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  • Volume 48,Issue 16,2024 Table of Contents
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    • >Review·Perspective
    • Retrospect and Prospect of Artificial Intelligence for Electric Power System —From Domain Intelligence to General Intelligence

      2024, 48(16):1-17. DOI: 10.7500/AEPS20231226004

      Abstract (289) HTML (551) PDF 1.94 M (1107) Comment (0) Favorites

      Abstract:In the background of rapid development of new power systems, the deep coupling between massive multi-source heterogeneous information and diverse business brings significant challenges such as strong complexity and randomness in the power system operation. Concurrently, accelerating the construction of a flexible and intelligent new power system is a crucial strategy for energy development. There is an urgent need to establish a technology system of artificial intelligence for electric power system (AI EPS) that is intelligent, self-adaptive, and secure, in order to promote the intelligent transformation and development of the new power system. This paper reviews and summarizes the evolution and current research status of AI EPS technologies. It analyzes the technical framework, principles, and key technical methods for the new generation of AI EPS, which is based on pre-trained multimodal large models. The application schemes for power large model technology in the scenarios such as perception prediction, dispatching and control decision-making, and operation planning are proposed. The technical challenges and application bottlenecks faced by electric artificial intelligence based on power large models are discussed. Finally, the application of electric artificial general intelligence technology is summarized and prospected.

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    • >Basic Research
    • Analysis of Forced Low-frequency Oscillation in Single-stage Grid-connected Photovoltaic System Based on Virtual Synchronous Generator Control

      2024, 48(16):18-29. DOI: 10.7500/AEPS20240425008

      Abstract (142) HTML (209) PDF 2.58 M (537) Comment (0) Favorites

      Abstract:In the grid-connected photovoltaic system, the maximum power point tracking (MPPT) control will introduce massive interharmonics to DC-side voltages, which may cause forced oscillations. Traditional research focuses on the relationship between the frequency of disturbance sources and the frequency of the inherent weak damping mode of the system, with less attention paid to the effect of converter control characteristics on exacerbating the forced oscillation. This paper takes the virtual synchronous generator (VSG) control as an example to construct a power input-output frequency-domain closed-loop transfer function model of the single-stage grid-connected photovoltaic system. It is pointed out that the interharmonics introduced by the MPPT control will cause forced low-frequency oscillation in the single-stage grid-connected photovoltaic system based on the VSG control. Then, the proposed model is compared with the the forced oscillation analysis method based on the traditional second-order time-domain differential equation. The equivalence between the proposed closed-loop transfer function and the traditional second-order differential equation is verified. Thus, the influence of disturbance sources and converter control characteristics on the forced oscillation can be analyzed from the perspective of the frequency-domain input-output relationship. Finally, the influence mechanisms of the VSG overshoot effect and dynamic coupling effect of active and reactive power on the forced oscillation are analyzed. The characteristics of the influence of the VSG control overshoot and power coupling on the exacerbation of forced oscillation are verified by simulation cases.

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    • Low-frequency Oscillation Analysis and Damping Control for Grid-forming Converters Based on DC Voltage Synchronization

      2024, 48(16):30-39. DOI: 10.7500/AEPS20230826001

      Abstract (114) HTML (98) PDF 3.12 M (712) Comment (0) Favorites

      Abstract:The grid-forming converters based on DC voltage synchronization cannot only stabilize DC voltage but also realize the self-synchronization of converters. However, the interaction between DC voltage dynamics and control loops may cause low-frequency oscillation of the system. To study the mechanism of low-frequency oscillation, the “active power-power angle” model for stability analysis is built with consideration of the DC voltage synchronization control loop and AC voltage loop. It is found that DC voltage synchronization introduces more phase lag compared with active power synchronization, while the AC voltage loop also introduces phase lag in the loop. As a result, the system loop gain has the -180° phase crossing in the low frequency range with a high magnitude, thereby triggering the low-frequency oscillation issues. Based on the analysis of the low-frequency oscillation mechanism, two phase compensation methods for DC voltage synchronization control are proposed. Through reshaping the system loop phase in the high-gain-amplitude area of the low-frequency range, the damping control and oscillation suppression in the low-frequency range can be realized. Finally, the simulation and the experiment verify the correctness of the theoretical analysis and the effectiveness of the proposed methods.

