Under unbalanced load condition, capacitor voltage balancing become a critical issue for muti-level dual-activebridge(DAB) dc-dc converters. The imbalance in load current can cause DC offset in the transformer secondary current and leakage current, as well as voltage imbalance across the output supporting capacitors. This, in turn, leads to uneven voltage stress distribution across the power devices, which can reduce the reliability of the converter. To address this challenge, this paper proposes a periodic sequence voltage-balancing control strategy for a 2/3-level Dual Active Bridge (DAB) converter based on Triple Phase Shift (TPS). With the modification of the voltage level for the primary side and the secondary side, the different power transfer of the upper and lower capacitors can be realized in voltage equalization period (VEP). Then, by inserting voltage equalization period (VEP) to the traditional power transmission periods (PTP), the capacitor voltage balance can be achieved under entire unbalanced load condition and even when there is no-load at one side of the dc poles. Moreover, considering the converter's efficiency and voltage balancing condition, the voltage equalization boundary and the design principle of PTP's number are analyzed. The proposed voltage-balancing strategy was validated through simulation and experimental studies on a 2/3-level Dual Active Bridge (DAB) converter platform. Under various load conditions, the strategy effectively reduced the voltage imbalance from 12% to less than 1%, thereby demonstrating its correctness and effectiveness.

ABSTRACT Boost‐type power factor correction (PFC) circuits are widely used in high‐power rectifiers due to their adjustable input current and continuous conduction mode (CCM) operation. However, the boost structure imposes high reverse voltage stress on the switching transistor. The three‐level boost PFC reduces this stress by adding a switch and splitting the DC‐side capacitor into two, but the inherent single‐phase power imbalance still introduces a twice‐line‐frequency ripple voltage, causing output fluctuation and input current harmonics. To address this issue, an improved three‐level boost PFC topology where two diodes are replaced with fully controlled switches is proposed in this paper, forming a half‐bridge active power decoupling circuit (APDC) on the DC side. By controlling these switches, counterphase AC voltage components are generated across the split capacitors, eliminating the second‐order harmonic DC voltage ripple. The paper analyzes the operating principles and control strategy of the improved circuit, compares its performance with the conventional topology through simulation, and validates the approach using an experimental prototype. Experimental and simulation results demonstrate that the proposed converter achieves a 91.7% fluctuation power suppression and a 40% DC voltage ripple reduction under rated operating conditions, while maintaining a power factor higher than 0.99 and low input current total harmonic distortion.

This article proposes a three-port converter utilizing five power channels. Four of these channels form an output-parallel multiphase interleaved LLC structure, designed to transfer the energy to high power and current loads. The other channel operates as an equivalent multilevel dual active bridge (DAB) converter, connects to a wide voltage range load and allows for bidirectional energy flow. The four LLC channels efficiently transfer power while significantly enhancing the soft-switching capability of the DAB channel. In addition, the high-voltage side switches operate with half voltage stress, which not only mitigates the challenges associated with soft-switching but also improves the overall reliability of the converter. To further optimize the performance, a modulation method is introduced for the proposed converter, enabling phase-shift control of four LLC channels and reducing the peak current in the DAB converter. Finally, a 3-kW prototype is developed to validate the effectiveness of the proposed converter. One port of the prototype has half of the input voltage stress, enabling it to handle a high input voltage of 750 V, while another port achieves a wide output voltage range of 40-200 V, and the third port delivers a high output current of 40 A.

This paper presents a novel three-level, three-power-channel hybrid DC/DC converter tailored for renewable energy-based medium-voltage DC (MVDC) systems. The proposed architecture integrates two LLC channels and one phase-shifted bridge (PSB) channel to simultaneously achieve wide output voltage range, high power transfer efficiency, and strong zero-voltage switching (ZVS) capability under fixed-frequency operation. Unlike traditional PSB converters that require large output filters, the inherent ripple suppression of the proposed topology eliminates the need for bulky inductors, improving power density. The LLC channels deliver the main power efficiently with full-range ZVS, while the PSB channel handles a minor portion of power to extend the voltage gain. Additionally, the converter significantly reduces voltage and power stress on the switches and resonant components. A 1 kW prototype was built and tested, demonstrating a wide output voltage range of 100-190 V and a peak efficiency of 96.7 %, validating the converter's effectiveness and practicality for MVDC systems.

To tackle issues such as leakage current, power fluctuation, and harmonic current in existing non-isolated grid-connected inverters, this paper proposes a non-isolated inverter with high gain, no electrolytic capacitors, a common-ground design, and power decoupling capability. By integrating a traditional boost converter with switched-capacitor technology, a structure is developed that directly links the grid's neutral point to the negative terminal of the DC power source, thereby eliminating leakage voltage and current. Owing to the time-division multiplexing of circuit components, the proposed inverter uses fewer components while achieving power decoupling and an extremely high boost ratio. This paper elaborates on the operating principles, working modes, and performance characteristics of this new topological structure.

