Investigation of variable DC link voltage operation of a PMSG based wind turbine with fully rated converters at steady state

The variable speed wind turbine with a multi-pole Permanent Magnet Synchronous Generator (PMSG) and Fully Rated Converter (FRC) is a popular choice for modern megawatt scale wind turbines. Two back to back 2-level pulse width modulated (PWM) voltage source converters are typically used for the FRCs. The two converters are interconnected through the dc link capacitors. The power converters are based on multi-chip IGBT modules. One way of increasing the inherent reliability of the FRC is to reduce the operating electrical and thermal stresses on these IGBT modules. A higher dc link voltage leads to high IGBT stresses. Higher dc link voltage is also a major contributor to cosmic ray induced failures. In the existing FRCs in wind turbine systems, the dc link voltage is maintained at a fixed value while the modulation indices of the two converters are varied. The investigation presented in this paper and authors´ previous work shows that this fixed dc link voltage is significantly higher than the minimum dc link voltage required at the majority of the operating conditions in the maximum power point tracking range. In the case study based on a multi-megawatt wind turbine, for generator speeds up to about 85% of the rated speed, the dc link voltage can be lowered to match the ac system voltage by setting the modulation index of the network bridge converter to the maximum value while the modulation index of the generator bridge is varied to match the generator voltage. Above this speed, the generator bridge converter dominates at certain network conditions where its modulation index needs to be at maximum while network bridge modulation index is varied to match the network voltage. Simulation results show successful operation of the system with minimum dc link voltages at steady state. A study on cosmic ray induced failure rate calculations shows higher reliability may be achieved when the FRC is operated with a variable dc link voltage optimised to minimum values.