Hybrid and modular multilevel converter designs for isolated HVDC–DC converters

Efficient medium and high-voltage dc-dc conversion is critical for future dc grids. This paper proposes a hybrid multilevel dc-ac converter structure that is used as the kernel of dc-dc conversion systems. Operation of the proposed dc-ac converter is suited to trapezoidal ac-voltage waveforms. Quantitative and qualitative analyses show that said trapezoidal operation reduces converter footprint, active and passive components' size, and on-state losses relative to conventional modular multilevel converters. The proposed converter is scalable to high voltages with controllable ac-voltage slope; implying tolerable dv/dt stresses on the converter transformer. Structural variations of the proposed converter with enhanced modularity and improved efficiency will be presented and discussed with regards to application in front-to-front isolated dc-dc conversion stages, and in light of said trapezoidal operation. Numerical results provide deeper insight of the presented converter designs with emphasis on system design aspects. Results obtained from a proof-of-concept 1-kW experimental test rig confirm the validity of simulation results, theoretical analyses, and simplified design equations presented in this paper. - 2013 IEEE.Scopu

A family of discontinuous PWM strategies for quasi Z-source nine-switch inverters

This paper proposes a new family of discontinuous PWM strategies to control the quasi-Z-source nine-switch inverters (qZS-NSI). The presented strategies provide buck and boost inversion capabilities, and suitable for common-frequency and different-frequency modes of operation. Accordingly, two different shoot-through (ST) approaches are introduced and compared. The first approach uses three-leg ST, while the second uses single-leg ST to reduce the number of switching commutations, therefore minimizing switching losses. Both approaches can be implemented using simple-boost (SB) and maximum boost (MB) control methods. The operating principles, performance criteria, and PWM modulator of each scheme are introduced. Compared to the conventional PWM technique for the same output voltage gains, the proposed schemes ensure continuous input current with minimum ripples, and the voltage stresses on the switching devices and capacitors could be reduced in the proposed MB control schemes. Moreover, the effective switching frequency of upper and lower switches of all schemes is fixed and could be reduced by 1/3 from the switching frequency of the conventional technique of the qZS-NSI, while only the single-leg ST schemes ensure minimum effective switching frequency of the middle switches. The proposed modulation strategies are digitally implemented and tested on the LAUNCHXL-F28379D DSP. The feasibility of the proposed modulation schemes is confirmed via simulation and experimental results, which show good agreement with the theoretical analysis. Moreover, the presented strategies can be applied to other types of Z-source NSIs.Scopu

Development of Modular DC-DC Converters for Low-Speed Electric Vehicles Fast Chargers

To increase the utilization of Low-Speed Electric Vehicles (LS-EVs), rapid recharging of the EV's battery pack turn out to be essential. This permits reduced charging times, greater vehicle utility, and broader adoption of LS-EVs. This paper presents a modular Input-Series Output-Parallel (ISOP) DC-DC converter for LS-EVs fast chargers. A generalized small-signal analysis applicable for any multimodule connection (Input-Series Input-Parallel Output-Series Output-Parallel (ISIP-OSOP) is introduced. The employed topology is a multimodule DC-DC converter based on Dual Active Bridge (DAB). Nonetheless, a single bridge is utilized at the primary side, and the modularity concept is applied to the high-frequency transformer and the second bridge where the connection of the modules is ISOP. In the presented system, 3-modules are employed where each module is rated at 1.5kWto achieve the desired power rating, which is 4.5kW. The charging process is achieved from a single-phase outlet. However, due to the high output current, a modular approach is required to avoid high losses. Uniform power-sharing is achieved through a direct output current sharing control, ensuring stability without the need for input voltage sharing loops, unlike the conventional ISOP converters. This is due to the fact that the proposed configuration uses only a single capacitor at the input side, avoiding the inherent instability problem caused by the output current sharing control. The controller is examined using a 3-module ISOP DC-DC converter, where the controlled current is following the reflex charging algorithm. Simulation results using the Matlab/Simulink platform are provided to elucidate the presented concept considering parameter mismatches, where the input voltage and the output current are equally shared among the three modules.Qatar Foundation;�Qatar National Research FundScopu

Modular isolated DC-DC converters for ultra-fast EV chargers: A generalized modeling and control approach

