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.

中文翻译：

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高频纹波电流视角下的固态变压器混合直流环节寿命延长与ESR监测一体化设计

混合直流链路电容器在级联两级系统中发挥着关键作用，例如为中压公用电网开发的固态变压器 (SST) 单元。电容器需要吸收宽频谱的纹波电流，这会引起老化应力。本文介绍了一种将设计中的寿命延长与状态监测 (CM) 相结合的技术。发现过大的高频纹波电流通常流经电解电容器（E-caps）。然后设计了一个专用的基于变压器的传感器，其初级侧电感充当外部阻抗，以降低 E-cap 环路中的高频电流。另一方面，在传感器次级侧测量的高频电流感应的电压可用作 CM 等效串联电阻 (ESR) 退化的指标。一体化设计具有复杂度低、纯无源性和通用性的特点，并阐述了传感器的通用测量原理。最后，构建了一个 SST 单元的 5 kW 原型，以验证所提出解决方案的性能。实验结果表明热应力和电应力显着降低，对应于 E-cap 寿命延长 40%，CM 模型可以准确跟踪 ESR 的变化。构建了一个 5 kW 的 SST 单元原型，以验证所提出解决方案的性能。实验结果表明热应力和电应力显着降低，对应于 E-cap 寿命延长 40%，CM 模型可以准确跟踪 ESR 的变化。构建了一个 5 kW 的 SST 单元原型，以验证所提出解决方案的性能。实验结果表明热应力和电应力显着降低，对应于 E-cap 寿命延长 40%，CM 模型可以准确跟踪 ESR 的变化。