This study focuses on the further implementation of the Self-Consistent Leader Inception and Propagation Model -SLIM-to evaluate the probability of thunderstorms to self-trigger upward lightning. Three tall structures have been analysed: the Morro do Cachimbo station in Brazil, the KNBN FM antenna in Rapid City in the United States and the Säntis tower in Switzerland. Topographic and meteorological data have been used used as input for the evaluation of the physical conditions required for triggering upward lightning under stationary thundercloud fields. It has been found that the calculated probability of storms self-triggering upward lightning agrees well with the observations for the Morro do Cachimbo and the Säntis Towers. This results agrees with previous analyses performed before with SLIM for the Gaisberg and a wind turbine and a protection mast in Uchinada, Japan. However, it is found that the probability of self-triggered upward lightning cannot be calculated with the described procedure for structures where other-triggered upward lightning is dominant such as for the towers in Rapid City. It is suggested that self-triggered upward lightning cannot take place in storms in which upward lightning can be initiated instead by other nearby lightning events.

Plasma diagnostics is a key tool to support the further development of plasma-induced chemical conversion of greenhouse gases (such as CO2) into high-value chemicals. For this reason, spectroscopic and electric measurements of low current (below 1.7 A), stationary arc plasmas in CO2 at atmospheric pressure with addition of N-2 or H2O are reported. High-speed photography, imaging emission spectroscopy and time-resolved electrical measurements are used to obtain time-space resolved gas temperatures as well as the electric-field current characteristics of the discharge. It is found that the lowest average electric field in a CO2 arc plasma at atmospheric pressure is similar to 20 kV mm(-1) at a current between 0.8 and 1 A. If the current decreases below this level, the arc remains in vibrational-translational (VT) equilibrium by increasing the electric field. However, VT equilibrium conditions can be only maintained until a threshold minimum current of 0.33 +/- 0.05 A, at which the arc transitions into a non-equilibrium condition with further increasing electric fields (reaching 68 +/- 15 V mm(-1) at 0.03 A). The addition of N-2 or H2O did not influence the electrical characteristics of the CO2 arc within to the tested mixtures. However, there is only a significant decrease in the electric field of the formed transition arcs and the threshold minimum current in the presence of N-2. The spectra of the low-current CO2 arc is found to be dominated by emission from the C-2 Swan band system and the O I 777 nm triplet peak. However, the CN band dominates the spectra even when small amounts (0.5 wt%) of N-2 is present in the plasma. The gas temperature at the axis of the CO2 arc plasma decreased slightly with decreasing current, from an estimated 7000 K at 1 A down to 6300 K at 0.4 A. The thermal radius of the arc is estimated to be larger than 1.2 mm, more than two times larger than the optical radius obtained from the emitted radiation. The addition of N-2 and H2O (up to 7 and 9 wt% respectively) lead to only to a 500 K decrease in the axial arc temperature.

The dark period in long air gap discharges has been assumed in the literature as the time between consecutive streamer current pulses when ionization and luminosity are absent. These dark periods are also present in natural lightning, in processes such as the inception and propagation of upward positive leaders, the development of needles, as well as transient luminous events in the upper atmosphere. Only recently, faint luminosity has been observed during dark periods, challenging this assumption. This paper aims to study any possible electrical activity during dark periods by means of experiments supported by computer simulation. Therefore, an experimental platform, including low-current measurements, Schlieren and standard photography as well as ultraviolet (UV)-photon detection was used to observe the electrical-optical-thermal characteristics of the dark period. A complementary numerical model was used to estimate the streamer space charge spatial distribution and its drift during dark periods. It is found that faint UV and visible light during the dark period is emitted exactly at the location of the low-density streamer stem channel. This process is accompanied by the generation of an electronic current in the order of hundred microamps to milliamps. The simulation results show that this ionization activity occurs due to strong reduced electric fields in the residual stem channel above 112 Td, which is mainly determined by a combination of applied voltage, space charge distribution, and localized heating. Thus, the presented results show that there is indeed ionization activity during dark periods in long air gaps, which maintains a continuous glow-like discharge.

Nanofluids (NFs) with their superior electrical and thermal properties are promising alternate liquid dielectric in power transformers. Alongside the electric field, the magnetic field is also an integral part of power transformers. It accounts for power transfer through flux linkages and due to the design constraints, flux leakages exist in every transformer. This leakage flux interacts with the liquid insulation incessantly. This article aims at studying the magnetic response of the conductive, semi-conductive, and insulating nanoparticles (NPs) and their NFs prepared using mineral oil (MO) and vegetable oil (VO) as the base liquids. Quasi-static and dynamic magnetic fields are applied to the NPs and their NFs to magnetically characterize them by using a vibrating sample magnetometer (VSM) and an electron spin resonance (ESR) spectrometer. Magnetic susceptibility and resonance field are obtained from the magnetic response studies.

