Distribution system protection solution for distributed power access
1. Research Background
The integration of distributed generation (DG) into power distribution systems has introduced new challenges in protection and control. Traditional protection schemes, designed for radial networks with unidirectional power flow, struggle to adapt to the bidirectional and variable nature of DG-connected systems. As a result, the selectivity and sensitivity of conventional protection devices are compromised. Moreover, DG units typically connect to the grid through power electronic interfaces, which alter fault characteristics depending on their control strategies. This makes it difficult to apply standard ultra-high voltage protection methods in distribution systems with high penetration of DG. To address these issues, a novel protection scheme based on transient polarity comparison has been proposed. This method utilizes fault transient components, which reflect the inherent fault behavior of the grid rather than the type or capacity of the power sources involved. By focusing on these transient features, the protection system can overcome the limitations caused by differences in DG and system short-circuit currents. Additionally, the high-frequency components of faults occur within milliseconds, making them suitable for fast-acting protection. With advancements in microprocessors and sensor technology, this approach enables high-speed protection that meets the dynamic needs of modern distribution networks, while also supporting coordination with low-voltage ride-through capabilities of DG units.2. Transient Polarity Comparison Protection Principle
Transient polarity comparison protection leverages wavelet transform to extract specific frequency band information from fault-induced high-frequency signals. This technique allows for accurate identification of fault locations by comparing the polarity of the transient signals at different points in the network. The core of this protection method is the polarity of the high-frequency component of the transient current. A cross-correlation function is used to assess the similarity between two transient signals. If the signals from both ends of a line show strong positive correlation, it indicates an internal fault. Conversely, if they exhibit strong negative correlation, the fault is considered external. This principle ensures rapid and accurate fault detection across all types of faults. Furthermore, the protection method is adaptive. The sampling rate, frequency bands extracted via wavelet transform, and fault determination time are interrelated. Depending on the hardware capabilities, the system can adjust its frequency band selection, allowing for flexible and efficient operation. As the frequency band increases, signal attenuation becomes more pronounced, enhancing the accuracy of the polarity comparison-based protection mechanism.3. Integrated Protection for Multi-Point Access Distribution Systems
As more renewable energy sources are integrated into distribution networks, the need for advanced protection schemes has grown. Modern distribution systems must accommodate both microgrids and large-scale DG units, creating a more interactive and distributed active network. To achieve this, centralized integration into medium-voltage distribution systems is essential to maximize renewable energy utilization. Recent advances in distributed sensing technologies, particularly in intelligent substations, have laid the foundation for the implementation of integrated protection systems. Synchronization technologies based on global positioning systems (GPS) and time-based communication between intelligent electronic devices further enhance the reliability of these systems. Based on the transient polarity comparison principle, an integrated protection system combining regional centralized control and local protection can be developed. Each busbar is equipped with an integrated protection unit (IR), which uses local and adjacent protection data to protect individual equipment and lines. These units also aggregate multi-point information to perform coordinated protection, enabling fast fault location, backup protection, and overall system automation. When a fault occurs, the high-frequency components propagate throughout the network. Current transformers detect a consistent polarity pattern—those pointing toward the fault have the same direction, while those facing away show opposite polarity. This allows each integrated protection unit to determine the relative position of the fault. By analyzing data from all units, the exact fault location can be identified. If a bus fault is detected, all connected circuit breakers are tripped. For line faults, only the affected circuit breaker is triggered, isolating the fault area effectively. This system also supports backup protection using polarity comparison data, ensuring reliable and coordinated fault handling.4. Conclusion
The transient polarity comparison protection method proposed in this paper offers significant advantages over traditional approaches. It is not affected by the capacity of distributed generation or differences in short-circuit currents, improving the speed and accuracy of fault detection. The integrated protection scheme based on this principle enables rapid fault location and isolation in distribution networks with high DG penetration. This advancement enhances the ability of active distribution networks to integrate renewable energy sources and ensures the stability and reliability of power supply in the future.Compared with large complex thin - wall castings, civil products have lower requirements on casting quality. However, for the latter, shorten the production cycle, improve the production efficiency of the problem becomes more prominent. The gelation process of common silica sol mainly depends on the dehydration and drying of silica sol, which takes longer time than the gelation of chemical hardening ethyl silicate. Ethyl silicate shell using ammonia dry each layer can be completed in 2h, and the final hardening of silica sol generally takes more than 12h, for some deep holes and other difficult to dry parts of the need for a longer time. At the same time, because the Investment Casting shell needs to be made in layers, each layer needs to be fully dried, to ensure that the lower shell immersion coating will not cause the problem of remelting off, and immersion coating itself, water will be immersed in the dried shell, resulting in a long overall drying cycle. It is a schematic diagram of the production cycle of silica sol shell investment casting under general conditions. As can be seen from the figure, shell making time accounts for more than 50% of the whole casting production cycle. To shorten the delivery time and shell making cycle is the core of the problem. The key factors to shorten the shell-making period can be divided into two aspects: internal cause and external cause. The main internal cause is the characteristics of the binder, and the external cause is the drying condition.
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