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Pulse width modulation (PWM) techniques can be classified into continuous pulse width modulation (CPWM) and discontinuous pulse width modulation (DPWM) types. The switching loss of power devices in DPWM converters is lower than that in CPWM converters. Lower loss could reduce the junction temperature fluctuation in converters of wind turbine generator system (WTGS) and may result in longer power devices lifetime. However, employing DPWM scheme under all WTGS operation conditions will lead to power quality concern. To solve this problem, a new hybrid modulation scheme which combines the CPWM and DPWM methods for WTGS converters is presented in this paper. In the presented hybrid modulation method, two modulation schemes are switched back and forth according to the wind speed in the wind farm site. The performance of the presented modulation scheme is verified and compared with that of other PWM schemes through a case study of 1.2 MW WTGS in long-term mission profiles. The results show that the lifetime of power devices with the presented hybrid approach is longer than that with the CPWM, and is shorter than that with the DPWMs. Moreover, the power quality of the power converters with the hybrid modulation scheme can be guaranteed in all operation conditions, which may not be achieved with DPWMs.

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Power converters are crucial components of the wind turbine generator system (WTGS), and the reliability of wind power converters is partly susceptible to the wind speed at the wind sites. The existing literatures, which evaluate the effect of wind speed on lifetime of power semiconductors, mostly compare lifetime of power semiconductors in the different wind farm and hardly obtain relationship between wind speed distribution and lifetime of power semiconductors. In this paper, a lifetime evaluation approach of the power semiconductors in WTGS considering the probability distribution of wind speed is firstly presented, and then the relationship between the probability of wind speed and the lifetime consumption of power semiconductors is obtained. The results show that the great majority of the lifetime of power semiconductors is consumed when the wind speed is equal to and higher than the wind speed which is close to the rated wind speed, however, the probability of this wind speed range is much small. Based on this observation, a new idea for extending the lifetime of wind power converters through modifying the control scheme is presented in this paper. The effectiveness of the proposed control scheme is verified through a case study.