Reactive Power Control to Improve Reliability of High Wind Power Converters Connected in Parallel

Introduction

The demand for high power wind turbine systems (WTSs) of multi-MW range is increasing and at the same time a promising renewable energy source. Many topologies for the WTS have been developed in the last years. The parallel connected full-rated back-to-back converters are widely adopted in commercial products due to the fact that the converter’s capacity is expandable and has a higher reliability in comparison to other topologies. Typically, over 20 years of lifetime is required in high power converters for industries and reliability is one of the key importance with the significant reduction of cost for maintenance and operation. In this paper, the reactive power control strategy for a generator-side parallel connected full-power converter with improved reliability is proposed.

Approach

Many researches have revealed that the lifetime of switching devices such as IGBTs is closely associated to thermal behavior of the average junction temperature and especially the junction temperature variation. The proposed approach improves the reliability by minimizing the temperature variation during wind gusts. This can be achieved by circulating a modified reactive power between the parallel-connected converters. The change of thermal distribution and the junction temperature variation can be achieved by controlling the parallel-connected converters with an EtherCAT communication.

Main body of abstract

Before describing the impact of reactive power on thermal behavior, it should be noted that the reactive current of generator is typically controlled at a certain value under normal conditions. Due to this reason, the reactive power control of a single converter of the generator-side does not have the same flexibility as parallel connected converters, because the reactive power can be widely controlled by interchanging the reactive power between the parallel connected converters which does not have a general effect to the generator control.
The reactive power control is divided into the over-excited and the under-excited cases. Both cases have different impacts to the thermal distribution of the switching devices. In the over-excited case the total losses are reduced, which causes a smaller temperature increase, rather the under-excited case leads to a higher temperature increase. This feature can be used to stabilize the temperature variation during a wind gust. So, the temperature would be increased or decreased dependently on the varied wind speed. When the wind speed is changed at under-excited operation, a proper reactive power will heat up the switching devices of the converter, which leads to a minimization of the temperature variation, but results a higher average temperature. Meanwhile, the over-excited converter experiences a comparable temperature distribution with the no-reactive power.
Finally, it can be concluded that the reliability is improved by introducing a new approach for the reactive power control which minimizes the stress of the temperature variation, while accepting a higher average junction temperature.

Conclusion

The parallel connected converters are widely adopted in the high power WTSs since they feature the expandable capacity and the higher reliability. Moreover, they can support the wide range of reactive power control, which changes the thermal distribution of the switching devices of each converter. As a result, the junction temperature variation that is the most influential factor to the reliability of the semiconductors can be minimized by the proposed method and, therefore, the lifetime can be improved according to the Coffin-Masson lifetime model.


Learning objectives
After this research has been introduced, the clear impact of reactive power control on the thermal behavior can be derived and the centralized control through the EtherCAT protocol can be achieved. Finally, it is expected that the results can contribute an improvement of the reliability in other applications such as in a central controlled wind farms, the smart transformer directly connected to renewable energies and in high power wind turbine converters.