DC Prestress Research Articles

Hindered phenolic antioxidant grafting on tailoring the DC electrical characteristics of polypropylene cable insulation

AbstractThe authors focus on the impact of melt‐free radical grafting with hindered phenolic antioxidants (AO3052) on the electrical properties of polypropylene (PP) for DC cable insulation. The DC conductivity, space charge distribution and breakdown characteristic tests of grafting‐modified PP are performed by comparing unmodified PP. The results demonstrate that the grafting of antioxidants can effectively suppress space charge injection, owing to the deeper trap sites at the grafting molecule. The breakdown strength of the grafted PP is significantly enhanced from 30°C to 90°C and especially achieves a 5.3%–6.7% increase after the same DC‐prestressed time at 90°C. The surface electrostatic potential and molecular orbitals of the grafted PP are calculated. Simulation shows that the antioxidant introduces multi‐level local state traps that can effectively trap the injected space charge, thus decreasing the destruction of molecular chains by electrons and increasing the breakdown strength level. In conclusion, antioxidant grafting modification can improve the breakdown characteristics with or without DC prestress, and thus it appears to be promising in the application of PP‐insulated cables.

Breakdown Voltage of Polymeric HVDC Insulation at DC Stress and Superimposed Lightning Impulse Voltages

Space charge formation makes it difficult to predict the resulting electric field distribution within HVDC cable insulation.The main purpose of the work presented here has been to experimentally study how the lightning impulse breakdown strength of thin (0.1 mm thick) insulation foils of polypropylene (PP) and Polyethylene Terephthalate (PET) are affected by DC prestress, polarity of the impulse voltage and the temperature.Tests were performed using parallel plate electrodes and prior to DC prestressing the lightning impulse breakdown strengths were found to be approximately 17.5 kV for samples of both PP and PET. Prior to superimposing lightning impulse breakdown voltages, a DC voltage of 8 kV was applied for 60 seconds. The effect of DC prestressing was to significantly increase the impulse breakdown strength during aiding and reduce the breakdown strength during opposing impulse polarity. This strongly support the assumption of homo-charge formation close to the metal electrodes during DC stress.The results showed higher and more temperature dependent charge accumulation in samples of PET. It is indicated that at 22 °C the homo-charge formed during DC stressing approximately reduced the electric field at the electrodes from 100 to 48 kV/mm in the PP samples and from 80 to 22 kV/mm in the PET samples, respectively

The Estimation of Lightning Impulses Superimposed onto Pre-Stressed DC Breakdown Voltages Using the Leader Propagation Method

The impulse breakdown attributes are of rudimentary importance in the insulation coordination in the gas-insulated switchgear (GIS). The critical voltage waveshape for the insulation is the surge over voltages. Usually, such over-voltages will emerge on the electrical equipment while these are burdened under DC or AC working voltages. Therefore, during relatively small-time intervals, the undermined devices are exposed to surge voltages that are superimposed upon the already running DC or AC voltages. From a safety point of view, and for providing reliable power to the consumer, we need to study the breakdown characteristics under such over-voltages, with reference to normal operating voltages (DC or AC). The breakdown mechanism of the GIS under compressed SF6 gas is known to be controlled by a stepped leader propagation method. Although different experimental studies have been conducted by many researchers for different experimental conditions under DC, AC, or impulse high voltages alone, not much research has been performed, in a systematic way, to model and estimate the breakdown voltages of the GIS under such superimposed conditions. This paper presents a systematic model, using a leader propagation technique for the estimation of breakdown voltages for complex voltage conditions, i.e., a lightning impulse (LI) superimposed on pre-stressed DC for different experimental conditions. The estimated values are in good agreement with the measured experimental results.

Effects of Hindered Phenolic Antioxidants on Space Charge and Breakdown Properties of Polypropylene

This paper focuses on the effects of hindered phenolic antioxidants AO300 and AO736 with similar molecular structure on the electrical properties of isotactic polypropylene (iPP). Experiments of conductivity, space charge distribution, and DC breakdown strength after the hetero-polarity DC prestress are performed. The experimental results show that these two antioxidants can increase the conductivity-temperature coefficient, reduce the accumulation of space charge and improve the DC breakdown strength after the hetero-polarity DC prestress of iPP. By calculating the molecular orbitals of these two antioxidants, it is found that the antioxidants introduce local states in the forbidden band of iPP matrix to capture high-energy charges. Besides, the bond dissociation energy (BDE) of the hydrogen-oxygen bond in AO736 is lower than that in AO300, representing a stronger ability of proton transfer, which scavenges the radicals, inhibits the growth of microvoids and prevents the process of breakdown. It is concluded that the improvement of space charge and breakdown properties is attributed to the coupling effects of deep trap sites and radicals scavenging introduced by antioxidant. This research shows that AO736 has potential applications in PP insulation.