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    • Iterative Solving and Morphological Analysis of Small-signal Stability Region for Hybrid AC/DC System with MMC-HVDC

      2024, 48(16):40-50. DOI: 10.7500/AEPS20230615004

      Abstract (61) HTML (122) PDF 1.94 M (521) Comment (0) Favorites

      Abstract:With the development of HVDC transmission technology, regional power systems will expand into large-scale hybrid AC/DC systems. Therefore, it is necessary to build a large-scale hybrid AC/DC system model that is able to accurately depict both AC and DC dynamics to study the dynamics and stability of the hybrid system. Considering the detailed dynamics from both AC and DC sides, this paper establishes a state-space model of the asynchronously interconnected hybrid AC/DC system with modular multilevel converter based high voltage direct current (MMC-HVDC). The specific analytic solution form is given and a multi-layer iterative trust-region algorithm (MITRA) is derived. The proposed algorithm builds the mapping relation between the points in the decision variable space and the equilibrium points. Combining with the boundary identification based on the Hopf bifurcation, this paper proposes a multi-iteration method for solving the small-signal stability region boundary. After verifying the model accuracy by using the Simulink simulation, the stability region of the hybrid AC/DC system is plotted with the generator output as the decision variable. By using the eigenvalues and participation factors, the mechanisms of DC instability and AC instability as well as the influences of control strategies and parameters on the region boundary are all quantitatively studied. The stability regions of the multi-region hybrid system and the AC system are compared and analyzed. The results show that the introduction of the converter station and the DC network may cause the stability region boundary of the multi-region system to present an approximate piecewise linear topology characteristic determined by multiple constraints.

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    • Transient Stability Analysis of DC Distribution System Considering Duty Cycle Limiter

      2024, 48(16):51-58. DOI: 10.7500/AEPS20240202003

      Abstract (68) HTML (49) PDF 2.07 M (1470) Comment (0) Favorites

      Abstract:Under the condition of high power disturbance, the limiter is prone to the saturation phenomenon of duty cycle, and the DC power distribution system will be degraded from closed-loop control to open-loop response and transient instability. Therefore, this paper carries out the transient stability analysis of DC power distribution system considering the duty cycle limiter. Firstly, this paper uses Sigmoid function to simulate the nonlinear characteristics of duty cycle limiter, and establishes a large-signal model of DC distribution system which is closer to the engineering reality. Secondly, the estimated attraction domain taking into account the influence of the duty cycle limiter is characterized by the Takagi-Sugeno (TS) fuzzy modeling method. Then, the variation of the estimated attraction domain with different control parameters reveals the transient instability phenomenon of the DC distribution system induced by duty cycle saturation. Finally, for the limitations of the existing transient stability improvement methods due to the duty cycle limiter, an optimization design method of control parameters is proposed to provide a scheme for the control parameter selection to improve the transient stability of the system. The effectiveness of the theoretical analysis is verified by the RT-Box hardware-in-the-loop experiment.

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    • Analysis on Characteristics of DC Dynamical Power and Frequency Response for AC-DC-AC Asynchronous-interconnection System Under Quasi-synchronous Control

      2024, 48(16):59-67. DOI: 10.7500/AEPS20230815004

      Abstract (45) HTML (52) PDF 2.70 M (507) Comment (0) Favorites

      Abstract:The asynchronous system interconnected by DC transmission can realize quasi-synchronous operation of the asynchronous system and achieve the goal of real-time sharing of frequency regulation resources by implementing additional control on the DC transmission. However, in the engineering practice of quasi-synchronous control, unreasonable control structure or parameter configuration can lead to unexpected power fluctuations in the system. First, a dynamical power and frequency response analysis model of the quasi-synchronous system is constructed based on the additional quasi-synchronous DC control. Then, the influence of DC control parameters on power oscillations is analyzed based on this model. Furthermore, the similarities and differences between power fluctuations caused by quasi-synchronous control and traditional angle oscillations are analyzed. Finally, the analysis and verification are conducted by using the power fluctuations that occurred during the quasi-synchronous experiment process, and recommendations for suppressing DC power fluctuations are proposed, along with a discussion on quasi-synchronous control.

