DC Leakage Current Research Articles

Ferroelectric Al1−xBxN–GaN heterostructures

This report demonstrates Al0.93B0.7N thin films grown epitaxially on n-type GaN (0002)/c-plane sapphire substrates by reactive magnetron sputtering at 300 °C. At 200 nm film thickness, the Al0.93B0.07N layers exhibit partially relaxed substrate-induced epitaxial strain, a 0.16° wide (0002) rocking curve, in-plane crystallographic registry, and sub-nanometer surface roughness. Electrically, the stack shows robust hysteresis over three frequency decades, a remanent polarization of ∼125 μC/cm2, a strongly frequency dependent coercive field, highly uniform dc leakage currents, and endurance >106 field cycles. This report validates possibilities for ferroelectric nitride integration into conventional III-nitride heterostructures with high crystalline fidelity, high electrical resistivity, and persistent hysteresis. Such materials are synthesizable at thermal budgets and temperatures compatible with back-end-of-the-line boundary conditions.

Performance Assessment of Frequency Selective Grounding for Grid-Connected Photovoltaic Systems

Photovoltaic (PV) panels and their grid-connection circuitries have to be grounded to comply with various standards and industrial codes. Objectives of grounding a PV system include limiting ground currents and potentials, supporting the operation of protective devices therein, and preventing the accumulation of static charges on PV panels. These objectives can be translated into design constraints for an impedance to connect a PV system to the ground. This paper presents the design and performance assessment of a frequency selective grounding (FSGR) for grid-connected PV systems. The proposed FSGR is composed of a parallel <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$R$</tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$L$</tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$C$</tex-math></inline-formula> circuit, which provides a resistive path for dc currents and a low capacitive impedance path for harmonic currents. The FSGR is designed and experimentally tested for a 2 kW, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$3\phi$</tex-math></inline-formula> , laboratory grid-connected PV system under fault and non-fault conditions. The performance of the proposed grounding is also compared with the solid grounding under similar conditions to further assess its capabilities. Experimental results show that the FSGR can reduce ground currents, limit ground potentials, and reduce the dc leakage currents. These observed features are complimented with minor effects on the operation of protective devices in a grid connected PV system.

A Sensor for the Ultraweak DC Current Covered by Strong AC Currents

In the field of insulation monitoring of high-voltage power equipment, the dc leakage current is an important indicator to evaluate the insulation state. However, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{A}$ </tex-math></inline-formula> -level dc leakage currents are usually mixed with mA-level power-frequency ac currents, bringing obstacles to the accuracy of the dc measurement. In this regard, a new magnetic modulator with a suppressing structure is proposed. Based on the fact that Lenz law only keeps valid for a changing magnetic flux, the ac suppressing structure is developed to enclose the original magnetic modulator. Then, the factors of the suppression structure’s performance, including weakening the ac magnetic flux and reducing the dc measurement sensitivity, have been analyzed. And the performance is found to be closely related to the magnet dimensions, the magnetic permeability, and the suppressing winding turns. Finally, the improved prototype current sensor for the converter transformer valve side bushing has been developed, and experiments are conducted to verify its dc accuracy and ac suppressing performance.

태양광발전원 계층별 전기적 특성에 대한 위해성 영향 연구

The photovoltaic(PV) system is a representative renewable energy source, and the share of power generation is increasing. The power generation is based on the power converter for linking with commertial power system. So the DC and the AC coexist in the PV system. In the DC section, the DC leakage current was measured and it can be caused by the parallel resistance among the parasitic resistances that occur due to the limitation of uniform doping of the PN junction of the semiconductor. And it is directly affected by the irradiation. In the AC section, the AC leakage current was measured and it can be caused by the leakage path through the stray capacitance. And it is affected by the output power and the module temperature which limits to increase the output power and AC leakage current. Based on these electrical characteristics, the risk was demonstrated in the 50 kW PV system.

