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This paper presents an investigation of the underlying mechanism behind the interface charge barrier between oil and oil immersed paper. Two types of oils and two types of cellulose papers are selected in the study. The interface charges of the samples are measured by the pulsed electro-acoustic (PEA) method and the zeta potentials of different oil-paper combinations are also measured. The results show that the amount of interface charge changes with the type of oils and papers, and is much larger than the Maxwell-Wagner-Sillars (MWS) polarization charge. The difference in the amount of interface charges cannot be attributed to the difference in the conductivity and permittivity of the materials. It is suggested that the barrier for charge transport across the interface, which is formed due to the electric double layer, should be taken into consideration to explain the interface charge behavior in oil-paper insulation. [ABSTRACT FROM AUTHOR]

Oil-paper insulation of higher voltage winding and traction winding in on-board traction transformers frequently withstand large AC fluctuation voltages mainly combined with AC and inter-harmonic components. This paper presents the characteristics of partial discharge (PD) of oil-paper insulation under superimposed inter-harmonic AC voltages. The experimental results show that the addition of the inter-harmonic to the fundamental sinusoidal component results in variation of peak voltage and dv/dt along the phase. This superimposed inter-harmonic AC voltages has a significant impact on PD including partial discharge inception voltage (PDIV), breakdown voltage, PD repetition rate, and phase resolved partial discharge (PRPD). In addition, a dynamic characterization method of PD statistical parameters is proposed to determine the developing trend. The critical values and inflection points of PD are obtained. Besides, the theoretical mechanism is expounded to better interpret and understand the influence of inter-harmonic component on PD properties. These methods will provide the dynamic datasets and more information for insulation condition assessment of on-board traction transformer. [ABSTRACT FROM AUTHOR]

In this paper, the temporal space charge dynamics under both temperature and voltage polarity reversal of an epoxy resin impregnated crepe paper are characterized by pulsed electroacoustic method. It is found that the space charge injection and the interfacial traps located between the epoxy and paper plays an important role in determining the space charge dynamics. The results reveal that below 353 K, the calculated interfacial charge density is consistent with the Maxwell-Wagner (MW) model, while above 353 K, it is smaller than the measured value. The time constant during charging is much smaller than that predicted by the MW model and the space charge injection showed polarity effect, which leads to the interfacial charge densities under polarity poling voltage not being equal. It is proposed that the interfacial traps and space charge injection should be responsible for the observed space charge dynamics. [ABSTRACT FROM AUTHOR]

Transformer oil, insulating paper, high voltage direct current (HVDC) bushings and on-load tap changer (OLTC) are essential parts of converter transformers. When the power flow of the HVDC system is reversed or a sudden outrage occurs, the charge field will deform the dielectric internal electric field, thus easily leading to the occurrence of discharge or insulation damage. In this case, it is necessary to carry out the characterization of the oil?paper interface to avoid these consequences. This paper analyses the interface charge accumulation and dissipation characteristics of an oil?paper insulation system under DC and polarity reversal voltages. In this paper, induced electrification was used to charge the composite oil?paper insulation. The amplitude of DC and polarity reversal voltages applied in this paper was 4 kV, and paper with thicknesses of 0.08 and 0.13 mm was used. This research offers a theoretical exploration to optimize the transformer oil? paper insulation structure and prevent damage to its insulation systems. From the research on different layers and thicknesses of paper, it can be concluded that the decay is fast in the initial period and then becomes slower with the lapse of time until finally the decay curve tends to become flat. With an increase in the number of the paper layers, the interface charge density decreases. So in real life multilayer paper is used to avoid failures owing to its structural weakness. Regarding the DC polarity reversal effect, increasing the thickness of the paper is equivalent to adding paper layers. The dissipation rate of the positive interface charge is quicker than that of the negative one. In the polarity reversal test, the dissipation rate becomes smaller as the reversal time gets longer. [ABSTRACT FROM AUTHOR]

