The Cascaded H-bridge energy storage converter (CHBESC) integrates energy storage components within cascaded multilevel converters to deliver short-term high power to the stator of ultra-high-speed linear motors. This integration effectively mitigates the impact on the power grid. However, during the acceleration and braking of the ultra-high-speed linear motor, the frequency of the CHBESC output voltage varies with the motor speed. This variation causes the frequency of secondary harmonic currents in the submodules to fluctuate accordingly. To address this issue, this paper establishes an analytical model for secondary harmonic voltage and current over a wide frequency range. The study investigates the fluctuation patterns of these harmonics under different control and circuit parameters, providing theoretical guidance for the design of circuit parameters and controllers. Simulation results validate the accuracy of the model and the effectiveness of the parameter design guidance.

Low switching frequency of power semiconductor devices is expected to decrease losses in the cascaded H-bridge (CHB) inverter with supercapacitor and dc–dc stage. This paper proposes a switching frequency reduction method accompanied by a hybrid modulation strategy (HMS) for the CHB inverter. By optimizing the comparison output between the reference wave and the triangular carrier, unnecessary state jumps in the switching signals for the H-bridge can be avoided when rotating the switching mode of the H-bridge in the HMS. The proposed method can reduce the average switching frequency among the H-bridges without side effects on capacitor voltage balance and ac outputs. Finally, its effectiveness is demonstrated through simulation results of five SMs based on MATLAB/Simulink.

Energy balancing control in cascaded multilevel energy storage inverters (CMESIs), which consist of distributed energy storage devices across power modules (PMs), poses a significant challenge. Existing studies have identified limitations and shortcomings in achieving energy balance through differential power distribution to individual PMs. This paper presents a comparative analysis of two established energy balancing control strategies to assess their respective regulation capabilities for the CMESIs. The effectiveness of the selected strategy is verified through the simulation results, showcasing its potential to achieve energy balance in the CMESIs. The findings contribute to the advancement of energy balancing control techniques, offering valuable insights to enhance system performance and stability in practical applications.

The study focuses on the interphase power imbalance problem in the cascaded multilevel energy storage inverter for ultra-high-speed linear motor propulsion (UHSLMP) systems. The traditional interphase energy balancing methods, which rely on fixed frequency and amplitude, are limited due to the continuous variation of supply voltage amplitude and frequency, traditional zero-sequence voltage injection methods with constant values are inadequate. To address this challenge, this paper analyzes power calculation and definition considering the time-varying AC supply and establishes a phasor model that accounts for the time-varying amplitude and frequency. Based on this model, a novel zero-sequence voltage injection method is proposed to tackle the interphase energy balance problem under varying amplitude and frequency conditions. Simulation results demonstrate the effectiveness of the proposed method.

The high-speed linear induction motor (LIM) has multiple parameters coupling and fast time-varying characteristics, which easily leads to current and thrust control instability and further restricts speed improvement. To address this issue, this paper proposes a decoupling mathematical modelling method to accurately characterize the time-varying parameter feature of LIMs. Subsequently, an indirect magnetic field orientation control strategy, consisting of the voltage feedforward control algorithm and a PID feedback control algorithm with time-varying parameters, is proposed based on the mathematical model. The simulation results demonstrate that the proposed method exhibits better stability in the fast changes of parameters, and the speed of the LIM is superior to the traditional methods.

Hardware-in-the-loop (HIL) real-time simulation of cascaded H-bridge inverter (CHBI) with supercapacitor and DC-DC stage is one of the important links in the development of its controller. This paper presents a full-scale HIL simulator for the CHBI. Firstly, the discrete mathematical model of the CHBI is derived and implemented by utilizing several field programmable gate arrays (FPGAs) as the solving units. Each FPGA is equipped with up to twelve fiber optic communication interfaces, allowing one-to-one connection between the simulator and its controller. The simulation capacity of the presented simulator can be easily expanded through incorporating more FPGAs and combining data communication. Finally, the effectiveness of the presented simulator for the CHBI is verified through the gridconnected charging HIL real-time simulation results.

