Abstract:In recent years, extreme natural disasters and potential geopolitical conflicts have seriously threatened the stable and reliable operation of distribution networks. Traditional dynamic recovery areas (DRA) overlook the recovery potential of vast grid following resources and fail to clarify the multi-action stages of multi-inverter coordinated operation. A heterogeneous DRA partitioned dynamic coordinated operation framework is proposed. Firstly, grid-forming, grid-following, and hybrid grid-forming and following DRAs are defined. Multi-key action stages for fault recovery, including partition establishment, mode switching, and grid synchronization, are introduced. Then, based on the operational requirements of each stage, a dynamic collaborative control method for distributed converters and soft open point is designed. In secondary control, voltage/frequency-free recovery and power sharing variables are introduced to achieve stable operation within the partition, while seamless interaction variables for smart switch (SSW) and grid-connection switches are introduced to ensure smooth cross-partition interaction. Finally, a simulation model is built in MATLAB/Simulink to verify the feasibility and effectiveness of the proposed framework and control design. The results show that the proposed method can establish DRA voltage and frequency, equally distribute internal resource power, flexibly respond to commands from SSW and grid-connection switch, fully exploit the coordinated recovery potential of heterogeneous resources, and enhance the operational resilience of the distribution network.

Abstract:Non-isolated flexible interconnected distribution networks as the research object are explored, focusing on the adaptability issue of typical grounding fault detection methods when single-phase grounding faults occur in AC systems due to zero-sequence transfer characteristics. Firstly, The zero-sequence equivalent topology network for non-isolated soft open point (SOP) is established, with transmission equations for zero-sequence components proposed to quantitatively analyzed their magnitude when transmitted to the non-fault side. The suppression effects of zero-sequence components are evaluated in various scenarios. Then, an adaptability analysis of typical protections is conducted for interconnected systems with low-current, low-resistance connections and hybrid low-current/low-resistance grounding methods. Results show that in non-isolated flexible interconnected systems, zero-sequence components transmission to the healthy side may cause protection maloperation or false alarms. Finally, a typical non-isolated flexible interconnected distribution network model is established in PSCAD to verify the adaptability analysis of protections in different interconnected systems, and the theoretical analysis is confirmed to be correct. The findings provide a theoretical basis for protection configuration in non-isolated flexible interconnected distribution networks.

Abstract:Flexible interconnection and dynamic power flow regulation in distribution networks are achieved by soft open point (SOP), optimizing the configuration and scheduling of distribution resources. To improve the load voltage during asymmetric ground faults, it is proposed that positive and negative sequence currents are output by the converter during faults. This provides voltage support and enhances fault ride-through capability. However, actual operating conditions make open-loop optimization schemes susceptible. Poor performance is shown by traditional negative sequence voltage-based closed-loop support. In this paper, the conventional negative sequence voltage outer loop is improved by controlling the amplitude of negative sequence current to suppress the negative sequence voltage. The phase of the negative sequence current is adjusted to optimize the suppression effect. To enhance the inverter's capacity utilization and limit active power fluctuations, a comprehensive scheme is proposed for limiting the positive and negative sequence current amplitudes. A short-circuit fault simulation is constructed using PSCAD simulation software, based on the actual parameters of the distribution network. The simulation results show that the proposed control strategy significantly enhances voltage performance during fault conditions, improving the low-voltage ride-through capability.

Abstract:With the continuous development and application of new energy power generation technologies, distribution networks are gradually becoming a hub platform with functions of transmission, distribution, storage, and trading. The operating pressure on distribution networks is increasing. Flexible interconnected devices (FIDs) provide a solution for interconnection of distribution networks, enhancing the power flow regulation capability and fault isolation capability of distribution networks. However, the integration of new energy power generation represented by photovoltaics, along with the retirement of traditional large-inertia synchronous generators, has led to increasingly prominent issues such as reduced overall grid inertia and poor frequency stability. The existing FIDs do not meet the demand for grid inertia support. To address the above issues, this paper proposes a solution based on energy storage-based FIDs combined with grid-forming control represented by virtual synchronous generators (VSG). On the basis of meeting the flexible regulation requirements of distribution networks, this solution realizes inertia support for the AC grid and reveals the intrinsic relationship between virtual synchronous generator control and droop control. Considering that the VSG strategy is prone to overcurrent during fault conditions, a feedforward suppression strategy based on virtual voltage construction is proposed. This strategy can achieve overcurrent suppression in FIDs under fault conditions through power feedforward and voltage feedforward. Finally, the effectiveness of the proposed strategy in power allocation control and low-voltage ride-through is verified through MATLAB/Simulink simulation.