Single lithium-ion batteries typically have a lower voltage, so multiple batteries are often connected in series to achieve the required voltage level. To address the voltage imbalance caused by variations between individual cells, a fast active battery equalizer is proposed in this article. Compared to other active balancing circuits, the proposed balancing circuit utilizes a multiwinding transformer, requiring only two magnetic components, and each individual battery is controlled by a single switch, which reduces the circuit cost. Additionally, the proposed converter allows energy exchange between any positions of the battery string through the coordination of two balancing paths. The LC series resonant tank ensures that all switches operate under soft-switching conditions during the balancing process, thereby improving efficiency. Furthermore, a variable group control strategy is introduced to further increase the balancing current magnitude, enhancing the overall balancing speed of the converter. Finally, a 100 kHz resonant frequency, 24-cell voltage balancing prototype validates the effectiveness of the proposed balancing circuit.

Voltage equalizer is essential for mitigating the inconsistencies of series-connected lithium-ion batteries in electric vehicles to ensure operation life and work safety. To keep voltage balancing, voltage equalizers are widely adopted in the battery string application. However, traditional voltage equalizers have high circuit costs and large sizes due to the need for a large number of MOSFETs and isolated gate drivers. Moreover, as the voltage difference between the batteries in the series decreases towards the end of the equalization process, the equalization current also decreases, which can affect the speed of voltage equalization. To overcome these problems, an isolated multiple half-bridge converter with a multiport transformer is proposed. The balancing circuit has two power transfer paths, allowing for energy transfer among multiple cells simultaneously, which could achieve a significant equalization current even when the voltage difference is close to zero. In the proposed equalizer, only one MOSFET is needed for one cell and two cells can share one transformer winding. Hence, it is more applicable for a large number of battery stacks, such as electric vehicles. Finally, to improve the equalization speed and efficiency, three equalization control strategies, i.e., average voltage, predicted voltage, and boundary voltage control strategies are introduced and verified by a 100 kHz 24-cell voltage equalization prototype.

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.

To achieve ultrawide output voltage range, this paper proposes a novel isolated LLC resonant converter with three coordinated power channels. The proposed converter features three working conditions and encompasses five operation patterns, allowing for the adjustment of quality factor for a wider gain range. With more power channels, the converter can reduce the stress of the resonant tank and switches to further improve the higher power level and achieve the soft switching. To ensure smooth gain transition and a wider gain range, the turns ratio of the transformer is designed by considering three different conditions. Finally, a 1 kW prototype was built to verify the effectiveness of the proposed topology. The results confirm the eight times output voltage range of the converter and a peak efficiency of 96%.

AbstractLLC resonant converter has the features of low noise, high efficiency and power density, which is suitable to be integrated into DC microgrid. Due to severe stresses in the resonant tank, achieving fast and safe transient performance has been challenging for resonant converters. With less switch number and voltage stress, half‐bridge LLC resonant converter with full‐bridge rectifier (H‐F LLC resonant converter) are more suitable for high‐voltage application. To improve the transient response of the converter, this paper proposes a method based fixed center state trajectory for the converter to achieve soft start‐up. To reduce dynamic response time during start‐up, each step of fixed center state‐trajectory is designed optimally for the shortest path with symmetrical current limitation. In addition, the proposed method is also applicable to full‐bridge LLC converters. Moreover, according to the state‐plane analysis, load stepping‐up and stepping‐down transient state trajectories with the shortest path are carried out as well. To minimize the adverse effect of digital delay, fast load transient control based on 3D look‐up table is given based on FPGA‐EP3C25E144I7, which would also significantly reduce the computation requirement of the hardware controllers. The experimental results are verified on a 380V/12V LLC converter prototype at 130 kHz.

Due to the inherent double-line(2ω) frequency ripple caused by the instantaneous power unbalance between dc-and ac-side, dc source damage and ac-side power quality deterioration become a challenge to the unified power quality conditioner (UPQC) operating in the island mode. In this paper, a novel active power decoupling method is proposed for a single-phase half-bridge non-isolated UPQC in islanding mode. By utilizing the spare series-side half-bridge as the power decoupling circuit and coordinately working with dc capacitors, the double-line frequency voltage ripple at the DC side can be reduced effectively without any extra circuit. Moreover, by adopting the proposed method, the power ripple reduction performance still can be maintained with smaller dc capacitance, which could further improve the power density of the UPQC system. Simulation and experimental results show the correctness and effectiveness of the proposed method. More than 75% of DC voltage ripple is cut down, while 64% capacitance can be reduced with acceptable voltage ripple.

