Metallized Film Caps in SSTs: Design & Selection

In solid-state transformers (SSTs), the DC-link capacitor is an indispensable key component. Its primary functions are to provide stable voltage support for the DC link, absorb high-frequency ripple currents, and serve as an energy buffer. Its design principles and lifetime management directly impact the overall system efficiency and reliability.

I. Design Priorities
Designing a DC-link capacitor is a systems-level engineering task that requires balancing electrical performance, thermal management, and reliability.

• Accurate Capacitance Calculation: Capacitance value is not “the larger, the better.” It must be determined based on allowable DC-side voltage ripple—particularly the second-harmonic component common in three-phase SPWM rectifiers—and the acceptable voltage droop coefficient. Moreover, with the increasing operating frequencies of modern solid-state      transformers (SSTs), high-frequency ripple currents have become a critical factor that must be accounted for during design. A useful reference is the asymmetric operating condition–based design method proposed in a patent by China Electric Power Research Institute.

• Multiphysics Co-Design: High-performance capacitor design demands integrated consideration of coupled electro-thermal-magnetic effects. For instance, internal element geometry and layout should be optimized to minimize equivalent series resistance (ESR) and thermal resistance, ensuring efficient heat dissipation and preventing localized overheating that accelerates aging.

II. Lifetime Management Strategies
Extending capacitor lifetime and accurately predicting remaining useful life (RUL) are crucial for enhancing overall system reliability.

• From “Reactive Replacement” to “Proactive Management”: Researchers at Chongqing University proposed an innovative approach that integrates lifetime extension with real-time health monitoring. By leveraging the sensitivity of capacitor health indicators (e.g., ESR) to high-frequency ripple currents, real-time aging assessment becomes feasible. Furthermore, circuit-level designs enabling spontaneous current  balancing among parallel capacitor banks in hybrid DC links can significantly extend total service life.

• In-Depth Failure Mechanism Analysis: Harmonics severely degrade capacitor lifespan. Studies show that high harmonic content accelerates electrochemical corrosion of metallized films (causing rapid initial capacitance loss) and may break chemical bonds in polypropylene dielectric films, compromising insulation performance. Therefore, lifetime prediction models must incorporate the synergistic acceleration effect of DC electric fields combined with harmonic stress.

III. Selection Guidelines
Beyond standard datasheet parameters, the following aspects deserve attention during component selection:

• Technology Path: In high-reliability applications such as flexible HVDC transmission, metallized film capacitors have become the dominant choice due to their self-healing capability and long operational life. Chinese manufacturers like XD Group have mastered this technology, offering products with high voltage/current endurance and low impedance.

• Localization Trend: Notably, domestic substitution of DC-link capacitors is a clear strategic direction. Localization reduces costs and mitigates supply chain risks—especially under geopolitical or trade tensions, where reliance on imported critical components could lead to severe price surges or even shortages.

IV. Conclusion

• System-Oriented Design: Never treat the capacitor as an isolated component. Instead, embed it within the full SST system and perform co-simulation and optimization across electrical, thermal, and  magnetic domains.

• Cutting-Edge Approaches: The research frontier is shifting from passive capacitor design toward “active” architectures with embedded health-monitoring capabilities, as well as advanced integrated design methods for DC-link capacitors in multi-port SSTs—dramatically improving system intelligence and reliability.

• Rigorous Validation: For mission-critical applications, accelerated aging tests under realistic operating conditions—especially combined DC voltage and harmonic stress—must be conducted to validate both lifetime models and component selection.