With the increasing penetration of renewable energy sources such as solar and wind power, integrating renewable generation with power electronic converters, energy storage, and hydrogen production technologies has become an effective approach to address grid integration challenges. During the analysis process, accurate modeling of the Proton Exchange Membrane (PEM) electrolyzer is particularly important. Existing PEM electrolyzer models mainly rely on mathematical formulations, which often fail to accurately represent the actual operating characteristics in system-level simulations. In addition, the limited equivalent circuit models reported in the literature and also cannot adequately reflect the impedance characteristics of the electrolyzer. Therefore, this paper employs electrochemical impedance spectroscopy (EIS) to measure the impedance characteristics of a PEM electrolyzer using a dedicated experimental platform. Based on the measured data, a novel equivalent circuit model incorporating the electrolyzer's polarization characteristics is developed. Simulation results demonstrate that the proposed model shows good agreement with the real behavior of the electrolyzer.

The half-wave-shaping based modular multilevel converter (HWS-MMC) is a lightweight hybrid multilevel topology that combines the advantages of conventional two-level converters and modular multilevel converters. By synthesizing sinusoidal half-wave voltages and unfolding them through low-frequency commutation, the converter achieves AC–DC conversion with fewer submodules. In a back-to-back (BTB) configuration, the topology is well-suited for AC–AC conversion, offering improved power density and simplified control decoupling. This paper studies the operating principles and modeling of the BTB-HWS-MMC, and proposes an asymmetric phase-shift modulation strategy for wide-range energy balancing. A coordinated control scheme is developed, including grid current control, module capacitor and DC link voltage control, AC voltage control, and submodule balancing. RT-Lab results validate the proposed converter and its control strategy.

To facilitate highly efficient parallel scalability of multiple battery packs in battery energy storage systems (BESSs), this paper proposes a topology and control strategies of partial-power-conversion parallel-connected BESS with dual DC buses. This topology combines a dual active bridge (DAB) converter with a series resonant dual active bridge (SR-DAB) converter to significantly reduce system losses and improve efficiency through partial power conversion. Building on this topology, a state of charge (SOC) balancing control strategy is integrated into the single-phase control of the SR-DAB converter to achieve the SOC balancing among battery packs. To overcome the low efficiency of parallel-connected battery packs under light-load conditions, a switching control strategy is proposed. A weighted hysteresis control strategy is introduced for scenarios with stable bus power over extended periods, maintaining the SOC difference between active and inactive battery packs with in an acceptable range, thereby ensuring consistent state of health (SOH) across all battery packs. Hardware-in-the-loop (HIL) experiments performed on an RT Box platform confirm that the proposed topology and SOC balancing control effectively equalize the SOCs of multiple parallel-connected battery packs. The proposed topology is experimentally validated using a setup comprising a 320 V DC bus and an 80 V balancing bus. Efficiency measurements further confirm that the switching control strategy significantly improves the performance under light load conditions. Finally, the potential for further improving overall system efficiency through flexible bus-voltage regulation is discussed.

As a core equipment of the low-frequency transmission system (LFTS) of offshore wind power, the impedance model of the modular multilevel matrix converter (M3C) is essential for system stability analysis. Due to the direct connection of M3C to two AC systems with different fundamental frequencies, the conventional multi-harmonic linearization method used for MMC modeling, which employs single fundamental-frequency as the frequency interval, cannot precisely describe the harmonic characteristics of M3C. In this paper, based on the comprehensive harmonic analysis of M3C, a multi-harmonic vector model in the frequency domain is established, which incorporates the dual-fundamental-frequency (DFF) coupling harmonics. Based on this, the multi-harmonic small-signal models of the M3C power stage and control loops are analyzed, and the precise sequence impedance model of M3C is developed. Finally, through frequency-sweeping experiments and comparison with a conventional model neglecting DFF coupling harmonics, the accuracy of the proposed M3C impedance model is verified. Real-time simulation results confirm the model's capability to precisely assess the stability of M3C-grid interactive system. In addition, the impact of key control parameters on the M3C's impedance characteristics and system stability is investigated, providing guidance for enhancing system stability through impedance reshaping.

