Positive DC Voltage Research Articles

Growth and partial discharge characteristics of DC electrical trees in cross-linked polyethylene

In the present study, the growth and partial discharges properties of DC electrical trees in cross-linked polyethylene (XLPE) were investigated under positive and negative DC voltages. The obtained results revealed that the voltage rise exerted significant influences on growth properties of DC trees. In either polarity, DC electrical trees grew rapidly during rising voltage and maintained the rapid growth for a certain time after DC voltage reached a constant value. The growth rate of positive DC electrical trees was quite faster than that of negative ones, and channel breakdown mainly occurred in positive ones. Although the growth rates of positive and negative DC electrical trees were different, their shapes were similar (both were branch-like electrical trees with one or two main branches). Self-healing usually occurred in negative DC electrical trees, thus electrical tree branches gradually disappeared under the application of steady DC voltage or when the voltage was off. Partial discharges mainly occurred at the beginning of electrical tree growth; however, the amount of partial discharge was relatively small, and it gradually reduced even during the rapid growth of electrical trees, consequently, almost no partial discharges were found when the voltage was constant. Finally, a discharge-avalanche theory was proposed to explain the growth properties of DC electrical trees.

Study on Surface Charge Accumulation Characteristics of Resin Impregnated Paper Wall Bushing Core Under Positive DC Voltage

As a critical component of a high-voltage direct current (HVDC) transmission system, resin impregnated paper (RIP) wall bushing has become a weak point because of its surface charge accumulation. This paper studies a model RIP wall bushing core designed by the equal capacitance method. The stationary resistive field along the gas–solid interface of the RIP wall bushing core is investigated theoretically by a gas model, which considers the non-linearly field-dependent volume conductivity. The results show that the gas conductivity along the core surface tends to be an arched distribution from the high-voltage conductor to the end shielding screen. The surface charge mainly accumulates at the turning point of the radius, which may threaten the core’s insulation. Then, the surface charge is obtained through a measurement system, where the experimental results are highly consistent with the simulation results. Considering the time constant of charge dissipation is nearly 15 min, it would be better to measure the surface charge on one axial direction of RIP wall bushing core after each voltage application. The simulation and experimental results of this paper can guide the design of a RIP wall bushing core.

Understanding the dielectric and mechanical properties of self‐passivated Al–epoxy nanocomposites

Epoxy nano passivated aluminium composites with optimised size and filler contents were fabricated. Variation in contact angle and surface roughness is insignificant with increasing filler into nanocomposites but it showed a drastic reduction on corona ageing. Water droplet initiated corona inception voltage (CIV) is high under the negative DC voltage followed with positive DC and AC voltages. The bandwidth of ultra-high frequency signal generated due to water droplet initiated corona discharge lies in the range of 0.5–1.2 GHz. Surface potential measurements have shown that the decay in the potential was fast initially, and it became slower and sluggish subsequently. The trap energy density versus trap depth plot exhibits shallow traps and deep traps at around 0.8 and 0.87 eV, respectively. Permittivity, conductivity and loss factor have increased with an increase in the filler content in nanocomposites. The bulk resistance and capacitance of samples were determined for obtaining the equivalent parallel RC circuit model. Incorporation of nanofillers increases the glass transition temperature and reduces the tan δ with increasing frequencies as evident from dynamic mechanical analysis studies. A direct correlation is observed between the plasma temperature measured through laser-induced breakdown spectroscopy spectra and hardness of the material.

Implementation of Single Phase Multimode Matrix AC-DC Converter

This paper proposes a novel topology of multimode matrix AC-DC converter which gives multiple DC outputs using a single AC input. The generated multiple outputs appearing across load has both polarity (Positive and negative DC voltages). The control scheme developed for this converter is consisting of Op_Amps and AND gates and IGBTs are used as power devices. The multiple dc output are synthesized using sinusoidal pulse width modulation technique. A prototype model of proposed converter has been implemented and resultant voltage and current waveforms are analyzed. It has been observed that experimental and simulation results are promising for industrial applications with variable voltage.

