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Compared with multilevel space vector modulation, level-shifted multicarrier sinusoidal pulsewidth modulation (SPWM) for nested neutral point-clamped (NNPC) inverters requires much less calculations and implementation efforts, but the existing conventional SPWM cannot suppress the severe flying-capacitor (FC) voltage ripples under low output-frequency conditions. In order to solve this problem, a multicarrier-based PWM strategy with complete voltage balance control (VBC) capacity is proposed in this paper. For NNPC inverters operating under low output-frequency conditions, two preprocessing steps, the zero-sequence injection and the modulation-signal rearrangement, are proposed before the comparisons between level-shifted carriers and modulating signals. These preprocessing steps make the average currents flowing through FCs zero, which is beneficial to greatly reduce the FC voltage ripples under low output-frequency conditions. Together with additional VBC, the proposed modulation can assure the FC voltages balanced under the entire output-frequency conditions. Principles and implementation processes of proposed modulation techniques and VBC methodsare elaborated in detail and compared with conventional modulation methods. Both simulation results from a 5-MVA system and experimental results based on a 5-kVA platform are presented to verify the effectiveness. [ABSTRACT FROM AUTHOR]

Four-level nested neutral-point clamped (NNPC) inverters experience severe voltage balancing problems under low output-frequency conditions when the conventional four-level space vector modulation (4L-SVM) technique is applied, which limits their wider application. In order to get a complete control of the flying-capacitor voltages, and to alleviate the calculation burden of conventional 4L-SVM, a simplified but effective 4L-SVM technique along with the voltage balance control (VBC) function is proposed in this paper. For low modulation indexes, zero and small vectors are selectively utilized and distributed to force the average charging and discharging time of flying capacitors to be equal to each other during every switching period, so that voltage ripples are greatly reduced to be within an acceptable level under low output-frequency conditions. For high modulation indexes, the simplification of conventional 4L-SVM for NNPC inverters into two-level SVM is proposed to reduce the computation complexity. Principles of the proposed 4L-SVM and VBC are elaborated in detail. The steady-state, transient-state, and VBC performances are investigated and compared with the case where the conventional 4L-SVM is applied. Simulation results of a 5 MVA inverter system and experimental results of a scaled-down 5 kVA inverter system are presented to validate the proposed method. [ABSTRACT FROM PUBLISHER]

Flying capacitor photovoltaic (PV) inverters have been widely discussed in the literature. However, the relevant leakage current issues have not received much attention. In this paper, the modulation techniques for a transformerless three-phase flying capacitor PV inverter are investigated for the leakage current suppression. First, the theoretical analysis of the system common mode model and leakage current is presented. Second, three kinds of conventional carrier-based modulation techniques are evaluated, and results indicate that they fail to effectively suppress the leakage current. Third, a new carrier modulation and logic function is presented. It is able to maintain the constant common mode voltage, and consequently, the leakage current can be significantly reduced. Finally, the experimental tests are conducted on a transformerless three-phase flying capacitor PV inverter to verify the effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

The study proposes a non-isolated three-phase integrated battery charger (IBC) based on electric vehicle drivetrains that have two permanent magnet synchronous motors with shafts coupled via a torque coupler. The windings of both machines are used as input filter inductances after reconfiguration and connected to a three-phase grid in charging mode. Their existing traction inverters are operated as a three-phase charger that controls the charging power from the grid. The total torque produced on the shaft during operation is analysed and a strategy to eliminate it is proposed. The strategy not only maintains a zero-average torque but also eliminates the pulsating torque component on the shaft during operation. The topology allows conversion of the existing drive to an IBC through minor reconfiguration and provides an opportunity to reduce the effective THD injection into the grid via interleaving for machines with low winding inductance. A power balancing control is also proposed to reduce the second harmonic on the DC power output due to machine non-ideality. Simulation and experimental results validate that the topology can be used as both a battery charger and as a distributed resource, while the resultant torque on the shaft remains eliminated by the proposed strategy. [ABSTRACT FROM AUTHOR]