Abstract:The flexible multi-state switch (FMSS), as a new generation of flexible interconnection device, enables flexible interconnection between different feeders in distribution networks by replacing traditional tie switches. However, the influence of zero-sequence current on the stable operation of the FMSS under asymmetrical fault conditions has not been fully considered in existing research on FMSS control mode transitions. This leads to AC oscillation in the DC voltage before the asymmetrical fault is cleared. To address this issue, a multi-mode control transition strategy based on zero-sequence suppression is proposed for flexible interconnection devices. Firstly, the smooth transition between PQ control mode and U_dcQ control mode during faults is improved by the introduction of a steady-state inverse model. Subsequently, the smooth transition among PQ control mode, U_dcQ control mode, and Vf droop control mode during faults is improved by the introduction of state tracking control. Finally, the impact of the zero-sequence current component on the DC voltage during asymmetrical faults is reduced by the introduction of a zero-sequence current suppression strategy based on a proportional resorant (PR) regulator. A three-port FMSS simulation model is established in MATLAB/Simulink to verify the proposed control strategy. Simulation results demonstrate that the proposed control mode transition strategy effectively reduces DC voltage fluctuation and AC voltage/current phase offset under various fault conditions.

Abstract:The busbar, serving as a critical power transmission component in power electronic converters, fulfills essential functions including interconnection of power devices, capacitors, terminals, and insulation. To mitigate parasitic parameters and device stresses, converter circuits must be integrated through busbars. This paper focuses on the ANPC topology composed of a 15 kV SiC metal oxide semiconductor field effect transistor (SiC MOSFET) and a series-connected 6.5 kV Si insulated gate bipolar transistor (Si IGBT), investigating optimized busbar design through dimensional arrangement, layer stacking sequence, and terminal positioning. A three-dimensional electromagnetic model of medium-voltage multi-device integrated busbars is established using finite element simulation software. Parametric analysis is conducted to optimize device spacing and layer structures, proposing a busbar layout strategy tailored for hybrid ANPC topologies. Simulation results demonstrate that the optimized design effectively reduces system parasitic while validating reasonable electric field distribution under high-frequency switching conditions. Experimental tests on a prototype platform confirm that the optimized busbar exhibits superior insulation performance at critical nodes and enhanced overall reliability compared to conventional designs.

Abstract:Low-voltage flexible direct currect (LV-FDC) technology represents a vital technical approach to addressing the issues of heavy overloading and power supply reliability in end-user distribution networks. The core technological foundation of LV-FDC systems lies in their efficient operation under steady-state conditions and rapid switching control during transient states. This paper analyzes and summarizes the typical decentralized networking modes of LV-FDC systems, encompassing system topologies, connection schemes, and voltage levels. Aiming at the heavy overloading issue in transformer zones caused by uneven spatial and temporal load distributions, a set of steady-state load balancing control logic is proposed. For the issue of system operation mode switching during fault transients, an adaptive switching control based on local voltage information is proposed, which reduces the system's dependence on communication and achieves seamless switching between operating modes through the design of the slave control unit's control loop. Furthermore, an integrated design for steady-state and transient operation control based on master-slave control is analyzed in the context of multi-terminal flexible interconnection applications, ensuring efficient coordination and reliable operation of the system under both steady and transient conditions. Finally, the correctness of all control logic strategies is verified through PSCAD time-domain simulations and RT-LAB hardware-in-the-loop experiments.

