The necessity of solid-state breakers in modern traction power systems
By Anju Upadhyay
This is an excerpt from the full article. The full version can be downloaded below.
Electrified railways constitute the backbone of modern transport networks, valued for their low emissions, high efficiency, and ability to carry large passenger and freight volumes. As global railway systems evolve, with increasing interoperability, new high-speed rail projects, and diversified electrification schemes, traction power systems face growing complexity. These systems rely heavily on robust protection devices, particularly circuit breakers, to ensure reliability, safety, and uninterrupted operation. Conventional traction networks in Europe use a mix of AC (15–25 kV) and DC (750 V–3 kV) supplies, resulting in varied protection requirements. With the rapid rise of advanced power electronics (IGBTs, SiC MOSFETs, and GaN devices) traditional electromechanical protection devices increasingly struggle to meet the demands of modern traction environments. This context has driven the development and adoption of Solid-State DC Circuit Breakers (SSDCBs) as next-generation protection solutions.
Overview of Traction Power Systems
Traction systems can be divided into two categories:
- Infrastructure-side systems (substations, feeders, network protection)
- Rolling stock-side systems (converters, motors, onboard protection)
European networks must balance historical electrification choices with modern standards such as EN 50123, EN 50152, and railway EMC and environmental standards. Circuit breakers mechanical, hybrid, or solid-state—play a central role in maintaining interoperability and safety across this fragmented landscape.
Evolution of Semiconductor Technology in Rolling Stock
Rolling stock relies on semiconductor-based converters to regulate energy transfer from overhead lines or third rails to traction motors. Semiconductor technology has undergone significant transformation:
- SCRs improved early traction motor control but suffered from thermal stress, EMI, and harmonics.
- GTOs allowed higher power but required complex gate drives, intensive cooling, and had low switching speeds.
- IGBTs in the 1980s–1990s revolutionized traction drives with faster switching, simpler control, and higher reliability.
- SiC MOSFETs now offer further gains in efficiency, reduced losses, and lighter converters, accelerating adoption in high-speed trains.
- GaN transistors are emerging for auxiliary converters, though voltage limitations restrict their use in main traction circuits.
As switching frequencies increase (tens of kHz), power electronics create sharp transients, making fast and selective protection essential.
Modern traction systems face several critical protection challenges:
- Coordination in multi-converter architectures
Multiple onboard converters connected to a shared DC bus make selective fault isolation difficult. Slow protection can propagate faults or cause unnecessary tripping. - High-frequency transients
SiC/IGBT switches produce steep dv/dt and di/dt transients that exceed the reaction speed of mechanical breakers. - Bidirectional current from regenerative braking
Reverse current flow complicates relay settings and may cause mis-trips if protection is not directionally adaptive. - DC arc fault persistence
In DC networks, the absence of natural current zero-crossings causes arcs to persist. Mechanical breakers cannot extinguish them quickly. - EMI and communication issues
High-frequency switching produces EMI, interfering with digital relays and protection coordination networks. - Reliability and maintenance
Electromechanical breakers degrade due to vibration, temperature cycling, and frequent load changes. - Power quality and harmonics
Converter-generated harmonics cause resonances, overvoltages, and relay malfunctions. - Inductance of traction lines
High line inductance leads to voltage drops, current ripple, resonance, reduced regenerative braking efficiency, and EMC issues.
Why solid-state circuit breakers?
Solid-State DC Circuit Breakers (SSDCBs) directly address the shortcomings of mechanical and hybrid breakers.
Technical Comparison Mechanical circuit breaker / Hybrid circuit breaker / Solid-state circuit breaker
| Feature | Mechanical CB (MCB / HSCB) | Hybrid CB | Solid-State CB (SSCB) |
| Interruption Speed | 5–15 ms | 1–2 ms | 50–200 µs (ultra-fast) |
| Arc Formation | Yes, requires arc chute | Reduced (mechanical + semiconductor) | None (fully electronic) |
| Bidirectional Current Support | Limited / difficult | Possible with design complexity | Native, fully bidirectional |
| Fault Selectivity | Low (slow + less precise) | Medium | High (fast, precise, electronic control) |
| Suitability for High dv/dt / di/dt from SiC/IGBT | Poor | Medium | Excellent |
| EMI Robustness | Low | Medium | High (monitoring integrated) |
| Maintenance Needs | High (mechanical wear, contacts erosion) | Medium (mechanical part still present) | Very low (no moving parts) |
| Thermal Losses (Conduction) | Almost none | Low | Higher (semiconductor conduction losses) |
| Lifetime | Short–Medium (mechanical fatigue) | Medium | Long (electronics + no wear) |
| DC Arc Quenching | Difficult, slow | Improved | Instant, no arc |
| Size & Weight | Medium–Large | Medium | Smallest (depends on cooling) |
| Compatibility with Regenerative Braking | Limited, slow response | Good | Excellent (bidirectional + fast) |
| Integration with Protection Coordination | Poor | Moderate | Excellent (programmable logic) |
| Response to High-Frequency Transients | Slow, may mis-trip | Better | Best (μs response) |
| Environmental Impact (SF₆) | Some HV versions require SF₆ | Often SF₆-free | Naturally SF₆-free |
| Cost | Lowest | Medium | Highest (but TCO lower due to reliability) |
| Typical Use in Traction | Legacy systems, HSCBs, infrastructure | Some rolling stock + DC substations | Modern converters, batteries, DC links, rolling stock protection |
- Mechanical breakers are robust and cheap but slow, maintenance-heavy, and unsuitable for modern power electronics.
- Hybrid breakers improve speed and reduce arcing but still have mechanical wear and slower fault clearing.
- SSCBs deliver unmatched speed, precision, and reliability—ideal for SiC-based traction converters and battery-electric locomotives—but have higher conduction losses and cost.
Applications of SSDCBs
In rolling stock SSDCBs provide
- Microsecond-level fault interruption
- Bidirectional current management (ideal for regenerative braking)
- Compact form factors suited for constrained onboard spaces
- Integration with smart diagnostic and monitoring systems
This enhances equipment protection, improves uptime, and enables predictive maintenance.
In traction infrastructure such as substations and feeder lines, SSDCBs:
- Prevent voltage oscillations from high line inductance
- Quickly interrupt DC faults without arcing
- Improve selectivity across parallel feeders
- Enhance reliability of DC traction networks
Astrol’s solid-state DC breakers advantages
- 8–10 µs interruption speed
- Fault current capacity up to 20 kA
- Modular, compact enclosures (507 × 507 × 673–1017 mm)
- Liquid cooling for higher current ratings (air cooling possible)
- System-independent integration
- DNV, Lloyd’s Register, and CCS certifications
These characteristics make them suitable for both maritime, traction and land-based DC systems.
Conclusion
Solid-State DC Circuit Breakers represent a transformative leap in the protection of modern traction systems. As railways increasingly adopt advanced semiconductors, higher switching frequencies, and bidirectional energy flows, traditional mechanical breakers no longer meet operational and safety requirements. SSCBs offer ultra-fast fault interruption, superior selectivity, reduced maintenance, and seamless integration into both rolling stock and infrastructure. With global environmental regulations accelerating the transition away from SF₆-based equipment, SSCBs provide a future-proof, sustainable, and highly reliable solution. As electrified rail networks continue to expand and modernization accelerates, solid-state protection technologies will play a central role in enabling safer, smarter, and more efficient traction power systems.
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