Full-bridge submodule (FBSM) capacitance of FBSM based modular multilevel converter (FB-MMC) can achieve a significant reduction with boosted ac voltages, which makes it a promising candidate in voltage-source-converter based high- voltage-direct-current (VSC-HVDC) scenarios in terms of cost reduction and dc short circuit faults tolerance. With consideration of third-order harmonic voltage injection (THVI), the characteristics of the FBSM voltage fundamental frequency and second-order harmonic fluctuations are analyzed in detail under steady-state and single-line-to-ground (SLG) faults conditions. Both of the optimal phase and amplitude of the injected third-order harmonic voltage are derived constrained by the same modulation margin, based on which the optimized THVI method is proposed. Under the same fluctuations conditions, the energy storage requirements comparison between the FB-MMC with and without the THVI method are also carried out. The feasibility and validity of the proposed THVI method are verified by simulations on a ±200 kV FB-MMC system and experiments on a laboratory-scale platform. With the same fluctuations as the FB-MMC without THVI, both of the simulation results and the experimental results indicate that the FBSM capacitance of the one applying the proposed THVI method can be reduced about 16%.

The three-level neutral-point-clamped inverter used in the variable frequency system has the characteristics of a wide frequency range, and the PWM strategy needs to be designed for the low-frequency region with low modulation index and the high-frequency region with low carrier ratio, respectively. The traditional carrier-based PWM (CBPWM) is simple to implement, but in the low-frequency region, CBPWM will cause narrow pulse and voltage distortion; while in the high-frequency region, CBPWM can only achieve the synchronized modulation with half-wave symmetrical (HWS) and three-phase symmetrical (TPS) waveforms when the carrier ratio is an odd integer multiple of three, and it cannot realize the square-wave modulation. To solve the defects of CBPWM, this paper proposes improved CBPWM (ICBPWM) I, which can avoid narrow pulse and generate line voltage accurately at any low modulation index, ICBPWM II, which can ensure the voltage to meet TPS and HWS when the carrier ratio is an arbitrary integer multiple of three, and ICBPWM III, which can achieve square-wave modulation, separately. On this basis, by using ICBPWM I, ICBPWM II, and ICBPWM III in combination, a hybrid ICBPWM strategy with wide frequency range is further proposed. Finally, the proposed strategy is verified by simulation and experiment.

Compared to the conventional half-bridge sub-module (HBSM) based modular multilevel converter (HB-MMC), the full-bridge SM (FBSM) capacitance of FB-MMC can be reduced significantly with consideration of negative voltage state of the FBSM, which has benefits to cost and size reduction. This paper analyzes the characteristics of the FBSM voltage fundamental frequency and second-order harmonic fluctuations of the FB-MMC without and with zero sequence voltage injection (ZSVI) comparatively. The analyses show that ZSVI has almost no impacts on the FBSM voltage fundamental frequency fluctuations, while the second-order harmonic components become 5/6 times of the one without. Based on these analyses, with the same fluctuation constraint as the case without ZSVI, the FBSM capacitance dimensioning method is also presented. The correctness of the presented analyses is verified by comparative simulation results on a ±200 kV FB-MMC system and experimental results on a laboratory platform. The simulation and experimental results indicate that the FBSM capacitance of the FB-MMC can achieve a reduction about 10% with ZSVI.

A decentralized control method for power module (PM) capacitor voltage and switching frequency balancing in modular multilevel converter (MMC) is presented in this paper. The proposed method is composed of a central controller, six arm controllers and several group controllers. The group controller is only in charge of the PM capacitor voltage balancing within a corresponding group and the arm controller is in charge of the PM capacitor voltage balancing and PM switching frequency balancing among groups within an arm. The key advantages of the proposed method are the reduction in the number of the PM capacitor voltages that need to be transmitted and sorted in each group controller as well as the reduction of the communication burden between the group and arm controllers. The proposed method is also highly effective under PM faults. The voltage ripple factor and the PM switching frequency of the proposed method are almost identical to the conventional centralized methods. The RT-LAB based real-time control hardware in the loop (HIL) test results of a ±350 kV/1000 MW MMC system verify the correctness and effectiveness of the proposed method.

This paper proposes an improved deadbeat predictive thrust control (IDPTC) for linear induction machine (LIM) drives based on online parameter identification. First, to achieve fast thrust dynamic response with low thrust ripples, a DPTC method is induced based on the reference primary flux vector calculator and discrete-time LIM model. Second, a novel online magnetizing inductance identification is designed based on back electromotive force model reference adaptive system (MRAS) and linear extended state observer. Compared to the conventional MRAS identification strategy, there is no pure integration and differential operation in both reference and adaptive models for the proposed method, so that integral initial values, dc bias and high-frequency-noise amplification problems can be solved. Then, to improve the robustness of DPTC against magnetizing inductance mismatch, the proposed online parameter identification is further combined with the DPTC method, in which the flux can also be estimated as an intermediate variable without conventional parameter-based flux observer. Finally, comprehensive simulation and experiments have been conducted on one 3 kW arc induction machine, showing that the proposed method can effectively eliminate the influence of magnetizing inductance mismatch on the control performance and significantly improve the parameter robustness compared to the existing DPTC methods.