Abstract:The soft normally open point (SNOP) is gradually applied in distribution networks due to its flexible power regulation ability. However, owing to the access of a large number of distributed generations (DG), the regulation ability of SNOP is limited by line capacity constraints. A definition of the sensitivity of SNOP to the line active power margin regulation for distribution systems is proposed in this paper as an evaluation indicator for SNOP's regulation ability, and the allocation optimization model for SNOP is established accordingly. On this basis, two evaluation indicators, namely node voltage margin and line power margin, are introduced to construct a distribution network operation optimization model with the objective of maximizing comprehensive operation margin. The model is converted to a second-order cone programming model, which is efficiently solved using MATLAB. Finally, the proposed model and solution method are validated on modified IEEE 33-node system, further indicating that the proposed method can maximize the regulating ability of SNOP, and the optimized control strategy can achieve operational safety and economy of the distribution network.

Abstract:To address issues such as reverse power flow, voltage fluctuations, and insufficient power supply capacity resulting from the increased penetration of distributed generation in distribution networks, an optimized control strategy for active distribution substations incorporating a three-terminal intelligent soft open point (E-SOP) for energy storage is proposed. Firstly, the topology of E-SOP is thoroughly analyzed, and a corresponding mathematical model is established to lay the foundation for subsequent optimization control. Secondly, a site selection and planning model for the E-SOP based on voltage-power sensitivity is proposed to determine its optimal installation location. On this basis, a multi-objective optimization model is developed, aiming to minimize comprehensive costs and voltage deviations, to configure the capacity of the E-SOP. This model is transformed into a second-order cone programming model using conic relaxation techniques and solved iteratively through a particle swarm optimization algorithm. Finally, simulations on an IEEE 33-node flexible interconnected system validate the effectiveness of the proposed strategy, and further simulations on an IEEE 69-node system confirm its applicability and superiority. The results indicate that, compared to traditional systems without E-SOP interconnection, the proposed strategy reduces voltage deviation by 2.24%, daily average system losses by 50.41%, and overall costs by 21.74%, making it suitable for distribution systems of various scales.

Abstract:With the increasing load and a large number of new energies connected to the grid, the flexible DC interconnection system is widely used. The flexible DC interconnection system based on sagging control strategy can realize automatic regulation of power and voltage according to sagging characteristics, but the transmission capacity of the system is limited by the capacities of different converter stations and requirement of voltage quality. So it is difficult to make full use of the transmission capacity of the system. An adaptive sagging control strategy for capacity optimization of flexible DC interconnection systems is proposed in this paper. By the proposed strategy, a sagging coefficient correction loop is added to gradually adjust the sagging coefficient of the corresponding converter station when the voltage or power is close to the limit. By the correction loop, the power can continue to increase without exceeding the limit. In addition, in order to reduce the influence of droop coefficient change on the traditional control and optimize the energy consumption characteristics of the system, a mathematical model is established with the goal of minimizing the adjustment rate of the power flow and the energy consumption of the power generation of the system. Then the non-dominated sorting in genetic algorithm (NSGA-Ⅱ) is used to solve the model and the optimal correction gradient of each converter station can be obtained. Finally, PLECS is used to build a four-terminal flexible direct interconnection model which verifies the effectiveness and feasibility of the proposed strategy.

Abstract:To address the stable operation issue of AC/DC hybrid microgrids in islanded mode, a unified droop control strategy based on soft normally open point (SNOP) is proposed. This strategy utilizes a normalization method to combine the frequency droop characteristics of the two feeders with the DC voltage droop characteristics, based on the power balance relationship of the system. The system's operating state is determined by the AC frequency and DC voltage, and SNOP uniformly adjusts the frequency of the AC subgrid and the voltage of the DC subgrid. This ensures that the AC and DC microgrids can evenly share the total power change of the hybrid microgrid, maintaining the AC frequency and DC voltage within the permissible range of the system. Additionally, to prioritize the nearby integration of distributed energy resources and avoid unnecessary actions of SNOP, a deadband adjustment is designed. Then, the AC and DC microgrids can utilize distributed energy resources within each subgrid, thereby reducing the frequent interaction of SNOP on both the AC and DC sides. Finally, the effectiveness of the proposed unified droop control strategy is verified by MATLAB/Simulink simulations.

