3-Stage SST Architecture
System Overview
The proposed solid-state transformer architecture employs a modular 3-stage topology designed for bidirectional power flow, high efficiency, and grid service capability. Each stage is optimized for specific voltage levels and functionality, leveraging wide-bandgap semiconductors to achieve high power density and efficiency.
- Modular architecture for scalability and redundancy
- Galvanic isolation via medium-frequency transformer
- Wide-bandgap devices for high-frequency operation
- Integrated fault tolerance and protection
Stage 1: Three-Level NPC Rectifier
Topology Analysis
The neutral-point-clamped (NPC) rectifier provides AC-DC conversion with reduced voltage stress on semiconductors, lower harmonic distortion, and improved efficiency compared to two-level topologies. The three-level structure enables direct connection to medium voltage grids without series connection of devices.
\[ V_{dc} = \sqrt{2} \cdot V_{LL} \cdot m_a \] where \( m_a \) is the modulation index (typically 0.9-0.95)
Component Selection
| Component | Specification | Selected Device | Rationale |
|---|---|---|---|
| Main Switches | 3.3 kV, 450 A | Wolfspeed CAB450M12XM3 | Low Rds_on, high temp operation |
| Clamping Diodes | 3.3 kV, 300 A | SiC Schottky Diodes | Zero reverse recovery |
| DC Link Capacitor | 20 kV, 5 mF | Film Capacitor Bank | Low ESR, high ripple current |
| Input Filter | LCL Configuration | L1=2mH, C=50μF, L2=1mH | THD < 3%, resonance damping |
Interactive Design Tool
Stage 2: Dual Active Bridge with MF Transformer
DAB Operation Principle
The dual active bridge provides bidirectional power transfer with galvanic isolation through a medium-frequency transformer. Power flow is controlled via phase-shift modulation between primary and secondary bridges, enabling soft-switching operation across a wide load range.
\[ P = \frac{n \cdot V_1 \cdot V_2}{2\pi f_s L_{lk}} \cdot \phi \left(1 - \frac{|\phi|}{\pi}\right) \] where \( \phi \) is the phase shift, \( L_{lk} \) is leakage inductance
MF Transformer Design
Core Selection
- • Material: Nanocrystalline (Vitroperm 500F)
- • Core Type: C-Core Configuration
- • Effective Area: 45 cm²
- • Window Area: 120 cm²
- • Operating Flux: 0.8 T @ 50 kHz
Winding Design
- • Primary: 28 turns, Litz wire 1000×0.2mm
- • Secondary: 3 turns, copper foil 0.5mm
- • Turns Ratio: 9.33:1
- • Leakage Inductance: 25 μH (designed)
- • Insulation: Nomex, 50 kV BIL
Stage 3: LV Output Converter
Configuration Options
DC Output (DCFC Mode)
Regulated DC bus for EV fast charging
- • Output: 400-1000 VDC adjustable
- • Current: up to 500 A
- • Ripple: < 1% peak-to-peak
- • Protocol: CCS/ChaoJi compatible
AC Output (Grid-Tie Mode)
Three-phase inverter for grid connection
- • Output: 480 VAC, 3-phase
- • THD: < 3% at rated load
- • Power Factor: 0.8 leading to lagging
- • Grid Services: Volt-VAR, frequency support
GaN Device Implementation
The LV stage utilizes GaN HEMTs for high-frequency operation with minimal switching losses. The enhanced electron mobility and low gate charge enable switching frequencies up to 200 kHz, reducing passive component size and improving dynamic response.
| Parameter | GaN HEMT | Si MOSFET | Improvement |
|---|---|---|---|
| Switching Frequency | 200 kHz | 50 kHz | 4× |
| Switching Loss | 0.2 mJ | 0.8 mJ | 75% reduction |
| Power Density | 8 kW/L | 3 kW/L | 2.7× |
System Integration & Control
Hierarchical Control Architecture
Level 2 - Converter Control (DSP): Current/voltage loops, phase-shift control, MPPT
Level 3 - System Control (ARM): Power management, grid services, communication
Protection & Fault Management
- Overcurrent protection with di/dt detection
- Overvoltage clamping and crowbar circuits
- Thermal monitoring with predictive derating
- Ground fault detection and isolation
- Arc flash mitigation systems
Experimental Validation
Prototype Specifications
| Parameter | Design Target | Measured Value | Status |
|---|---|---|---|
| Power Rating | 350 kW | 352 kW | ✓ Achieved |
| Peak Efficiency | 98.5% | 98.3% | ✓ Close |
| Power Density | 5 kW/L | 4.8 kW/L | ○ Optimizing |
| Input THD | < 3% | 2.4% | ✓ Achieved |
| ZVS Range | 20-100% | 25-100% | ✓ Achieved |