Broadband Complex Permittivity and Electric Modulus Spectra for Dielectric Materials Research

Measurement of the complex permittivity of a substance in a wide frequency range, for example, from 10 mHz to 100 kHz, provides useful information on the dielectric properties of the substance. In this regard, some of such dielectric properties can be analyzed more clearly if we pay attention to the frequency spectra of electric modulus, which is a reciprocal of complex relative permittivity. Especially, the imaginary part of the electric modulus can be a powerful tool for dielectric materials research. In this paper, this fact is demonstrated clearly by showing abundant examples acquired by the author's laboratory. First, the equations of electric modulus that hold for various dielectric relaxation processes, including their derivation, are shown in an easy‐to‐understand manner. After that, for several insulating polymers, actual measurement examples of broadband complex permittivity spectra are shown, up to frequencies of about 100 kHz and a much broader range up to several THz. In the first example of showing permittivity spectra, polyamide exhibits an incredibly high relative permittivity value of 106 at high temperatures such as 200 °C at a frequency as low as 10 mHz. This is a result of the phenomenon often called electrode polarization. In other words, if electric charge carriers accumulate in an insulator facing a nearby electrode and have the polarity opposite to that of the electrode, the permittivity of the insulator goes up. In such a case, the dielectric loss factor usually shows a very high value in the low‐frequency range and decreases in inverse proportion to the frequency. This reflects the fact that the charge carriers are transported, and the conductivity can be derived from this frequency dependence much more easily with apparently higher reliability than measuring the dc leakage current. Furthermore, it is shown that the coefficient of thermal expansion can be estimated from the permittivity spectra relatively accurately for several polymers that satisfy the necessary condition. Next, taking examples in electric modulus spectra measured in important insulating polymers such as engineering ones, it is demonstrated that the processes of dielectric polarization and relaxation, which are difficult to see in permittivity spectra, can be recognized obviously. Moreover, the effects of the addition of nanosized or microsized fillers to epoxy resin on its thermal property and filler‐induced inhomogeneity are analyzed using the electric modulus spectra measured in these samples. The effects of thermal aging treatment and simultaneous aging treatment with heat and radiation on the degradation of cross‐linked polyethylene are also analyzed by paying attention to the frequency spectra of its imaginary electric modulus. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

240$^\circ$-Clamped PWM Applied to Transformerless Grid Connected PV Converters With Reduced Common Mode Voltage and Superior Performance Metrics

Transformerless voltage source inverters (VSIs) are one of the popular topologies for photovoltaic (PV) grid-connected applications due to the lowest component count and simple design. Because of the absence of galvanic isolation in such systems, problem of electromagnetic interference (EMI) due to high leakage current is highly pronounced. Recently various reduced common mode voltage (CMV) PWM (RCMV-PWM) methods which address these issues of CMV, and leakage current have been proposed. These schemes typically function at the expense of increased total harmonic distortion (THD), higher switching loss, high DC link current stress along with limited modulation index range. In this work, 240<inline-formula><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> clamped PWM (240CPWM) is selected as an ideal candidate for grid connected VSIs in terms of all round performance of switching loss, THD, DC link current stress, CMV and leakage current. 240CPWM is a relatively new space vector based PWM method which can attain much superior performance by avoiding the zero states. After a brief overview of the 240CPWM concept, the paper provides a detailed comparison of 240CPWM with conventional space vector PWM (CSVPWM) and discontinuous PWM (DPWM1) methods in terms of each of the above metrics. A 3-phase 208 V 3 kW silicon IGBT based hardware prototype is built to validate the performance of 240CPWM and compared with other popular schemes under grid-connected mode. Experimental results show a 66.7&#x0025; reduction in the peak CMV and 50&#x0025; reduction in leakage current with the proposed scheme as compared to CSVPWM. Also, THD, DC link current stress and inverter switching loss are greatly reduced as an added advantage using the proposed method. A new, combined performance index is proposed to compare the performance of different PWM schemes, and it is shown that the 240CPWM achieves the best value for this index among the PWM methods studied.

