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The smart transformer (ST) is a potential solution for an upgrade of the electric distribution grid, which enables to provide services to the grid and dc connectivity. However, the power electronics within the system are challenged by high reliability requirements. One possible solution to increase the reliability is to employ prognosis to predict the failures and avoid down times of the system. Traditional maintenance scheduling is based on the remaining useful lifetime (RUL) of the individual components or the forecasted failure probability. For a further increase of the time to the next maintenance, it is desired to have similar wear out of all components, which need to be maintained or exchanged. In this article, it is proposed to route the power internally in a modular power converter consisting of a cascaded H-bridge connected to dual active bridges in order to influence the RUL of its building blocks. Therefore, a thermal stress based wear-out control is designed for addressing the processed power dependent failures of the devices in the building blocks of the ST. Compared to the conventional power routing methods, the impact of the proposed system-level control considering electrical and thermal parameter variations is demonstrated using Monte Carlo analysis. [ABSTRACT FROM AUTHOR]

Smart transformers (STs) are challenged by high-reliability requirements. Particularly, modular STs consist of a large number of power semiconductors, and unevenly distributed thermal stress results in different remaining useful lifetimes of the devices in the different building blocks. This problem can be addressed with discontinuous modulation for the medium-voltage-side converter, but results in a coupled thermal stress with the isolated dc–dc cells. For overcoming this drawback, this article proposes an advanced discontinuous modulation, which enables to manipulate thermal stress in different cells independently. The algorithm is analyzed with respect to its capability in thermal stress manipulation and validated experimentally with junction temperature measurements. In addition, this article investigates the impact of the algorithm on the overall system's efficiency as well as potential lifetime of the power semiconductors in the different building blocks. [ABSTRACT FROM AUTHOR]

Reliability and availability are crucial in smart transformers (ST), and power routing in a modular design can reduce thermal stress of power semiconductors and thereby prevent failures. A desired feature of power routing is the independent control of the thermal stress within the ST architecture, which also includes the maximum unbalanced loading of the medium-voltage side converter. The currently employed algorithms, including the multi-frequency power routing, obtain an inherent coupling of the thermal stress between medium-voltage alternating currents (MVac) building blocks and low-voltage direct current (LVdc) building blocks as well as a limitation in the maximum imbalance for the loading of the MV side converter. For overcoming these limitations, discontinuous pulsewidth modulation is combined with power routing for decoupling the thermal behavior of the MVac and the LVdc building blocks and increasing the maximum achievable imbalance of the MV side converter. The algorithm is analyzed and implemented on an experimental test bench with junction temperature measurement. In addition, a study case is carried out to demonstrate the impact of the proposed method on the lifetime of different building blocks. [ABSTRACT FROM AUTHOR]