We perform direct numerical simulations (DNS) of the hyperviscous Navier–Stokes equations in a periodic box. We consider values of the hyperviscosity index h=1, 2, 8, and vary the hyperviscosity to obtain the largest range of lengthscale ratios possible for well-resolved pseudo-spectral DNS. It is found that the spectral bump, or bottleneck, in the energy spectrum observed at the start of the dissipation range becomes more pronounced as the hyperviscosity index is increased. The calculated energy spectra are used to develop an empirical model for the dissipation range which accurately represents the bottleneck. This model is used to predict the approach of the turbulent kinetic energy k to its asymptotic value, k∞, as the hyperviscosity tends to zero.

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Direct numerical simulation of homogeneous turbulence with hyperviscosity

With the construction and promotion of the Ubiquitous Power Internet of Things (UPIoT), it is an increasingly urgent challenge to comprehensively improve the recognition accuracy of the gas-insulated switchgear (GIS) partial discharge (PD), and to incorporate the model into UPIoT intelligent terminals supported by edge computing in embedded systems. Therefore, this paper proposes a novel MobileNets convolutional neural network (MCNN) model to identify the GIS PD patterns. We first construct the PD pattern recognition classification datasets by means of experiments and FDTD simulation, and also preprocess images via binarization processing. After constructing the MCNN model, depthwise separable convolutions and an inverse residual structure are adopted to deal with the vanishing gradient of the deep convolutional neural network (DCNN) in the GIS PD pattern recognition process. Then, through the graphics standardization process, the MCNN model is trained and tested. The whole training process is visualized by Tensorboard. Compared with other deep learning models and traditional machine learning methods, MCNN particularly stands out in recognition accuracy and time consumption with a 96.5% overall recognition rate and merely 7.3 seconds in training time. This research explores how to optimize the model by improving the recognition accuracy, and by reducing its computing load, storage space and energy consumption for better incorporation into intelligent terminals in the UPIoT context.

/pdf/a-mobilenets-convolutional-neural-network-for-gis-partial-2ykv06ix6l.pdf

A MobileNets Convolutional Neural Network for GIS Partial Discharge Pattern Recognition in the Ubiquitous Power Internet of Things Context: Optimization, Comparison, and Application

Health monitoring of gas insulated apparatus is critical to improve the reliability and to reduce the life cycle cost. Among the various methods for monitoring, optical sensing developed fast in recent years due to their highly sensitive and anti-electromagnetic interference behaviour. This study reviews the optical monitoring methods for partial discharge, gas concentration and temperature of gas insulted apparatus. In addition, the challenges of sensor development are carefully discussed here. The future prospects of optical monitoring of gas insulated apparatus are also analysed to inspire new ideas.

Optical sensing in condition monitoring of gas insulated apparatus: a review

The growing application of gas insulated switchgear (GIS) in electric power system asks for higher reliability of epoxy (EP) insulators, thus it is important to seek effective methods to improve the operating reliability of GIS insulators. On the basis of the 110 kV GIS insulator, this study studied the aluminium oxide (Al2O3) settlement in the curing process and the stress distribution of the horizontally laid GIS. Besides, the risk coefficient was defined to evaluate the operating reliability of insulators laid by different methods (I–V). Some conclusions are shown below: the Al2O3 settlement in the curing process causes the insulator density to have a large variation from 2.13 to 2.32 g/cm3. The Al2O3 content has a great influence on the mechanical performances of EP/Al2O3system. The first principle stress is localised on the upper interface between the insulator and the conductor, which is the main threat to the mechanical damage of insulator. The laying method V appears to be optimal with a pretty low risk coefficient of 31.5. In actual application, for the horizontally laid GIS, the insulator should be laid with the mould gate downward to improve its mechanical reliability.

Mechanical stress distribution and risk assessment of 110 kV GIS insulator considering Al2O3 settlement

Optimizing GIS partial discharge pattern recognition in the ubiquitous power internet of things context: A MixNet deep learning model

It is important to estimate the current magnitude and charge amount of a partial discharge (PD) in a gas-insulated switchgear (GIS). In this study, electromagnetic fields generated by a PD in an 84 kV-class three-phase GIS for testing have been measured with a voltage probe installed on the outer surface of an insulation spacer of the GIS tank, and the corresponding simulations have been carried out using the three-dimensional finite-difference time-domain (FDTD) method. FDTD-computed time-domain voltages across spacers agree reasonably well with the corresponding measured ones, and the current magnitude of PD and its charge amount have been estimated.