Effect of regular and core shell nanoparticles on tuning the electrical properties of polypropylene for high voltage insulation

This research investigates the effect of the addition of regular titanium dioxide (TiO2) and core-shell titanium dioxide coated with Silicon dioxide (TiO2@SiO2) nanoparticles for tailoring the electrical properties of polypropylene (PP) nanocomposites. Nanoparticles with and without surface treatment were added into PP through the solution blending method. Sixteen different compositions were formulated by varying types, concentrations, and surface treatment of nanoparticles. Nanocomposites were characterized by Fourier transform infrared spectroscopy, scanning electron microscope, AC, DC breakdown tests, leakage current, and volume resistivity. The addition of both, regular and core-shell nanoparticles, significantly improved the electrical properties of PP. It was also observed that core-shell nanoparticles revealed better electrical properties as compared to regular nanoparticles. Nanoparticles with surface treatment exhibit superior results as compared to those without treatment. This is because of the lower surface energy of surface-treated nanoparticles causing a uniform dispersion inside the polymer base.

Operation of a transformerless three‐phase PV inverter using multiple bidirectional choppers

AbstractA 50‐kW class three‐phase photovoltaic (PV) inverter typically applies a transformerless circuit configuration due to its high efficiency and small volume/size. On the other hand, the conventional PV inverter may suffer from the deterioration of solar panels due to a dc leakage current flowing through the stray capacitance of the solar panels. To solve this problem, this paper presents a transformerless three‐phase PV inverter consisting of multiple bidirectional choppers, which prevents the dc leakage current by maintaining a negative PV array voltage with respect to the ground potential to a dc voltage that is zero or higher. In addition, the size/volume of an ac filter can be reduced by applying the so‐called phase‐shifted PWM to the inverter. In this paper, the operating principles and control method of the inverter are discussed and the validity of the control method is verified by performing computer simulation using the “PSCAD/EMTDC” software package and conducting experiments using a 150‐V downscaled model.

Evaluation of insulation condition detection methods applied to DC XLPE cable

At present, with the construction and operation of new energy grid-connected and offshore wind power projects in China, DC XLPE cables have been widely used. Under the operating conditions, there are mainly two feasible technical means of DC cable insulation state detection, partial discharge (PD) and leakage current. However, it is still a blank in the world to apply it to the validity and condition of DC cable operation detection. Based on the semi-conductive layer residual defect model of XLPE cable, the characteristics of PD and leakage current under different voltage values and polarities are studied by setting up an experimental platform for detecting PD and leakage current of DC XLPE cable. The results show that both PD and leakage current can detect cable defects, among which PD is easier to detect cable defects under negative polar voltage and higher voltage. Leakage current characteristics change obviously under positive polar voltage. When applied voltage is lower than rated voltage, obvious changes can also be measured. In this paper, the feasibility of live detection of leakage current of DC cables under operating conditions is proposed, which is of great value to improve the detection capability of DC XLPE cables.

Unusual behavior of dislocations freshly-introduced under Schottky contact in GaN

The impact of the dislocations introduced by local plastic deformation on the electric properties of low-ohmic n-GaN was investigated. It was found that the freshly introduced dislocations gave rise to increase of the dc leakage current, to changes in frequency dependence and in the shape of current-voltage, ac conductance and capacitance voltage characteristics of stripe-like Au-Schottky diodes. Besides, a rapid recovery of the dislocation-induced changes of the electric properties to their initial state was observed after several hours at room temperature indicating the impact of the presence of depletion region of Schottky diodes on the dynamic properties of freshly introduced dislocations.

Stray Current DC Corrosion Blind Spots Inherent to Large PV Systems Fault Detection Mechanisms: Elaboration of a Novel Concept

This paper elaborates on a new issue that should progressively receive attention by photovoltaic (PV) installation industry and utility-management entities. It specifically introduces stray current corrosion blind spots that are inherent to PV systems' grounding and associated dc ground fault detection mechanisms. These blind spots arise as the existing thresholds for dc leakage currents have been based on other issues, such as fire or personnel safety. Leakage current may cause accelerated corrosion on PV supporting metallic structures as well as on third-party metallic utilities that may be present in the vicinity of PV installations. The impact of dc stray current corrosion is recognized by the stakeholders across the world in the dc traction community and codes and standards have been developed to provide designers and utility companies with a corrosion-management strategy that defines a level of corrosion risk which is acceptable across infrastructures. This paper outlines the theoretical background that will assist the understanding of stray current corrosion by PV plant developers, designers, and owners and raise awareness with the utility distribution network owners.