Implementation of Single Phase Multimode Matrix AC DC Converter

This paper proposes a novel topology of multimode matrix AC-DC converter which gives multiple DC outputs using a single AC input. The generated multiple outputs appearing across load has both polarity (Positive and negative DC voltages). The control scheme developed for this converter is consisting of Op_Amps and AND gates and IGBTs are used as power devices. The multiple dc output are synthesized using sinusoidal pulse width modulation technique. A prototype model of proposed converter has been implemented and resultant voltage and current waveforms are analyzed. It has been observed that experimental and simulation results are promising for industrial applications with variable voltage.

Investigation on dielectric and mechanical properties of epoxy reinforced with glass fiber and nano-silica composites

The composite materials of different filler concentration were fabricated by reinforcing the glass fiber and silica nanoparticles into epoxy to understand the variation in surface wettability, water droplet initiated discharge, dielectric parameters, surface charge, space charge and mechanical properties. Nanofiller inclusion has shown the enhanced surface properties viz. water droplet initiated corona inception voltage (CIV) and hydrophobicity. CIV is high under negative DC voltage followed with positive DC and AC voltage. It is realized that the dielectric constant is increased with the addition of nano-silica. The surface potential has decayed exponentially irrespective of the applied voltage polarity and the type of the sample, whereas, the 3 wt% sample showed the highest decay rate for an applied voltage. Silica filled composites have shown better space charge accumulation and decay characteristics initially and then deteriorated rapidly when the filler concentration is increased. The optimised filler concentration is found considering the minimal space chare accumulation, decay rate and the optimised electric field for the space charge analysis. Further, the optimised field conditions were devised for the optimum filler concentration for the prolonged life span and reliability of the insulation material. The mechanical storage modulus has increased with the addition of nanofiller. The plasma temperature calculated from the LIBS spectra has shown a direct correlation with the surface hardness of the material.

PD pulse waveform and charge distribution in oil-pressboard insulation with needle-plate model at positive DC voltage: Their characteristics and relationship

A better understanding of the complicated characteristics and interrelationships between the charge distributions and partial discharge (PD) pulse waveforms occurring in oil-pressboard insulation can be used to characterise the PD mechanism as a basis for the optimisation of the insulation structure to minimise PD activity. These characteristics were studied by using a non-inductive resistor to measure the PD pulse waveforms produced in oil-pressboard insulation by a needle-plate at positive DC voltage and through additional simulations based on bipolar charge transport and hydrodynamic drift-diffusion models. It was found that the PD pulse waveforms are unimodal, and with increased applied voltage, their rising and falling edges and equivalent times decrease, and their apparent charge amplitudes and equivalent frequencies increase. These parameters have a saturation state at high voltages (>45 kV) and the oil streamers have voltage-dependent shapes and evolution velocities (vs); the higher the voltages, the higher the values of vs, and the wider are the streamer bodies. Accumulated charge can form a shield layer that hinders streamer evolution. Analysis of the PD mechanism in terms of the relationships between charge distribution and PD pulse waveform reveal that the rising and falling edges of a PD pulse correspond to streamer propagation in the oil gap and charge accumulation at the pressboard surface (i.e., the charging process in the pressboard), respectively. The peak value of the PD pulse waveform is shown to be related to the conduction current in the coupling loop as reflected by the current density at the streamer head.

Electrical Tree Characteristics in Polypropylene Under Impulse Superimposed DC Voltage in LN2

During the operation of the high-temperature superconducting (HTS) dc cable, due to the operation of the power electronic device, the cable insulation is subjected to the repeated impulse overvoltage while being subjected to the rated dc voltage, which easily generates the electrical tree and causes insulation failure. Polypropylene, the primary substrate for polypropylene laminated paper, faces a working environment in liquid nitrogen (LN 2 ). The insulation performance of polypropylene in a low-temperature environment may change and affect its degradation process, threatening the safe and stable operation of power system. In this paper, a needle-plane electrode system was used to study the electrical tree characteristics in LN 2 . The results show that the tree morphology changes with different impulse frequency under dc-impulse voltage. With the increase in the positive dc voltage, the treeing process is accelerated. However, with the increase in the negative dc amplitude, the tree length decreases. With the impulse frequency increasing, the tree length increases significantly.