Hydrogen is a clean energy carrier with broad application potential. This study focuses on improving hydrogen production efficiency in a proton exchange membrane (PEM) electrolyzer system that integrates a photovoltaic (PV) array, a battery energy storage system, and the electrolyzer. The PV array is interfaced with the electrolyzer through a buck converter using a maximum power point tracking (MPPT) algorithm to ensure maximum energy harvesting. A key contribution of this work is the integration of a battery system through a dual-active-bridge (DAB) converter. The DAB converter employs a multilayer perceptron (MLP) model to dynamically regulate the electrolyzer current and maintain optimal operating efficiency. An adaptive energy management strategy is further proposed to address solar irradiance fluctuations and enhance long-term operational stability. The MLP model is developed in Python and embedded into a PLECS simulation environment. The simulation results verify the effectiveness of the proposed control approach and efficiency optimization scheme. Throughout the simulation period, the PEM electrolyzer sustains an optimal efficiency of 69.9% under maximum PV power output. A limitation of this study is that the efficiency model is derived from the literature and does not yet consider all operational factors, indicating the need for refinement in future work.

To optimize the efficiency of the dual DC bus partial-power-paralleled battery energy storage system, this paper analyzes the efficiency characteristics and high-efficiency operating region of the topology, and proposes an efficiency optimization control method. Specifically, the efficiency characteristics are first analyzed according to the operating principles of the topology. Current-stress and RMS current models are then developed for both the bus voltage control unit (BVCU) and the balancing unit (BU), based on which the system’s high-efficiency operating region is derived. By combining the efficiency characteristics with the identified high-efficiency operating region, an automatic system-efficiency optimization algorithm based on dc-bus voltage regulation is proposed. Moreover, an indirect system-efficiency evaluation approach is adopted, which does not require any additional sensors or hardware cost. Finally, experimental results demonstrate that the proposed efficiency optimization control strategy effectively improves the operating efficiency of the parallel battery energy storage system without introducing extra cost, while having no adverse impact on system operational stability.

The Modular Multilevel Converter (MMC) technology has matured significantly, and its startup process, which is essential for normal system operation, can be divided into two phases: uncontrolled charging and controlled charging. After the uncontrolled charging phase, the submodule capacitor voltages have not yet reached the rated level, requiring specific control strategies for voltage elevation. Existing methods mainly leverage the controllable characteristics of submodules during this phase, regulating voltage by adjusting the number of blocked submodules. It is important to note that the charging process must strictly avoid risks of overvoltage and overcurrent. To address this, this study proposes a novel constant-current pre-charging control strategy: DC-side phase unit blocking is implemented to simulate a constant DC source for the converter, while AC-side arm unit blocking is used to create a three-phase symmetrical AC source with a constant amplitude. This strategy combines ease of operation with broad applicability across systems. Simulation results based on MATLAB/Simulink verify that the proposed method effectively achieves constant-current startup control for both the DC and AC sides of the MMC.

Sparse modular multilevel converters (SMMCs) are a new type of lightweight high-voltage large-power AC/AC converter that significantly reduces the number of components compared to modular multilevel matrix converters (M3Cs). This study proposes a fault ride through a control strategy for SMMC to address the issues of arm energy imbalances and valve-side overvoltage, which occur during asymmetric faults on the low-frequency side. First, we establish models of the energy deviation of the arms under asymmetric short-circuit faults on the low-frequency side of SMMC. We also study the influence mechanism of the control strategies on the arm energy imbalance during faults. On this basis, an arm energy balancing strategy based on zero-sequence voltage injections combined with AC voltage control is proposed; this can achieve arm energy balance and suppress the negative sequence current and overvoltage of the SMMC. Finally, we construct a simulation model of an offshore wind power low-frequency transmission system based on the SMMC. The simulation results show that the proposed energy balance strategy can realize the stable operation of the low-frequency transmission system (LFTS) under asymmetric faults on the low-frequency side, that the maximum capacitor voltage deviation during the fault does not exceed 10% and that capacitor voltage returns to normal 0.25 s after the fault occurs.

