Utilizing hybrid electric propulsion systems in the next generation of aircrafts with elevated DC voltage level has raised the concern about the degradation of insulation subjected to partial ...discharge at high altitude and harsh environment. In this work, the surface discharge behavior of commonly used high-temperature insulation materials in aviation systems (FEP, ETFE, and PEEK) under positive and negative DC and ramp voltage was studied, including the pulse waveform, frequency spectrum, PD count and magnitude. Pressure changes as a significant factor influencing surface discharge phenomenon was thoroughly studied. Dust figure technique was employed after voltage ramping tests for both positive and negative polarities at low pressure to investigate the surface charge accumulation and surface discharge traces, and unveil the involved physical mechanisms at different stages of streamer propagation. The content of this paper provides a reference for surface discharge studies and evaluation of high temperature materials for medium voltage direct current power distribution in the future aviation systems.
Noncontact flow measurement methods have been popular among hydrographers for their convenient and safe operation in recent years. Ultrahigh frequency (UHF) radar is an essential flow measurement ...instrument that is normally deployed on the river bank to monitor flow velocity and discharge. However, when the river flow is too high, some frequency points of the Doppler spectrum are aliased. The frequency point in the aliased first-order peak region may contain more than one current signal, which cannot be distinguished by the previous radial current extraction methods. The consequent poor radial current decreases the estimation performance of cross-sectional surface velocity and river discharge. This letter first interprets the mechanism of the aliasing phenomenon according to the relationship between Bragg frequency and Doppler shift. Then, a radial current extraction method based on the classification of Doppler frequency points is proposed to solve the aliasing problem. Experimental results demonstrate that the proposed method outperforms the other two radial current extraction methods in calculating discharge with a reduction in a root-mean-square error (RMSE) of 364 and <inline-formula> <tex-math notation="LaTeX">574 \mathrm {m^{3}/s} </tex-math></inline-formula>, respectively.
This paper presents an experimental study of the fundamental phenomena of progressive creeping discharge as well as the influences of pressboard ageing and voltage amplitude under divergent AC ...voltages. A ramp-stress test is used to determine the partial discharge inception voltage (PDIV) and the flashover voltage of oil/pressboard interface versus pressboard ageing. Constant-stress tests are carried out to investigate the partial discharge (PD) behaviors, the pressboard surface temperature, and the oil-dissolved gases of progressive creeping discharges. It is found that the pressboard ageing affects only the interface PDIV. The progressive creeping discharges concentrate at the triple-junction and comprises probably oil corona discharges and surface discharges in pressboard dimples. If no pressboard damage is present, the progressive discharges exhibit in general a constant PD pattern but a diminishing intensity, and nevertheless can be sustained for the severely-aged pressboards under strong voltages. The underlying causes are correlated with the competing effects of pressboard ageing on interface moisture and the triple-junction field intensification. Gas analyses indicate the hydrogen is the principal faulty gas and the Duval's Triangle diagnoses the progressive creeping discharges as either low-energy or high-energy discharge fault. No pressboard overheating is observed during the discharges.
During the operation of high voltage (HV) motors, the end-winding surface can be contaminated by oil or dust, resulting in a type of intense discharge: surface tracking, which gradually develops into ...phase-to-phase or phase-to-ground faults. To improve our understanding of the development process and characteristics of surface tracking on the end-winding, an accelerated aging test was performed on 10kV stator coils to induce surface tracking under electrical and oil contamination stresses. To understand the evolution of surface tracking, the partial discharge (PD) was collected and the characteristics of the corresponding phase resolved partial discharge (PRPD) patterns and pulse sequence analysis (PSA) patterns were analyzed. Combined with finite element analysis (FEA) simulations, the development process of surface tracking was interpreted. It is established that the electrical field of dry band with high resistivity on the contamination is much higher than other contaminated area via FEA simulation, causing electrical breakdown of the adjacent air and discharge. Such continuous and intense discharges finally leave on the end-winding surface a carbonized channel, forming a tree like structure. This paper presents results of changes in PRPD patterns and PSA patterns during different stages, together with the surface potential distribution after tracking generation. The research of this paper is devoted to provide a reference for PD monitoring and fault diagnosis of HV motors.
Insulator pollution flashover occurs frequently and is harmful to the power transmission and distribution network. But the randomness of the pollution flashover and the complex influencing factors ...make it difficult to be predicted. The objective of this paper is to study the mechanism of the discharging on insulator surfaces and design a the calculation method which derives flashover probability. By calculating the distorted electric field in the initial streamer discharge quantitatively and combining it with the equivalent circuit of the insulator surface, the initial discharge voltage is derived. By analyzing the change in power input and dissipation in the process of the arc discharge, we propose the criterion on arc development and extinction, and the critical flashover conductivity of the insulator is obtained. Based on the probability density function, the behavior of the factors on the critical flashover conductivity is described, and the probability of insulator pollution flashover is calculated under different environmental conditions. In the validation experiment, the measured initial discharge voltage and pollution flashover ratio are close to the calculated values. The methods proposed in this paper could be used in the quantitative calculation of the initial discharge voltage and the prevention of the pollution flashover on insulators.