The paper presents the application of the Cole-Cole model for determining the runs of the return voltage of the paper-oil insulation sample through a computer simulation. Different insulation conditions were simulated through an adequate selection of parameters a and T of the Cole-Cole model. For calculation purposes, a series - parallel substitute scheme, obtained through approximation of discrete characteristic operator impedance Za of the model applied, was used. The diagnostics of the simulated insulation condition was based on the analysis of a few selected parameters of the return voltage, obtained from time characteristics. [ABSTRACT FROM AUTHOR]

This paper presents an experimental work investigating the failure data of oil-impregnated paper under pulsating DC voltage. The failure data of the oil-impregnated paper specimens were obtained through constant voltage stress tests; the Weibull distribution was used to estimate the statistical distribution of failure data. The exponential function (EF) and the inverse power function (IPF) were used to modify the Weibull distribution of failure data, so as to obtain two dual-variable Weibull failure distributions (WFDs), which were used to analyze failure data of oil-paper specimens obtained under pulsating DC and individual DC and AC voltages. Their adequacies were evaluated by using failure data obtained through the constant stress tests under various voltages. Both EF-modified and IPF-modified WFDs show more adequacy for failure data of oil-paper insulation under DC and pulsating DC voltages than AC voltages. The EF-modified WFD was more adequate than IPF-modified WFD in estimating the statistical distribution of failure data. [ABSTRACT FROM PUBLISHER]

The oil-paper insulation of valve-side windings in a high voltage direct current (HVDC) converter transformer is subjected to both complicated voltage and severe thermal stresses. Partial discharge (PD) occurring in oil-paper bounded cavities is influenced by voltage stress type as well as temperature. This paper presents PD behaviors in an oil-paper bounded gas cavity under pulsating DC voltages. The voltage across the gas cavity and PD inception voltages at different test conditions are analyzed. Influences of temperature on statistical PD distributions and parameters are also presented. A numerical model is used to interpret PD behaviors under pulsating DC voltages and various temperatures. Experimental results and theoretical statements show that the temperature exhibits a great influence on the behaviors of cavity discharge under pulsating DC voltages. Volume resistivity, gas pressure, charge decay time constant, and electron generation rate are the primary parameters influencing PD behaviors at different temperatures. [ABSTRACT FROM PUBLISHER]

The oil-impregnated paper, as a vital part in the oil-paper insulation system, serves as the main insulation medium in converter transformer suffering various electric fields. However, the accumulated surface charge on the paper would accelerate the degradation of insulation system and may lead to surface flashover. Direct-fluorination is a method to change the chemical components of polymer matrix thus could affect the surface property of the dielectric material without the alternation of bulk property. This paper presents a method to modify the surface structure of the paper applied in transformers. The paper used in the experiment was treated using the F2/N2 mixture with a volume fraction of 10% for the F2 gas at a pressure of 0.05 MPa in a reaction vessel for 2, 5, 10 and 15 min, respectively. The chemical elements, functional group and physical morphology of the paper before and after modification were measured with energy dispersive X-ray spectroscopy (EDAX), attenuated total reflection infrared (ATR-IR) and scanning electron micrographs (SEM), respectively. Surface and volume resistivity of the fluorinated paper were measured. Furthermore, the surface charge property of the oil-impregnated paper was measured under various positive and negative voltages. Obtained results showed that the fluorination had an obvious influence on the chemical property and surface structure of the paper leading to the reduction of surface and volume resistivity. The treatment was shown to reduce the initial potential and cause acceleration in charge decay process. The trap distribution of the paper surface presented alternation as well. [ABSTRACT FROM PUBLISHER]

