Fast and accurate DC fault-current evaluation is crucial for the design and protection of multi-terminal MMC– HVDC grids, especially under low-impedance metallic pole-to-pole (P2P) short circuits with steep current rise. This paper proposes a semi-analytical EMT-oriented fault calculation method for an n-terminal DC grid in which a metal-oxide varistor (MOV) is connected in parallel with the outlet smoothing reactor to realize source-side energy suppression. In this paper, the term shunt-MOV reactor denotes an outlet smoothing reactor whose terminals are shunted by an MOV. The key challenge is to embed the nonlinear MOV-shunted smoothing reactor into a sparse nodal matrix framework without sacrificing computational efficiency. To this end, an odd-symmetric MOV characteristic is represented by a piecewise-affine PWL companion, where both the conductance term and the affine current-injection term are explicitly stamped into the EMT nodal equation. A one-shot (iteration-free) segment selection rule based on a predictor voltage is further adopted so that each time step requires only one sparse linear solve, with an optional single corrective reselection when needed. The method is specialized to a three-terminal star MMC–HVDC grid with bipolar stacking and a near-end low-impedance metallic P2P fault. EMT validations confirm that the MOV-shunted smoothing reactor effectively limits the fault current and reduces its rise rate, and that the proposed semi-analytical calculation closely matches the EMT benchmark for the limited fault current. The verified source-side limiting capability suggests a reduced energy stress on downstream DC protection devices, such as DC circuit breakers, pending device-level sizing assessment.

In this paper, grid-forming and reduced-order modeling of a novel pumped hydro energy storage-integrated wind power generator (PHES-WPG) is developed and validated. The PHES-WPG makes use of Hopf oscillator-based DFIG grid-forming and mirror-principle-based reduced-order modeling approaches. For the pumped-hydro storage, the wind and water towers in hilly regions are combined as upper reservoir, and the nearest lake is used as lower reservoir. Rather than directly connected to the grid, the doubly fed induction generator (DFIG) rotor-side AC-DC-AC system is taken as a conversion system of permanent magnet synchronous machine (PMSM)-based hydraulic storage system. By utilizing DFIG kinetic energy and installing supercapacitors at DC-links, combined matching and Hopf oscillator controls are further developed to enable better amplitude-phase motion of the GFM-DFIG. This leads to the self-synchronization of GFM-DFIG with external power grid and facilitates its faults-ride through capacity. Furthermore, a reduced-order modelling approach of the PHES-WPG based on the mirror principle is developed to analyze the inertia and primary frequency responses in a convenient manner. To accelerate the simulation speed, a comprehensive transients-dynamics simulation framework of the PHES-WPG is implemented. Through the comparisons of variant approaches and parameters, the effectiveness of the advanced GFM and reduced-order modelling method is validated. The advantages of the proposed PHES-WPG are verified via diverse studies that include tolerated fault currents, scheduled dispatching capacities, primary frequency regulations as well as simple economics analysis.

Nitrogen/tetrafluoromethane (N2/CF4) has emerged as an effective refrigerant mixture, which plays a significant role in various high-Tc superconducting (HTS) power devices and energy systems. Several studies have documented the superior performance of such zeotropic binary mixtures in heat transfer and energy conservation compared to pure liquids. However, boiling heat transfer characteristics of liquid mixtures on the molecular scale are not fully understood, especially the mechanisms associated with the role of the additive in regulating thermal properties. Here, we performed the molecular dynamics simulations for the pure N2 and the mixtures containing 20 and 40 mol% of CF4 to probe into the boiling heat transfer process on an ideal copper substrate. Analyses suggest that the boiling process of the N2/CF4 mixture shares similar features with the pure N2, but the binary refrigerant manifests advantages at higher substrate temperatures. Specifically, the additive CF4 delays the onset of film boiling, and the operational temperature range could be enlarged by almost 40% compared to pure N2 in terms of the 40 mol% mixture. Moreover, the mixture with CF4 additive maintains a small interfacial resistance even if the substrate temperature exceeds the critical value of film boiling for pure N2, highlighting the potential to use such mixtures for devices that may suffer from high heat flux levels. Finally, analyses regarding the mixture composition and the solid-liquid interactions confirm the essential role of the additive CF4 in mitigating the mismatch of the vibrational density of states at a high substrate temperature, which may reveal the mechanisms concerning the CF4 improving the heat transfer performance of the original N2. These findings provide a better understanding of the advantage of N2/CF4 at high substrate temperatures, and they lay a foundation for designing the cooling systems of HTS apparatuses.