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    • Frequency Response Estimation Method Considering Low Voltage Ride-through Recovery Difference of Wind Farms Under Large Disturbance

      2024, 48(16):68-78. DOI: 10.7500/AEPS20230914005

      Abstract (88) HTML (52) PDF 3.33 M (459) Comment (0) Favorites

      Abstract:With the increasing penetration rate of the wind power, multiple wind farms entering into the low voltage ride-through (LVRT) at the same time will bring significant impacts to the power system, resulting in temporary drop in active power output, rapid drop in power grid frequency and even the risk of triggering under-frequency load shedding. It is necessary to quickly estimate the frequency curve of the power grid under the temporary power disturbances to develop corresponding control measures. However, due to the difference in power recovery within wind farms, LVRT curves present different shapes. If this impact is ignored, there will be significant estimation errors in the frequency response curve, and traditional time-domain simulation based on detailed model has low efficiency. Hence, a disturbance power analysis method considering LVRT recovery difference of wind farms is proposed. The active power disturbances caused by single-unit wind turbine at each stage of LVRT are described by segment functions, and the total temporary disturbance power of the system is accurately estimated based on energy conservation principle. Considering the frequency regulation characteristics of the wind farm and the virtual inertia effect, an optimized model for system frequency response suitable for power girds with high-proportion renewable energy is proposed. The differental evolution algorithm is used to identify the parameters of the system transfer function based on actual data, and then the frequency response trajectory is fitted. The applicability and reliability of the proposed method in different scenarios are verified by the simulation of IEEE 39-bus system and simplified system for actual power grid

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    • System Frequency Response Model of Deloaded Wind Power System Considering Working Point Shifting

      2024, 48(16):79-87. DOI: 10.7500/AEPS20230826003

      Abstract (62) HTML (46) PDF 2.42 M (629) Comment (0) Favorites

      Abstract:With the increasing grid-connected capacity of wind power, using deloaded wind turbines (WTs) to participate in the primary frequency regulation is of great significance in safeguarding system frequency stability. However, the working point of the WT shifts with its rotational speed change, which results in the actual capacity of the primary frequency regulation of the WT smaller than the desired value. Therefore, the interaction between the power tracking control of deloaded WTs and the integrated inertia control after a frequency fault is firstly analyzed, and the basic principle that the working point shifting of the WTs leads to the weakening of the frequency regulation effect is pointed out. Secondly, the wind power-frequency transfer function is derived based on the rotor motion equation of the WTs, and the system frequency response (SFR) model is established considering the working point shifting of the deloaded WTs. The least square method is also used to reduce the order of the proposed SFR model, and an analytical formula of the system frequency nadir is deduced. Then, in view of the shortcomings of virtual inertia control, such as power backflow during the frequency recovery and noise amplification in differential links, it is proposed to replace the integrated inertia control with droop control. Meanwhile, in order to make the deloaded WTs obtain the desired frequency regulation effect, the control parameters of the WTs are modified based on the frequency nadir and the steady state frequency indices, and calculation methods for the two equivalent droop coefficients are proposed. Finally, a four-machine two-region model is established on the MATLAB/Simulink platform to verify the effectiveness of the proposed order reduction method and the correction method.

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    • Comprehensive Inertia Frequency Regulation Control of Variable-speed Pumped Storage Unit Considering Asymmetric Speed Boundary Constraints

      2024, 48(16):88-98. DOI: 10.7500/AEPS20230627004

      Abstract (48) HTML (147) PDF 2.35 M (400) Comment (0) Favorites

      Abstract:When the frequency of the power system drops, the variable-speed pumped storage (VSPS) unit can greatly reduce its speed and quickly release the rotor kinetic energy to provide inertia support for the system. However, the existing frequency regulation strategies fail to effectively take the advantage of the VSPS unit. In order to make full use of the rotor kinetic energy, a comprehensive inertia frequency regulation control strategy of the VSPS unit is proposed, which takes into account the asymmetric speed boundary constraints. First, based on the active power support characteristics and asymmetric speed constraints of the VSPS unit, the optimal frequency control structure suitable for the VSPS unit is derived, which includes two virtual inertia control links with different time constants. Then, the function of each inertia control link is analyzed and the related parameters are set. On this basis, considering the operation characteristics of the VSPS unit and the frequency regulation demands of the power system, the asymmetric comprehensive inertia control strategy of the VSPS unit is designed, through which the unit can provide greater inertia support to the system when the speed is reduced. Simulation results show that the proposed frequency regulation strategy can give full play to the advantages of the VSPS unit and maintain system frequency stability.