The insulation system in a transformer degrades over time and it needs periodic monitoring for uninterrupted operation of the power system network. Moisture content (MC), acid number, and dissolved gases are the important parameters that establish the aging and degradation status of the transformer liquid insulation. This work focuses on using high-frequency signals nonintrusively for estimating the degradation of the transformer liquid insulation by considering the MC and acid content. A novel nonintrusive and nondestructive testing method using an S-band pyramidal horn antenna is introduced in this work to estimate the degradation of the liquid insulation with respect to aging duration. The scattering parameter S-11 of the liquid insulation is first characterized and measured in an anechoic chamber to avoid electromagnetic interference (EMI). These tests are performed initially on laboratory-aged mineral oil (MO) samples and a prediction curve of vertical bar S-11 vertical bar as a function of moisture and acid number is obtained. vertical bar S-11 vertical bar of the in-service transformer oil samples is then compared with the prediction curve and the degradation status is predicted. It is found that the chosen frequency band is able to differentiate the aging status of MO accurately. This technique can be used for establishing the degradation status of any type of liquid insulation. A miniature version of the proposed technique can be used for portable and faster condition assessment of insulating oils.

Spatial and time-resolved characteristics of a single surface dielectric barrier discharge (sDBD) actuator are experimentally and numerically investigated. The paper also focuses on the efficiency of sDBD actuators used as flow-control devices. The motivation is the need for developing a cost-effective way to optimize the balance between control performance and actuator power consumption. The study considers the steady state as often employed in experiments as well as the transient regime. Experimental methods to obtain the active power are revisited, and for the first time, the commonly used simplified phenomenological Suzen–Huang model (SHM) is used for the computation of electrical characteristics. The SHM represents fair qualitative features of the starting vortex. However, it fails when time-resolved velocity profiles are compared. Results show that even with an optimized parametrical analysis of the “tuned” plasma variables, the model is not able to fully reproduce the induced wall-jet neither spatially nor temporally. Furthermore, it underestimates the power consumption by more than 80%. The intrinsic challenge of accurately measuring the alternating current of the DBD and the instantaneous mechanical power, together with the failure of representing time-resolved velocity profiles and the underestimated electric power by the model, highlights that a better phenomenological model including gas dynamics and electric characteristics or using a fully coupled physical plasma model is required.

In this paper the electromagnetic compatibility (EMC) considerations made to the direct lightning current measuring system that is part of the Johannesburg Lightning Research Laboratory (JLRL) are presented. The output voltages from two series connected shunts and the sending of their analog signals downward a tower to the measuring system, makes this measuring setup particularly critical regarding immunity against the strong effects of the lightning current.

Enhanced active resonant (EAR) dc circuit breakers (DCCBs) are a novel type of DCCB that use a discharge closing switch as interruption medium. A technical limitation of discharge closing switches is the minimum voltage across the main gap required for successful triggering. A novel commutation process creating the minimum voltage internally is proposed, which allows to simplify the EAR DCCB configuration and to reduce its component count. In the prototype, the discharge closing switch is implemented with a TVG. Experiments show that the TVG can be triggered reliably down to a voltage of 50 V and that the discharge in the TVG is highly oscillatory at low current. The originally proposed EAR DCCB configuration has to be tuned such that the commutation to the TVG succeeds at low current. Conversely, the novel commutation process decouples the minimum voltage from the current level by adjusting the triggering delay. This allows reliable commutation irrespective of the operating conditions. It is shown that the novel commutation process does not adversely affect dc interruption. Proactive commutation operation and auto-reclosing strategies are demonstrated.

A 3-dimensional macroscopic discharge model in mineral oil is introduced. The process of streamer propagation and branching is simulated between two energized electrodes. The propagation of individual streamer branches in mineral oil is evaluated by using simple physical considerations based on electrostatic calculations performed throughout the discharge development. The simulation is performed by using commercial finite element method (FEM) software in a simulation application. The morphology and the potential gradient change along the streamer perform a good consistency with experimental results. In the paper, the first attempt to validate the model with a "single" guided streamer filament creeping on a glass dielectric under step voltages is presented.

Enhanced active resonant (EAR) dc circuit breakers (DCCBs) are a promising set of recently proposed DCCB concepts. As other DCCBs, EAR DCCBs still require a fast mechanical switch. The requirements on the actuator of the mechanical switch depend on the DCCB concept and the dc grid and are derived here for an EAR DCCB. Thomson-coil actuators (TCAs) can open and close mechanical switches sufficiently fast to satisfy the requirements. This work studies experimentally a TCA system with active damping for an off-the-shelf industrial vacuum interrupter used as mechanical switch in an EAR DCCB. The prototype is explained in detail, and extensive measurement results are presented, showing that active damping must be perfectly timed to be effective. A novel Thomson-coil (TC) driver is proposed and studied experimentally, which operates the TCA more efficiently by recycling energy during the actuation. Moreover, the novel TC driver reduces the capacitive storage by 50% and allows for faster recharging with lower charging current. Finally, the autoreclosing and proactive commutation operation of the TCA system and the interruption capability of the prototype EAR DCCB are demonstrated experimentally.