The superconducting fault current limiter (SFCL) improves the power grid safety by restricting the drastically increased current when the circuit fault occurs. However, the SFCL suffers from a long recovery time and can hardly recover to the superconducting state before the breaker attempts to reclose, which hinders its practical application. This paper presents the implementation of the rapid-recovery SFCL with efficient heat dissipation even if encountering a large fault current. This is enabled by thermal performance regulation of the working coolant through adding tetrafluoromethane (CF4) into the liquid nitrogen (LN2). The proposed mixed coolant features a high critical heat flux that suppresses the onset of film boiling. Controlled experiments were performed with the fabricated current-limiting coil immersed in the N-2/CF4 mixed coolants containing 0, 20 and 40 mol% of CF4. Measured results regarding both quenching and recovery processes are reported, demonstrating an 85.4% reduction in recovery time for the 40 mol% composition compared to the conventionally used pure LN2. Analyses of the electrical parameters of the coil verify its effective quenching performance in the proposed immersion cooling system with an overall current limiting rate of over 55%. These findings lay a foundation for designing the rapid-recovery-type SFCL and facilitating its application in power systems.

In this paper, a Hopf oscillator-based weakly nonlinear oscillator is proposed to present its electromagnetic model, and then the matching control analysis framework is given from viewpoint of large-scale power system level. As internal core, the Hopf oscillator considers angular frequency and voltage amplitude as inputs, and it is used to emulate sinusoidal waveforms of electromagnetic transients. Correspondingly, approximated analytical solution of the oscillator and precise control are derived. Furthermore, angular frequency as output is generated by inertia motion, and voltage amplitude is emulated by virtual generator exciter. On this basis, the matching control framework is presented including inertia and primary controls and so on. Finally, the proposed control and matching approaches are verified by employing isolated IEEE 9-bus power system with 100 % grid forming inverters.

Nitrogen/tetrafluoromethane (N2/CF4) has attracted wide attention for its reliable dielectric performance and potential to be utilized as a coolant and insulator of power apparatus. Several studies have investigated the basic properties of N2/CF4, but dielectric responses of such a mixture under extreme electric fields are not fully understood, primarily on a molecular level. Here, we modeled the N2/CF4 complex and calculated the molecular properties under different external electric fields through computational quantum chemistry methods. Results suggest that the electric-field-induced polarization can be apparently observed in both positive and negative direction cases; the deformed electronic structure leads to an increased dipole moment and a narrowed energy gap. In addition, the molecular vibrational modes are sensitive to external electric fields, resulting in the splitting and shifting of IR spectra. Moreover, the external electric field also affects the excited-state properties of N2/CF4, and the emerged absorption peaks under extreme electric fields were analyzed in detail. These findings provide a better understanding of the N2/CF4 properties under external electric fields and lay a foundation for designing the electrical insulation systems of the power apparatus.

Liquid nitrogen/tetrafluoromethane (LN _2 /CF _4 ) mixture with a wide liquefied temperature range of 50 to 100 K might be a promising coolant for high- T_c superconducting (HTS) apparatus. However, certain features have not yet been clarified before further application, particularly the insulation properties regarding the composite insulation system consisting of such a cryogenic mixture and polypropylene laminated paper (PPLP). In this paper, AC and DC breakdown experiments and simulations of LN _2 /CF _4 - PPLP composite insulation system are carried out with various molar fractions of each constituent part taken into account using sample cables insulated by the foregoing composite insulation. Results indicate that such a mixture/PPLP composite insulation system possess a superior breakdown strength compared to pure LN _2 /PPLP system; LN _2 /CF _4 can therefore be an advantageous choice for HTS power apparatus.

