Windowed PWM: a Configurable Modulation Scheme for Modular Multilevel Converter Based Traction Drives

This article introduces a modulation technique for modular multilevel converter (MMC) in variable speed traction drives for electrical transportation referred as windowed pulsewidth modulation (W-PWM). The windowed PWM (W-PWM) is derived by blending the principles of operation of conventional modulation schemes for MMC based on the nearest level control (NLC) and on PWM with the aim of combining their inherent strengths and offering a higher degree of flexibility. This can reduce switching losses compared to classical PWM schemes and lower the current harmonic distortion compared to NLC schemes. The window in which the PWM is applied can be seen as an additional degree of freedom that allows a dynamic optimization of the performance of the traction drive depending on its operating characteristics. The performance of the W-PWM technique is assessed in this article for several operating conditions and compared with conventional schemes based on NLC and on the phase opposition disposition PWM with both numerical simulation and experimental verification on a small-scale prototype. Results demonstrate the flexibility of the W-PWM and its potential for applications in electrical traction drives. © 1986-2012 IEEE.acceptedVersio

Comparative Analysis of Modulation Techniques for Modular Multilevel Converters in Traction Drives

The spreading of electric vehicles led to a renewed interest for innovative traction drives. In this context, an electric traction drive based on a Modular Multilevel Converter and integrating batteries in each module has been recently proposed. This converter concentrates into a single unit, the control of the traction motors, the balancing and energy management of the battery cells and the vehicle recharge functions. By using different modulation techniques it is possible to optimize different aspects like efficiency or THD of output current and voltages. Performance and efficiency in balancing, traction and recharging operations have been analyzed for two modulation techniques: near level modulation and phase disposition PWM. The near level modulation reduces switching losses while the second one better follows voltage references reducing current THD and, consequently, conduction losses. In this paper an innovative modulation technique is proposed and its performance in terms of current THD and global losses are compared with the other two modulation strategies already proposed.acceptedVersio

Resynchronization of Islanded Virtual Synchronous Machines by Cascaded Phase and Frequency Controllers Acting on the Internal Power Reference

Virtual Synchronous Machines (VSMs) have the capability to operate in grid connected or in islanded mode in the same way as synchronous machines. Furthermore, VSMs can inherently support seamless transition from grid connected to islanded mode. However, the reconnection of a VSM-based islanded to the grid requires a dedicated synchronization controller. The synchronization process should align the frequency and phase angle of the islanded system to the grid voltage to avoid abrupt transients in the currents when closing the breaker connection to the grid. This paper presents a simple and flexible control structure for grid resynchronization of a VSM operating in islanded mode. The scheme is based on two cascaded regulators operating on the phase and frequency difference between the VSM and the grid to generate an offset in the power reference. Starting from a simplified analytical model of the islanded system, a tuning procedure ensuring a controlled resynchronization transient is derived. Numerical results confirm the intended operation of the synchronization scheme with the associated tuning algorithm and validate their robustness against variations in the initial conditions of the synchronization process and to the virtual inertia value.acceptedVersio

Impact on Power System Frequency Dynamics from an HVDC Transmission System With Converter Stations Controlled as Virtual Synchronous Machines

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