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    • Capability Assessment and Application of Fast Frequency Response Load Participating in Inertia Auxiliary Service

      2024, 48(16):99-108. DOI: 10.7500/AEPS20230825001

      Abstract (65) HTML (62) PDF 2.07 M (851) Comment (0) Favorites

      Abstract:The increasing proportion of renewable energy has led to the reduction of the power system inertia level. To prevent rapid frequency drop after fault in the low-inertia system, it is urgent to explore new inertia response resources. Fast frequency response load (FFRL) can flexibly adjust the load level to provide emergency support according to the rate of change of frequency after the power grid fault. An assessment method for FFRL inertia support effect is proposed based on the system frequency response model of power system with FFRL participation. An optimization model for FFRL participation in inertia auxiliary services is established considering the inertia service compensation mechanism of different market entities. The calculation results have verified the accuracy of the proposed assessment method and the improvement effect of the participation of FFRL in inertia auxiliary services on system operation economy and system frequency safety and stability. Compared with traditional thermal power units, FFRL can provide more inertia response with less response capacity, and flexibly transform according to its response strategy and control method, thereby more accurately responding to the grid requirements for its own inertia level.

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    • Credible Aggregation Optimization and Adaptive Decentralized Coordination Method for Large-scale Inverter Air Conditioning

      2024, 48(16):109-122. DOI: 10.7500/AEPS20231102004

      Abstract (53) HTML (65) PDF 7.63 M (447) Comment (0) Favorites

      Abstract:Inverter air conditioning (IAC) becomes an important flexibility resource for power systems due to its considerable potential of regulation, but its characteristics of massive volume, heterogeneity and privacy sensitivity make centralized regulation less applicable. The existing weak-centralization methods pay more attention to the improvement of computational efficiency, and do not consider the feasibility of the results and the adaptability of the disaggregation, which easily leads to the actual load distribution crossing the boundary and the difficulty of convergence. Therefore, based on the IAC physical model, the individual credible boundary is obtained by constructing the conservative energy scenario to ensure that the cluster scheduling curve is always in the adjustable range. Then, in order to distribute the load nimbly, an adaptive deaggregation strategy based on hierarchical distributed architecture is proposed, which enables the cluster to accurately track the scheduling target and minimize the utility loss in each period. The control process is lightweight and non-parametric, and the feasible sub-optimal solution can be obtained quickly with very few iterations. After the data is summarized by layers, the dimension is reduced, which avoids individual privacy disclosure and is more practical. Finally, in order to further tap the potential of real-time regulation, the IAC cluster is used to carry out three-phase load balancing, creating the application scenario, and through the “iterative adjustment” way to smooth the load and photovoltaic fluctuations, to achieve a complete unity of energy optimization, distribution and balance. The case results verify the efficiency of the proposed method and provide a new idea for flexible mining of massive IAC.

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    • Continuous Power Flow Considering Multiple Control Modes and Operational Constraints of High Proportion of Renewable Energy

      2024, 48(16):123-131. DOI: 10.7500/AEPS20231210002

      Abstract (61) HTML (113) PDF 1.83 M (519) Comment (0) Favorites

      Abstract:The voltage stability problem of power grids with high proportion of renewable energy is prominent, and continuous power flow is an important tool for voltage stability analysis. The current continuous power flow of power grids with voltage source converter (VSC) power supply does not fully consider the various control modes and operational constraints of the VSC, resulting in insufficient accuracy of voltage stability margin calculation. In this paper, a continuous power flow model and calculation method considering the output current constraint and internal potential constraint of the VSC with renewable energy integration, constant voltage control and reactive power-voltage droop control of grid-forming renewable energy and constant reactive power control of grid-following renewable energy are proposed, and the switching logic of control mode and bifurcation point type identification method caused by constraints are given. The results of a case with the modified IEEE 39-bus system show that the proposed model and algorithm improve the applicability of the traditional continuous power flow method to the static voltage stability analysis of power grids with high proportion of renewable energy.

