SST Voltage Challenges: Topologies & SiC Tech Mga Hamon sa SST Voltage: Mga Topolohiya & Teknolohiya sa SiC
Isa usa sa mga core nga challenge sa Solid-State Transformers (SST) mao ang insuficiente nga voltage rating sa usa ka single power semiconductor device aron mas direktang mahatagan ang medium-voltage distribution networks (e.g., 10 kV). Ang pag-address sa ingong limitasyon sa voltage wala gisangpot sa usa ra nga teknolohiya, apan labi na "combination approach." Ang main strategies makakategorisaray ngadto sa duha ka klase: "internal" (pinaagi sa device-level nga teknolohikal ug material innovation) ug "external collaboration" (pinaagi sa circuit topology).
1.External Collaboration: Pag-solve pinaagi sa Circuit Topology (Ang Kasagaran ug Maturanong Approach Karon)
Ini ang kasagaran nga maoy mas reliable ug gi-aplikar na sa medium- ug high-voltage, high-power applications. Ang core idea niini mao ang "strength in unity"—gamiton ang series connections o modular combinations sa multiple devices aron mag-share sa high voltage.
1.1 Device Series Connection
Principle: Multiple switching devices (e.g., IGBTs o SiC MOSFETs) gisunod sa series aron mag-collectively withstand high voltage. Ini sama sa pag-connect sa multiple batteries sa series aron makab-ot og mas taas nga voltage.
Key Challenges:
Dynamic Voltage Balancing: Tungod sa minor nga parameter differences sa mga devices (e.g., switching speed, junction capacitance), ang voltage dili mas evenly distribute sa mga devices sa panahon sa high-speed switching, posible nga maka-overvoltage ug failure sa usa ka device.
Solutions: Gikinahanglan ang complex nga active o passive voltage balancing circuits (e.g., snubber circuits, gate control) aron mas enforce ang voltage sharing, nag-increase sa system complexity ug cost.
2. Multilevel Converter Topologies (Kasagaran nga Choice para sa SST Karon)
2.1 Principle: Ini usa ka mas advanced ug mas high-performance nga "modular series" concept. Iginenera niini ang stepped approximation sa sine wave gamit ang multiple voltage levels, aron ang bawg switching device mogamit lamang sa bahin sa total DC bus voltage.
2.2 Common Topologies:
Modular Multilevel Converter (MMC): Usa sa pinaka favor nga topologies alang sa medium- ug high-voltage SSTs. Gitukod niini gikan sa daghang identical submodules (SMs) gisunod sa series. Bawg submodule adunay typical nga capacitor ug daghang switching devices. Ang devices mogamit lamang sa voltage sa submodule’s capacitor, effective nga solb-an ang voltage stress issue. Ang advantages mao ang modularity, scalability, ug excellent output waveform quality.
Flying Capacitor Multilevel Converter (FCMC) ug Diode-Clamped Multilevel Converter (DNPC): Usa usab ang commonly used multilevel structures, apan nag-become structurally ug control-wise complex isip ang number sa levels nag-increase.
Advantages: Fundamentally solb-an ang voltage rating limitation sa individual devices, significantly improve ang output voltage waveform quality, ug reduce ang filter size.
3. Input-Series Output-Parallel (ISOP) Cascaded Structure
Principle: Multiple complete, independent power conversion units (e.g., DAB, Dual Active Bridge) gisunod sa series sa ilang inputs aron mag-withstand high voltage ug parallel sa ilang outputs aron deliver high current. Ini usa ka system-level modular solution.
Advantages: Bawg unit adunay low-voltage standard module, simplifying design, manufacturing, ug maintenance. High reliability (failure sa usa ka unit dili moguba sa overall system operation). Highly suitable sa modular design philosophy sa SST.
4. Internal Reinforcement: Device-Level Technological Innovation (Future Development Direction)
Ini nga approach fundamentally address ang issue gikan sa perspectives sa materials science ug semiconductor physics.
4.1 Use of Wide-Bandgap Semiconductor Devices
Principle: New-generation semiconductor materials sama sa silicon carbide (SiC) ug gallium nitride (GaN) adunay critical breakdown electric fields an order of magnitude higher kaytud sa traditional silicon (Si). Ini means nga ang SiC devices makakab-ot og mas taas nga voltage ratings sa same thickness compared sa Si devices.
Advantages:
Higher Voltage Rating: Ang single SiC MOSFET kasagaran makakab-ot na sa voltage ratings above 10 kV, kontra sa silicon IGBTs nga typically limited sa below 6.5 kV. Ini enables simplified SST topologies (reducing the number of series-connected devices).
Higher Efficiency: Ang wide-bandgap devices offer lower conduction resistance ug switching losses, allowing SSTs to operate at higher frequencies, thereby significantly reducing the size ug weight sa magnetic components (transformers, inductors).
Status: High-voltage SiC devices kasagaran hot topic sa SST research ug considered a key enabling technology for future disruptive SST designs.
4. 2 Superjunction Technology
Principle: Advanced technique sa silicon-based MOSFETs nga introduce alternating P-type ug N-type pillar regions aron alter ang electric field distribution, thereby greatly improving voltage blocking capability while maintaining low on-resistance.
Application: Primarily used sa devices with voltage ratings between 600 V ug 900 V. Applied sa low-voltage side or lower-power sections sa SSTs, but still insufficient for direct medium-voltage applications.
5. Comparison
| Solution Approach | Specific Method | Core Principle | Advantages | Disadvantages | Maturity |
| External Collaboration | Device Series Connection | Multiple devices share the voltage | Simple principle, can be realized quickly | Difficult dynamic voltage sharing, complex control, high reliability challenge | Mature |
| Multilevel Converter (e.g., MMC) | Modular sub-modules are connected in series, each module bears low voltage | Modular, easy to expand, good waveform quality, high reliability | Large number of sub-modules, complex control, relatively high cost | Current Mainstream / Mature | |
| Cascaded Structure (e.g., ISOP) | Standard conversion units are connected in series at input | Modular, strong fault tolerance, simple design | Requires multiple isolation transformers, system volume may be large | Mature | |
| Internal (Device Innovation) | Wide Bandgap Semiconductor (SiC/GaN) | The material itself has a high breakdown electric field, and the voltage withstand is inherently strong | High voltage withstand, high efficiency, high frequency, simplified topology | High cost, driving and protection technology is still developing | Future Direction / Rapid Development |
| Super Junction Technology | Optimize the internal electric field distribution of the device | Performance improved compared to traditional devices | There is an upper limit on voltage withstand level, difficult to cope with medium voltage | Mature (used in low-voltage field) |
How to address the voltage rating limitations of power semiconductor devices in SSTs?
The most practical and reliable solution at present is to adopt multilevel converter topologies (especially Modular Multilevel Converters, MMC) or cascaded input-series output-parallel (ISOP) structures. These approaches, based on mature silicon-based devices, circumvent the voltage rating bottleneck of individual devices through sophisticated system-level architectures.
The fundamental solution for the future lies in the maturation and cost reduction of high-voltage wide-bandgap semiconductor devices, particularly silicon carbide (SiC). Once realized, SST topologies can be significantly simplified, enabling a leap forward in efficiency and power density.
In actual SST research and development, multiple technologies are often combined—for example, employing an MMC topology using SiC devices—to achieve optimal performance and reliability.
