A Study on the Lifetime of Q2L-MMC-DAB’s Switches for Wind Turbine Applications

This paper studies the lifetime of semiconductor switches of a dual-active-bridge (DAB) DC-DC converter for wind turbine applications. Quasi-two-level operating modular multilevel converters (MMC) are used as the building blocks of the DAB converter. One of the established lifetime models is used for the lifetime estimation of the switches. Measurement data of an onshore wind turbine for three hundred days is used as the mission profile. It is shown that the short-term thermal cycles (cycles with frequency in the range of switching frequency) are detrimental to the lifetime estimation of the auxiliary switches of the MMCs' submodules. Thus, neglecting the short-term thermal cycles will overestimate the lifetime of the auxiliary switches by several orders of magnitude. On the other hand, these cycles will not affect the lifetime of the bypass switches considerably. It is also shown that the thermal stress on the secondary-side auxiliary switches is more severe than the primary-side ones. It is suggested that two parallel devices should be used for the secondary-side auxiliary switches; as a consequence, a reasonable lifetime is achieved for the secondary-side auxiliary switches.