Understanding the influence of ambience on thermal ageing of natural ester liquid

Ambience during thermal ageing of ester liquid has a high impact on corona inception voltage (CIV) under AC and DC voltages. The CIV is high with ester liquid thermally aged in nitrogen ambience and is the least with air aged. CIV is high under negative DC voltage followed by positive DC and AC voltages. CIV reduces with increase in harmonic voltage and the reduction is predominant with harmonic voltages with higher total harmonic distortions. Corona activity radiates ultra-high-frequency (UHF) signal with its dominant frequency at 1 GHz. Phase-resolved partial discharge analysis carried out using UHF signal indicates that corona discharges occur at the peak of the voltage waveform. The magnitude of discharges is less in thermally aged liquid but the number of discharges increases. Fourier transform infrared spectroscopy analysis indicates the formation of carbonaceous particles with thermal ageing. Thermal ageing of ester liquid causes to form new chromophoric moieties causing red shift of absorption band in ultraviolet-visible spectra. Fluorescence spectroscopy traces the characteristic variations due to thermal ageing, which can be adopted for condition monitoring.

Flow visualization and mechanisms of three-electrode sliding discharge plasma actuator

The unsteady flow characteristics induced by a three-electrode sliding discharge plasma actuator under different actuation modes were analyzed by ensemble averaged and phase-locked particle image velocimetry. The discharge morphologies, voltage–current waveforms, and particle image velocimetry data in continuous mode were compared to clarify the performance modification mechanism of the pulsed three-electrode sliding discharge. The particle image velocimetry results revealed that deformation of the electromagnetic field around the additional electrode caused by applying a high DC voltage triggers changes in the induced flow field. When the three-electrode sliding discharge plasma actuator is actuated in continuous mode, a strong accelerated wall jet and homogeneous discharge region covering the whole gap between the two exposed electrodes are generated. The large discharge extension mainly results from the accelerated drift of positive ion particles created during the positive half cycle, while negatively ionized particles have a significantly larger impact on the induced velocity production process. In the pulsed mode, when a positive high DC voltage ( VDC = 18 kV) is applied to the additional electrode, both the size and magnitude of the induced vortex structures increase, and highly accelerated regions are periodically generated. When V DC = −18 kV, the induced velocity field evens out, the accelerated region becomes less obvious, the intensity of both the primary and secondary vortices decreases, and the vortex structure dissipates faster, owing to the turbulent motion of ionized particles. An additional positive DC component attracts the negatively ionized particles during the negative half cycle, improving the velocity and intensity of the stream-wise vortices, which is very attractive for flow control applications.

Electrical treeing initiation and breakdown phenomenon in polypropylene under DC and pulse combined voltages

In this paper, the effects of DC voltage, pulse voltage and pulse frequency on the electrical treeing characteristics in polypropylene (PP) under DC and pulse combined voltages were investigated. The rise time of the pulse voltage is 50 μs. The DC amplitude range is from −25 kV to +25 kV, and the pulse amplitude range is from −35 kV to +35 kV. The pulse frequency range is from 400 to 800 Hz. The results show that the electrical tree structures change with different DC amplitudes, pulse amplitudes and pulse frequencies. With the increase in the positive DC voltage, the treeing process is accelerated. However, with the increase in the negative DC amplitude, the breakdown time decreases first and then increases. This phenomenon is attributed to the different charge transport processes under different DC amplitudes. With the pulse amplitude and frequency increasing, the growth rate and accumulated damage increase significantly. The pulse amplitude and frequency affect significantly the breakdown time. The pulse voltage with the same polarity as that of the DC voltage is more likely to cause breakdown than the pulse voltage with opposite polarity of that of the DC voltage because of the larger effective values of the composite voltages.

PD classification in oil-pressboard insulation by simulating a needle-plate model with +DC voltage

The DC voltage related partial discharge (PD) classification in solid dielectrics under a uniform electric field has been studied, but it is more complicated in liquid-solid composite insulation under a highly non-uniform electric field. Therefore, PD pulse waveforms and PD channel shapes in oil-pressboard insulation were investigated by simulating a needle-plate model with a positive DC voltage via pulse current method. The elements of PD can be classified in order of ionizing intensity strength as corona, tree-like PD, single-streamer PD, or multi-streamer PD according to the discharge channel shape. The corona pulses are sharply oscillating, whereas the other types of PD can generate unimodal pulses with no over-zero oscillation. A finite element analysis reveals the PD types are related to the charge quantity at the oil-pressboard interface, which influences the streamer evolution by distorting the space electric field. This study uncovers the physical process during PD in oil-pressboard insulation and provides basis for explaining the PD mechanism.

