PhD Diego VELAZCO: “Condition and Health Monitoring of Modular Multilevel Converter for HVDC Grids”

Abstract

The growing power and energy demand, as well as the integration of new renewable sources into the electrical system and the long distances between electrical consumers and producers are pushing the actual AC grid to evolve. Hence, HVDC has emerged as a promising solution for upgrading the power transmission infrastructure and meet the demanding requirements and future needs of the electrical grid. Moreover, HVDC is nowadays the preferred technology for bulk power transmission and the integration of large-scale renewable sources such as offshore windfarms.

One fundamental component of HVDC systems is the converter station. At the ends of an HVDC line, the converter station is responsible for interfacing the DC link with the AC grid. Among the different types of technology for converter stations VSC have gained in relevance in the last few decades, as they provide improved functionalities when compared to the traditional converters employed for HVDC applications. Concerning VSC topologies, the MMC has emerged as the most advantageous topology and the main candidate for future HVDC projects.

The MMC is a scalable converter composed mainly of stacks of SMs that allow its operation. Each SM is composed itself by a large storage capacitor and IGBT modules used for the insertion or the bypass of the capacitor. As power transmission is a critical application, HVDC systems are subjected to very demanding reliability and availability requirements and the MMC is not the exception. This thesis addresses the condition monitoring and the prognosis of the remaining useful life of IGBT modules in a HVDC-MMC application, as these elements were identified as one of the weakest components present in converter applications.

A physics-of-failure approach is chosen for the prognosis scheme. This scheme uses the thermal loading of the semiconductor devices of a HB-SM for making computations of the lifetime consumption in the evaluated period. Then, from this lifetime estimation the remaining useful life can be inferred. Additionally, the prognosis scheme allows the calculation of the reliability of the HB-SM by considering the reliability functions of its composing elements. Consequently, this calculation is used for computing the redundancy needed for attaining the target availability values with the help of a Markov Chain and Monte Carlo simulations.

Another important subject addressed in this thesis is the condition monitoring of IGBT modules. For this purpose, a Power Cycling testbench was developed. The was chosen as the parameter for tracking the evolution of the degradation state of the power device throughout the cycling tests. Consequently, an online measurement board was conceived and developed. In order to get more relevant insights of the cycling protocol applied, a strategy for the online estimation of the junction temperature of IGBT devices was developed and it is based on the use of a Kalman filter. This strategy also enables to perform an estimation of the degradation of the state-of-health of the IGBT by analyzing the Thermo-Sensitive Electrical Parameters.

Director of thesis: M. Guy Clerc, Professeur des Universités (UCBL)

Co-director of thesis: M. Emmanuel Boutleux, Maître de conférences (ECL)