Ye Li

Photovoltaic (PV) power exhibits inherent intermittency and rapid fluctuations, which pose challenges to the stable operation of off-grid hydrogen production systems. Proton exchange membrane water electrolysis (PEMWE) is commonly used to mitigate such fluctuations, but the mismatch between fast PV variations and electrolyzer response may affect durability-related operating conditions. This paper proposes a durability-aware capacity ratio design method for PV–PEMWE systems under rapid PV power fluctuations. Based on the dynamic response of the electrolyzer, durability-related constraints, including voltage limits, overpotential thresholds, and voltage variation rates, are incorporated into a feasible-region formulation for capacity configuration. A robustness margin index is further introduced to evaluate the tolerance of different configurations. Simulation results indicate that inappropriate capacity ratios may lead to violations of durability-related limits under rapid PV fluctuations. The proposed method identifies feasible capacity regions that help maintain operation within durability-related limits. Compared with a conventional configuration outside the feasible region, the levelized cost of hydrogen (LCOH) is reduced from €6.79/kg to €5.64/kg, corresponding to a reduction of approximately 16.9% under the studied conditions. The proposed method provides a theoretical and practical basis for capacity planning of off-grid PV hydrogen production systems.

The inherent intermittency and volatility of photovoltaic (PV) power cause rapid fluctuations on the DC bus. In off-grid PV hydrogen production systems, proton exchange membrane electrolysis (PEMWE) is used to smooth these power fluctuations. However, a temporal mismatch exists between rapid PV variations and PEMWE capacity. This allows power disturbances to affect internal components of PEMWE, accelerating degradation. This paper establishes a link between the electrical dynamics and durability of PEMWE. A dynamic overpotential response model is developed to characterize PEMWE behavior under PV power fluctuations. A PV/PEMWE capacity ratio configuration method is then proposed, incorporating PV power fluctuation and PEMWE durability constraints to define the feasible sizing region. The simulation results demonstrate that the proposed method suppresses PV power fluctuations and extends electrolyzer lifespan for more simple durability constraint calculations. Consequently, the levelized cost of hydrogen is reduced by 16.9%. This work provides a theoretical basis for efficient sizing in off-grid PV hydrogen production systems.

The inherent intermittency and volatility of photovoltaic (PV) power cause rapid fluctuations on the DC bus. In off-grid PV hydrogen production systems, proton exchange membrane electrolysis (PEMWE) is used to smooth these power fluctuations. However, a temporal mismatch exists between rapid PV variations and PEMWE capacity. This allows power disturbances to affect internal components of PEMWE, accelerating degradation. This paper establishes a link between the electrical dynamics and durability of PEMWE. A dynamic overpotential response model is developed to characterize PEMWE behavior under PV power fluctuations. A PV/PEMWE capacity ratio configuration method is then proposed, incorporating PV power fluctuation and PEMWE durability constraints to define the feasible sizing region. The simulation results demonstrate that the proposed method suppresses PV power fluctuations and extends electrolyzer lifespan for more simple durability constraint calculations. Consequently, the levelized cost of hydrogen is reduced by 16.9%. This work provides a theoretical basis for efficient sizing in off-grid PV hydrogen production systems.

Driven by the global pursuit of carbon neutrality, green hydrogen produced from renewable energy has emerged as a promising pathway for decarbonization. Among the various application scenarios, DC off-grid photovoltaic (PV)-proton exchange membrane (PEM) hydrogen production is a representative solution. However, PV generation typically exhibits low utilization efficiency, and the volatility of photovoltaic output poses a serious challenge to the efficient and stable operation of DC off-grid hydrogen production systems and the lifespan of equipment. To improve system efficiency and economic performance, this study quantitatively evaluates lifetime degradation in a proton exchange membrane electrolyzer (PEMEL) and lithium-ion batteries (LIBs) based on their operating mechanisms. Building on this evaluation, an optimization model that minimizes the levelized cost of hydrogen is built, and PEMEL gas safety, start-up/shutdown and overload operation, equipment safety and capacity limitations, as well as system power balance are taken into account as constraints. Accordingly, a coordinated operation optimization strategy for DC off-grid PV-PEM hydrogen production considering efficiency and lifetime degradation is proposed. Finally, using typical regional solar irradiance and ambient temperature data, the optimal operating performance under different external conditions is obtained and benchmarked against representative existing schemes, demonstrating the feasibility and advantages of the proposed method.

As the capacity and penetration of distributed photovoltaics (PVs) in distribution networks continue to increase, the ability of downstream PV sources to supply fault currents after reverse faults also progressively grows. This trend reduces the sensitivity of line current protection, potentially leading to maloperation. Implementing direction protection criteria can effectively mitigate the risk of reverse maloperation in PV-integrated distributed networks. To this end, the applicability of the conventional power direction criterion is evaluated in this study. The analysis demonstrates that, at power frequency, PV sources exhibit capacitive impedance, whereas line impedance is inductive. As a result, direction criteria based on measured impedance angle characteristics become increasingly prone to maloperation with higher levels of PV penetration. This study further investigates the distinct transient voltage-current polarity characteristics after forward and reverse faults under PV contributions and develops a novel direction protection scheme based on transient polarity comparison. Finally, the feasibility and superiority of the proposed method are validated through comprehensive hardware-in-the-loop testing .