Influence of fine particulate matter on the variation of surface morphologies of conductors subjected to positive DC voltages

A study on the effect of fine particulate matter on the surface morphologies of conductors subjected to high positive dc voltages has been undertaken. A global distribution pattern of particulate matter was observed in the shape of parallel chains consisting of particle agglomerations. Based on a coupling model of electric and force fields, the dynamic progress of particles near conductor surfaces was investigated and the effects of forces exerted on the particles were analyzed. The experimental observations agreed well with calculated results. It was found that the interactions between particles played a critical role in the formation of agglomerations and parallel chains. The surface roughness and the corona discharge intensity of the conductors increased significantly as a result of the variation of surface morphologies.

Contribution of surface charges on high‐voltage DC silicon rubber insulators to DC flashover performance

In this study, the results of measurement for DC flashover characteristics of silicon rubber insulators in the presence of surface charges and in terms of geometric characteristics have been examined and analysed. In the carried out experimental tests, the surface of different insulators was charged by an external corona source and both the metal end fittings were kept grounded while the needles have been connected to a certain applied negative and positive DC voltages and then surface electric charge measured. A series of flashover tests have been carried out on the charged insulators under positive DC voltages to investigate the effects of surface charges. The experimental results revealed that positive electric charges reduced flashover performance while negative charges increased the flashover voltage level. Also, the experimental modelling of DC flashover of charged insulators regarding the ratio of shed spacing to shed depth and specific leakage distance in terms of surface electric charges, revealed that flashover voltage gradient of the charged insulator is most affected by specific leakage.

Streamer characteristic and breakdown in a mineral oil and a synthetic ester liquid under DC voltage

This paper presents experimental studies of streamer propagation and breakdown in a mineral oil and a synthetic ester liquid under both positive and negative DC voltages. A point-plane electrode configuration with a point tip radius of 10 μm was used. Characteristics of streamer including length, propagation velocity and shape were analysed based on shadowgraph images obtained at a gap distance of 10 mm using a multi-channel ultra-high speed camera. Under positive polarity, the streamers in both liquids propagate in the 2nd mode; the stopping length of the synthetic ester is longer than that of the mineral oil at the same applied voltage level. However there is no obvious streamer propagation (less than 10% of the gap distance) under negative polarity even when the applied voltage approaches to the breakdown voltage; the streamers in both liquids stay in the 1st mode. Breakdown tests at various gaps of 2 mm, 5 mm, 10 mm, 20 mm and 30 mm were also investigated. 50% breakdown voltages of the synthetic ester liquid are lower than those of the mineral oil at all the gap distances investigated under both polarities.

A novel electrical method to measure wire tensions for time projection chambers

We present a novel electrical technique to measure the tension of wires in multi-wire drift chambers. We create alternating electric fields by biasing adjacent wires on both sides of a test wire with a superposition of positive and negative DC voltages on an AC signal (VAC±VDC). The resulting oscillations of the wire will display a resonance at its natural frequency, and the corresponding change of the capacitance will lead to a measurable current. This scheme is scalable to multiple wires and therefore enables us to precisely measure the tension of a large number of wires in a short time. This technique can also be applied at cryogenic temperatures making it an attractive solution for future large time-projection chambers such as the DUNE detector. We present the concept, an example implementation and its performance in a real-world scenario and discuss the limitations of the sensitivity of the system in terms of voltage and wire length.