Fiber optic ultrasonic sensors, which are highly resistant to electromagnetic interference, are expected to be implanted inside electrical equipment to achieve high-sensitivity partial discharge (PD) detection. However, the influence of the fiber optic rings on PD parameters remains unclear. In this paper, a three-dimensional finite element model of the needle plate discharge is established. The effect of the fiber ring on the ultrasonic characteristics of PD is systematically investigated. A parametric analysis is used to investigate the effect of the fiber ring's diameter and height on the ultrasonic characteristics. The effect of fiber optic rings on PD characteristics is investigated through PD tests. The results indicate a time delay in the propagation of acoustic signals compared to electrical signals, typically at the microsecond level. As the diameter of the fiber ring increases, the maximum vibration velocity at the P-point initially increases and then tends to stabilize, but it always remains lower than the value observed without the fiber ring. Additionally, increasing the height of the fiber ring results in a decrease in the maximum vibration velocity at the P-point. The presence of the fiber ring does not affect the PD inception voltage, but it does influence the apparent discharge-phase spectra. This paper provides a theoretical and experimental basis for the application of fiber optic sensing technology in PD monitoring.

AbstractThe mechanisms governing electric field and partial discharge (PD) under stress cone dislocation remain unclear. This study employs the finite element method to explore the relationship between the electric field, externally applied voltage, and length of dislocation. Subsequently, the correlation between voltage and electric field is determined. The relationship between voltage and PD is established through PD tests conducted under stress cone dislocation conditions. By considering the influence of voltage, the association between the electric field and PD is established, revealing the mechanism of PD initiation by electric field distortion in joints. The findings suggest that the lifetime and reliability were increased by wrapping semi‐conductive self‐adhesive tape around the exposed cross‐linked polyethylene (XLPE) insulation layer. The electric field at a specific location is approximately proportional to the applied voltage. However, the slope is influenced by the composite insulation interface and the radial distance of the copper conductor. When crossing the interface between the XLPE insulation layer and the accessory insulation layer, the slope decreases significantly. The threshold for electric field PD is determined by examining the electric field distribution and the PD inception voltage during testing. When the voltage is sufficiently high, significant electric field distortions can occur at multiple points in the joint, potentially leading to concurrent PDs at several locations.

The modular multilevel matrix converter(M3C)is the core equipment of the low frequency transmission system(LFTS).The two AC systems with different frequency that M3C is connected to are directly coupled in the converter arms.And the arm electrical quantities contain not only the integer harmonics of the fundamental frequencies of both AC sides,but also the non-integer order harmonics generated by their interactive coupling(called differ-frequency coupling harmonics).Ignoring the dynamic DC voltage of the submodule and the differ-frequency coupling harmonics and taking the fundamental frequency as the frequency interval,the multi-harmonic linearization modeling method is difficult to accurately describe the harmonic characteristics of M3C,thus reduces the accuracy of the impedance model.Moreover,it cannot reveal the mechanism how the DC voltage dynamic of the submodule and the harmonics affect the impedance characteristics.In this paper,based on the harmonic analysis of M3C and considering the differ-frequency coupling harmonics,a multi-harmonic vector model in frequency domain is established firstly.Through analyzing the phase sequence and transfer relations of the multi-harmonic small signals of the M3C 9 arms,the M3C power stage model can be reduced to that of one arm,so the impedance modeling complexity is reduced.On this basis,the impedance model of M3C is developed including the effects of control loops coupling that can accurately reflect the impact of DC voltage dynamic and harmonic characteristics.Finally,the accuracy of the M3C impedance model is verified through RT-Lab real-time simulations and sweeping experiments,and the key factors affecting the impedance characteristics are analyzed.