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    • Optimal Scheduling Method for Integrated Energy Systems with Hydrogen Based on Deep Reinforcement Learning

      2024, 48(16):132-141. DOI: 10.7500/AEPS20240102002

      Abstract (169) HTML (122) PDF 3.23 M (511) Comment (0) Favorites

      Abstract:In order to achieve carbon reduction targets, the combination of hydrogen energy and integrated energy systems has become one of the most potential development directions. Aiming at the problems such as the insufficient flexibility of scheduling strategy of hydrogen integrated energy system and difficulty in solving multi-objective optimization of complex systems, an optimal scheduling method for hydrogen integrated energy systems based on deep reinforcement learning is proposed. First, the variable operation condition model of coupled equipment is used to construct a wind-solar-hydrogen-cooling-heat-electricity integrated energy system, and expand the joint energy supply space of equipment. Secondly, considering the system operation cost, carbon emissions, system self-supply balance and renewable energy utilization rate, a multi-objective optimization model is built based on the optimal solution distance to stimulate the exploration of the agent. Then, the deep reinforcement learning algorithm is optimized by time segment characterization to enhance the estimation accuracy of the system state change. Finally, a simulation case is designed based on the measured data of the source and load. The results show that the proposed method can effectively improve the scheduling flexibility of the hydrogen integrated energy system, fully tap the carbon emission reduction potential of hydrogen energy, and realize the dual optimization of scheduling economy and environmental protection.

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    • >Application Research
    • Remote Offshore Wind Power Transmission Scheme Based on Diode Rectifier Unit and Modular Multilevel Converter with Black-start and Grid-forming Capability

      2024, 48(16):142-153. DOI: 10.7500/AEPS20231118001

      Abstract (105) HTML (69) PDF 3.94 M (536) Comment (0) Favorites

      Abstract:Due to the characteristics of low cost, small size and weight, and high reliability, the transmission scheme using the diode rectifier unit (DRU) at the sending end and the modular multilevel converter (MMC) at the receiving end has attracted much attention in the field of offshore wind power transmission. Since DRU cannot form an AC power grid and provide reverse active power, this paper proposes a remote offshore wind power transmission scheme based on DRU-MMC with black-start and grid-forming capability. The proposed scheme adds conventional small-capacity voltage-source auxiliary converters, energy-dissipating units, and necessary fast switches on the basis of DRU-MMC transmission technology. With the multiplexing of DC marine cables and the topology reconstruction by the fast switch switching operations, the low-voltage circuit for the black-start power supply and the high-voltage circuit for wind power transmission are constructed respectively, and the online-switching between the black-start condition and the power transmission condition is realized. The grid-forming control strategies of the auxiliary converter in different operation stages are proposed to solve the formation of the offshore AC power grid, the reactive power compensation and harmonics suppression of the DRU. Moreover, the restriction of the DRU-MMC transmission scheme on the type of wind farm is eliminated by the proposed scheme. Finally, the superiority and the effectiveness of the proposed scheme are verified by means of economic analysis and case simulation.

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    • Topology and DC Fault Ride-through Strategy of Embedded Modular Multilevel Converter with High-/low-voltage Valve Connection