Experimental Assessment on Air Clearance of Multiple Valve Unit Considering Switching Impulse and DC Superimposed Switching Impulse

Multiple valve unit (MVU), which converts AC to DC and DC to AC, is one of the key elements of high voltage DC (HVDC) transmission. Therefore, the insulation design of MVU against overvoltage should be considered for the stable and reliable operation of HVDC transmission system. Especially, the air clearance of MVU should be calculated based the switching impulse, since it is fatal to MVU in terms of electrical insulation. However, the previous studies were limited to wave front, and the air clearance of the switching impulse is specified only for an ultra-high voltage (UHV) above 750 kV. As a result, it is difficult to calculate the air clearance of MVU which must endure for a switching impulse under 750 kV. In addition, when the switching impulse introduced while the MVU is in normal operation, it is superimposed to DC and creates the most severe situation, but the studies on such subjects are also insufficient. Therefore, as a fundamental step to calculate the air clearance of MVU, the dielectric characteristics of switching impulse and DC superimposed switching impulse in air have been investigated. The experiments on switching impulse showed that the critical flashover voltage was varied according to the curvature of electrode in the gap distance, up to eight times of the electrode radius. However, beyond that gap distance, the critical flashover voltage became similar, regardless of the radius of electrodes. In case of the superimposed experiment, it was performed according to DC pre-stress level and the polarities of switching impulse. The results were most severe when the positive switching impulse was superimposed on the positive DC, and the peak voltage at which flashover occurs was independent of DC pre-stress.

Simulation of Influence of DC Pre-Stress on Space-Charge Characteristics of Cross-Linked Polyethylene in Inhomogeneous Field

Space charge is one of the main causes of electrical tree initiation in high-voltage cable insulation. To investigate the influence of DC pre-stress on the space-charge distribution of cross-linked polyethylene (XLPE), a two-dimensional needle-plate electrode model is established in this study. Based on the bipolar charge-transport model, the space-charge distribution characteristics of the needle-plate electrode model under DC pre-stress are simulated and analyzed. The space-charge distribution characteristics are compared with that of the grounded DC electrical tree initiation properties. The work in this study illustrates that the pre-stress time and value affect the space-charge distribution characteristics. There is a certain relationship between space-charge distribution characteristics and electrical tree initiation characteristics. The distribution range of the DC grounded electrical tree is very similar to that of space charge. Both have a similar distribution shape and increased trend under different pre-stress times, and both have an obvious polarity effect.

Electrical tree initiation in XLPE cable insulation under constant DC, grounded DC, and at elevated temperature

Electrical tree initiation behavior was investigated in XLPE insulation samples made from a HVDC cable by applying constant DC and grounded DC voltages under different temperatures. No trees appeared when a constant DC voltage of −60 kV was applied for 12 days at room temperature or 20 days at 90°C, and the tree initiation time was estimated to be more than 200 days by Bayes statistical processing of zero-failure data. In the grounded DC voltage test, −15 kV prestress could trigger a tree, and under −35 kV, the tree initiation rate reached 100 %. With the increase of DC prestress, both the initiation rate and tree length increased. Grounded DC trees were branch trees with dark-colored channels, and a similar material damage could be caused by the same DC prestress. The connection of a grounding resistor could reduce the damage of sample caused by the grounded DC voltage, and the tree initiation rate declined with the increase of the resistance. According to the experimental results, the tree initiation time under constant DC voltage was not significantly shortened at 90°C, whereas tree initiation under grounded DC voltage was promoted by the increase of temperature. From 25 to 70°C, the tree branches became longer and denser, and extended in more varied directions. The observations are explained by the injection, transfer, trapping and detrapping of space charges, and charge detrapping is believed to be the cause of material damage.

