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With the development of dc renewable power generation and dc power consumption, more and more attention has been paid to residential low-voltage dc (RLVDC) power systems in recent years because of their advantages in conversion efficiency and control flexibility. Meanwhile, there are still some obstacles and challenges in the practical application of RLVdc due to the lack of standardization. One of the most important issues in RLVdc standardization progress is the voltage level standard. In this article, a comparative study on the performance of different RLVdc system voltage levels is presented. The main purpose of the article is to provide a voltage level selection guidance for energy prosumers and equipment manufacturers interested in RLVdc systems. The voltage levels commonly used in academia and industry, from 220 to 750 V, are compared and analyzed considering their influence on compatibility, topology, power supply capability, and safety. The article contains a research surveys, market statistics, power system simulations, and calculations with simplified models. The results of the comparative study show that ±375 V is optimal for the RLVdc system in cases with large power ratings and voltage levels below 240 V are suitable for RLVdc systems with special safety requirements. [ABSTRACT FROM AUTHOR]

Due to the independent controllability and fast power regulation capability, the High Voltage Direct Current (HVDC) system could be a prospective technology to provide multiple ancillary services to the system besides conventional bulk power transmission. However, with the increase of False Data Injection Attacks (FDIAs) on PMU data, the HVDC system could have the wrong response once the collected data that the HVDC system relied on is attacked, thus threatening the system operating security. How to ensure the security of the PMU-based HVDC ancillary service control become an urgent issue. To mitigate the risk, this paper proposed an HVDC attack-defense control based on the FDIAs detection method. Firstly, the Squeeze-Excitation based Double Convolutional Neural Networks (SE-DCNN) is proposed to realize fast identification of the attacking frequency type based on the time and frequency domain signals. The duration time of FDIAs is detected by the local outlier factor. Then, utilizing the results from SE-DCNN, HVDC ancillary service control framework is reorganized and an HVDC attack defense control is proposed for suppressing the potential influence of various types of FDIAs on the HVDC system ancillary service. Different experiments results demonstrate that the proposed method has the ability to significantly mitigate the frequency deviation and oscillation under the FDIA. [ABSTRACT FROM AUTHOR]

Urban power grids are facing many operational and expansion challenges to meet further demand growth and increased reliability requirements. Advanced transmission technologies have been considered by the electric utilities to effectively increase the utilization of existing infrastructure and operational flexibility. The focus of this paper is on VSC-HVDC technologies for urban power grid enhancement and modernization. First, a potential technical scheme is proposed for converting an existing AC circuit to DC operation, which could boost the power transfer capability of the critical transmission corridor and increase network operational flexibility. Second, this paper proposes three operation modes for the VSC-HVDC interties in urban power grids corresponding to normal, emergency and island operating conditions, respectively. An integrated, adaptive emergency control strategy (AEC) is proposed that can enable adaptive power flow responses of the VSC-HVDC intertie under varying system operating conditions and critical contingencies. The flexibility and effectiveness of the proposed operational principles of urban VSC-HVDC intertie and the corresponding control strategies are verified in PSCAD/EMTDC using a realistic urban power grid in China. [ABSTRACT FROM AUTHOR]

• The small-signal stability of current vector control and power synchronization control under different system strengths is studied. • The key factors of disturbance in the process of control mode switching are analyzed. • Based on the principles of current vector control and power synchronization control, a smooth switching control strategy for PV grid-connected converter is proposed. • An advanced smooth switching control strategy with complete process frequency support by the power correction is proposed. The continuous increase in the penetration of renewable energy sources such as photovoltaics (PV) brings operational challenges to the traditional distribution network. With a large-scale PV integration, the power system behaves as a weak grid, which leads to more frequent and wider changes in system strength. In order to improve the system stability, the PV system is required to operate stably while providing necessary system support under different operating conditions. However, there are two issues for the PV system needs to address to satisfy this requirement. Nowadays, there are two types of control strategies for PV systems: the grid-following (GFL) control and the grid-forming (GFM) control. The two control strategies differ in inertia support capability and control stability under different operating conditions. To guarantee the grid connection stability as well as the operating stability under AC systems with different strengths, the smooth switching between the GFL and GFM is an urgent topic. In addition, how to maintain the system frequency support performance during the control switching of the PV system is also a challenge. In this paper, an advanced smooth switching control strategy is proposed to enhance the operational stability and flexibility of the PV system while maintaining the frequency support performance during the switching process. Firstly, the small-signal stability of typical control strategies of GFL and GFM under different system strengths is compared to analyze the necessity of the control switching between the two control modes. Then, a smooth switching control strategy is proposed to realize the smooth switching when AC grid strength changes. More importantly, to improve the system frequency performance and realize continuous frequency support during the switching process, a power correction function is developed on the smooth switching control strategy. The effectiveness and universality of the proposed control strategy are verified by case studies on a modified IEEE 33 bus distribution network with high PV system integration. [ABSTRACT FROM AUTHOR]

• A coordinated frequency regulation scheme for asynchronous AC grids is proposed. • The proposed scheme is a communication-free method. • A sample and hold component is implemented to enhance the inertia response. • A frequency safety control is proposed to improve the system security. • Two measures are used to reduce the coupling effect among asynchronous AC grids. With the increasing penetration level of renewable power, the power systems' inertias are decreasing. To ensure that the frequency is above the threshold of the under frequency load shedding, one effective way is to share the spinning reserves among asynchronous AC grids through HVDC systems. In this paper, a coordinated frequency regulation scheme that enables spinning reserves sharing is proposed for such hybrid AC/DC grids. This scheme is a communication-free method that consists of the droop control, the inertia emulation control and the frequency safety control. The combined response of the three controls can increase not only the frequency nadir but also the settled frequency of the disturbed grid. In addition, the hysteresis comparators and the power injection limits are adopted in the frequency regulation scheme to reduce the coupling effect among AC grids. Thus, under small and harmless disturbance, the frequency regulation scheme will be inactive; the disturbance in strong AC grids would not affect the weak AC systems. The frequency regulation scheme coordinates well with the automatic generation control of the grids. The feasibility and effectiveness of the proposed frequency regulation scheme are verified through simulation studies. [ABSTRACT FROM AUTHOR]