Abstract:The high voltage alternating current/direct current (HVAC/DC) system has a complex structure and a wide variety of components, so the reliability assessment using traditional methods bears huge computational burden. To improve the efficiency of HVAC/DC system reliability assessment, a HVAC/DC system reliability evaluation method based on an improved point estimation method is proposed in this paper. The proposed method analyzes the structure of various components of a typical HVAC/DC system and leverages the Dirac function to smooth the involved discrete random variables, so that the discrete component status can be generally tackled in common with the load, wind power and other continuous random variables using the point estimation method. On top of that, a non-linear program of minimum load shedding is embedded into the point estimation method to efficiently summarize kinds of reliability indices via finite-time optimal power flow calculation. Finally, a modified HVAC/DC test system based on the IEEE 14 system is utilized to demonstrate the usability and limitations of the proposed method. The results show that the proposed method can effectively reduce the calculation amount of HVAC/DC system reliability evaluation under certain conditions, and has good accuracy and efficiency performance.

Abstract:The five-phase induction motor, known for its simplicity among multiphase induction motors, offers benefits such as low noise, minimal torque ripple, and high load capacity, rendering it suitable for applications demanding high power and reliability. To counter the vulnerability of speed encoders to external disturbances, a model reference adaptive system (MRAS) without speed sensor vector control system is devised for five-phase induction motors to enable speed identification and achieve sensorless control. Subsequently, a dual-parameter identification system is developed based on MRAS to address the dependency of the traditional MRAS's magnetic flux reference model on stator resistance values, allowing for the simultaneous identification of speed and stator resistance with adaptive rate calculations for each parameter. The simulation of this dual-parameter identification system using Simulink demonstrates excellent speed identification performance across a range of speeds and load torques, with minimal overall identification errors and steady-state fluctuations of around 9.7 r/min. When encountering abrupt changes in stator resistance under transient load conditions, the identification error remains below 5%, showing errors of 8.7% at half-load and 13.7% at full-load.

Abstract:Thyristor controlled phase shifting transformer (TCPST) is a new type of power flow control equipment. Due to its series and shunt coupling topology and special connection to the power grid, it is necessary to consider the handling strategy in case of power grid faults. Based on the structure and principle of TCPST, the influence of grid fault on TCPST is analyzed in this paper. It is clarified the distribution of fault current and characteristics of overcurrent and overexcitation within TCPST. On this basis, the control strategy of TCSPT for deal with grid fault impact is proposed, and the fault state identification criterion and control sequences for the TCPST is constructed. By identifying the faults within and outside the zone of TCPST and the recovery state of power grid faults with the electrical quantities, the out of service and auto-restart strategy for TCPST are realized. The temporarily withdraw method for TCPST is used to protect equipment safety in the event of severe power grid faults. Finally, some simulations are conducted for the proposed fault ride-through strategy of TCPST. It is shown that the reliable ride-through of TCPST during power grid faults can be achieved by using the strategy, which has a good application for feasibility and effectiveness of TCPST. It is benefit to take advantage of the potential of power flow regulation function of TCPST.

Abstract:In the conventional high voltage direct current transmission system with large-scale new energy access, when commutation failure or DC blocking occurs at the receiving end, the reactive power surplus at the sending converter station may cause overvoltage in the AC system. In order to suppress the overvoltage at the sending end, an overvoltage suppression device is proposed based on the principle of controllable energy dissipation. Firstly, the overvoltage mechanism in weak AC system under the fault of DC system is analyzed, and the topology and working principle of the controllable energy dissipation device are proposed. Secondly, the suppression effect and electrical stress of the controllable dissipation device on AC system overvoltage under typical fault conditions is simulated and studied. Then, the design of the controllable dissipation device is carried out, focusing on the potential distribution of the arrester and the the static and dynamic characteristics of electromagnetic repulsion of the trigger switch under different coil turns, the entry and exit control strategy of the dissipation device is proposed. Finally, the experimental verification of the energy dissipation device prototype is carried out, proving the correctness of the design. The energy dissipation device developed is successfully applied to the ±800 kV Lu-Gu high voltage direct current transmission project, and the operation results show that the controllable energy dissipation device can effectively suppress of AC overvoltage.