Distributed acoustic sensors (DASs) have the advantage of long-distance and distributed monitoring of vibration signals. However, there are few studies on the application of DAS for partial discharge (PD) monitoring of power equipment. There is also a lack of related theoretical and methodological studies. In this paper, the DAS-measured phase signals are theoretically analyzed from the frequency and energy perspectives, and the method of PD broadband signal coverage using spectrum and energy accumulation is proposed. Fiber optic rings are employed for the detection of PD signals, and the precise localization of these signals is achieved by calculating the DAS energy spectrum. The fiber optic ring is wrapped around the cable joint to achieve the detection and locating of the PD signal. The results indicate that the use of a fiber optic ring can realize the sensitive detection and locating of PD. The signal strength is maximum when the length of the fiber ring is equal to the spatial resolution of DAS. The sensitivity of PD detection in cable joints can reach 40 pC, and the simultaneous detection and localization of PD at different locations can be realized. The research can provide support for theoretical and experimental for the monitoring of PD in power equipment.

Distributed acoustic sensing incorporates multiple indicators, and there exists a mutually constraining relationship among these indicators. Different application fields have varying requirements for indicators. Therefore, indicator testing and comprehensive evaluations are crucial for engineering applications. In this paper, we conducted a theoretical analysis of key indicators, including frequency response, sensitivity, spatial resolution, sensing distance, multi-point perturbation, and temperature influence. The indicator test scheme was developed, and a test system was constructed. The test data were analyzed and compared in the time-frequency domain. A performance evaluation method for distributed acoustic sensing, based on the analytic hierarchy process, is proposed, and a comprehensive evaluation example focused on high-frequency applications is presented. The results show that the test scheme and method presented in this paper can accurately measure the upper limits of each indicator of distributed acoustic sensing. The proposed comprehensive evaluation method enables the assessment of sensor performance and applicability based on engineering practices. It addresses the challenge of evaluating distributed acoustic sensing with multiple indicators and offers an efficient approach for equipment development and engineering applications.

作为海上风电分频输电(fractional frequency transmission system,FFTS)的重要装置,模块化多电平矩阵变换器(modular multilevel matrix converter,M3C)的故障穿越是海上风电并网安全稳定运行的重要问题之一.为研究M3C故障穿越控制,首先基于M3C双dq坐标系下的数学模型和稳态控制策略,在MATLAB/Simulink中搭建仿真模拟电网发生三相对称故障的情况,分析M3C的暂态特性,并提出故障穿越策略.通过改变M3C低频侧交流电压指令,迫使风电场根据自身低电压穿越策略减少输出功率,从而降低M3C传输的有功功率;M3C工频侧采取有功平衡优先、剩余最大无功输出的控制策略以支撑网侧跌落电压;且根据系统对有功无功功率的需求分为有功无功兼顾、有功优先、无功优先3种场景的控制方案.仿真结果表明,所提策略可以根据不同的控制需求提升M3C故障穿越的能力,保证M3C各子模块电容平均电压不超过阈值,无需额外卸荷电路和通信即可实现电网故障期间的安全穿越,同时有利于维持风机持续并网运行和电网电压的恢复.

The modular multilevel matrix converter (M3C) is the core equipment for fractional frequency transmission system (FFTS). The AC-AC conversion of M3C leads to direct coupling of the ac electrical quantities at different frequencies,causing complex harmonic distribution. In order to analyze the harmonic characteristics of the M3C,the analytical expression of the sub-module capacitor ripple voltage is derived based on the operating principle of M3C at first. On this basis,the analytical formulas of the nine bridge arms currents harmonics are derived,taking into account the coupling of all four frequency components of the capacitor voltage. The relationships between the multi-frequency harmonic currents of bridge arm and the system currents on two sides are analyzed,as well as the key factors affecting the amplitudes of ripple voltage/harmonic current are discussed. The results show that in steady-state,the currents at frequencies ω1 and ω2 flow into the ac systems as the positive-sequence fundamental currents; the currents at frequencies 3ω1 and 3ω2 flow into the ac system as the zero-sequence components;the remaining harmonics are circulated in the converter. A zero-sequence current mitigation control strategy for the M3C is proposed. The accuracy of the theoretical harmonic analysis and the effectiveness of the control strategy are verified by simulations in Matlab/Simulink.

