Thermal load analysis and monitoring of doubly-fed wind power converters in low wind speed conditions

Abstract

State-of-the-art wind power technology allows multi-megawatt installations in low wind areas. Meanwhile, the present development is focused on aerodynamic improvement, enhanced efficiency, and reliability. Research is carried out on power electronics reliability issues of wind turbines installed on low wind speed sites. Reliability issues associated with power electronics arise as a result of the increasing power capacity of a single wind turbine, which also increases the power density of the module. The power converter as part of a doubly-fed induction generator (DFIG) based wind turbine (WT) is subjected to a considerable thermal stress, especially, in operation close to and at the synchronous operating point. Moreover, low wind speed conditions and a high turbulence level make the power electronics more vulnerable, thereby inducing degradation mechanisms and reducing its lifetime.

A comprehensive method to perform a mission-oriented reliability analysis of a wind power converter covers three areas of research, namely the wind speed characteristics, the DFIG operation, and the failure mechanisms of the power electronics. A lifetime estimation method for the insulated-gate bipolar transistor (IGBT) is implemented. The method comprises transformation steps from the turbulent wind speed profile, the wind turbine dynamic response, the corresponding power converter thermal stress, and the consumed lifetime estimation. The modelling results show that the thermal stress of the power converter is considerably affected by the low operating frequency of the rotor circuit and the bidirectional flow of the rotor power. The obtained results for the lifetime consumption indicate that an IGBT failure caused by the bond wire lift-off has a higher probability than a failure resulting from solder fatigue. Additionally, the strong influence of the site-specific wind characteristics on the lifetime consumption is demonstrated.

In the present research, application of a gradient heat flux sensor (GHFS) in power electronics is analysed. The GHFS, thanks to its thinness, can be attached between the power device base plate and the heat sink, and it provides direct heat flux measurements. The test results are compared with the modelled IGBT power losses, and a good accuracy is found between them. Furthermore, the GHFS is used to detect power device degradation, and consequently, a possible solution is proposed for the GHFS-based condition monitoring system implemented in the WT.