To investigate the characteristics of micro bridges and their interactions with partial discharge (PD) in oil-paper insulation under DC voltage, an experimental platform with two-copper-electrode models is established. Generation and growth of micro bridge impurities are observed. PDs in experimental process are detected and analyzed. The result shows that the generation and growth of micro bridges have three stages based on their quantity and shape. At first stage PD accelerates the generation of micro bridges. At second stage the grown bridges trigger stronger discharges. At the last stage a so-called "hazardous bridge" that triggers highly intensive PDs emerges, through which breakdown happens. Therefore, bridges and PDs promote each other to cause insulation failure. In addition, impurities at the second stage are observed to transform into pile-like, bridge-like or particle-like shapes. The transformation process is considered to relate with the change of composition force on fiber bundles. Besides, a "cluster discharging" phenomenon is observed at the second stage, which is associated with the generation and disappearance of micro bridge impurities. [ABSTRACT FROM AUTHOR]

Heretofore, unlike HVAC transformers, there is not much experience in both insulation design guides and test methods for ultra HV converter transformers. This special insulation phenomenon is very prominent due to the complication of voltage applied on valve-side windings of converter transformers. There is a severe lack of empirical research on partial discharge in oil-paper insulation under the combined ACDC voltage. The study in this paper provides a timely research into the issues. Nine possible configurations, which intent to simulate different defects in the transformer, have been investigated. The PD inception voltages varied ratios of combined AC-DC voltages were conducted. The discharge magnitude and its relation with the applied voltage for selected four common configurations were obtained during voltage increasing and decreasing respectively, which presented the PD initiating process and also reflected the insulation behavior of dielectrics. Besides, the Finite Element Analysis Soft was used to propose the effect of morphology of electrode surface (3 dimensional roughness is approximately 0.03 mm) on PD characteristics, which is very lacking in the previous references. The roles of DC and AC component in PD characteristics were discussed in the context of the relevant work in literatures. [ABSTRACT FROM AUTHOR]

In this paper, the rapid aging due to Partial Discharge (PD) activity was investigated in unaged and thermally aged oil-impregnated paper with a disk-shaped cavity between the sheets of paper. The PD inception voltage and the voltage for instantaneous breakdown were measured, and the time to breakdown was measured by applying voltages less than the instantaneous breakdown voltage. A comparison between time to breakdown of unaged and thermally aged paper shows no significant difference between thermally aged and unaged paper at sustaining PD activity. The time to breakdown as a function of applied electric field was plotted and compared for both cases showing points scattered around a line in log-log scale. Changes of the PD parameters, such as PD magnitude and PD repetition rate were analyzed from the beginning of PD activity up to the moment of breakdown. The results show that the number and magnitude of PD increase at the beginning of aging until they reach to a maximum value, then both quantities decrease slowly over time until the final puncture breakdown occurs. The results emphasize the importance of PD monitoring on real equipment with oil-impregnated paper as insulation system, such as power transformers, since a focus on the number and magnitude of PD at just the present time may mislead the interpretation. Dielectric spectroscopy measurements performed on unaged and thermally aged paper showed an increasing trend of ε" after the sheets of paper were exposed to thermal aging for longer time. Dielectric spectroscopy measurements performed on samples before and after exposure to PD activity showed a big change of ε", This change can be attributed to byproducts and ions produced by PD activity. [ABSTRACT FROM AUTHOR]

The effect of HV impulses on surface and cavity partial discharges (PD) of oil-impregnated paper (OIP) is investigated in this work, by subjecting specimens to impulse voltages superimposed on a continuous AC voltage. The superposition of transient and AC voltage is relevant to insulation systems in power-system equipment, which is exposed to transients such as lightning and switching impulses during its operation. For several different PD-generating defects in oil-impregnated paper, the PD behaviors before and after the impulse are compared. The influence of moisture content is studied by using OIP with moisture content <0.5%, 3.0% and 5.5%. The results show a distinctive behavior of PD due to the impulse for each defect and insulation condition, either by change in the PD rate and magnitude or by causing PD inception or extinction. Recordings of the voltage signals and associated PD measurements during and after the incidence of a high voltage transient are of potential use for condition assessment of the insulation of power transformers and their bushings, as a form of online diagnostics. [ABSTRACT FROM AUTHOR]

