High Voltage最新文献

Solid-state transformers (SSTs) have applications in medium-voltage direct current (MVDC) grids and compact power systems. High-frequency transformer (HFT) is the core component of SSTs. High levels of high frequency high dv/dt voltage stresses challenged the integrity of the galvanic insulation of HFTs. However, dielectric thermal runaway and resultant electrical failure mechanisms in epoxy resin (EP) cast insulation remain unclear. Dielectric heating of EP across varying voltages, frequencies, rising edges, duty cycles and DC biases were measured and corroborated by simulation. The thermal runaway threshold mainly depends on the tangency point of the loss generation and heat dissipation curves below the glass transition temperature. Observations reveal that thermal runaway does not directly cause breakdown; instead, thermal decomposition above 200°C triggers discharge and eventual failure. Simulations demonstrate that temperature rise mainly depends on the average field within the electrode region and inter-segment and inter-layer distances within the HFT winding definitively impact insulation thermal runaway. By applying different criteria for MV and high-voltage (HV) transformers, the reference electric fields for insulation design with unfilled and filled EP were obtained. For instance, limiting dielectric heating below 5 K at 50 kHz necessitates an RMS average field less than 0.44 V/mm, which is much lower than dry-type transformer conventions. The authors prove the necessity of re-evaluating the permissible field strength in HFT insulation design.

A dual-frequency (DF) dielectric barrier discharge, excited by the superposition of a 50 Hz low frequency (LF) and a 5000 Hz intermediate frequency (IF), is proposed to enhance discharge. The effect of the LF voltage component on the breakdown behaviour of the DF discharge during different periods has been studied both experimentally and numerically. The number of high-current pulses rises as the LF voltage increases. The statistical analysis shows that the number of the current pulse amplitude above 80 mA in the DF discharge reaches nearly 6 times that in the IF discharge. Additionally, the total discharge energy in the DF discharge is significantly higher than that in the IF discharge. The simulation reproduces the temporal variation of the breakdown behaviours in the DF discharge. The simulated results reveal that the maximal electric field strength of the breakdown process is greater in the DF discharge compared to the IF discharge during a half-period of DF. Finally, the comparison between the IF and DF discharges exhibits that the LF voltage regulates the accumulation of residual charged species on the dielectric surface after the breakdown by modulating the residual voltage between the air gap.

Electrical tree degradation is one of the main causes of insulation failure in high-frequency transformers. Electrical tree degradation is studied on pure epoxy resin (EP) and MgO/EP composites at frequencies ranging from 50 Hz to 130 kHz. The results show that the tree initiation voltage of EP decreases, while the growth rate and the expansion coefficient increase with frequency. Moreover, the bubble phenomenon at high frequencies in EP composites is discussed. Combined with trap distribution characteristics within the material, the intrinsic mechanism of epoxy composites to inhibit the growth of the electrical tree at different frequencies is discussed. It can be concluded that more deep traps and blocking effect are introduced by doping nano-MgO into EP bulks, which can improve the electrical tree resistance performance of EP composites in a wide frequency range.

Commutation failure (CF) is one of the most common issues in line-commuted converter-based high voltage direct current systems (LCC-HVDC), leading to critical power system security and stability problems. Accurate and rapid identification of CF is crucial to prevent subsequent CF in HVDC systems. However, the existing CF identification methods are lack of the required accuracy and speed. The relationship between bridge arm current, commutation voltage, and trigger pulse based on the commutation progress mechanism after the AC fault is analysed in this paper. A novel CF identification method is presented, which utilises the CF identification factor, enabling fast and high-accurate identification without relying on the value of the extinction angle. The proposed method offers a simpler and more efficient implementation in engineering practice. Finally, the effectiveness of the proposed method is verified in both the standard International Council on Large Electric systems HVDC model and the Hardware in Loop test using practical project parameters. The results demonstrate that the proposed method can accurately and fast identify CF.

Accurate calculation of short-circuit electromagnetic force is crucial for both mechanical strength check and the optimal design of gas-insulated transmission lines (GIL). Since the full 3D numerical simulation method is highly time-consuming, a novel lightweight 2D multi-slice electromagnetic field-circuit coupled method for computing transient electromagnetic force is proposed, where appropriate port voltage degrees of freedom (DoFs) are introduced for the solid GIL conductor terminals. When the transient magnetic field equations are combined with the constraint equations of circuit part, including nodal voltage and loop current DoFs, a direct field-circuit coupling scheme is thus derived. The proposed method can simultaneously consider the effect of interphase-shunts and ground wires, as well as the skin effect and proximity effect. It can accurately capture the transient electromagnetic characteristics of GIL spanning from several to tens of kilometers under different short-circuit conditions. The transient electromagnetic forces, as well as the induced voltages and currents of the enclosure, are analysed by the proposed method for both single-phase and three-phase enclosed GIL under various short-circuit conditions. The proposed method has the advantages of high accuracy and lightweight computational cost, and thus it is also suitable for conducting important simulation tasks such as mechanical strength checks during the design optimisation phase of long-distance GIL.