Comparative study of AC and DC inclined plane tests on silicone rubber (SiR) insulation

Currently there are no international standards for evaluating the tracking and erosion resistance of DC polymeric insulation under contaminated conditions. Researchers often modify the existing AC inclined plane test standards such as the IEC-60587 to accommodate DC voltage conditions but this has been reported to give various inconsistences. This paper presents comprehensive experimental results on inclined plane tests of silicone rubber (SiR) insulation at 3.5 and 4.5 kV AC and positive DC using intravenous (IV) system as the pollutant supply. The leakage currents (LC) were recorded throughout the entire tests. In addition, various physiochemical tests namely, Fourier Transform Infrared analysis, thermo-gravimetric analysis, scanning electron microscopy and energy dispersive spectroscopy were performed on the aged and unaged samples. Results show that DC LC is bigger (about three times) than that under AC for the same equivalent voltages. Furthermore, DC LC variations are less random and the average magnitudes increase with duration of voltage application compared with AC. The physiochemical analyses show that 3.5 kVrms AC and 3.5 kV DC aged samples have comparable chemical characteristics albeit with electrode corrosion elements detected on the DC aged samples. Under 4.5 kV DC the degradation becomes significantly more severe and unrepeatable. It is therefore concluded that at 0.3 ml/min, pollutant flow rate, 3.5 kV positive DC and 3.5 kVrms AC are comparable as test voltages for inclined plane accelerated ageing of SiR insulation.

Analysis and modeling of leakage current sensor under pulsating direct current

In this paper, the transformation characteristics of current sensor under pulsating DC leakage current is investigated. The mathematical model of current sensor is proposed to accurately describe the secondary side current and excitation current. The transformation process of current sensor is illustrated in details and the transformation error is analyzed from multi aspects. A simulation model is built and a sensor prototype is designed to conduct comparative evaluation, and both simulation and experimental results are presented to verify the correctness of theoretical analysis.

Pr-Fe co-doping induced room temperature multiferroic properties in SrTiO3 films

The structural, leakage current, ferroelectric and magnetic studies of the co-doped SPTFx (x=0.1, 0.2 and 0.3) films have been investigated. X-ray diffraction and atomic force microscopy reveal the microstructure of the samples. It is found that the dc leakage currents of the films are enhanced gradually with increasing Fe ions due to the variable valence of Fe ions and the increase of oxygen vacancies (OVs). More importantly, both the room-temperature ferroelectric and ferromagnetic properties are successfully realized in STO films, whose possible causes are discussed. This work provides an effective method for the induction of the multiferroic properties in paraelectric materials.

直流漏れ電流測定による水トリー劣化位置標定

In general, the degradation span of 6.6kV XLPE cable with plural connections is determined by DC leakage current measurement. This method needs to disconnect and reconnect the shielding layer and the outer semi-conducting layer at the cable connecting part, and hence it takes a lot of time and cost. We investigated a new method to locate the water tree degradation by DC leakage current. In 6.6kV XLPE cables the pulse-like leakage current called “kick” due to water tree degradation is often observed. Consequently, we detected the currents at the DC applying terminal and another terminal, and transformed them to the signals like a sine-wave having the phase difference exactly proportional to the time difference of arrival (TDOA) by using ceramic filters at 455kHz. The position of the water tree is calculated from TDOA obtained by the phase difference. In the experiments to locate the water tree using 6.6kV XLPE cable samples up to 85m long, the mean squared error was 2.2m. From the results, it is possible to locate the water tree in 6.6kV XLPE cable 2km long with an error of about 20m by optimizing the center frequency of the ceramic filter.

