In this letter, we propose a single-phase transformerless unified power quality conditioner (UPQC) with two legs. Owing to the limited quantity of switches, the half-bridge (HB) UPQC consists of a HB dynamic voltage restorer (DVR) and an active power filter (APF). The proposed topology maintains constant switching stress under all operational scenarios by combining an HB DVR with a full-bridge (FB) APF without increasing the number of switches. To address the issue of current compensation and voltage compensation coupling caused by the shared leg, a four-vector space vector pulsewidth modulation (SVPWM) is proposed for decoupling. With the proposed SVPWM for independent control reference calculation, the effects of dc voltage and load current on voltage compensation can be eliminated by the control strategy. Finally, the effectiveness of the topology and modulation strategy are verified by the experimental results.

Due to device losses and heat dissipation, single-phase rectifiers in high-speed railways are operated at low-switching frequencies. Harmonic resonance in the traction power supply substation is mostly caused by the low-frequency switching harmonics. In this article, a hybrid cascaded H-bridge multilevel rectifier is proposed to improve the overall performance by cascading a high-frequency switching cell. The added cell operates at full-range no-load with half the dc voltage of the low-frequency cell. The proposed asynchronous space vector pulsewidth modulation (SVPWM) provides equivalent switching frequencies in the low-voltage cell (LV _ cell) while maintaining power frequency in the high-voltage cell. The limitation of no redundant vectors, the LV _ cell's no-load operation, and the different voltage set values make voltage regulation challenging. An enhanced control strategy is proposed to maintain a stable voltage with wonderful dynamic performance. Simulation and experimental results show that better harmonic performance and lower power losses are achieved.

To ensure the voltage-balancing capability of the cascaded H-bridge multilevel rectifier (CHMR), the voltagebalancing space vector pulse width modulation (VB-SVPWM) is constructed by combining the minimum energy characteristic. However, the existing voltage balancing modulation strategy could fail under extreme conditions of unbalanced cell loading or when some cells are unloaded due to failure. The power transfer limitations of the sub-cell terminals determine the voltage balancing boundary (VBB) for the VB-SVPWM under several load scenarios. Extension methods are also considered for situations out of this VBB. Following the theoretical derivation and the relationship between the number of no-load cells, the load unbalance index, the number of cascaded cells, the modulation index, and the power factor, a wider voltage balancing boundary can be reconstructed by modifying these above variables. Simulations and experiments are conducted with a three-level, three-cell cascaded rectifier to validate the VBB of the proposed VB-SVPWM and its expansion.

As the challenge of unbalanced load and grid condition in power electronic transformer (PET) application system, the dc voltage balancing control is essential for cascaded H-bridge (CHB) rectifier. This article proposes a novel hybrid dc voltage balancing control strategy for three-phase, four-wire CHB rectifiers. To achieve interphase voltage equalization, d-q current control is applied to each phase. Without additional negative or zero sequence voltages injected, there is no grid current distortion introduced to the system. As the active and reactive power of each phase is controlled independently, interphase dc voltage balance can be maintained even in phase loss conditions. For the inner-phase unbalance problem, a modulation method combining real-time voltage level calculation with voltage sorting is adopted. Finally, experimental results with different unbalanced load and grid conditions verify the effectiveness of the proposed method based on a three-phase, four-wire, seven-level CHB rectifier platform; moreover, the dynamic performance is also improved with the proposed hybrid control strategy. The rectifier still can run for 12 s under phase loss conditions, which comply with the low voltage ride through (LVRT) grid codes defined in standard GB/T 36995-2018. When the grid returns to normal, the rectifier only needs 1.5 s to resume the three-phase operation.

The dual active bridge (DAB) converters offer high-power density and bidirectional transfer capabilities. The input series output parallel (ISOP) modular DAB converters are commonly used for isolated medium/high to low voltage conversion. To address the power unbalance problem caused by the differences among modules in the ISOP-DAB converter, this article proposed a sensorless power balance control method based on parameters estimation (SPBCM-PE) that does not require additional voltage/current sensors, additional auxiliary circuits, or specific system parameters. Compared to existing power balance methods for ISOP-DAB, the proposed load-independent method is compatible with multimodule converters and does not require load parameters, eliminating the need for real-time dynamic estimation. It is considered the balance of both the modules' input voltage and output current. To validate the proposed SPBCM-PE, this article designed a 220 V-24 V ISOP-DAB experimental platform with a neutral point clamped half bridge-full bridge topology and significant differences between modules. In the power range of 10-435 W, the experiments showed that the proposed method significantly improved input voltage and output current balance compared to the traditional method with system efficiency ranging between 87.2% and 94.3%. Overall, the proposed balance method offers an effective solution for achieving power dynamic balance in the full power range.

This paper introduces a single-phase transformerless unified power quality conditioner (UPQC) with two bridge arms. In contrast to traditional half-bridge UPQC, this system is comprised of a H-bridge active power filter (APF) and a half-bridge dynamic voltage restorer (DVR) coupled through a shared bridge arm. It can operate at low dc-link voltage during undervoltage and overvoltage conditions in the power supply, and is unaffected by the series converter compensation strategy. Additionally, an equivalent current model is employed to calculate the dc-link capacitor voltage expression. The results indicate that the voltage capacitor ripple of two-leg UPQC is reduced, leading to approximately one-third less capacitor requirement under equivalent load conditions. Finally, simulations confirm the accuracy of the ripple analysis and demonstrate the feasibility of the proposed topology.

The dual active bridge (DAB) converter is widely used in renewable energy power generation systems with wide input voltage characteristics. An inappropriate duty cycle will lead to a larger inductor root-mean-square (RMS) current and low efficiency. In this article, an efficiency-oriented optimized triple-phase-shift (OTPS) scheme is proposed for the DAB converter that can reduce the inductor rms current for a wide input voltage and realize zero-voltage switching (ZVS). The ZVS condition contains the direction and amplitude of the inductor current to make the ZVS area more accurate. In addition, the OTPS scheme also has the capability of voltage balancing without additional voltage equalizers under unbalanced load. The influence of deadtime on voltage balance is analyzed and a voltage balancing scheme with compensation of the duty cycle is proposed. To reduce resource occupation and operation time, an online implementation scheme of variable parameter control based on field-programable gate array (FPGA) is proposed. Without a look-up table, the sum of the operation time of the control module and modulation module is only 0.66 $\mu$ s, and the required memory bits are only 459 k. Both operation time and memory bits are reduced by more than 90% compared to the existing literature. Finally, the whole proposed process is verified by the experimental results.