Electrical Tree Initiation Properties in Cross-Linked Polyethylene Under DC-Impulse Composite Voltages

DC cables may be subjected to impulse overvoltage during operation, which may cause electrical tree inside the cable insulation. So, it is necessary to investigate the influence of impulse voltage on the insulation of dc cable. The initiation characteristics of electrical trees in cross-linked polyethylene under dc-impulse composite voltages were investigated in this paper. The influences of pre-stressed dc voltage, impulse voltage, polarity, and space charge accumulation on the initiation characteristics were analysed. The experimental results indicated that the impulse voltage would facilitate the electrical tree initiation and had an obvious polarity effect. Electrical tree initiation voltages under positive impulse voltages were lower than those under negative ones, which was independent of the polarity of the pre-stressed dc voltage. With the increase of the pre-stressed dc voltage, the initiation impulse voltages declined when dc and impulse voltages were of the same polarity. In the presence of opposite polarity, the initiation impulse voltage did not change obviously at first and then increased with the pre-stressed dc voltage.

Study on insulation characteristics of GIS under combined voltage of DC and lightning impulse

Operating experience shows that residual DC voltage remains in GIS bus bar for a long time after opening of disconnectors. This DC voltage will not only cause accumulation of charges and contaminants on the insulator surface, but will also easily lead to motion of free metallic particles in SF6 gas gap, weakening the insulation ability of GIS. At the time of disconnectors reclosing, impulse voltage could be generated in the bus bar and superimpose on the pre-existing DC voltage. As a result, GIS will be under combined voltage of DC and impulse. This paper experimentally studies the insulation characteristics of GIS under combined voltage of DC and lightning impulse. It is found that, for non-contaminated insulators, when DC and LI voltage are of the same polarity, flashover voltage under combined voltage is nearly the same as the flashover voltage under LI alone. However, when they are of the opposite polarity, flashover voltage decreases with increasing DC voltage. In cases there are metallic particle or powder around insulator, pre-stressed DC voltage will cause adhesion of contaminants on the insulator surface and lead to decrease of flashover voltage. Specially, for the particle-contaminated insulator, it is found that the surface charge accumulation caused by DC voltage will have big influence on the flashover voltage, and the most critical condition is when DC and LI voltage are of the opposite polarity. In addition, the breakdown characteristics of SF6 gas gap with free metallic particle are investigated. It is found that under pre-stressed DC voltage, particle will do standing motion or bouncing motion. And breakdown voltage becomes lowest when particle is doing standing motion. The results of the experiments show that residual DC voltage in GIS could deteriorate its insulation ability and the combined voltage of DC and impulse is found to be a critical condition for GIS insulation. Moreover, the combined voltage is shown to be more sensitive to detect some insulation defects in GIS than applying impulse voltage alone, especially for free metallic particles, which are commonly found in GIS but up to now no effective detection method is proposed. Therefore combined voltage of DC and impulse could be considered to serve as a supplement of field test for GIS.

Effect of DC Prestressing on Periodic Grounded DC Tree in Cross-Linked Polyethylene at Different Temperatures

Periodic grounded dc trees in cross-linked polyethylene under various dc prestressing times are investigated in the temperature range of 20 °C–80 °C. Space charge behaviors in the samples during dc prestressing are simulated based on the bipolar charge transport model. The results reveal that the dc prestressing time has different effects on the tree growth at different temperatures, which is because that the space charge behavior in the sample during dc prestressing is closely related to the dc prestressing time and the temperature, and it has both promotion effect and impedance effect on the tree growth. The moderate promotion effect and impedance effect under 160 s dc prestressing at 20 °C, and the strongest promotion effect and the weakest impedance effect under 240 s dc prestressing at 40 °C result in the largest tree lengths, widths, and accumulated damages under each corresponding condition. At 60 °C and 80 °C, the lengths, widths, and accumulated damages are the largest under 240 s dc prestressing, smaller under 80 s dc prestressing, and the smallest under 160 s dc prestressing, and the reason is believed to be the growth rates reversal during the tree growth. Also it is found that the dc prestressing time has significant effects on the tree shape at 60 °C and 80 °C, which is believed to be related to the wider charge diffusion range under longer dc prestressing time. The electrical tree characteristics are discussed in detail combined with the space charge behaviors.

