Thermally Compensated Discontinuous Modulation for MVAC/LVDC Building Blocks of Modular Smart Transformers

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.