In the switching process of segmented power supply linear motor (SPSLM), the thyristor can cause the switching current impact and lead to insufficient utilization of converter capacity. To reduce this current impact, the mathematical model of linear motor concerning the parallel connection and zero-state response is deduced, the influence of different voltage switching angles on the current of the converter is analyzed, and then a switching method is proposed by calculating the position of the mover and voltage phase angle to minimize the effect of parallel connection and the zero-state response of stator. The electromagnetic transient simulation is implemented in Matlab/Simulink platform, the simulation results verify that the proposed method can reduce 50% of the current impact by comparing it with the conventional method.

Cascaded H-bridge multilevel converter(CHBC) has become a highly attractive topology in high-voltage and large-capacity energy storage system(ESS). In a cascaded H-bridge multilevel energy storage converter(CHB-ESC), energy storage elements(ESEs) such as batteries and supercapacitors are distributed in each power sub-module(PM), and the energy balancing among the ESEs is of great importance. In a two-stage CHB-ESC, the voltage balancing of the DC-link capacitors and the energy balancing of the ESEs should be met simultaneously. Due to the limitation of the charging and discharging current of the ESEs, the large current on the AC side in high-power applications will cause large ripple on the voltage of the DC-link capacitor. In order to reduce the voltage ripple of the DC-link capacitor in the two-stage topology, this paper proposes a novel voltage balancing method of CHBC with supercapacitors ESS for capacitor voltage ripple reduction. The PMs rotation is realized by a multi-factor weight optimal sorting method based on the voltage value of the supercapacitors and the voltage value of the DC-link capacitors. While ensuring the voltage balancing among the supercapacitors of each PM, the voltage ripple of the DC-link capacitors can be effectively reduced. Finally, the effectiveness of the proposed method is verified by simulation results.

A dual active bridge series resonant converter (DABSRC) is proposed with the isolation of an air-core high frequency transformer (ACT) in this paper. It reduces the weight of the transformer without the magnetic core which improves the gravimetric power density of DABSRC. It adopts crossed figure-eight shaped windings to ensure high coupling capability of ACT. In addition, the simplified ACT T-type equivalent model is established. Based on this model, the soft-switching characteristics of DABSRC are also analyzed in this paper. Simulations on a 100 kW 1kV / 750V DABSRC verify the validity of the proposed ACT-DABSRC.

The pulse width modulation current source converter (PWM-CSC) has been regarded as one of many interesting circuit topologies in high-power systems. This paper analyzes the influence of system parameters on the power operating range of PWM-CSC, and designs the parameters for PWM-CSC applied to HVDC transmission system. A 250 kV/500 MW PWM-CSC based HVDC transmission system is built using PSCAD/EMTDC software, and the experimental platform was built to verify the correctness of the analysis and the feasibility of the design.

Due to the existence of tail current of IGBT, dual active bridge series resonant converter (DABSRC) cannot achieve true zero current switching (ZCS). In this paper, the efficiency of the converter is improved by using hybrid switch (HyS) and optimized switching pattern, SiC MOSFET and Si IGBT of the HyS are operated in segments depending on the on-state resistance of the devices, and the conduction loss and switching loss is optimized at the same time. The method proposed is verified by simulation, and the preliminary experiments prove the feasibility of the proposed strategy.

The cascaded H bridge-power electronic transformer (CHB-PET) has potential application in traction and smart grid, however, it is a great challenge to achieve fast electromagnetic transit because of numerous nodes and nonlinear devices and high switching frequency. The equivalent model of the submodule and three-phases CHB-PET with the detailed feature of the high-frequency circuit, which can reduce the size of the admittance matrix, eliminate the affection of switching state to admittance matrix, and alleviate the impact of the increasing number of submodules per arm on computing time, are proposed to speed up simulation of CHB-PET. The accuracy and computing performance are verified by comparison of reference and proposed models in Matlab Simulink. Simulation results show that the proposed model has good performance of accuracy under blocked mode and unlock mode, and has excellent performance of computing time with the increasing of the number of submodules per arm. It can significantly speed up the offline simulation of CHB-PET. Moreover, the proposed model has great potential for real-time simulation by CPU and FPGA hardware platform.

