In this paper, a railway power conditioner (RPC) based on a modular multilevel converter (MMC) with a split supercapacitor energy storage system (SCESS) is studied. In this case, the MMC-SCESS-based RPC could not only provide normal negative-sequence current compensation, but also reduce the impact of power fluctuations caused by the locomotive braking or startup on the electric grid. First, this paper analyzes the power flow patterns and deduces the reference circulating current under different operation modes of the MMC-SCESS-based RPC. Then, the control objectives of the MMC-SCESS-based RPC are divided into two categories—the balance control and the current tracking control. The balance control methods are developed for the submodule capacitor voltages and the state of charge of the supercapacitor, which are associated with the operation modes. To ensure the current tracking performance, a model-predictive direct current control method is presented for the MMC and the bidirectional energy converter. Finally, the effectiveness of the proposed control methods is verified by the experimental results of a downscaled prototype.

Modular multilevel converter (MMC) has become a promising topology applied in medium/high-voltage applications due to its advantages. However, there are several challenges in the MMC, such as the appearance of harmonic circulating current. In this paper, a novel two-stage model predictive control (TSMPC) is proposed to suppress the harmonic circulating current. First, the mathematical model of the MMC is discretized to describe steady-state objectives including output currents, circulating currents, summation, and imbalance of capacitor voltages. Then, an optimal control options set consisting of inserted numbers is built by the front stage of TSMPC, while the inserted number applied to the MMC ultimately is selected from the above-mentioned set by the back stage of TSMPC. The proposed TSMPC can not only improve the control performance of harmonic circulating current but also ensure the overall control performance of the MMC system. In addition, a simplified rolling optimization method is proposed to reduce computation from the aspect of simplifying each control option prediction. An optimized search space method is also proposed to design the control options set to be evaluated from the aspect of ensuring the output current tracking and harmonic circulating current suppression. Finally, the effectiveness of the proposed method is validated by experiment results.

Power amplifiers have a wide range of applications in civilian and military areas. In this article, a modular multilevel switching power amplifier (MMSPA) with high power and high output performance is investigated. Due to mismatched parameters or proper control signals among the modules, there may be significant temperature differences among the modules of MMSPA under long-term operations. Uneven temperatures among modules directly lead to premature aging and failure, which in turn affects the overall system reliability. Aiming at the thermal stress imbalance among modules of MMSPA, a model predictive control based on the thermal stress balance is proposed. First, the mathematical model is constructed for the MMSPA. Then, the junction temperature estimation method based on the power loss calculation is presented. On this basis, the finite set control switch state optimization and the switch distribution algorithm for thermal balance are presented. Finally, the proposed active thermal control is verified by the simulation and the experimental results.

Modular multilevel converter (MMC) is very popular in high power applications due to its attractive characteristics. The full bridge-based MMC featuring three-phase to single-phase direct ac/ac conversion is a potential solution for ac power supply (ACPS). In this paper, new modulated model predictive control (MMPC) methods are proposed for MMC-ACPS to improve the steady state multi-objective current tracking performance. In the proposed methods, modulated vectors sequence is employed, output voltage levels of upper and lower arms are combined and represented by vectors in the current increments plane first. Then, the plane is divided into eight sectors according to the predictive tracking errors of input current and circulating current. After determining sectors, a modulated vector sequence, consisting of one zero vector and two active vectors, is selected to eliminate these current tracking errors simultaneously at the end of each control period. Duty cycles of the three selected vectors are calculated based on the principle of multiple current tracking errors minimization. Optimized vector selection approaches are illustrated in detail for the proposed MMPC. Since only the adjacent nine vectors are utilized in the proposed methods, the calculation amount is suitable and the dv/dt of the output voltage is also limited. Finally, steady-state and dynamic performances of the proposed control methods are verified by experimental results.

Wide bandgap devices (SiC and GaN) bring the potential for a great increase in power density of power electronics, but also pose a challenge to the bandwidth of the current sensor because of the high operating frequency. Rogowski coil is suitable for high-frequency current measurement due to its characteristic of differential output, but the existing design is difficult to take into account the sensitivity and mutual inductance at the same time, and faces the inherent defect of droop distortion. This article proposes an optimal printed circuit board (PCB) Rogowski coil structure design method for SiC MOSFET power module and a novel sample-filter-hold (SFH) method to minimize droop distortion in integrators. First, the optimal parameters with multiobjective constrained optimization of the Rogowski coil current sensor for the SiC-MOSFET power module are obtained. Then, the influence factors of droop distortion of the sensor in buck converters and inverters are quantitatively analyzed for the first time and an SFH suppression method is proposed, which can eliminate the offset caused by droop distortion reliably and accurately. Finally, the experimental results demonstrate that the proposed Rogowski coil design method and the SFH method for lower droop distortion achieve better low-frequency performance compared to the commercial Rogowski coil.

Soft switching technology is an effective way to reduce the switching loss of the single-phase inverter to improve efficiency. Because of the ease of implementing soft switching, inverter works in boundary conduction mode (BCM) is widely used. On the contrary, BCM causes wide variation in switching frequency and large inductor current ripple. In this article, a fixed switching frequency hybrid modulation method for the inverter is proposed, which can achieve zero-voltage switching without changing the switching frequency and reduce the peak value of inductor current. The modulation achieves good tracking of the output current with different loads by switching mode between the triangular mode and the trapezoidal mode accurately. For implementing modulation, the operation states of the inverter with different conditions are analyzed in detail, the number of operation states is deduced and the state machine about mode switching is summarized. In addition, a single-phase inverter prototype is designed to verify the feasibility and effectiveness of the proposed modulation. Compared with BCM, the efficiency of the inverter at both light load and full load is increased.