Dielectric properties of bismuth niobate films using LaNiO3 bottom electrode

Bi3NbO7 (BNO) thin films were deposited on Pt/TiO2/SiO2/Si (100) and LaNiO3 bottom electrode substrates at room temperature from the polymeric precursor method. X-ray powder diffraction was used to investigate the formation characteristics and stability range of the tetragonal modification of a fluorite-type solid solution. The results showed that this tetragonal, commensurately modulated phase forms through the intermediate formation of the incommensurately modulated cubic fluorite phase followed by the incommensurate-commensurate transformation. LaNiO3 (LNO) bottom electrode strongly promotes the formation of high intensity (111) texture of BNO films. The dielectric constants of the films increased from 192 to 357 at 1 MHz with the bottom electrode while the leakage current behavior at room temperature of the films decreased from 10−7 to 10−8 A/cm2 at a voltage of 5 V. The reduction of dc leakage current is explained on the basis of relative phase stability and improved microstructure of the material. The capacitance density of 75 fC/µm2, dielectric loss of 0.04 % at 1 MHz, and breakdown strength of about 0.30 MV/cm is compatible with embedded decoupling capacitors applications.

Measurement-based electrical characterization of through silicon vias and transmission lines for 3D integration

Measurement-based electrical characterization of through silicon via (TSV) and redistribution layer (RDL) is of great importance for both fabrication process and system design of 3D integration. This paper presents the electrical measurements and analysis of TSV and double-sided RDL test structures, from DC to high frequency up to 40GHz. TSV shows great dependence of DC resistance and leakage current on fabrication process. An inverse V-shaped C–V curve is presented between adjacent TSVs in N-type silicon substrate, from −10V to 10V. In the high frequency characterization, two methods are proposed and applied to extract resistance and inductance of a single grounded TSV. Individual transmission loss of TSV, RDLs on top and bottom surface of silicon substrate are calculated, and corresponding circuit parameters thereof are extracted to characterize their electrical properties precisely.

High Temperature Dielectric Properties of Polyimide/Boron Nitride Nanocomposites: Nanoparticle Size and Filler Content Effects

An interesting way to improve high temperature polyimides (PI) electrical properties by filling it with boron nitride nanoparticles (BN) is proposed in this study. The filler content (from 0 to 60 vol.%) and the average diameter size (35 nm and 120 nm) of these nanoparticles have been varied. The preparation of the patented PI/BN nanocomposite films was performed following an optimized process in order to obtain a good dispersion of the nanoparticles in the films. The nanocomposite films were electrically characterized in the range 200–350 °C. In the case where the nanoparticles had the smallest size, the DC conductivity and leakage currents decreased from 4 to 6 orders of magnitude when the PI/BN films were compared to the neat PI films. The improvement of this property was obtained by a small amount of nanoparticles (1.6 vol.%). Results also show an increase in the dielectric strength of 1 MV/cm for a filler content of 42 vol.%. In the case where the nanoparticles had a larger size, a small improvement was seen on the DC conductivity and leakage currents. In contrast, the breakdown field was lower than the one of the neat PI, due to aggregate formation within the films.

Electrical design of through silicon via (lee, m., et al; 2014) [book review

This is a very interesting and well written book on a hot topic nowadays in signal and power integrity: the study of three dimensional integrated circuits (3D IC) with particular emphasis on the analysis and design of Through Silicon Vias (TSV). TSV, in very basic terms, is a metal pillar inside the silicon (Si) wafer with a silicon oxide coating (‘liner’) to block the DC leakage current between the metal pillar and the silicon wafer.

Fabrication and electrical characteristics of a novel interposer with polymer liner and silicon pillars with ultra-low-resistivity as through-silicon-vias (TSVs) for 2.5D/3D applications

Motivated by the desire of process simplicity and feasibility for 2.5D/3D integration, a novel interposer technique with polymer liner and silicon pillars of ultra-low-resistivity as through-silicon-vias (TSVs) is proposed in this paper. Silicon pillars with ultra-low-resistivity, instead of conventionally electroplating copper posts, are utilized as vertical via conductors, and low-k polymer Benzocyclobutene, other than silicon dioxide (SiO2), is used to form liners. Fabrication techniques and electrical characteristics of the proposed interposer are illustrated. Test vehicles are successfully fabricated and their electrical characteristics including DC resistance and leakage current are measured. The results show that the DC resistances of the proposed TSVs are averaged at 5.94, 2.68 and 1.72 Ω for silicon pillars with diameters of 10, 20 and 30 μm respectively. Also, with a DC bias voltage of 10 V, the leakage current between TSV to silicon substrate is as low as 6.79 pA. These elementary results illustrate the simplicity, feasibility and high reliability of the proposed interposer structure.