Abstract
This paper shows a preliminary study about the output voltage modulation of a modular battery system based on a seven-level cascaded H-bridge inverter used for vehicle propulsion. Two generally known modulation techniques, pulse width modulation (PWM) and fundamental selective harmonic elimination (FSHE), are extensively compared for such an innovative modular battery system inverter considering EVs’ broad torque-speed range. The inverter and the battery losses, as well as the inverter-induced current THD, are modeled and quantified using simulations. At low speeds, if the modulation index M is below 0.3, FSHE induces a high current THD (>>5%) and, thus, cannot be used. At medium speeds, FSHE reduces the drivetrain losses (including the battery losses), while operating at higher speeds, it even reduces the current THD. Thus, an individual boundary between multilevel PWM and FSHE can be determined using weightings for efficiency and current quality. Based on this, a simple hybrid modulation technique is suggested for modular battery system inverters, improving the simulated drive cycle efficiency by a maximum of 0.29% to 0.42% for a modeled small passenger vehicle. Furthermore, FSHE’s high speed dominance is demonstrated using a simple experimental setup with an inductive load.
Highlights
The two-level inverter topology is predominantly used in today’s electric vehicles (EVs) [1,2]
The battery packs in a cascaded H-bridge inverter are intermittently conducting the corresponding phase current, which is illustrated in Figure 7a for the third converter module when using multilevel pulse width modulation (PWM) and fundamental selective harmonic elimination (FSHE) as depicted in Figures 4 and 5, respectively
As can be seen, when exceeding a relative fundamental frequency of 0.047, the WTHD3 value using PWM becomes higher than when using FSHE and, the current quality (THDI ) or the converter induced torque ripple using FSHE should be theoretically improved for higher fundamental frequencies, e.g., high speed
Summary
The two-level inverter topology is predominantly used in today’s electric vehicles (EVs) [1,2]. In [11,22,23,24] the authors have investigated the energy efficiency of multilevel propulsion inverters using only multilevel PWM for the entire operating range of a vehicle (considering variable output frequency and low modulation indices). Besides multilevel PWM or space vector modulation (SVM) [21], fundamental frequency switching techniques as nearest-level control [25] or selective harmonic elimination (SHE) [26] are used to synthesize MLIs’ desired output voltage. These reduce the switching losses, but induce low-order voltage harmonics. In extension to [1], the analysis of this paper considers the ohmic battery losses, the drive cycle losses are quantified and FSHE’s high speed dominance is demonstrated using a simple experimental setup with an inductive load
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