以交联聚乙烯(XLPE)绝缘电缆为例,分析了绝缘介质微观结构对载流子在不同电场下运动规律的不同影响,讨论了交/直流电缆绝缘介质中的电场分配机制,在综合考虑微观载流子与陷阱的耦合机制中,进一步讨论了影响绝缘稳定性与电缆运行稳定性的主要因素.基于此,总结了交/直流电缆绝缘及其运行特性的主要区别,结果表明:电缆绝缘介质微观载流子在工频交流电场和稳恒电场下的不同响应,决定了交/直流电缆绝缘特性和运行特性的显著差异,空间电荷是影响直流电缆绝缘特性的主要因素,而局部放电是交流电缆绝缘老化的主要影响因素.该研究为交/直流电缆绝缘在结构设计中的不同技术考量以及电缆的运行维护等提供了理论依据.

The cooling of cryogenic liquid fuel in a superconducting cable is an important technical means to realize efficient energy and power transmission. However, there are only few types of cryogenic liquid fuels that can maintain the superconducting strip below the critical temperature, and little is known about the insulating properties of fuels other than liquid hydrogen. Besides, cryogenic liquid fuels are flammable and explosive, and a safe operating procedure is essential to conduct the discharge tests. Based on the trinitrotoluene equivalent method, the safety distance of the discharge test is determined, and the safety precautions are summarized in this article. Taking the molar ratio of methane to propane as 3:1, the breakdown and flashover voltages of liquefied small molecular alkanes were studied, and the test of long-time withstand voltage was also carried out. At last, the discharge parameters and waveform, as well as the surface state of the polymer after flashover in the fuel, were analyzed. The breakdown field strength of cryogenic liquid fuel is 22.9 kV/mm, and the flashover field strength is 15.9 kV/mm. The reduction of breakdown strength is not found in the test of long-time withstand voltage. Therefore, it has some advantages for the liquid fuels to directly cool the superconducting strips from the cost and electrical performance. This study aims at the superconducting energy pipeline cooling by liquefied natural gas, and it can also provide technical references for the electrical property and safety research of other fuels.

Resistive type superconducting fault current limiter (R-SFCL) is one of the most promising candidates to suppress fault currents in power grids. However, the violently boiled cryogen liquids, commonly liquid nitrogen, occurred due to fault currents, poses a crucial cryogenic insulation challenge for R-SFCL. Here we report the design and development of a Cryogenic Insulation Supporter Module (CISM) for a 220 kV R-SFCL. First, we present the design and test of the prototype Cryogenic Insulations Supporter (CIS). The developed CIS passed the high voltage tests under operating conditions. Following on this, we then present the design of CISM, which consists of 14 CISs, with the length and outer diameter of 12 m and 2.5 m, respectively.

In a cryogenic liquid medium, shock waves will be formed by the expansion of compressed gas that has been vaporized due to the injection of a large amount of energy. This may severely threaten the reliability of high-Tc superconducting (HTS) apparatus. Several studies have documented the destructive forces of evaporating nitrogen produced by arc energy. However, the properties of and propagation mechanisms relating to pressure waves in liquid nitrogen have yet to be understood. The aim of this study was to clarify the evolution of pressure waves within an enclosed pipeline and reveal the effects of several factors such as the pipe size and the injected energy on the shock-wave impact using explosion dynamics simulations. The results provide evidence for the strengthening of shock waves due to multiple reflection and superposition. In addition, analysis of the pressure impulse and effective strain reveals that, in the case of moderate injected energy, the overall shape of the inside wall of the pipe will remain unchanged except at the points closest to the explosion center; in contrast, the ends of the pipeline may suffer from more severe deformation. Finally, the calculations suggest that the shock-wave impact increases almost linearly with the injected energy, and in logarithmic coordinates, the pressure is inversely proportional to the explosion distance. These findings provide a better understanding of the characteristics and propagation patterns of shock waves in liquid nitrogen, and they lay a foundation for evaluating the safety of HTS cables and energy pipelines.

