Active thermal control of power electronic modules in smart transformer applications
The Smart Transformer (ST) is a possible solution to obtain intelligent nodes in the electrical grid, which can be used for the grid management and increase the capacity for the integration of renewable energy sources. A problem for the application of the ST in the distribution grid is the expected lower reliability in comparison with the traditional transformer. To address this problem, the knowledge of power system, power electronics and reliability is combined in this work. Following the "Physics of failure" approach, the most frequently failing components are identified, their load profile in the electrical distribution grid is analyzed and finally solutions are developed to improve the reliability. The power semiconductors are found to be the most prone to fail components and most of their failure mechanisms are found to be affected by thermal cycling. For this reason, thermal stress analysis is performed for the three-stage ST. As an opportunity to increase the reliability, active thermal control is introduced, which is a software based solution for the reduction of the thermal stress during operation. The existing approaches from literature are reviewed and categorized into control of the power converter losses and the control of the device loading. For increasing the reliability by control of the power converter losses, one algorithm is introduced and validated for hard switching power converters and one algorithm is introduced for soft switching power semiconductors. Controlling the thermal stress of modular building blocks in a modular power converter, referring to power routing, is proposed. The capability of the algorithm is investigated analytically for series connected and parallel connected modular building blocks. For the validation, the influence of the power routing on the loading of the single cell is demonstrated experimentally for series connected, parallel connected and medium frequency transformer coupled cells in modular power converters.