Analysis of the production mechanism of H2O2 in water treated by helium DC plasma jets

In this paper, helium plasma jets are used for water activation to explore the production mechanism of H2O2 in the water. Either positive or negative DC voltage is used for the plasma excitation. It is found that the concentration of aqueous H2O2 induced by the positive plasma jet is ~4 times larger than that by the negative plasma jet with similar average discharge current. Three production pathways of aqueous H2O2 are considered important in literature, including the electrolysis, the dissolution of gaseous H2O2, and the combination of aqueous OH. However, the first two pathways are found to have similar contributions on H2O2 production for the positive and negative plasma jets. It is deduced that the OH combination in the surface layer of the plasma-activated water is most responsible for the concentration difference of aqueous H2O2 between the two plasma jets.

About the influence of temperature and humidity level on pulse shape parameters of positive DC corona discharges in air

The increasing importance of HVDC transmission systems requires the development and evaluation of suitable basic approaches regarding the partial discharge diagnosis. Although diverse promising approaches are identified, recent research is focused on standard atmospheric conditions. Due to the fact that this is just partly consistent to real operating conditions, additional research is required. Focusing on the time domain analysis of corona discharges, occurring under positive DC voltage stress in air, a measurement method for investigating the influence of varying atmospheric quantities is presented. Measurements are carried out for five different relative humidity levels in the range of 20 % to 95 % and for four different temperature levels in the range of 20 °C to 65 °C. As characterizing pulse shape parameters, the rise time, the pulse width and the fall time are determined as well as the apparent charge. The gained values are compared to each other and reconciled with physical processes.

DC electrical tree initiation in silicone rubber under temperature gradient

In this paper, initiation behaviors of DC electrical tree in silicone rubber were studied under different voltage polarities and temperature gradients (temperature of needle and ground electrodes ranged from 20 to 120 °C). Electrical tree initiation probability, tree shape, and tree length were obtained via the statistics. The results show that polarity effects, where the positive DC voltage is easier for trees to initiate, appear evidently under various temperature situations. As for the temperature gradient, tree initiation voltage decreases first and then returns to a larger value with the increasing needle temperature when the ground electrode temperature is fixed at 20 °C. Both the probability of multiple-branch tree and corresponding tree length increase and subsequently decrease. However, tree initiation voltage decreases alone and tree length increases with the rise of ground electrode temperature when the needle temperature is fixed at 20 °C. Homo charge injection becomes easier with increasing needle temperature following the Schottky-Richardson approximation under ramped DC voltages. In addition, electric field strength at the tip decreases when the needle temperature is higher than ground temperature, since the conductivity is sensitive to the local temperature and electric field. It is believed that the needle temperature and the temperature gradient dominate tree initiation behaviors, where the former affects the collisional ionization and partial relaxation of silicone rubber chains, and the latter decides electric field distribution in the bulk.

Influence of moisture on the interface charge of oil–pressboard composite insulation under DC voltage

The distortion of the electric field in the oil–pressboard composite insulation caused by the accumulation of the interface charge is detrimental to both the insulation design and operation of converter transformers. The influence of moisture content on the surface charge accumulation of oil–pressboard insulation under DC voltage was studied in this study. In accordance with the Kerr electro-optic effect, the electric field strengths in transformer oil and the surface charge density were acquired after applying the positive and negative DC voltages in three oil–pressboard insulation models with different moisture content, respectively. The resistivities of the oil and pressboard in three models, namely Model 1# with 3.8–4.2 ppm moisture in oil and 0.35–0.37% moisture in pressboard, Model 2# with 7.6–7.9 ppm moisture in oil and 0.79–0.82% moisture in pressboard and Model 3# with 14.9–15.4 ppm moisture in oil and 1.39–1.42% moisture in pressboard, was also measured. The results indicate that: (i) as negative charges in oil accumulated on the pressboard surface in a much greater speed than the positive ones, the electric field in transformer oil under negative DC voltage decreases more rapidly with time than that under positive DC voltage; (ii) the increase of the moisture content in both oil and pressboard, under either positive or negative DC voltage, leads to the decrease of both the electric field strength in transformer oil and the charge density with time; and (iii) the increase of moisture content could not only decrease the resistivity of both oil and pressboard, but also the ratio of the resistivity between the pressboard and the oil. On the basis of the Maxwell–Wagner theory, the decrease of the ratio between the pressboard and oil could lead to the decrease of the interfacial charge density, leading to the slow transient process of the electric field in transformer oil under DC voltage.