In comparison with the AC and DC working voltages, the effect of switching impulse voltage on insulation is very different because of its fast-rising rate and large amplitude. This paper mainly focuses on the characteristics of PD in oil-impregnated paper insulation under switching impulse voltage. First, a robust experimental platform was set up including the impulse power source, test samples with uniform gas and oil gap, and the signal acquisition and processing system. PD characteristics in the air and oil gap are analyzed and compared involving the time-domain waveforms, the variation tendency of the average amplitude and numbers of PD pulses, the occurrence time and the instantaneous value of impulse corresponding to the first PD. Based on the dielectric barrier discharge (filament) theory, a parallel microcircuit model was established to explain the similarities and dissimilarities, and to evaluate the gap discharge intensity. The occurrence and development of PD under switching impulse voltage of typical oil-impregnated paper defect were studied systematically with simulation, experiments, and modeling. [ABSTRACT FROM AUTHOR]

The oil-impregnated paper, which is the major section in converter transformer, plays a considerable role in the reliability and safety of power grid. However, the insulation degradation will be accelerated by the accumulation of charges that are on the surface of oil-impregnated paper. Therefore, the operating overvoltage occurs and irremediable damage will appear. Direct fluorination is an effective method of modifying the chemical components of polymer matrix so as to have effects on the surface characteristic of the insulating material without reforming the bulk properties. In this research, for studying effects of fluorination on charge behavior on the surface of oil-impregnated paper under the application of DC and impulse voltage, the F2/N2 compound, which the volume fraction for the F2 gas occupied 10 % and the pressure was 0.05 MPa in a reaction kettle for 5, 30 and 60 min respectively, was applied. Meanwhile, the chemical bond groups and the surface morphology of the fluorinated specimen were tested. Besides, the DC voltage amplitude was 4 kV and the applied time was 4 min. The impulse frequency was 500 Hz and the impulse voltage amplitude was 1, 2, 3 and 4 kV, respectively. The charge properties under various situations were investigated. From the research under the application of different fluorination time, impulse voltage amplitudes and polarities, conclusions can be drawn that, fluorination has significant effects on the chemical structure properties and surface characteristic of oil-impregnated paper. Furthermore, the initial potential can be reduced and the decay process of surface charge can be accelerated with different fluorination time increasing from 0 to 30 min. Nevertheless, the tendency is the opposite when the fluorination time continuously extends from 30 to 60 min. The trap distribution also exhibits alternation through the fluorination. [ABSTRACT FROM PUBLISHER]

Due to its excellent performance, oil-paper insulation has been widely used in many power apparatuses including converter transformer of high voltage direct current (HVDC) system. Space charge accumulation phenomena may significantly distort local electric field inside transformer and cause oil-paper insulation overstressing. As a result of long term operation, oil-paper gradually degrades. In most severe cases, the breakdown of insulation system may occur. In this work, the effect of space charge on breakdown characteristics of aged oil-paper insulation is investigated. Oil-paper samples with different aging states are obtained by thermally accelerated aging for various periods of time, categorized by the degree of polymerization (DP) of 1107 (new), 725, 504, 376 and 241, respectively. Experimental results show that the breakdown voltage of oil-paper samples without pre-stressing remains almost constant with thermal degradation. After the homo-polarity pre-stress, when the DP value is higher than ∼300 (from 1107 to 376), the breakdown voltage has an increasing tendency with an increase in thermal degradation. On the contrary, the breakdown voltage presents a decreasing tendency after the hetero-polarity pre-stress. When the DP is lower than ∼300 (DP241), the changes of breakdown voltage does not remain the same trend as previous samples after pre-stressing. Meanwhile, space charge distributions in different aged samples are measured by the pulsed electro-acoustic (PEA) method, and the trap parameters are correspondingly evaluated. It has been experimentally demonstrated that special attention should be paid to space charge accumulation and aging state of oil-paper insulation, which may lead to severe insulation accidents. [ABSTRACT FROM AUTHOR]

