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The aim of this work is to obtain a reliable estimation of the remaining lifetime of power-electronic modules. Lifetime models provide information on the fatigue durability of power modules under repetitive loading circumstances. However, the thermo-mechanical stresses that a device is exposed to during operation are different than the ones evaluated to create the lifetime models. Rainflow counting provides a tool to analyze and evaluate the stress content of a randomly varying stress waveform, but counting stress cycles while the device is operating is challenging. In this paper, implementation challenges for an online rainflow-counting method are presented, and solutions to overcome them are discussed. An algorithm for online rainflow counting is implemented, and numerical results from tests on experimental device-temperature data are presented.

The prospect of future multivendor HVDC systems has led to concerns about interoperability between converter terminals based on different technologies. This paper investigates the interoperability of converter terminals in a small-scale experimental setup, consisting of two 2-level (2L) Voltage Source Converters (VSCs) and two Modular Multilevel Converters (MMCs) with different number of half-bridge sub-modules in each arm. The laboratory setup is utilized to demonstrate interoperability of a 2-level VSCs and an MMCs when operated in parallel on the dc-side as well as in parallel at a common connection point in an ac grid. Experimental results are presented for the investigated 4-terminal configuration with one 2L VSC controlling the dc voltage and all other converters controlling their active power flow. The presented results give an additional empirical confirmation that topological differences between properly designed converter terminals should have limited influence on system level operation and system interoperability of MT HVDC systems.