Although the probability of occurrence of station internal ac grounding faults in modular multilevel converter (MMC) based high-voltage direct-current systems is low, they may lead to severe consequences that should be considered when designing protection systems. This paper analyzes the characteristics of valve-side single-phase-to-ground (SPG) faults in three configurations of MMC systems. Fault responses for symmetrical monopolar MMCs are first studied. Upper arm overvoltages and ac side non-zero-crossing currents arising from SPG faults in asymmetrical and bipolar configurations are then investigated. DC grounding using an LR parallel circuit is employed to create current zero-crossings which will enable the operation of grid side ac circuit breakers. The theoretical analysis is verified through simulations performed in PSCAD/EMTDC, with simulation results and the theoretical analysis showing a good agreement. The studies in this paper will be valuable for the design of protection systems for station internal ac grounding faults.

With large scale of wind turbines connected to high impedance AC grid, transient synchronizing stability of PLL (phase locked loop) based WTC (wind turbine converter) during LVRT (low voltage ride through) is increasing concerned. However, the existing research is usually carried out at the assumption of constant current injection. The influence of dynamic current injection adjusted by LVRT control is neglected. Yet, in high impedance AC grid, the dynamic current injection significantly affects the terminal voltage tracked by PLL. Then LVRT control has much impact on the transient synchronizing stability, which is concerned especially in this paper. First, an novel equivalent motion model is developed, in which PLL is modelled in the form of rotor motion and LVRT control adjusts its equivalent driving force similar as governor in synchronous generator. Then based on the developed model, accelerating-decelerating area method is employed to reveal the transient stability mechanism from energy's transformation and dissipation. Transient stability region method is employed to present the visualized stability region. Furtherly, the influence of K-factor, grid impedance and grid voltage sag are studied from the influence of them on accelerating-decelerating area and stability region. Finally, simulated results are presented to verify the analytical results.

One of the most critical faults affecting modular multilevel converter (MMC) based bipolar high-voltage direct-current (HVDC) transmission systems is the single-phase-to-ground (SPG) faults between the converter transformer and the valve. However, half-bridge (HB) and full-bridge (FB) based MMCs exhibit a different behavior following such a fault and, thus, converter protection should be addressed in a different manner for each configuration. For HB-MMCs, an SPG fault at the valve-side leads to a severe overvoltage on the submodule (SM) capacitors in the converter upper arms and to grid-side non-zero crossing currents. Although FB-MMCs only exhibit overvoltage, these are more severe than for their HB counterparts. To address this problem, this paper presents a protection strategy considering thyristor bypass branches placed in parallel with upper arms of FB-MMCs. By employing this configuration, the upper arm overvoltage in the faulted converter is mitigated and remote converters can be quickly blocked using their local protection schemes. For completeness, the effectiveness of the strategy is verified through time-domain simulations in PSCAD/ EMTDC. The studies in this paper demonstrate the effectiveness of the presented protection scheme for station internal faults occurring in FB-MMCs in bipolar HVDC systems.

In the context of harsh emission control and ecological environment protection, the shipping industry is transforming and upgrading towards greening, decarburization, and electrification. Battery-powered all-electric inland ships have been attracting increasingly attention. However, its initial investment cost is much more expensive than a traditional diesel-driven mechanical ship because lithium-ion batteries are currently expensive. Hence, a suitable battery size and efficient energy management strategy for ship sailing are very important for a battery-powered ship. In this paper, a novel joint optimization method of the sailing speed and battery capacity, which considers the interaction between battery size and sailing speed as well as multiple operation factors, such as freight demand and battery life, and port electricity price, is proposed to fully exploit the battery-powered ships’ application potential. Moreover, a joint optimization model of the sailing speed and battery energy consumption model considers the battery-powered ship’s characteristics and waterway characteristics. Next, a solution algorithm for the proposed joint optimization model is established to achieve joint decision-making regarding the sailing speed and battery size. Finally, case studies are conducted to demonstrate the flexibility and effectiveness of the proposed method. The results show that the proposed method can obtain the optimal sailing speed and the corresponding battery capacity synchronously when the actual transportation scenario is fixed. Moreover, the battery initial investment cost can be effectively reduced with the prosed method.

In order to achieve carbon peak and neutrality goals, many low-carbon operations are implemented in ports. Integrated energy systems that consist of port electricity and cooling loads, wind and PV energy devices, energy storage, and clean fuels are considered as a future technology. In addition, ports are important hubs for the global economy and trade; logistics optimization is also part of their objective, and most port facilities have complex logistics. This article proposes an energy–logistics collaborative optimization method to fully tap the potential of port-integrated energy systems. A logistics–energy system model is established by deeply examining the operational characteristics of logistics systems and their corresponding energy consumption patterns, considering ships’ operational statuses, quay crane distribution constraints, and power balances. To better represent the ship–energy–logistics optimization problem, a hybrid system modeling technique is employed. The case of Shanghai Port is studied; the results show that costs can be reduced by 3.27% compared to the traditional optimization method, and a sensitivity analysis demonstrates the robustness of the proposed method.

Crops are the most important ground cover on slope farmland and have a significant impact on the soil erosion. But soil erosion on slope farmland is also affected by many other factors, such as topography and rainfall. In order to explore the effect of crop growth on soil erosion on different slope gradient of slope farmland, and analyze the interaction of crop growth and slope gradient on soil erosion, this study used artificial simulated rainfall to observe the runoff rates and soil loss amounts under different slope gradients for maize, soybeans, and winter wheat in different growth stages. Results showed that crops and slope gradient both significantly affected production and development of slope runoff. Compared with bare land, mean runoff rate on slopes was reduced by 24%, 32%, and 94% respectively, and sediment yield was decreased by 44%, 55%, and 99% respectively on maize, soybean, and winter wheat fields. Inhibitory effects of crops on slope runoff rate and sediment yield were enhanced with crop growth and decreased with increasing slope gradient. Crop growth and coverage could offset the impact of increasing slope gradient on runoff and sediment to some extent and reduced water and soil loss on slopes. Sediment yield was produced largely when the slope gradient was greater than 10 degrees on maize and soybean fields, but soil erosion was effectively inhibited when the slope gradient was less than 15 degrees on winter wheat fields. Crop planting can effectively reduce the impact of slope gradient on soil erosion, especially during the flourishing period of crop growth.