Renewable energy sources (RESs) are generally connected to the grid through power electronic interfaces, which generate electrical power instantaneously with little inertia. With the increasing penetration of RESs, the grid will gradually develop into a low inertia and underdamped power system, which results in serious grid frequency stabilization problems. The virtual synchronous generator (VSG) is an emerging technology that mimics the operation characteristics of traditional synchronous generators (SGs). Virtual inertia and damping are therefore introduced, which help to stabilize grid frequency. This paper gives a comprehensive overview of the VSG. The basic operation principle of VSG is introduced and analyzed in depth. The key issues related to VSG are summarized and discussed, including hardware configuration, software control strategies, energy supporting methods, and typical applications.

The insulation failure is a bottleneck to restrict the safe operation and development of direct current (DC) superconducting power devices. The solid-liquid flashover test is an important evaluation index, but the previous researches mostly focus on engineering application, and less involve in the mechanism analysis. As a characterization method for defects existing in the dielectric, the trap has been applied to the study of flashover to occur polymer surface in liquid nitrogen (LN2) in recent years. However, the mechanism of trap action in solid-liquid flashover is still confusing. Therefore, this paper carries out the following research. Firstly, the flashover voltage and current of glass epoxy laminate sheet (G10) in LN2 were obtained through experiments and analysis using the Weibull model in this paper. Secondly, the flashover process of G10 was monitored. The changes of G10 surface morphology and chemical bond before and after flashover were also observed. Finally, the relationship between polymer flashover and trap in LN2 was analyzed based on the surface trap distribution test and theoretical calculation of the charge transport equation. The results are beneficial to guide the selection and performance improvement of dielectric materials and the design of insulating structures in superconducting devices.

PID control is the most mature control method in classical control theory. PID control is widely used in the electromagnetic bearing control system. However, how to design PID control parameters to reduce the adjustment time and overshoot of electromagnetic bearings after disturbance is still an issue that needs to be further investigated. In this paper, the influence of "natural" stiffness and "natural" damping on system stability is evaluated to select the value range of P and D control parameters, and the value range of the I parameter is determined by using the Rouse criterion. Based on the genetic algorithm with a novel fitness function considering both the system phase margin and closed-loop bandwidth, the PID controller parameters are optimized. The simulation results show that the overshoot and the adjustment time of the genetic algorithm optimized PID controller are much smaller than that of the conventional method.

Simulation concerning the field ionization and streamer propagation processes in liquid nitrogen under pulsed voltage is performed to investigate the discharging characteristics and the factors that may influence the discharging process. In order to have a better description of field ionization in liquid nitrogen, a computational quantum chemistry method is utilized for obtaining the relations between the ionization potential of nitrogen molecule and the electric field. Results indicate that the pre-breakdown process could be divided into four stages which are initiation, propagation, re-ignition and annealing. In addition, the temperature in liquid nitrogen during the discharging process is investigated which shows that the temperature of liquid nitrogen in a small region near the needle electrode would reach the global maximum close to 120 K whereas along the discharge path there exists an area where the temperature of liquid nitrogen would exceed 85 K after 250 ns under 200 kV pulsed voltage with a 2 mm needle-sphere gap. Finally, several factors that would impact the streamer propagation are discussed.

超导、空间环境模拟、生物医学与核聚变等领域广泛存在由冷却介质与聚合物材料组成的绝缘系统,放电失效问题往往存在于冷却介质、聚合物材料及两者之间的表/界面.在深冷温区,试验成本高、周期长、仪器缺乏以及安全隐患等因素严重限制了绝缘特性与放电试验的开展、规律总结、理论验证和新理论的提出.该文综述了深冷温区下绝缘介质击穿、闪络的试验成果和理论进展,侧重规律的总结和放电特性的分析、现有机理的解释以及工作展望.特别地,该文分析老化因素对绝缘介质击穿特性的影响,论述固-液、气-固界面放电行为的研究和现有模型的局限性,探讨了空间电荷的输运机制及其在深冷温区的适用范围,最后总结和评述深冷温区电荷、陷阱的测量方法和结果.该文内容有利于低温绝缘理论的发展和工程问题的解决,可为低温绝缘结构设计与优化、材料选型以及新材料的研发提供参考.