AC and DC pre-stressed electrical trees in LDPE and its aluminum oxide nanocomposites

Resistance of pure low density polyethylene (LDPE) and its aluminum oxide nanocomposites (up to 3.0 wt%) to degradation by electrical treeing under AC stress and DC pre-stress is analyzed. The experiments were carried out on wire-plane electrode specimens before and after exposure to thermal and DC electro-thermal ageing at 80 °C. The obtained results showed enhanced resistance of the nanocomposites to electrical tree inception under AC stress and the tree inception voltage (TIV) increased with nanoparticles content. It has been shown that there was an improved partial discharge (PD) resistance in the nanocomposites compared to the unfilled LDPE. The results also showed that the AC TIV in the nanocomposites consistently increased with the ageing and especially the DC electro-thermally aged specimens had about 30% higher the AC TIV as compared to the unaged material. This effect is attributed to significantly reduced mobility of charge carriers in the nanocomposites. The DC pre-stressed electrical trees generated in the investigated materials were of filamentary-branch structure and the branch channels content increases with the addition of nanoparticles. The mean tree number of the DC pre-stressed electrical trees decreased in the LDPE and its nanocomposites while the mean maximum tree length increased with the ageing treatments. It is postulated that material recrystallization and a very high electric field level on the wire electrode during the DC pre-stressed electrical tree test are the main reasons for the observed effects.

Flashover tests on air gap of ±800kV DC transmission line under composite DC and switching impulse voltage

This paper deals with the air gap insulation of ±800 kV DC transmission systems under composite DC and switching impulse voltage. The gaps studied were representative of ±800 kV DC transmission line, as rod-to-plane and conductor-to-tower gaps. The flashover voltages of the gaps at an altitude of 2100 m were determined for a pure switching impulse and composite voltages of DC and switching impulse. It is found that there are clear increase in the composite flashover voltage of both rod-to-plane and conductor-to-tower gaps, compared with those in the gap that only impulse voltage was applied. From the test results, the flashover voltage increases by 17-23%in the rod-to-plane gap and 5-10% in the conductor-to-tower gap, when the DC voltage of 800 kV is pre-stressed. The standard deviations have little obvious variation under the pre-stressed DC voltage and its average value is 4-4.5% in the tests. Additionally, a 1,000,000 fps high speed CCD camera is used to observe the breakdown process for investigation on the influence of pre-stressed DC voltage on the conductor. The leader inception and breakdown process properties were studied. These data and explanation are useful for ±800kV DC insulation design and testing.

Electrical tree initiation in XLPE cable insulation by application of DC and impulse voltage

Electrical tree initiation behavior of XLPE cable insulation was investigated by application of DC voltage, impulse voltage, repetitive DC voltage, and impulse voltage after DC prestressing. Treeing test samples are made of XLPE specimens with a pin-plane electrode system. The needle inserted has a tip radius of 5 μm and a pinpoint angle of 30 °, and the pin-plane distance is 2 mm. No trees appeared when DC voltage up to 70 kV or -60 kV was applied on the samples continuously or repetitively. Impulse voltage with or without DC prestressing could initiate an electrical tree. The tree initiation ratio increased due to higher impulse voltage, increased DC prestress level and shorter rest time between DC prestress and impulse voltage. The measured initial tree length showed a proportional relation with the impulse magnitude, whereas it was almost independent of DC prestress level and the rest time. The observations are explained by the injection, accumulation, dissipation, trapping and detrapping of space charges in the region around the needle tip.

Effect of voltage reversal on space charge and transient field in LDPE films under temperature gradient

Voltage reversal will strongly affect the stability of cable in service, especially under temperature gradient due to the temperature rise of the cable. In this paper, the results of space charge profiles and field distortions in low density polyethylene (LDPE) films after dc pre-stress of 50 kV/mm for 2 h under temperature gradient and then during different voltage reverse polarities or during different voltage reverse periods were presented. The test results show that the charge accumulation and field distortion are different under different voltage polarities due to the dependence of charge injection on electrode materials. In this work, under the Al / SC (Semiconductive) electrode system with a temperature difference of 40°C, the maximal transient field occurs near the low temperature Al electrode when polarity varies from negative to positive while near the high temperature SC electrode when polarity varies from positive to negative. In addition, the faster the voltage reversal at room temperature the higher the maximal transient field, but the faster the voltage reversal the lower the maximal transient field under temperature gradient. Moreover, the maximal field appears during voltage inversion at room temperature while appears at pre-stress under temperature gradient.

An Experimental Study on the Effect of DC Bias on Streamer Initiation and Propagation in a Dielectric Liquid under Impulse Voltage

The role of composite voltage waveforms on prebreakdown processes was studied at atmospheric conditions in a small point-plane gap with a synthetic ester Midel 7131 fluid, representing a model for the insulation in power electronic components. Shadowgraphic imaging and photomultipliers to record emitted light were used to measure streamer initiation voltages, streamer propagation lengths and velocities. The applied voltages were step voltages (100 ns rise time) with or without a dc bias. Results have demonstrated the presence of space charges in the vicinity of electrodes. With a step voltage with the same polarity as the dc prestress, initiation voltages increased and propagation lengths were reduced. For a polarity reversal for positive steps the initiation voltages were reduced and propagation lengths increased as expected, while for negative step the opposite was seen. The latter is explained by an electron "cleaning" effect of the positive dc prestress reducing possibilities for electron avalanches. It is shown that streamers may be initiated by a sudden turn off of a dc stress.