When faults occur in the distribution network accessed with photovoltaic (PV) sources, the performance of the traditional power direction identification criterion is affected by the PV low-voltage ride-through (LVRT) control strategy. The paper analyses the distribution characteristics of negative-sequence voltage and current after forward and backward phase-to-phase faults, and the change characteristics of the positive-sequence measured impedance angle after three-phase faults, and accordingly proposes a composite fault direction identification method applicable to distribution networks with high proportion of PV sources. The proposed method makes use of the negative-sequence voltage magnitude to distinguish phase-to-phase and three-phase faults. For phase-to-phase faults, the direction identification is achieved based on the negative-sequence current magnitude, while for three-phase faults, the operation range of traditional power direction criterion is adjusted to improve the operating reliability. Further, the threshold selection of the proposed method is discussed. Theoretical analysis and simulation test show that, compared with the traditional power direction criterion, the proposed method has outstanding performance in systems with high proportion of PV sources, especially for the faults with high transition resistance.

Proton exchange membrane fuel cells (PEMFC) integrated with renewable energy sources face risks from external electrical short-circuit (ESC) faults. ESC faults not only threaten the power system side but also adversely impact internal physical parameters, such as temperature and gas diffusion. The resulting abnormal dynamics of internal physical quantities (e.g., thermal and gas conditions) within the PEMFC stack can affect safe and reliable system operation. However, little research focuses on the characteristics of external electrical faults and their multi-physics impact on the internal response of PEMFC systems. Therefore, this paper first analyzes the external electrical characteristics and internal multi-physics response of PEMFCs after an ESC fault, using an equivalent circuit model of the PEMFC system integrated with renewables. On this basis, this paper proposes a method to evaluate the safe operation boundary of the studied system post-ESC fault, considering PEMFC internal temperature and gas safety limits. Finally, the analysis is verified on simulation and experimental platforms, and key operating factors are investigated. Experimental and simulation results show excellent agreement, with an error of less than 1%, demonstrating the method’s effectiveness for safety analysis. The results indicate that clearing faults within minutes can prevent membrane thermal damage. This research provides valuable theoretical insights for developing safe operation strategies for the PEMFC system in renewable energy integration.

The application scenarios of high-voltage direct current systems are becoming increasingly diverse, featuring complex and variable topologies as well as boundary conditions. Existing single-ended protection methods for dc lines cannot ensure selectivity of operation in the absence of line boundaries. The pilot protection can identify faults without relying on line boundaries, but it generally depends heavily on precise system parameters or requires a cumbersome threshold value selection process. This study analyzes the transient impedance phase-frequency characteristics of dc systems after faults under three typical scenarios, i.e., no boundary, dc reactor boundary, and dc filter boundary. Based on the polarity characteristic of high-frequency transient power during forward and backward faults, a novel directional pilot protection for dc lines is proposed. Compared to existing methods, this approach does not depend on system parameters or boundary elements, and eliminates the need for threshold setting, significantly reducing the complexity of engineering implementation and maintenance. Finally, a prototype of the fast directional pilot protection device is developed based on the proposed method, and extensive hardware-in-loop testing is conducted on the real time digital simulator platform to validate its operation performance.

The protection is one of the most important techniques for the modular multilevel converter (MMC) high-voltage dc (HVdc) system. The existing single-ended dc line protections are mainly based on the boundary characteristics, which are not suitable for the condition with no boundary element on the line terminal. In this article, the variation characteristics of different electric variables with fault distance changing are analyzed in the MMC-HVdc system with no boundary reactors. On this basis, a novel segmented-change-of-voltage-based single-ended protection algorithm is proposed, which is suitable for the system with no boundary element. In addition in the designed scheme, the nearby dead zone problem and return-to-inactivated problem are researched specifically. According to the proposed method, a protection device is developed, and the corresponding hardware-in-loop experiment tests are carried out to validate the properties of the developed protection device.

DC circuit breaker (DCCB) is one of the most important technologies for safe and reliable operation of dc grid. And the hybrid DCCB (HCB) is obviously promising in high-voltage application area, because it combines both the advantages of mechanical DCCB and solid-state DCCB. At present, insulated gate bipolar transistors (IGBTs) are generally used for configuring the main breaker branch in the HCB. Compared with IGBTs, thyristors have the merits of ability to bear higher surge current, higher reliability, and lower cost, thus being more economical and reliable when used to replace IGBTs. Right now, several topologies of thyristor-based HCBs have been proposed, but the economy, reliability, control complexity, as well as the operation performance, can still be improved. In this article, a novel passive thyristor-based HCB is proposed. Compared with existing IGBT-based HCBs, the proposed HCB has significant advantages on economy and reliability, due to the application of thyristors for replacing IGBTs. In addition, in the proposed scheme, the required thyristors count is reduced and the additional power supply for capacitor precharging is saved, so the investment and control complexity are both cut down. The feasibility and superiorities of the proposed HCB are validated with scaled-down experiment test and 500-kV-level simulation case study.