This article addresses the decoupling control design for the cascaded power electronic transformer (PET) in input-series–output-parallel connection. Due to the coupling and interaction between the submodules and between substages, in essence it is a complex nonlinear system. A nonlinear control strategy based on feedback exact linearization (FEL) is developed to decouple all the control objectives so as to realize the dc-link and output voltages fast stabilizing and the submodule balancing without coupling and interactions. Based on the separation of the ac current time-scale and dc voltage time-scale, the FEL control law is derived, and the specific tuning process of control parameters is given for desired control bandwidths. Taken the dynamic influences of the current-loop, filter, and delay into account, further analyses are made and compared with a dual active bridge (DAB) balancing control-based strategy to show the improvement on decoupling effect and dynamic performance of the proposed control. Finally, the simulation and experimental results verify the theoretical analysis, which exhibit better dynamic features and minimal coupling effect under the condition of bidirectional power changes and parameter inconsistency and uncertainty.

This paper presents an input-series hybrid dual active bridge (DAB) converter for efficiency improvement. The proposed converter is achieved by integrating a series resonant tank (SRT) into a single phase-shift (SPS) controlled DAB that shares one H-bridge at the secondary side. By using the super-resonant state of the LC tank, the phase correction of the secondary side H-bridge current can be obtained to realize zero voltage switching (ZVS), zero current switching (ZCS), and full load ZVS for the primary side bridges. In addition, the circulating current is reduced when compared to that of the traditional input-series DAB converter based on SPS control, which improves the overall efficiency. The system modelling and parameters design procedures are provided. Finally, a 1-kW experimental prototype was built, and the soft-switching performance was tested. The obtained results verify the effectiveness of the proposed topology and demonstrate a significant efficiency improvement.

负荷灵敏度在线辨识是一种新的负荷建模方法,在对含有电力电子变压器(power electronic transformer,PET)的低压微电网负荷灵敏度在线辨识方法及应用进行深入研究的基础上,首先建立了AC/DC/AC型PET的数学模型,并给出了PET实现负荷灵敏度在线辨识的基本方法,分析了分布式电源的接入对辨识结果的影响;在此基础上,提出了一种基于PET电压/频率协调控制的微电网柔性负荷控制策略.仿真结果表明,在线辨识方法可以实时获得负荷模型参数,依据所获负荷模型,PET通过跟踪系统柔性减载指令,实现系统柔性控制与管理,为电网柔性负荷控制提供技术支持.

级联式电力电子变压器为多变量耦合的复杂非线性系统,该文基于微分几何非线性控制理论,提出电力电子变压器非线性综合控制策略.根据系统多端口及内部变量控制目标,首先建立级联式电力电子变压器多变量仿射非线性模型.其次,基于微分几何多输入多输出精确线性化原理,对其非线性坐标变换和状态反馈律进行推导,并采用二阶系统极点配置法进行控制参数设计,得到系统非线性综合协调控制策略.该策略可将系统精确线性化,从而简化和明确控制参数整定过程,按需求调节各变量控制带宽,提高大扰动时系统多目标动态响应;且可直接实现子模块均衡控制.最后,通过仿真和实验与传统控制方法进行对比,验证了所提控制策略的有效性及优越性.

快速切除负荷是电力系统在故障后保持稳定的一种常用措施.针对以电力电子变压器为核心的直流微电网,提出一种基于负荷灵敏度在线辨识的直流低压切负荷决策方法.通过控制电力电子变压器直流端电压小扰动实现负荷电压特征系数在线辨识,获得系统中各节点负荷功率对电压变化的灵敏度;然后将辨识得到的负荷特征系数与微网中各调压单元的下垂控制特性曲线相结合,准确评估各节点负荷的切除对直流微网电压的影响程度,从而快速判定负荷切除节点.仿真结果表明,在包含风/光微源、储能及多种特性负荷的直流微网中,当交流主网侧故障或分布式电源输出功率受限使直流电压低于下垂调压范围时,按照所提考虑负荷灵敏度的电压评估式判定负荷切除节点,系统直流母线电压与各调压单元工作模式能够按理论判定值运行,验证了所提方法的有效性.此外,该方法还可克服系统负荷时变性的难题,并具有原理简单、计算方便等特点.