      2024, 48(16):154-164. DOI: 10.7500/AEPS20231116001

      Abstract (50) HTML (29) PDF 2.96 M (491) Comment (0) Favorites

      Abstract:In the ultra-high-voltage and long-distance DC transmission system using the half-bridge modular multilevel converter (MMC) with high-/low-voltage valve connection, the ultra-high-voltage DC circuit breakers required for fault clearance are immature and costly. Aiming at the above problems, this paper proposes an embedded MMC topology with high-/low-voltage valve connection. According to the basic principle of full-bridge sub-module operation, an external embedded bridge arm and an internal embedded energy transfer branch are designed to reverse the voltage polarity of the MMC with the high-voltage valve. The converter station of the half-bridge MMC can realize the non-blocking DC fault ride-through with a lower cost than that of the hybrid MMC. Based on the switching sequence of each branch and the switching time of different devices during fault clearing, the fault ride-through control strategy is designed. By analyzing the current and voltage stress of the device in each stage, the basis for device selection and number configuration is given. Finally, the effectiveness of the proposed control strategy is verified by PSCAD/EMTDC simulations. The device cost and operation loss of the proposed strategy are compared with the existing MMC schemes, which shows the proposed strategy is more economical.

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    • Fast Analysis Method for Power System Harmonic Resonance Based on Adaptive Step Sizes

      2024, 48(16):165-173. DOI: 10.7500/AEPS20231122003

      Abstract (56) HTML (44) PDF 3.76 M (480) Comment (0) Favorites

      Abstract:With the rapid development of power system, its scale is getting larger and the number of nodes is increasing. As the two most widely used harmonic resonance analysis methods, the frequency scanning method and modal analysis method urgently need to be improved on the computational efficiency. Based on traditional methods, the frequency scanning method based on the adaptive step size and the modal analysis method based on the adaptive step size are proposed, respectively. First, by setting corresponding criteria to adaptively change the scanning step size, the scanning interval can quickly reach the vicinity of the resonant frequency point, thus greatly reducing the computational cost. Then, the quadratic interpolation method is used near the resonant frequency point to determine the resonant frequency, thus guaranteeing the calculation accuracy of the resonant frequency. Finally, the IEEE 14-bus, IEEE 39-bus and IEEE118-bus systems are taken as examples for analysis. The results show that compared with traditional methods, the proposed two methods can greatly improve the calculation speed while ensuring the accuracy of the resonant frequency, verifying the accuracy and efficiency of the proposed methods.

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    • Transient Characteristics of Small Resistor Exit in Flexible Grounding System and Implications for Transient Faulty-line Selection Methods

      2024, 48(16):174-183. DOI: 10.7500/AEPS20230828003

      Abstract (71) HTML (55) PDF 1.65 M (448) Comment (0) Favorites

      Abstract:When a high-resistor grounding fault occurs in a flexible grounding system, the protection during the parallel stage of neutral point small resistor often refuses to activate, so it is still necessary to consider the impact of the transient characteristics of small resistor exit on the existing transient faulty-line selection methods. The exit of the parallel small resistor is accompanied by a certain transient transition process. For this process, a transient equivalent circuit for a single-phase grounding fault in a flexible grounding system is established. The characteristics of transient electrical quantities of the system when the parallel small resistor exits, the impact of different fault conditions on the amplitude of transient electrical quantities, and the constraint relationship between transient zero-sequence current and transient zero-sequence voltage are analyzed. The impact of transient quantities after the parallel small resistor exits on the stability of the transient faulty-line selection methods is clarified. When the small resistor exits during a high-resistor grounding fault in a flexible grounding system, the generated transient electrical quantities will cause misoperation in the transient amplitude comparison method, transient polarity selection method, and transient power direction method, but the transient zero-sequence current projection method for the line selection is correct. The simulation verifies the accuracy of the results.

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    • Time-domain Directional Components for Transmission Lines of Inverter-interfaced Renewable Energy Plant

      2024, 48(16):184-191. DOI: 10.7500/AEPS20230921005

      Abstract (60) HTML (46) PDF 1.87 M (435) Comment (0) Favorites

      Abstract:The transient fault characteristics of inverter-interfaced renewable energy (IIRE) may lead to the misoperation of existing directional component for distance protection with near-end faults of transmission line. Based on the characteristics of positive sequence reactive current output during IIRE faults, a fault directional criterion suitable for the transmission line of IIRE plant is proposed. By comparing the Euclidian distance between the calculated value of positive sequence reactive current at the installation of protection and the reference value, the forward and reverse faults are distinguished. The simulation results show that the proposed directional component can accurately judge the fault direction in the near area of protection in various fault scenarios

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