Audible noise from transmission lines is mainly generated by corona discharge from defects and water droplets on the conductors' surface. Applying a superhydrophobic coating to the conductors can ...reduce corona discharge, thereby reducing the level of audible noise produced. In this paper, corona discharge and noise measurements from an untreated conductor and a conductor coated with superhydrophobic coatings were carried out in AC electric fields and continuous water spray. The coated conductors show higher partial discharge inception voltages (PDIV) than uncoated ones. PD magnitude and PD per cycle of the coated conductors are lower compared with uncoated conductors, especially at electric fields above 14 kV/cm. A linear correlation between overall sound pressure and total PD magnitude is established. The behavior of corona discharges from single droplets with various sizes and positions and associated droplet vibration were investigated and the local electric field enhancement was analyzed by finite element analysis to examine the influence of droplet size and position on electric field distribution. Superhydrophobic coatings result in reduced PD magnitude and number and therefore reduce audible noise generated. This is attributed to small sessile droplets forming rather than large pendant droplets.
In order to study the effect of surface morphology of dielectric materials on features of dielectric barrier discharge (DBD) in air at atmospheric pressure (AP), electrical characteristics and ...optical emission spectrum are obtained in a 2-mm air gap with quartz glass and ceramic used as the dielectric layer (the capacitance of the used dielectrics is similar). The transition of discharge mode is observed for different dielectric material and surface morphology. Results show that with varying dielectric material and surface morphology, air atmospheric pressure dielectric barrier discharge (APDBD) presents three discharge modes, including a filamentary mode, a homogeneous mode, and a mixed mode of them. Further investigation reveals that the homogenous mode is in a Townsend discharge regime, which is realized in a proper surface morphology of 96% Al 2 O 3 ceramic. Moreover, optical emission spectrum is obtained, which mainly consists of atomic lines of nitrogen and molecular bands originating from the second positive system of N 2 (<inline-formula> <tex-math notation="LaTeX">\text{C}^{3}\Pi _{\mathrm {u}}\to \text{B}^{3}\Pi _{\mathrm {g}} </tex-math></inline-formula>). From N 2 (<inline-formula> <tex-math notation="LaTeX">\text{C}^{3}\Pi _{\mathrm {u}}\to \text{B}^{3}\Pi _{\mathrm {g}} </tex-math></inline-formula>), molecular vibrational temperature is investigated as a function of surface roughness (<inline-formula> <tex-math notation="LaTeX">R_{\mathrm {a}} </tex-math></inline-formula>) of the different dielectric materials. Result reveals that there is a minimal vibrational temperature when the discharge is in the homogeneous mode. With quartz glass used as the dielectric, the vibrational temperature decreases first and then increases with increasing <inline-formula> <tex-math notation="LaTeX">R_{\mathrm {a}} </tex-math></inline-formula>. The minimum value appears at <inline-formula> <tex-math notation="LaTeX">R_{\mathrm {a}} </tex-math></inline-formula> of 215 nm. Besides, the discharge current is the most concentrated in time, and the discharge looks the most homogeneous under this circumstance.
Atmospheric pressure low-temperature plasmas produced by dielectric barrier discharge (DBD) provide a promising approach for civilian application of pulsed power technology. In this paper, repetitive ...nanosecond pulses were generated using a magnetic compression solid-state pulsed power generator, and the rise time and pulse duration of the nanosecond pulse are ~30 and 70 ns, respectively. The DBD in open air is created using two kinds of electrodes, i.e., water and metal electrodes. The electrical, luminous, and optical characteristics of the DBDs under these two electrodes are studied and compared. The experimental results show that no filaments are observed and the discharge is homogeneous when water electrodes are used. The DBD still behaves in a filamentary mode when the discharge gap is extended to 4 cm in the case of metal electrodes. The results are validated by fast images taken by an intensified charge-coupled device camera. In addition, some discussion about the experimental results is presented. Improvement of discharge uniformity is due to the effect of resistive stabilization using water electrodes.
Direct and indirect influence on microorganisms of non‐thermal atmospheric‐pressure DBD in air is compared in terms of bacterial inactivation rates. It is demonstrated that direct influence of the ...plasma, where charged particles contact bacteria directly, produces inactivation much faster than the indirect treatment, where plasma afterglow is delivered to the bacteria with a gas flow through the plasma region. This leads to an important conclusion that the effect of charged particles on plasma plays the essential role in interaction with living organisms, although synergy with longer living active molecules and atoms as well as UV radiation generated in plasma and at the surface of tissues may also play a role in inactivation.
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