Converters are the essential devices in the energy transformation process from the wind generators to the grid. Power electronic semiconductors have a high share of failures in the power electronic converters. Therefore, the reliability based on lifetime of power semiconductor generates more and more attention. This thesis presents a comparison of different active control methods to control the lifetime of power semiconductors in wind turbines. This investigation is implemented through simulations on a 10 MW direct drive permanent magnet wind generator with three-level neutral point clamped converters. The study shows that both switching frequency control and reactive current control can improve semiconductor lifetime to certain extents. As for which control method is more effective depends on the surrounding wind environment.
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Stability and security of the grid are two vital aspects of energy supply. To keep the grid stable, it is necessary that the electricity plant can control and protect the mechanism. In the past, traditional plants could meet these requirements. Nowadays, the share of sustainable power sources in total electricity generation has become so important that these sources must keep the grid stable. A significant part of these requirements is the Low Voltage Ride Through (LVRT) capability of the sustainable energy plant. When some large loads are connected to the grid or as a result of grid faults like lightning strikes or short circuits, short–term voltage dip may occur. This low voltage dip condition plans to be simulated and the behaviours of Brushless Doubly-Fed Induction Generator (B-DFIG) should be improved in this situation. This thesis is going to realize this goal that after the improvement, when low voltage dip occurs, it is not necessary to disconnect the generator from the grid.

The thesis is organized as follows: Firstly, the B-DFIG modelling is implemented, including the steady state modelling by equivalent circuits and the dynamic modelling by differential equations. The steady and transient state characteristic of B-DFIG can be studied by equivalent circuits and dynamic models respectively. Secondly, the control algorithm is added to the power electronic converter to control the machine in a stable and responsive manner. Thirdly, simulations of both full voltage and low voltage dip conditions are done to compare B-DFIG behaviours between these two different simulation conditions. Unexpected performance like large transient current will occur under low voltage dip condition. Fourthly, the LVRT control algorithm is added to the controller of B-DFIG for improving the behaviours of B-DFIG in low voltage dip condition based on some constraints and assumptions. Finally, experiments are executed to verify this result.

Index Terms — Brushless Doubly-Fed Induction Generator, Low Voltage Ride Through, security, stability ... Read more