Finite-difference time-domain simulation of partial discharges in a gas insulated switchgear

The effect of a shield wire, which is a bare conductor buried above an insulated metallic cable, on lightning-induced currents on the cable has been studied using the three-dimensional (3-D) finite-difference time-domain (FDTD) method. In the 3-D FDTD model, a 1-km-long insulated cable is buried at depth 0.6 m in 1850-Ω·m ground, and a 1-km-long shield wire is installed 0.3 m above the cable (at the depth 0.3 m from the ground surface). A 1-km-long vertical lightning return-stroke channel, attached to a 30-m-high tower or flat ground, is located 25 m away from the cable and the shield wire. The validity of this model has been shown by comparing the FDTD-computed lightning-associated currents on the cable and the shield wire with the corresponding measured currents associated with rocket-triggered lightning. The FDTD-computed results show that the presence of the shield wire reduces the induced current on the cable and the voltage across the outer insulation layer of the cable, as expected, but the series resistance of the shield wire has a negligible effect on this reduction. The presence of 30-m-high strike object slightly reduces the cable current. The ground permittivity and the return-stroke speed have negligible effects on the cable current, but the higher the ground resistivity, the higher is the cable current. The increase of bonding points between the cable sheath and the shield wire increases the cable current, but decreases the voltage across the outer insulation layer of the cable.

Effect of Shield Wires on the Lightning-Induced Currents on Buried Cables

Transitions of natural convection in a vertical fluid layer contained in a tall cavity with differentially heated side walls are investigated numerically and theoretically. Steady-state solutions are calculated by solving the steady-state equations numerically and their stability is investigated by applying the linear stability theory. It is found that the bifurcation structure of the steady-state solution consists of smooth transition and saddle-node branches and that the change in flow pattern occurs without any instabilities along the smooth transition branch, whereas transitions from the saddle-node branch to the smooth transition branch may happen which accompany the change of flow pattern.

Transitions of natural convection in a vertical fluid layer

Transition routes of natural convection in a vertical fluid layer contained in a tall cavity with differentially heated side walls are investigated theoretically on the assumption of two-dimensional flow field. Steady-state solutions are calculated by solving the steady-state equations numerically, and their stability is investigated by applying the linear stability theory. Periodic-state solutions are also obtained by numerical simulations. Bifurcation diagram of the steady- and periodic-state solutions is obtained and the routes of transition of the natural convection reported so far are interpreted according to the diagram. It is found that the bifurcation diagram is very complicated in a limited range of the Rayleigh number, whereas it is quite simple out of the range. The dependence of heat transfer between the two vertical walls on the flow pattern of the convection is also discussed.

Transition Routes of Natural Convection in a Vertical Fluid Layer

The protective effect of a shield wire against direct lightning flashes to a buried power cable has been clarified in this paper. The finite-difference time-domain (FDTD) method is used in the investigation. The FDTD model considers a 1-km-long insulated cable protected by a shield wire, and a lightning strike right above them. The model has been validated by comparing its results with experimental results available in the literature. The computed results show that the presence of the shield wire can effectively prevent the cable from being directly struck by the lightning flash. The voltage across the cable insulation at the striking point depends on the ground resistivity (the higher the resistivity, the higher the voltage), but this dependence is not linear. It is also shown that the soil surface ionization is relevant to the current delivered to the shield wire and to the voltage developed across the cable insulation. On the other hand, the soil ionization around the shield wire has negligible effects. The intrinsic resistance of the shield wire does not influence significantly the peak voltage across the cable insulation, but it influences its duration (the higher the intrinsic resistance, the longer the voltage duration).

Hiroki Tanaka

Papers

Finite-difference time-domain simulation of partial discharges in a gas insulated switchgear

Effect of Shield Wires on the Lightning-Induced Currents on Buried Cables

Transitions of natural convection in a vertical fluid layer

Transition Routes of Natural Convection in a Vertical Fluid Layer

Protective Effect of Shield Wires Against Direct Lightning Flashes to Buried Cables