Thermal Cycling in Converter IGBT Modules with Different Cooling Systems in Pitch- and Active Stall-Controlled Tidal Turbines

This paper compares active and passive cooling systems in tidal turbine power electronic converters. The comparison is based on the lifetime of the IGBT (insulated gate bipolar transistor) power modules, calculated from the accumulated fatigue due to thermal cycling. The lifetime analysis accounts for the influence of site conditions, namely turbulence and surface waves. Results indicate that active cooling results in a significant improvement in IGBT lifetime over passive cooling. However, since passive cooling systems are inherently more reliable than active systems, passive systems can present a better solution overall, provided adequate lifetime values are achieved. On another note, the influence of pitch control and active speed stall control on the IGBT lifetime was also investigated. It is shown that the IGBT modules in pitch-controlled turbines are likely to have longer lifetimes than their counterparts in active stall-controlled turbines for the same power rating. Overall, it is demonstrated that passive cooling systems can provide adequate cooling in tidal turbine converters to last longer than the typical lifetime of tidal turbines (>25 years), both for pitch-controlled and active speed stall-controlled turbines.