Electric Vehicles (EVs) play a significant role in the reduction of CO2 emissions and other health-threatening air pollutants Accordingly, several research studies are introduced owing to replacing conventional gasoline-powered vehicles with battery-powered EVs. However, the ultrafast charging (UFC) of the battery pack or the rapid recharging of the battery requires specific demands, including both: the EV battery and the influence on the power grid. In this regard, advanced power electronics technologies are emerging significantly to replace the currently existing gas station infrastructures with the EV charging stations to move from conventional charging (range of hours) to UFC (range of minutes). Among these power electronics conversion systems, the DCDC conversion stage plays an essential role in supplying energy to the EV via charging the EV's battery. Accordingly, this paper aims to present possible architectures of connecting multiple Dual Active Bridge (DAB) units as the DC-DC stage of the EV fast charger and study their Small-Signal Modeling (SSM) and their control scheme. These are, namely, Input-Series Output-Series (ISOS), Input-Series Output-Parallel (ISOP), Input-Parallel Output-Parallel (IPOP), and Input-Parallel Output-Series (IPOS). The control scheme for each system is studied through controlling the output filter inductor current such that the current profile is based on Reflex Charging (RC). The main contribution of this paper can be highlighted in providing generalized SSM as well as providing a generalized control approach for the Input-Series Input-Parallel Output-Series Output-Parallel (ISIP-OSOP) connection. The generalized model is verified with three different architectures. The control strategy for each architecture is studied to ensure equal power sharing, where simulation results are provided to elucidate the presented concept considering a three-module ISOS, IPOP, ISOP, and IPOS DC-DC converters.Qatar Foundation; Qatar National Research FundScopu

Quasi two-level PWM operation of a nine-arm modular multilevel converter for six-phase medium-voltage motor drives

This paper proposes a hybrid converter for medium-voltage six-phase machine drive systems that mixes the operation of a traditional two-level voltage-source inverter and the modular multilevel converter (MMC) to enable operation over a wide frequency range. Topologically, the proposed converter consists of nine arms resembling two sets of three-phase MMCs with three common arms, yielding a nine-arm MMC with a 25% reduction in the number of employed arms compared to a traditional dual three-phase MMC. The multilevel property of a standard MMC is emulated in the proposed converter, however on a two-level basis, resulting in a stepped two-level output voltage waveform. The proposed converter has a reduced footprint with advantages of small voltage steps, modular structure, and ease of scalability. Further, it is able to drive high-power six-phase machines within low operating frequencies at the rated torque. The operating principle of the converter is elaborated, and its modulation scheme is discussed. The features of the proposed converter are verified through simulations and experimentally

A modular multilevel voltage-boosting Marx pulse-waveform generator for electroporation applications

In order to overcome the limitations of the existing classical and solid-state Marx pulse generators, this paper proposes a new modular multilevel voltage-boosting Marx pulse generator (BMPG). The proposed BMPG has hardware features that allow modularity, redundancy, and scalability as well as operational features that alleviate the need of series-connected switches and allows generation of a wide range of pulse waveforms. In the BMPG, a controllable, low-voltage input boost converter supplies, via directing/blocking (D/B) diodes, two arms of a series modular multilevel converter half-bridge sub-modules (HB-SMs). At start up, all the arm's SM capacitors are resonantly charged in parallel from 0 V, simultaneously via directing diodes, to a voltage in excess of the source voltage. After the first pulse delivery, the energy of the SM capacitors decreases due to the generated pulse. Then, for continuous operation without fully discharging the SM capacitors or having a large voltage droop as in the available Marx generators, the SM capacitors are continuously recharged in parallel, to the desired boosted voltage level. Because all SMs are parallelly connected, the boost converter duty ratio is controlled by a single voltage measurement at the output terminals of the boost converter. Due to the proposed SMs structure and the utilization of D/B diodes, each SM capacitor is effectively controlled individually without requiring a voltage sensor across each SM capacitor. Generation of the commonly used pulse waveforms in electroporation applications is possible, while assuring balanced capacitors, hence SM voltages. The proposed BMPG has several topological variations such as utilizing a buck-boost converter at the input stage and replacing the HB-SM with full-bridge SMs. The proposed BMPG topology is assessed by simulation and scaled-down proof-of-concept experimentation to explore its viability for electroporation applications

Symmetrical nine-phase drives with a single neutral-point: common-mode voltage analysis and reduction

Power converters generate switching common mode voltage (CMV) through the pulse width modulation (PWM). Several problems occur in the drive systems due to the generated CMV. These problems can be dangerous to the insulation and bearings of the electric machine windings. In recent years, many modulation methods have been developed to reduce the CMV in multiphase machines. Symmetrical nine-phase machines with single-neutral are considered in this paper. In this case, conventional PWM uses eight active vectors of different magnitudes in combination with two zero states in a switching cycle, and this generates maximum CMV. This paper proposes two PWM schemes to reduce the CMV in such a system. The first scheme is called active zero state (AZS). It replaces the zero vectors with suitable opposite active vectors. The second scheme uses ten large active vectors during switching and is called SVM-10L. Compared with conventional strategies, the AZS reduces the peak CMV by 22.2%, and the SVM-10L reduces the peak CMV by 88.8%. Moreover, this paper presents a carrier-based implementation of the proposed schemes to simplify the implementation. The proposed schemes are assessed using simulations and experimental studies for an induction motor load under different case studies