查看更多>>摘要：To mitigate negative impacts of communication time delay, the DC voltage of offshore converters is usually employed as input signals of existing communication-free frequency support schemes of offshore wind farms (OWFs). However, the offshore DC voltage varies with the output power of OWFs, this distortion will deteriorate the frequency support performance. To address this issue, an improved communication-free coordinated control (ICFCC) scheme of voltage source converter based multi-terminal direct current (VSC-MTDC) integrated OWFs for frequency support is proposed. The ICFCC scheme involves the onshore frequency variation into the DC voltage deviation once frequency events occur, and an estimator is proposed to obtain the onshore DC voltage in offshore VSC stations to provide fast frequency support and reduce the distortion influence. Meanwhile, to obtain better support performance, the frequency support power is allocated reasonably by regulating droop coefficients adaptively, more power will be injected via the station closer to the disturbing bus. After frequency support, the offshore wind turbines will recover their rotor speed with asymptotic control to reduce the second frequency drop. Case studies are carried out on the 4-terminal and 5-terminal test systems, respectively, while considering the parameter uncertainties and noise disturbance. Different control methods are compared to illustrate the effectiveness of the ICFCC scheme.
查看更多>>摘要：Insufficient primary frequency control during severe disturbances requires the implementation of effective under frequency load shedding (UFLS) measures to prevent system collapse. The integration of non-dispatchable renewable energy sources like wind and solar, which are known for their variability, poses challenges to frequency management due to decreased system inertia. Consequently, there is a need for a comprehensive UFLS relay scheme tailored to renewable energy-integrated systems, aiming to enhance reliability and stability while minimizing unnecessary load shedding during under frequency events caused by unpredictable renewable sources. In this research paper, we present an optimal and robust multi-stage UFLS scheme designed for two operating points: base-load and peak-load. The suggested scheme considers different levels of wind power penetration and incorporates various contingency scenarios. The proposed UFLS scheme assumes that wind turbines are equipped with auxiliary controllers to actively participate in the power system frequency control task. To optimize the scheme, a genetic algorithm (GA) is employed, and the performance is evaluated through simulations using MATLAB and DIgSILENT/PowerFactory link. The results of the study demonstrate that the involvement of wind turbines in the frequency response significantly improves the performance of the UFLS scheme and reduces the total amount of load shedding required. This research provides valuable insights into addressing the challenges posed by renewable energy integration and offers a potential solution to enhance the reliability and stability of power systems in the presence of renewable sources.
Amir Haji-MohammadiMajid Sanaye-PasandSeyed-Alireza Ahmadi
693-704页
查看更多>>摘要：The fault current signal measured by digital relays contains the decaying direct-current (DDC) component and some higher-order harmonics in addition to the fundamental component. One important challenge of phasor estimation algorithms is the removal of the DDC component, as the presence of this component causes errors in the estimation of the fundamental component magnitude, which can result in malfunction of distance relays. To solve this problem, this paper proposes a new method based on solving the differential equations. Using this method, the fault current signal can be decomposed into minor components and its fundamental component can be properly extracted. After extracting the fundamental component, the discrete Fourier transform algorithm is used to estimate the current phasor. The proposed algorithm is firstly evaluated for different mathematical signals. Then, in the dynamic simulations performed using PSCAD/EMTDC software, the algorithm is employed as a phasor estimation function for a protection distance relay. Real fault data evaluations are also provided. In all cases, the algorithm is compared with some recent phasor estimation methods. The results of comparisons and evaluations confirm the speed and accuracy of the suggested algorithm in removing the DDC component and accurately estimating the fundamental component magnitude.
查看更多>>摘要：With the growing integration of renewable energy sources, Low Voltage (LV) distribution networks are experiencing increased demands to accommodate a diverse range of renewables. As a potential solution to alleviate the strain on existing distribution networks, the adoption of Low Voltage Direct Current (LVDC) distribution networks is under consideration. However, establishing reliable and selective protection techniques remains a significant challenge. Several protection algorithms focus on significant fault currents, fixed threshold settings, and high sampling frequencies for effective coordination, which often face limitations in low fault current scenarios or require costly data acquisition techniques. Therefore, this paper presents a novel centralized Post-Transient Derivatives (PTD) based protection scheme, which Leverages only the sign of Post-Transient Voltage Derivative (PTVD) and Post-Transient Current Derivative (PTCD). This approach addresses the selectivity constraints of existing voltage and current based solutions, particularly when lower sampling frequencies are applied. The proposed scheme is validated against various fault scenarios on an LVDC test network simulated in PSCAD/EMTDC and verified using an RT-Box based hardware test bed.
查看更多>>摘要：A ranking algorithm is proposed to aid the screening or initial assessment of different gases for their thermal interruption capability in high voltage gas-blast circuit breakers. The algorithm is directly based on the gas material properties without involving any complicated arc simulation. Despite that it does not explicitly consider the influence of product design, the ranking results based on the proposed algorithm show good alignment with laboratory obtained thermal interruption data of five gases and commercial-scale test data for three promising SF6 alternative gases. An in-depth analysis of the influencing factors and mechanisms responsible for the differing thermal interruption abilities of gases has been carried out, showing that the dominance of the arc column size near current zero in the arc cooling process is a natural consequence of the gas properties, especially the product of density and specific heat at constant pressure and electrical conductivity over the temperature range of 500K to 15,000K.
查看更多>>摘要：This paper introduces an evaluation method for gassing materials used in low-voltage circuit breakers and identifies key factors affecting arc characteristics. Firstly, sixteen types of gassing materials were prepared using PA6 and PA66 as matrices, with various flame retardants and reinforcements as additives. Breaking experiments were conducted in both low-frequency and direct-current circuits to assess these materials in terms of arcing and post-arc phenomena. Material characterization techniques were then employed to analyze the optical absorption and pyrolysis properties of the selected gassing materials. Finally, based on the physical mechanisms of interaction between arcs and gassing materials, a detailed mechanistic analysis and theoretical explanation were provided, integrating material characterization and microscopic modeling. The results suggest that the proposed evaluation method effectively identifies one or two materials with optimal overall performance from the 16 types tested. Superior materials are characterized by organic molecular fillers and high radiation absorption, which enhance arc energy absorption and increase ablation gas generation. Hydrogen-containing gases (e.g., H2, CH4, H2O) and gases with high dissociation energy (e.g., CO2, N2) are crucial for arc extinction. This paper also highlights two major challenges facing current gassing materials, suggesting areas for further research.
NETL
NSTL
IEEE