Effects of electrostatic discharge high-field current impulse on oxide breakdown

Stress testing is performed in two stages, a high-field prestress test followed by an electrostatic discharge (ESD) event, which induces high-field current impulse stress. dc and impulse high-field prestress sources are separately applied to generate different formations of bulk oxide traps, near-interface oxide traps (border traps), and interface traps. Experimental results indicate that the dc prestress testing induces many more interface traps and border traps in the metal-oxide-semiconductor capacitor structure than the impulse prestress testing. Additionally, an anomalous turnaround degradation of oxide breakdown subjected to the following ESD impulse stress is observed and attributed to the effect of border traps. Border traps cannot communicate with interface traps and silicon substrate during high-field current impulse stressing, and therefore cannot emit trapped charges instantaneously. Consequently, these trapped charges provide a negative electric field decreasing the Fowler-Nordheim stress current and therefore reducing the degradation of the oxide breakdown.

Particle-initiated Breakdown Characteristics around Spacer under Lightning Impulse Voltage Superimposed on Pre-stressed DC

This paper deals with the effect of the magnitude and polarity on the particle initiated breakdown characteristics around a simple shaped spacer in quasi-uniform field gap under lightning impulse voltage superimposed on pre-stressed DC voltage. The results are discussed on the dust figure technique and also on the breakdown voltage / delay time characteristics, while keeping in view the magnitude of the pre-stressed DC applied voltage. Consequently, it was found that the surface charges accumulated on the spacer in the vicinity of the particle tip under the pre-stressed DC voltage play an important role in the particle initiated breakdown phenomenon.

Factors affecting the dynamics of wire particles in gas‐insulated systems

AbstractThis paper introduces a theoretical study of the significant factors that affecting the dynamics of wire particles in a horizontal coaxial gas‐insulated transmission line (GITL) and/or gas‐insulated switchgear (GIS) systems. The dynamics of the particle is investigated using a computational algorithm and under different voltage waveforms, gas pressures and coating thicknesses. The simulated voltage waveforms are the 60 Hz AC, single and successive switching impulses (SI), and bipolar‐oscillatory and damped impulses. All the applied voltage waveforms are simulated with and without the DC prestress, except for the AC case. These voltages represent the actual waveforms that occur in practice. The present work highlights the dangerous situations of the particle dynamics and their methods of mitigation. Moreover, the investigated dynamic behaviours of such defect under AC energization give excellent signs for the recognition by the acoustic detection technique.

Depressed DC Breakdown Phenomena near Curie Temperature of BaTiO3-Based Ceramic Capacitors

In this paper, temperature-dependent dc breakdown in BaTiO3-based multilayer ceramic capacitors (MLCs) is studied. We measured dc breakdown fields (BDFs) at temperatures ranging from room temperature to 250°C. According to the change in the breakdown characteristics with temperature, the breakdown process can be regarded as consisting of three regions: room, Curie and high temperature regions. An abnormal depressed BDF was observed in the Curie temperature region. After analyzing the dc prestressing effect on the dc BDF, it was found that the homo effect of space charges plays an important role during dc breakdown in the room and Curie temperature regions. In the high temperature region, the thermal breakdown tends to dominate the breakdown process of MLCs. In addition, based on a structure controlled Schottky conduction process, we combined the thermionic emission process with the influence of barrier height on space charge accumulation. By relating the combined result with the experimental results, it was concluded that the abnormal depression in the dc breakdown is consistent with the change in barrier height caused by the phase transition of the BaTiO3-based ceramic insulator near its Curie temperature.

The breakdown mechanism of poly-p-xylylene film. Prestress effects on the breakdown strength

Impulse and dc breakdown strengths of 4 /spl mu/m thick poly-p-xylylene (PPX) films were 5.9 and 4.1 MV/cm, respectively. They were independent of temperature in the temperature range from /spl minus/60 to 60/spl deg/C. dc prestressing for a long time (t/sub p/=60 s) reduced impulse breakdown strength for both the same and the opposite polarity. But dc prestressing for a short time (t/sub p/ >