Nitrogen / carbon tetrafluoride (N2/CF4) may have a feasibility for use as the coolant or insulator of power apparatus but few studies have been carried out to figure out the characteristics of it. In order to have a better understanding of such mixture, some molecular features and properties concerning N2, CF4 and their compound are calculated through computational quantum chemistry methods and discussed in this paper with the emphasis focusing on molecular surfaces and intermolecular interactions. Concretely, the phase equilibrium curves are predicted according to several parameters obtained through the analysis of molecular surfaces. Furthermore, surface electrostatic potentials (ESPs) are studied and the mutual interpenetration distance was obtained. In addition, in order to have a further understanding of intermolecular interactions between N2 and CF4, the density of states (DOS) and charge decomposition analysis (CDA) are discussed as well as an analysis of electron density difference (EDD) and quantum theory of atoms in molecules (QTAIM). Finally, a prediction of electric strength is performed and it could be found from the prediction that N2/CF4 has an electric strength superior to N2 but inferior to CF4.

Y A superconducting energy pipeline is an integrated system for fuel delivery and power transmission. In the energy pipeline, direct current (DC) power is carried by superconducting tapes with almost no loss. Moreover, liquefied natural gas (LNG) is used not only as one kind of clean energy but also as a cooling medium. However, the lack of data on the electrical insulation characteristics of LNG (especially the resistance to breakdown and flashover) restricts the development of a superconducting energy pipeline. Therefore, this paper focuses on solving a series of problems (such as gas tightness, overpressure, and high voltage) in LNG insulation testing, obtaining the electrical failure data of multicomponent LNG, and analyzing the behavior of electric breakdown and flashover in LNG. The test results show that LNG as a dielectric cooling medium has remarkable performance in withstand voltage to replace the liquid nitrogen/liquid tetrafluoromethane (LN2/LCF4) binary mixtures. The research could offer key technical support for the superconducting energy pipeline and also provide a reference for the study of electrical properties on other cryogenic fuels.

将超导输电技术与液化天然气(LNG)管道输送技术相结合,形成能同时输送LNG与电力的能源管道,不仅可以节约能源通道,还可以利用LNG冷却超导电缆,提高能源输送效率和经济性,是一种极具前景的能源输送方式.在国家"智能电网与装备"重点研发计划的支持下,开展了超导直流能源管道的基础研究.该文主要介绍超导直流能源管道的基础研究项目近一年多的进展情况,主要包括:LNG混合工质的低温液固转变机理及传热流动特性,电力/LNG一体化输送动态稳定性判据,为安全性与故障演化分析而搭建的实验平台及初步实验结果,以及10m/10kV超导直流能源管道原理样机的研制与实验情况.

With the continuous improvement of the performance of High temperature superconducting(HTS) materials and cryogenic technology, HTS power cable has been developed rapidly. Many HTS cable projects in liquid nitrogen(LN2) temperature region have been carried out all over the world. Our current research is that HTS cable transmits high-power electric energy in the HTS energy pipeline based on the cooling sharing. The DC current transmitted by the superconducting DC cable is from the output of the converter station rectifier. In the actual working condition, different rectifier topological structure correspond to specific AC harmonics. Therefore, the influence of specific AC harmonics and joint resistance on the parameters of HTS cable should be considered in the design stage in order to realize the current homogenization transmission in each layer of the cable core. The current homogenization design is more important for the safety of HTS cable in energy pipeline. In this paper, the influence of specific AC harmonics on the inductance and mutual inductance of each layer of superconducting cable is analyzed, and the corresponding current distribution of each layer of AC harmonics is obtained. The critical current characteristics of Bi-2223 HTS tape at different magnetic field angles in the temperature range of 85-90 K are experimentally studied. The electromagnetic and structural parameters of 1 kA HTS DC cable in the temperature range of 85-90 K are optimized by ant colony algorithm with considering AC harmonics are obtained.