Zero vector modulation method for voltage source inverter operating near zero output frequency

A new zero vector modulation (ZVM) method is developed for usage in voltage source inverters operating near zero output frequency. In automotive applications during hill-holding maneuver a condition occurs requiring the electric drive system to produce high output current at zero or near zero output frequency. This mode of operation represents an increased stress for the VSI, since, depending upon the chosen modulation strategy, individual switch losses can exceed those occurring in normal operating modes. In the proposed ZVM method, the additional degree of freedom, choice of zero vectors, is used to distribute conduction losses among the inverter switches in the leg carrying the largest current. The zero vectors are altered periodically, at relatively low frequency (10 Hz-100 Hz). The method utilizes the fact that near zero speed, the average output voltage is small, and the time for which a zero vector is applied is significant. The method was tested experimentally on a 600 V, 400 A_rms inverter equipped with thermal-couples for the junction temperature measurements. For the tested inverter, the ZVM method enabled increased current rating at zero output fundamental frequency compared to commonly use discontinuous PWM method by approximately 15% and by about 30% compared to the space vector PWM modulation method with the same switching frequency.