Oil-paper insulation system is an extraordinarily essential system of converter transformers as well as HVDC bushings. When the system changes its state, such as switching the thyristors, operating over-voltage appears, and the charge will deform the dielectric internal electric field, thus easily leading to the occurrence of discharge or insulation failure, so charge accumulation and dissipation on oil-paper insulation under pulse voltage need more investigation. This paper studies the surface charge behavior on double-layer oil-paper insulation under pulse voltage and inducing electrification was used to charge the oil-paper composite. Studies of various voltage amplitudes, pulse frequencies, numbers and polarities showed that the decay is fast initially, and then becomes slower with the time. Eventually the dissipation of charge behaviors tends to be flat. With the increase of voltage amplitudes, the surface potential rises, and less positive charges are accumulated on the surface than negative charges. The increase of the pulse frequency causes the initial values of surface potential a little smaller and the charge to dissipate more quickly. Also, with the increase of pulse numbers, the initial value gets bigger and the surface charge would be slowly dissipated. From the tdV/dt and decay time curves, the figures of positive voltage usually have two peaks, while there is only one peak in most of the curves under the negative one. With the increase of voltage amplitudes, the charge will dissipate more quickly. Additionally, the peak value and the characteristic time increase with the reduction of pulse frequency and the addition of numbers. [ABSTRACT FROM PUBLISHER]

Oil-paper insulation system is an extraordinarily essential system of converter transformer as well as HVDC bushing. This paper studies the interface charge accumulation and dissipation characteristics of oil-paper insulation system under DC voltage. In this paper, an induced electrification was used to charge the oil-paper samples. The amplitudes of the DC voltage varied from 1 to 4 kV and the paper was 0.13 mm in thickness. From the research of different voltage amplitudes and charging times of the paper, conclusions can be drawn that the decay is fast initially, and then becomes slower with the lapse of time. At last the dissipation trend tends to be flat. With the increase of the voltage amplitudes, the surface potential rises more rapidly, and higher voltage would also cause the charge to dissipate more quickly. Longer charging times would cause the charge to dissipate a bit more slowly. The decay rate of the negative voltage is a little smaller than that of the positive charge. Moreover, when it comes to the effect on the interface charge of the paper layers, the surface potential is more proportional to the increase of paper layers. When the amplitude of the voltage applied in the test is small or the charging time is short enough, the interface charge can be hardly accumulated on the bottom layer. In addition, the tdV/dt and the decay time curves under the positive voltage usually have two peak values, while there is only one peak value in most of the curves under the negative condition. [ABSTRACT FROM PUBLISHER]

Space charge behavior is important in the oil-immersed-paper insulation in converter transformers for HVDC transmission. This paper reports an investigation of the space charge property in samples with two layers of oil-immersed-paper and oil. Based on pulsed electroacoustic (PEA) technique, a waveform recovery method was proposed to get rid of the effects of interface and anisotropy on PEA result, and the space charge distributions in the two-layer samples under different voltage and oil thicknesses were measured. It?s found that the charges with the same polarity as oil side electrode were accumulated at the interface and gradually increased with the increase of applied voltage. However, the positive charge charges always appeared in the oil under positive and negative voltages. Moreover, the charges in samples increased with the increase of applied voltage, while the amount of interface charges decreased with the increase of oil thickness. Due to the existence of interfacial charges, the electrical field in oil-immersed-paper was distorted with its maximum close to the interface. It is indicated that due to the different mobility properties of charges in paper and oil and the interface trap state, the interface between paper and oil plays an important role in charge formation and decay. [ABSTRACT FROM PUBLISHER]