Full-bridge submodule (FBSM) voltage ripples can achieve a significant reduction of FBSM-based modular multilevel converter (FB-MMC) with boosted ac voltages. However, the FBSM voltage ripples increase substantially when single-line-to-ground (SLG) faults occur, which may cause damage to the safe operation of FB-MMC. This paper analyzes the FBSM voltage ripples characterization of the FB-MMC under SLG faults conditions. It is found that the fundamental frequency component, which is minor in steady-state operations, becomes dominant in the event of SLG faults leading to a dramatic increase of FBSM voltage ripples. Based on these analyses, a circulating current injection (CCI) method is proposed in order to suppress the main components of the FBSM fundamental frequency fluctuations. Both of the presented analysis and the effectiveness of the proposed CCI method are verified by simulations on a ±200 kV FB-MMC system.

For medium- to high-voltage ac-ac conversion applications, direct ac-ac modular multilevel converters (MMCs) show advantages on power density compared to indirect ac-ac (ac-dc-ac) MMCs. However, direct ac-ac MMCs suffer from branch energy divergence when the operation frequencies of the input and the output are equal. This paper investigates the three-phase to single-phase direct ac-ac MMC under equal frequency operation condition and proposes a novel control strategy to balance its branches' energies. The effectiveness of the control strategy is validated by the simulation results.

For medium- to high-voltage AC/AC conversion applications, direct AC/AC conversion scheme by adopting the modular multilevel converter (MMC) shows remarkable advantages on costs and efficiency compared with the AC/DC/AC solution which employs a back-to-back MMC configuration. However, direct AC/AC MMC suffers from branch energy instability when conversion frequencies of input and output are equal. This paper presents a sept-branch MMC (SB-MMC) topology for direct AC/AC conversion applications. The feature of this SB-MMC is its capability of direct AC/AC equal frequency conversion from three-phase to single-phase and vice versa. Balanced branch energy and unity power factor at grid side can be achieved simultaneously under various load conditions. Operating principle as well as control strategy of the SB-MMC are described. Simulation on a 27.5kV/50MVA SB-MMC model for railway power supply is carried out. Simulation results validate the effectiveness of the SB-MMC and the proposed control strategy as well.

Hybrid transmission systems associated with line commutated converters (LLC) and voltage source converters (VSC) have gradually become a particular solution for high voltage direct current (HVDC) transmission. The full-bridge submodule (FBSM) based modular multilevel converter (FB-MMC) has gained significant attention for its dc voltage reversal and fault ride through capabilities. However, large capacitance is essential for high power transmission of FB-MMC, increasing cost and size rapidly. This paper analyzes the FBSM voltage fundamental frequency and second-order harmonic fluctuations of the FB-MMC with zero sequence voltage injection. The design principles of the FBSM capacitance are also presented compared with the one without zero sequence voltage injection under equivalent voltage ripple conditions. Finally, comparative simulation results on a FB-MMC system verify the correctness of the analyses.

Modular Multilevel Converter (MMC) is considered a promising circuit topology for high voltage and high power applications. There is a trade-off between submodule (SM) capacitor voltage balancing and switching frequency. In this paper, an adaptive SM capacitor voltage balancing method is proposed based on that different arm currents have different charging or discharging capabilities. The algorithm can reduce switching frequency without affecting capacitor voltage balancing. The threshold current selection and average switching frequency calculation of power device are presented which provide theoretical basis to system design. Finally, the simulation results validate the effectiveness of the proposed method.