Abstract:The large-scale integration of massive heterogeneous distributed new energy has brought great challenges to the safe and stable operation of power system. It is of great significance to carry out research on operational risk assessment considering the uncertainty of distributed new energy. Firstly, the key influencing factors of the dynamic response characteristics of the distributed new energy equivalent model are analyzed, and the online dynamic equivalent modeling method of distributed new energy is proposed to determine the comprehensive load model and the state of distributed new energy that meet the requirements of online risk assessment. Then, according to the output characteristics of distributed new energy and multiple types of security and stability impact factors, clusters are divided, and the number of deterministic scenarios participating in safety and stability analysis is reduced by reducing the dimensionality of uncertainty variables. On this basis, the control measures and risk indicators that meet the requirements of safe and stable operation are calculated for different confidence intervals, and the different consequences of high probability low loss and low probability high loss scenarios on the power system are quantitatively evaluated. Finally, the application case results of actual power grid show that the risk assessment method proposed in this paper not only improves the risk assessment accuracy and calculation efficiency of the power grid with distributed new energy, but also can more comprehensively reflect the real-time operation risk characteristics of the system.

Abstract:Aiming at the transient overvoltage problem in the new energy high-penetration area of the feeder due to DC bipolar blocking fault, a converter bus reactive power allocation capacity optimization scheme is proposed with the constraint that the transient overvoltage of the wind farm with the lowest safety margin in the feeder system is suppressed to below the target value. On this basis, a cutting scheme for suppressing transient overvoltage in the feeder system under bipolar blocking faults is proposed, which realizes the two-stage voltage stabilization control of the feeder system by prioritizing the cutover of wind turbine generators without high-voltage ride-through capability after the occurrence of faults. Finally, a simplified model of ±800 kV Tianzhong DC transmission system and its feeder grid is constructed based on the DIgSILENT simulation platform. The simulation results show that the proposed reactive power optimization allocation scheme and machine-cutting scheme can reduce the transient overvoltage of the target node by 0.12 p.u., and the accuracy can reach 95.56%. The proposed scheme has significant effect in suppressing the transient overvoltage caused by DC blocking, which can effectively prevent the new energy from large-scale chain off-grid and improve the stability and reliability of the system.

Abstract:Power to gas (P2G) technology converts electricity energy into natural gas, playing an important role in achieving a low-carbon economic dispatch in an integrated energy system. To solve the problem of underutilized oxygen in the P2G process and further reduce carbon emissions, a low-carbon integrated energy system that considers P2G oxygen-enriched improvement and combined solar energy utilization, handling carbon reduction from the following aspects is proposed. Firstly, the oxygen produced by P2G is mixed with CO₂ as an assistant combustion gas, and P2G utilizes captured CO₂ to produce natural gas for gas-fired units. Secondly, because the boiler efficiency is affected by the oxygen concentration, the optimal oxygen supply state for each period of the oxygen-consuming equipment is determined by a combined algorithm of genetic algorithm and Gurobi solver. Finally, the solar energy efficiency is improved by using mixed solar energy to reduce the use of fossil energy. By introducing oxygen-enriched and combined solar energy into the integrated energy system, a low-carbon economic operation model of the integrated energy system is constructed, and scenarios are set for comparison and verification. The simulation results show that the CO₂ emissions are reduced by 75.83% compared to the oxygen-enriched improvement before, and the total solar energy output is increased by 9.79% compared to the scenario without combined solar energy, indicating that the proposed model can effectively reduce carbon emissions and operating costs.

