Lifetime estimation for three level converter for wind energy application

The rated power of wind energy plants increases but generators and generator side converters still operate at low voltage level. This leads to high currents and so to large cable cross-sections and high cable losses. The change to medium voltage level overcomes these drawbacks but will make the use of multilevel converters necessary. The Neutral-Point-Clamped three level converter is a common multilevel converter. Important aspects about the converter are the power cycling stress and the lifetime analysis of the power semiconductors. This paper investigates the power cycling stress and the lifetime of the power semiconductors of three level converter topologies for wind energy application. For the investigation a real wind speed over time profile measured at FINO 1 station in the German North Sea has been used. Due to this it is possible to consider the influence of power cycling stress caused by the fundamental frequency of the generator as well as the power cycling stress caused by variations of the wind speed on the lifetime of the power semiconductors. For counting the total number of cycles, which includes the small fundamental frequency cycles and the large mission profile cycles, the Rainflow algorithm is used. For the fundamental frequency cycles only the maximum and the minimum temperature within a period of the fundamental frequency are extracted from the temperature over time profile. The counted cycles are used to estimate lifetime consumption due to fundamental frequency cycles and mission profile cycles.