I. Introduction
Dual-inverter configuration with two separated dc voltage supplies is one of the attractive approaches to provide a higher voltage for motor drives in electric vehicle applications, or large-power wind energy generation system [1]–[4]. Since each end of the stator windings of the open winding ac machine is directly connected to an inverter, usually the reference voltage vector will be split into two voltage vectors, two independent space vector pulse width modulations (SVPWMs) or discontinuous PWMs are adopted to determine the gate pulses for each inverter [5] . Thus, the switching frequency during a sampling period is doubled compared with a single inverter fed drive system. In order to reduce the switching frequency, an alternate-sub-hexagonal-center PWM switching strategy [6] and an alternate-inverter PWM strategy [7] –[8] are proposed. However, the dc-link voltages for the two inverters are restricted to be equal, and the output performance might be deteriorated when the two dc voltages are different. Thus, for the applications, such as the ship electrical system [9] and hybrid vehicles with energy storage [10], where two different dc-link voltages are usually employed, a more flexible modulation strategy is needed. In addition, the space vectors produced by the dual two-level inverter are identical to that of the three-level inverter [11]–[12], and each sector is divided into four triangle regions according to the magnitude and angle of the reference voltage vector, which increases computational complexities.