Abstract:The catalytic synthesis of methanol from hydrogen and carbon dioxide is the key to solving the technical problem of "production, storage, transportation, addition, and utilization" of hydrogen energy. This study highlights a mathematical model of generalized energy storage for methanol synthesis. It focuses on the design of a layered optimization and control scheme for a virtual power plant, focusing on source-network load-storage. The upper layer of the plant encompasses that of generalized energy storage for use with wind power and photovoltaics proposed, which can track several factors. Such factors are generation scheduling, the introduction of virtual power plants to consumers of the master-slave game theory, the use of genetic algorithms to hone the price of electricity at different times of the day, and responses to user demands, thus reducing the net load peak and valley differences. The lower layer is comprised of external power trading as the ultimate guarantee. This shall be combined with generalized energy storage to achieve balance between power supply and power demand, especially the Sinh Cosh optimization algorithm, to enhance the source storage and ensure that the virtual power plant operates under low carbon conditions. Finally, compared to different schemes, the proposed scheme can effectively increase the level of renewable energy consumption and promote regional decarbonization. Also, the proposed scheme can improve the comprehensive operational efficiency of virtual power plants.

Abstract:A high penetration photovoltaic distribution network contain numerous voltage-sensitive power electronic devices, which effect with each other, increasing the computational complexity of the node grid-connected critical capacity. Therefore, an analytical method for calculating the grid-integration capacity of distributed photovoltaic devices at feeder injection points is proposed, using a single-port reduced-order model, while analyzing the interactions effect between connected devices and the injection point. Firstly, based on the current injection balance equation of the distribution network, the grid characteristic function is defined to characterize the nonlinear relationship between the current and voltage at the infeed point. A single-port reduced model is used to simplify the grid characteristic function to reduce the computational complexity. Then, the equivalent parameters of the model are calculated by selecting some specific cross-sections of the grid characteristic function. The grid-connected critical capacity expression at the infeed point and quantify the interaction effect is analytically derived. Finally, a case study on the IEEE 33-node distribution network shows that the grid-connected critical capacity is improved to a greater extent when the photovoltaic equipment is connected to the adjacent location of the infeed point. Among the different distribution network topologies, radial distribution network has the strongest effect of access equipment with the infeed point, followed by the ring distribution network and the smallest in the two-terminal power distribution network.

Abstract:Traditional model predictive current control has been widely studied in the field of grid-connected inverter control due to its rapid response and multi-objective optimization capabilities. A data driven model-free predictive current control strategy is proposed to address the problem of control performance degradation caused by parameter mismatch in traditional model predictive current control. Firstly, the weighted average current method is used to reduce the order of the third-order LCL filter system, suppressing oscillations caused by LCL resonant frequency. Then, the ultra local model is adopted to simplify the traditional predictive current model, and a linear extended state observer is designed to estimate and compensate for system disturbances, thereby improving the accuracy of current prediction. Finally, the recursive least squares method is used to update the system model online based on system operating data, reducing the dependence of the control system on parameters. The simulation and hardware-in-the-loop experimental results demonstrate that compared to traditional model predictive current control, the proposed control strategy exhibits strong robustness and good steady-state performance under parameter mismatch conditions.

Abstract:Variable speed pumped storage units have obvious advantages in efficiency and adaptability under various operation conditions compared with traditional fixed-speed pumped storage units. Since the full-power converter is the key equipment in variable speed pumped storage units, it is of great significance to study the application of various inverter topologies in large capacity variable speed pumped storage units. Taking a 100-MW-scale variable speed pumped storage unit as an example, based on the existing devices, an evaluation index system is constructed from the aspects of cost, loss, volume, etc., and entropy method is used to weight the indexes. Four topologies are compared and analyzed in the design of the high-power variable speed pumped storage units with access voltage of 13.8 kV, including back-to-back three-level neutral point clamped (NPC) converter, back-to-back five-level stacked multicell converter (SMC), back-to-back modular multilevel converter (MMC) and modular multilevel matrix converter (M3C). The results show that the cost of back-to-back MMC is the lowest, the overall efficiency of M3C is the highest, and the occupation space of M3C is the smallest. In summary, back-to-back MMC and M3C are two preferred solutions for the full power converter of 100-MW-scale variable speed pumped storage units.