Conclusions
From this book, it can be concluded that power electronics will play more important role for the next generation of wind turbine systems. In this case, the thermal stress in the power semiconductors is a critical performance parameter because the con- verters need to carry all the generated power from wind turbines with very high power density. It is found that the thermal behaviors in wind turbine system could be rather adverse under either normal or faults conditions. On the other hand, it is also possible to improve the thermal behaviors of wind power converter by multiple methods like the control, modulation, modeling, and converter designs. The detailed conclusions are given as follows in several aspects:
Topologies and device for wind power converter
It is becoming more and more difficult for the traditional two-level converter topologies to achieve acceptable performances in the wind power application. Thereby, some multi-level and multi-cell converters with higher power handling ability are becoming promising for the next generation wind turbines. Regarding to the power semiconductor devices, press-packing IGCT and IGBT show significant improvement in respect to the thermal resistance and power density compared to the module packaging devices. Due to the rating limits on the market, paralleled connection of devices may be needed in order to achieve the required amount of power for the next generation wind turbines—this will modify the loss/thermal behaviors of device as well as the overall power density of converter.
Criteria and tools for the analysis of wind power converter
Due to the growing power and limited space in the wind power application, the thermal loading of the wind power converter is becoming significant at multi-MW level of power conversion. It has been calculated in this work that the thermal stress of power devices has close relationship with the reliability and cost of the converter.
Therefore, thermal stress analysis is crucial important for the next generation wind power converter system.
The loading of wind power converter is influenced by many factors which may involve multidisciplinary approaches of analysis under various time constants. Thereby, a framework with multi-domain models of the wind turbine system is established for the stress analysis. In this framework, the factors of mission profile, converter design, and converter controls are taken into account and they can be translated into the corresponding stress profile in the power semiconductor devices.
Electrical and thermal stresses under normal operation
The thermal profile of the power switching devices under steady-state wind speeds is an important tool which can be used either as a loading indicator for certain converter topologies or as a lookup table for the lifetime estimation. It was found that the wind speed variations will lead to severe thermal excursion of some power switching devices in the given 3L-NPC wind power converter. Meanwhile, the grid codes even under normal operation may change the delivered reactive power of the wind power converters and thereby also have impacts to the converter efficiency as well as thermal stress of the power devices.
By circulating the reactive power among paralleled converters in a wind farm or multi-cell converter system, it is possible to control the junction temperature and relieve the thermal excursion under wind gust operation, leading to higher reliability of the converter, while the increased thermal stresses to the other devices or paralleled converters are still acceptable.
Electrical and thermal stresses under fault condition
Depending on the types and severity of grid faults as well as corresponding LVRT control behaviors, the operating condition of the grid connected power converter is significantly different compared to the normal operating condition. It should be noted that some power devices under the LVRT operation of 3L-NPC converter may have even higher junction temperature than the most stressed normal operation.
According to the investigations, the thermal optimization target for 3L-NPC wind power inverter under extreme LVRT is to reduce the junction temperature in the NPC diode and inner switch. The proposed thermal redistributed modulation sequences, which all enable full neutral point potential control ability, can achieve more equal thermal distribution and relieving the hottest power devices under extreme LVRT operation of 3L-NPC inverter. The proposed thermal optimized modulation methods are especially feasible during the LVRT operation, where the modulation index is relative low and more redundant switching states can be utilized.
In the typical three-phase three-wire converter structure, the control freedom may be not enough to achieve satisfactory performances under unbalanced AC source condition. However, in the converter structure with zero sequence current path (4 or 6 wire system), two extra control freedoms coming from the zero sequence current can be utilized to extend the controllability of converter and to improve the performance under unbalanced AC source. By the proposed control strategies, it is possible to totally cancel the oscillation of both the active and reactive power, or to reduce the oscillation amplitude in the reactive power. Meanwhile, the current stress in the faulty phase is also relived compared to the typical three-wire system.