Jiawei He

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.

It is important to limit the dc fault current in voltage source converter-based high-voltage direct-current (VSC-HVDC) grids for fault ride-through and safe operation. At present, the most typical method of fault current limitation in VSC-HVDC grids is to install dc reactors directly in the system. However, this will lead to a drastic increase in the required capacity of the direct current circuit breaker (DCCB) metal oxide varistor (MOV), increasing the manufacturing difficulty of the DCCB and reducing its operation reliability. Meanwhile, the dc fault current clearing time is also markedly prolonged, meaning that the dc fault handling speed is reduced. To solve above problems, several dc fault current limiters (FCLs) based on controlled (full-controlled or half-controlled) power electronic switches have been proposed, which inevitably incur a high cost. In this article, a novel diode–thyristor hybrid dc FCL is proposed. It can effectively reduce the high requirement on DCCB MOV rated capacity imposed by the dc reactor and increase the dc fault current clearing speed. In addition, compared with existing dc FCLs with similar functions, its cost is much lower due to the hybrid design. Furthermore, when applied to multiterminal VSC-HVDC grids, an accurate parameter design method for dc FCLs is innovatively proposed based on the concept of optimization. Finally, simulation case studies and experimental tests are carried out to prove the feasibility and superiority of the proposed method.

As one of the most important component in mechanical and hybrid dc breaker (MCB and HCB), operating time of ultrafast vacuum switch (VS) determines the breaking speed of MCB and HCB. However, limited by the high capacitive storage energy requirement of Thomason-coil actuator (TCA), the low-efficiency and large-volume of operating mechanism are main restrictions for wide application of ultrafast VS. This article proposes a high-efficient and miniaturized operating method for ultrafast VS by recycling the residual energy in capacitor banks for sequential open- and close-operations (O-C). Mathematical model of ultrafast VS during the sequential O-C operation process is established, and the prototype with rated parameters of 3.6 kV/2.5 kA/1ms is developed for experimental verification. The research shows that by using the proposed method, the total required capacitive storage capacity for sequential O-C operation is decreased by 50%, from 2038 to 1019 J, resulting in the doubled operating efficiency. In addition, the cost and volume of the operating mechanism for ultrafast VS are decreased significantly.

The transient fault characteristics of photovoltaic (PV) grid-connected systems are highly dependent on the dynamic response of control links. The phase-locked loop (PLL) in the inverter control system plays an important role, and its dynamic output response will affect the PV transient fault characteristics. The existing method for PLL dynamic response calculation is based on the small-signal simplified model, and requires to previously measure the fault steady-state voltage phase at the point of common coupling (PCC). This limitation leads to inaccurate calculation and prevents the method from being applied in practical engineering. Here, to accurately describe the PV transient fault characteristics, an accurate model for calculating the PLL dynamic response independent of the fault steady-state voltage is established, and the model is further linearized to improve its computation efficiency. On this basis, a PV transient fault current calculation method that considers the PLL dynamic response is proposed. Compared with the existing method, the proposed method can more accurately calculate the dynamic response of the PLL and the fault transient characteristics of PV power source. Finally, simulation cases based on PSCAD/EMTDC are carried out to verify the accuracy of the proposed method and its superiority compared with the existing method.