Contents I. Introduction
Multilevel converters are popularly used for medium- and high-power conversion in motor drive applications, HVdc, photovoltaic systems, etc. [1]– [6]. Reduced device stress, fault tolerance, low electromagnetic interference issues, scalability, and low-switching-frequency operation are some of the merits of multilevel converters [7], [8]. The conventional two-level, standard, and cascaded multilevel inverter topologies [9] –[13] generating hexagonal voltage-space-vector structures (VSVSs) have limited linear modulation range for the variable speed operation. Neither higher switching frequency operation nor selective harmonic elimination techniques [14], [15] can achieve low-order harmonic suppression for full speed due to limited modulation range. Furthermore, overmodulation, especially during extreme six-step operation with a hexagonal VSVS, leads to significant low-order phase-voltage harmonics. To eliminate dominant fifth- and seventh-order harmonics, a dodecagonal space-vector structure was proposed for three-phase induction-motor (IM) drives [16]. Later, generation of a multilevel dodecagonal structure was achieved with an open-end winding machine configuration in [17]. The aforementioned topologies require two isolated and asymmetrical dc sources. By generating two-level and later multilevel dodecagonal VSVSs [18], [19] using topologies with single dc source, simple, and cost-effective four-quadrant drives are achieved. Superior harmonic performance throughout the full speed range and better linear modulation range utilization for variable speed drives application can be achieved with the generation of higher sided polygonal multilevel structures. A 24-sided polygonal VSVS generation using a single dc source proposed in [20], eliminates phase-voltage harmonics till 17th and 19th orders and extends linear modulation range very close to the base speed. In this paper, multilevel six-concentric 24-sided VSVS generation using a single dc source for variable speed drives is proposed for the first time. A finer sinusoidal voltage profile throughout the modulation range can be obtained using the multilevel structure when compared to a two-level 24-sided polygonal structure in [20]. A modular configuration by cascading basic inverter units is achieved by developing a multilevel 24-sided polygonal structure from space vectors of a dodecagon. A topology consisting of a flying-capacitor (FC) inverter fed with a dc source, cascaded with two low-voltage H-bridge (HB) cells fed with capacitors, forms the power circuit of the drive scheme. The proposed scheme has the following features.
The FC inverter handling high voltage switch at low frequencies throughout the variable speed operation and low-voltage HBs works as switched capacitors for filtering low-order harmonics.
The constructional feature of generating a multilevel 24-sided structure from dodecagons improves the drive availability with full power level operation and reduced harmonic performance even if any of the HB cell fails.
The proposed multilevel space-vector structure has a set of concentrically (radial) aligned 24-sided polygons. A space-vector-based modulation scheme using nonisosceles triangles for the multilevel operation is implemented.
The capacitors of low-voltage HB inverters are balanced during the pulsewidth modulation (PWM) operation. Also, the capacitor voltages are independently controlled, which improves the modularity in operation and isolating faults.
The linear modulation range utilization is almost equal to the base speed (99.42%) due to the 24-sided space-vector structure.