The recently-developed nested neutral-point clamped (NNPC) converter is a four-level converter for medium-voltage high-power applications. It has some merits such as operating over wide voltage ranges (2.4–7.2 kV) without switching devices in series and less diodes or capacitors when compared with classical four-level topologies. But it has severe voltage balancing problems under low output frequency conditions when conventional space vector modulation (SVM) or sinusoidal pulse width modulation (SPWM) is applied, which limits its wide applications. In order to solve this issue, a space virtual-vector modulation (SVVM) along with a voltage balance control (VBC) algorithm is proposed in this paper. It helps reduce the voltage ripples greatly and maintain them within an acceptable region under low output frequency conditions. Therefore, it guarantees even voltage stresses among switching devices and extends the NNPC converter's applications. Principles of the proposed SVVM and VBC algorithms are elaborated in detail, and the voltage balancing performance is investigated and compared with the cases where conventional SVM and SPWM with the same VBC are applied. The relationship curves between voltage ripples and flying capacitance, output power, and output frequency have been explored and compared for both conventional SVM and proposed SVVM. Simulation and experimental results are presented to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The development of high-power charging stations with fast chargers is a promising solution to shorten the charging time for electric vehicles (EVs). The neutral-point-clamped (NPC) converter-based bipolar-dc-bus-fed charging station brings many merits, but it has inherent voltage balance limits. To solve this issue, a voltage balance control (VBC) method based on a new modulation together with three-level (TL) dc–dc converter-based fast charger is proposed. Additionally, an effective VBC coordination between the TL dc–dc converter and the NPC converter is formulated. Through the proposed VBC coordination, the controllable balancing region is extended so that additional balancing circuits are eliminated. Meanwhile, the quality of the grid-side currents is improved as the NPC converter has more freedom to control currents. The low-frequency voltage fluctuations in dc buses are removed because the TL dc–dc converter performs most of the balancing tasks. Faster VBC perturbation performance is achieved due to higher available balancing current at TL dc–dc converter side. In addition, the voltage balance limits of both the TL dc–dc converter and the NPC converter are explored. The voltage balancing performances are compared when VBC is located at different sides. Simulation and experimental results are provided to verify the proposed VBC and the VBC coordination. [ABSTRACT FROM PUBLISHER]

The parallel three-level dc–dc converter has some merits over the conventional two-level buck converter when employed for fast chargers in a bipolar-dc-bus charging station. It can be directly connected to the bipolar dc bus, and can operate with the total neutral-point current eliminated under the 180°-interleaved mode, which guarantees balanced power consumption and helps decrease dc-bus voltage fluctuation. However, this incurs circulating currents, leading to increased losses and additional current stress to switching devices. To solve this issue, an integrated inductor is proposed in this paper. The inductor has an integrated magnetic core with both output and circulating windings placed on it, producing inductances to filter output currents and to attenuate circulating currents respectively. The circulating inductance can be flexibly adjusted to be much higher than the output inductance, so that the circulating currents can be well attenuated. The output and circulating inductances, along with the ratio between them, are deduced in detail. The current ripples are analyzed and compared under two operation modes with separate and integrated inductors, respectively. The weight, losses, and volume have also been compared between separate and integrated inductors. Both simulation and experimental results are presented to verify the effectiveness of the proposed integrated inductor. [ABSTRACT FROM PUBLISHER]

With the increasing popularity of electric vehicles, there is an urgent demand to shorten the charging time, so the development of high-power charging stations with fast chargers is necessary to alleviate range anxiety for drivers. The charging station based on the neutral-point-clamped (NPC) converter can bring many merits, but it has unbalanced power problems in the bipolar dc bus. To solve this issue, comprehensive dc power balance management (PBM) in conjunction with high-power three-level dc–dc converter based fast charger is proposed in this paper. The active dc power balance management (APBM) is proposed to assist the central NPC converter in balancing power so that the additional balancing circuit is eliminated; while the passive dc power balance management (PPBM) is proposed to eliminate the fluctuating neutral-point currents and to ensure the balanced operation of fast chargers. The principles of APBM and PPBM are researched, the efficient integration between them is studied, and the overall control scheme for the fast charger is proposed. The power balance limits of APBM are explored, while the circulating currents of PPBM are analyzed. Simulation and experimental results are presented to verify the effectiveness of the proposed fast charger with PBM functions. [ABSTRACT FROM PUBLISHER]