. In coastal or heavily industrial-polluted areas, special protective treatments—such as anti-corrosion coatings—can be applied to enhance the equipment’s resistance to environmental degradation.

4.2 Fault Handling Measures
4.2.1 Application of Rapid Fault Diagnosis Technologies
When a fault is detected in the neutral busbar circuit breaker, rapid fault diagnosis technologies should be employed to accurately identify the fault type and root cause. Intelligent diagnostic systems, combined with real-time operational data and fault characteristics, enable quick fault localization through data analysis and model-based calculations. For example, real-time monitoring and analysis of current and voltage parameters can help determine whether insulation failure, contact damage, or current transformer malfunction has occurred; vibration analysis can reveal mechanical issues in the operating mechanism.

4.2.2 Establishment of Rational Fault Handling Procedures
Detailed and rational fault handling procedures should be developed to ensure swift and effective response when failures occur. These procedures should include fault reporting, on-site inspection, fault diagnosis, repair planning, implementation of repairs, equipment testing, and acceptance verification. Throughout the process, strict adherence to safety protocols is essential to protect personnel and equipment. For instance, when addressing insulation faults, power must first be disconnected and stored energy discharged before inspection and repair; after component replacement, rigorous testing and acceptance checks must confirm that performance meets required standards.

4.2.3 Emergency Backup Equipment and Contingency Plans
To minimize the impact of neutral busbar circuit breaker failures on system operation, emergency backup equipment should be available, and comprehensive contingency plans should be formulated. In the event of a severe fault that cannot be promptly repaired, backup equipment can be rapidly deployed to restore normal system operation. Regular maintenance and testing of backup equipment are necessary to ensure it remains in good standby condition. The contingency plan should specify emergency response procedures, personnel responsibilities, communication protocols, and other key elements to enable orderly and efficient emergency handling.

5.Conclusion
During UHV converter valve blocking, neutral busbar circuit breakers face multiple fault risks—including insulation failure, operating mechanism malfunction, contact damage, and current transformer faults—all of which can significantly compromise the safe and stable operation of UHV DC transmission systems. By thoroughly analyzing the blocking mechanism of converter valves and the operational state of neutral busbar circuit breakers under such conditions, common fault types and their causes have been clearly identified, supported by detailed case studies. To effectively prevent and address these faults, preventive measures should be implemented in equipment selection and design, operational monitoring and maintenance, and environmental improvement. Simultaneously, fault handling strategies—including rapid diagnostic technologies, standardized repair procedures, and emergency backup systems—should be adopted to further enhance the operational reliability of UHV DC transmission systems.