With the widespread application of composite insulators in transmission lines, exploring the accumulation mechanism of pollution particles on composite insulator surfaces is of importance to ensure the safe and steady operation of the power system. Addressing the current theoretical shortcomings, this study categorises the accumulation process of particles on the insulator surface into three stages, namely ‘spatial motion’, ‘surface collision’, and ‘surface motion’. The motion and rotation velocities in a multi-physics field are calculated in the spatial motion stage. In the surface collision stage, a parameter called ‘neck height’ is introduced to determine the optimum mechanics theory, and the normal deposition criterion is established. For the surface motion stage, the sliding displacement and rolling displacement on the surface are calculated based on the rotation speed of the particles. A dynamic pollution accumulation model of the composite insulator is established based on the normal deposition criterion and tangential displacement. Finally, numerical simulations are performed by using the finite element method. Simulation results show that the proposed model agrees with the actual insulator pollution accumulation, and the deposition model is still applicable for various types of composite insulators operating in different applied voltages. The deposition probability of particles increases with the increasing particle size. In the surface motion stage, particle displacement increases with particle size and wind velocity.

A high-temperature superconducting (HTS) dynamo flux pump can inject DC currents into closed-loop HTS magnets without contact. It enables the realisation of current-lead-free or even through-wall charging systems for high-field applications such as nuclear magnetic resonance/magnetic resonance imaging (MRI) magnets, fusion reactors and accelerators. Researchers have proposed many simulation models to understand the working principle of HTS dynamos, few of which are in 3D because of converging problems. Therefore, the influences of many key 3D parameters in the HTS dynamo are scarcely reported. The authors propose an efficient 3D modelling method of the HTS dynamo based on the T-A formulation. The rotating magnets are modelled by a ring-shaped permanent magnet with space-time-variant remanent flux density to avoid moving meshes. This, together with the T-A formulation, makes the 3D model efficient and universal. The accuracy of the model is verified by the experimental instantaneous and time-integrated dynamic voltages. Using this model, the authors present systematic case studies to thoroughly explore the influences of the key parameters of a dynamo flux pump on the dynamic voltage and losses. The proposed modelling method and results could significantly benefit the design and optimisation of HTS dynamos for high-field magnets.

In order to accurately predict the content and variation trend of dissolved gas in transformer oil and guide the condition maintenance of power transformers, a combined prediction model based on multi-information fusion is proposed and its effectiveness is analysed. First of all, based on the possibility of pathological and missing historical sample data, a detection and filling method based on variable weight combination samples is established. Second, the authors propose two models. Aiming at the non-linear and non-stationary characteristics of gas content, a univariate decomposition prediction mode HBA-VMD-TCN which based on the Honey Badger algorithm, variational mode decomposition and time convolutional network (TCN) is established. Then the multivariate Informer prediction model is established for gas content affected by multiple variables. Third, the cross-entropy theory is used to determine the weight coefficients of the two models, and the multi-information fusion combined prediction model is formed. Finally, on the basis of the above, a method to determine the time step and the position information of the transition point adaptively in the process of prediction is proposed to further improve the prediction accuracy. The results show that, through a series of simulation experiments of model comparison and transformer anomaly prediction, the accuracy and effectiveness of the combined prediction model are verified.

Developing low-carbon gas insulated transmission and distribution equipment is essential to achieve the ‘carbon peak, carbon neutralisation’ or ‘net-zero’ in the power industry. Scholars have carried out various research on the eco-friendly insulating gas (such as C₄F₇N, C₅F₁₀O) and next-generation SF₆-free equipment to gradually reduce the utilisation of strong greenhouse gas SF₆.

The cover of this issue is inspired by the allusions of the “Year Beast” in Chinese traditional culture. It is said that in ancient times, every New Year's Eve there would be a murderous “Year Beast” attacking the world, and the people drove it away with firecrackers. These habits have gradually become the customs of the New Year. Coinciding with the Chinese Lunar New Year, the authors compare the “discharge” that threatens the safety of gas insulated equipment to the “Year Beast”. The traditional insulating gas SF₆ incarnates as an ‘immortal’ to protect the insulation safety of equipment for a long time. However, due to the greenhouse effect of SF₆, its use is gradually limited. At the same time, the eco-friendly insulating gas is like the ‘pupil’ that gradually jointly resist the discharge with SF₆ to ensure the safe and stable operation of power equipment.

In the future, the development of eco-friendly insulating gases and SF₆-free equipment will further ensure the safety of the power grid and protect the lights of all households. On the occasion of the Spring Festival, we wish all scholars and readers a happy New Year!