To meet the different charging or discharging requirements of batteries, this paper proposed an isolated three-port bidirectional resonant converter with additional notch filters. Only one transformer with multi-winding is adopted in the converter to simplify the structure. In addition, with the combination of the notch filter and the hybrid control, the impedance of each port can be controlled which realizes the decoupling of the output ports. Therefore, batteries with different states or different types of energy storage equipment can be charged at the same time. And voltage equalization can be also realized for the two output ports. Moreover, the converter can be operated with bidirectional power flow under ZVS condition, which is suitable for the application of V2G.

The harmonic suppression performance of active power filter (APF) under model predictive current control is obviously reduced, when the parameter exists a mismatch between the actual value in system and the nominal value adopted in the control. To suppress the influence of parameter mismatching, a model predictive control based on parameter identification with forgetting factor recursive least square (FFRLS) method is proposed for APF. The digital processing procedure of the FFRLS-based APF controller is detailed analyzed and designed. The identification error of inductance between identification value and the actual value is less to 6%, and the total harmonic distortion of grid current is significantly improved, especially a lower value of actual inductance. Simulation and experimental results show that the proposed method can not only effectively eliminate the influence of parameter mismatch on the control, but also result in better steady-state performance while retaining fast dynamic response.

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.

A three-phase to single-phase modular multilevel converter based advanced co-phase traction power supply (MMC-ACTPS) system is an effective structure to address the concerns of phase splitting and poor power quality of the conventional electrified railway. Due to the large number of MMCACTPS system modules, I/O resources and computing speed have high requirements on processors. Moreover, the module capacitor balance is challenging because the sorting time is too long when the traditional sorting algorithm for voltage balance is used. To solve the above issues, a digital implementation scheme of flexible power control strategy for three-phase to single-phase MMC-ACTPS system based on field programmable gate array (FPGA), which has sufficient I/O resources, has been proposed. Due to the parallel execution characteristics of the FPGA, the execution time of the controller and the modulator can be greatly reduced compared with a digital signal processor (DSP) + FPGA or DSpace. In addition, an improved sorting algorithm is proposed to reduce the sorting time and the implementation steps are analyzed. Finally, simulation and experimental results are presented to demonstrate the effectiveness and correctness of the proposed control strategy.

The single-phase unified power quality conditioner (UPQC) is widely used to improve power quality for sensitive end-users. However, the inherent instantaneous power difference between parallel and series converter will generate significant low-frequency dc-link voltage ripple and degrade the compensation performance of UPQC. The mechanism of dc-link voltage ripple generation and its influences on compensation voltage and current are analyzed in this article. To suppress the influence, a control strategy is proposed for the single-phase UPQC. For parallel converter, a notch filter is introduced in the outer voltage loop to prevent the voltage ripple entering the control loop, the specific order harmonics compensation is proposed in the inner-current loop to reduce the grid current harmonics. For the series converter, the dc-link voltage feedback is adopted to suppress the influence of voltage ripple on the compensation performance. Simulation and experimental results show that the grid current total harmonic distortion (THD) can be reduced effectively with the fixed dc-link capacitance compared with the traditional control strategy. More than half of the load voltage THD are also decreased by the dc-link voltage feedback control. Moreover, with the smaller dc-link capacitance, single-phase UPQC can maintain the better performance under the proposed control strategy.

In active power filter (APF), the output filter inductance mostly-changed with temperature variation and insulation aging. In order to suppress the influence of inductance parameter variation on the current compensation performance, online identification of the actual inductance is required. However, the traditional field-programmable gate array (FPGA)-based online parameter identification implementation requires lots of resources without reducing the identification accuracy. In order to reduce resource occupation while maintaining identification accuracy, an FPGA-based implementation of parameter identification with forgetting factor is proposed for APF. And time-sharing multiplexing method is adopted in matrix multiplication. The effectivity of the proposed implementation method is validated by simulation and experimental results. It shows that the error between the identification and the actual inductance decreases to 6%. When the output filter inductance changes, the THD of the grid current before identification is 2% to 10%, and the THD of the gird current after identification is always less than 3%.

High speed and heavy loads have become more prevalent in the traction power supply system recently. To ensure system operating stability, better power quality, and sufficient power capacity, improvements are needed over the conventional traction system. Inspired by the concept of a microgrid (MG), an AC co-phase traction MG system was proposed. Substations were connected to the traction grid as distributed generators operate in islanded mode. Droop control was adopted as the primary control to stabilize the system's operating frequency and voltage. Considering the operating features of the substation and locomotive load, a de-centralized secondary control strategy was proposed for AC co-phase traction MG system operation with enhanced resiliency. The proposed control strategy could increase system stability and prevent circulation currents between substations. Moreover, the proposed de-centralized coordination between substations does not rely on communication, which promotes the system's "plug-and-play" functionality. Stability analysis was undertaken and the proposed controller was proved to be exponentially stable. The dynamic response of the proposed controller was validated using comprehensive case studies in MATLAB/Simulink.