Integrated Design for Lifetime Extension and ESR Monitoring of Hybrid DC Link in Solid-State Transformer From the Perspective of High-Frequency Ripple Current

Hybrid dc-link capacitors serve a critical role in cascaded two-stage systems, such as the solid-state transformer (SST) units being developed for medium-voltage utility grids. The capacitors need to absorb ripple current in a wide spectrum, which induces aging stresses. This article presents a technique to combine lifetime extension in design with condition monitoring (CM). It is found that the excessive high-frequency ripple current typically flows through electrolytic capacitors (E-caps). A dedicated transformer-based sensor is then designed, and its primary-side inductance acts as an external impedance to reduce the high-frequency current in the E-cap loop. On the other hand, the voltage induced by high-frequency current measured on the secondary side of the sensor can be used as an indicator of equivalent series resistance (ESR) degradation for CM. The integrated design features low complexity, pure passivity, and versatility, and the universal gauging principle for the sensor is elaborated. In the end, a 5-kW prototype of an SST unit is built to verify the performance of the proposed solution. The experimental results indicate a noticeable reduction in thermal and electrical stress, corresponding to a 40% extension of the E-cap lifetime, and the CM model can accurately track the change of the ESR.