An oil-paper insulation system which will withstand DC voltage under normal conditions is a key component of converter transformers as well as high voltage direct current (HVDC) bushings. When the system changes its state, operating overvoltage appears. This kind of overvoltage will consequently affect the application of oilimmersed facilities and the safety of entire grid. Therefore, this paper studies the charge coupling behavior of double-layer oil-paper insulation under DC and pulse voltages. From the research of different pulse voltage amplitudes, pulse numbers and polarity, conclusions can be drawn that, with the increase of the pulse voltage amplitudes, DC and pulse voltages in the same polarity cause the absolute value of the initial surface potential to decrease and then rise; while in different polarities, the charge with the same polarity as the pulse voltage increases. Furthermore, for the same pulse voltage, the absolute initial value of the DC voltage applied in the same polarity is higher than that in different polarities. Considering the pulse number effect, it is obvious that the initial value increases with the increase in the pulse number, and then reaches saturation. Also, with the increase in pulse number, the decay rate decreases and thus the charge is slowly dissipated. From the tdV/dt and the decay time curves, the peak value increases with the increase in the pulse number, while the characteristic time varies by the polarity of the DC and pulse voltages. [ABSTRACT FROM AUTHOR]

The paper presents new diagnostic method applied to assess aging degree for paper-oil insulation used in professional electrical power device, primarily including medium and high power transformers. The described method is based on modified diagnostics using the recovery voltage effect occurring in dielectrics (RVM ? Recovery Voltage Methods). Modification of the RVM method applied so far, which is now used only to assess moistening degree for paper-oil insulation in power transformers, consists in a new parametric analysis of recovery voltage time characteristics. Simulation and experimental tests carried out in laboratory conditions for adequately prepared insulation samples have proven that the time-constant of recovery voltage decay (?R_MIN) is the most susceptible parameter as regards quantitative change in the function of aging of the examined insulation. This conclusion is also confirmed by statistical studies, in which a multi-parameter variance analysis method has been applied. An original equivalent circuit diagram of the insulation has been used to simulate aging degree of paper-oil insulation samples in numerical tests. The diagram is based on the Cole-Cole type dielectric response model. [ABSTRACT FROM AUTHOR]

A method of predicting the residual breakdown electrical field strength of an epoxy-mica paper insulation system is described. This method was developed by using data on service?aged stator bars from generators as well as accelerated aging model bars. Thermal aging, mechanical aging, and combined thermal and mechanical aging were done individually as accelerated aging. The partial discharge characteristics, dissipation factor, and breakdown voltage were measured on the model bars and service?aged bars from generators. From these results, the residual breakdown electrical field strength was predicted based on the partial discharge characteristics and dissipation factor. First, the dominant type of deterioration (thermal or mechanical aging) on a bar?s insulation was determined by the pattern of voltage?partial discharge magnitude characteristics. Then the residual breakdown electrical field strength was predicted by using a predicting equation for both thermal and mechanical aging. Each predicting equation uses dissipation factor tip-up (i.e. difference in dissipation factor values between 2 kV and rated voltage) and partial discharge inception electrical field strength. The accuracy of this method was confirmed using data measured on 112 service-aged stator bars from generators and 10 accelerated aged model bars. The 95% confidence interval level included 94% of the measured residual breakdown electrical field strength values. This result shows that the method enables precise predictions of the residual breakdown electrical field strength of an epoxy-mica paper insulation system. [ABSTRACT FROM AUTHOR]

Repeated impulses have long been considered to affect the insulation properties of electrical equipment. This interesting but rarely studied phenomenon may pose great risks to the reliable operation of power equipment. Therefore, in this work, we focus on the accumulative effect of repeated lightning impulses on the oil-impregnated paper (OIP) used in power transformers. Specifically, we study the effect of space charge on the accumulative characteristics of OIP. A repetitive impulse accumulating test platform was set up, and the tests were conducted to obtain the accumulative characteristics of OIP under repeated lightning impulses. The accumulative characteristics of OIP with various interval time were experimentally identified to study the influence of interval time on the accumulative effect. Then, space charge in OIP samples were investigated using the pulsed electro-acoustic technique. Charge behavior in OIP was analyzed under different conditions, i.e., changing the applied times, amplitude and interval time of the applied lightning impulses. Charge injection and transport occurred during lightning impulse accumulation. The increased application of lightning impulses, high applied voltage, and short interval time contributed to the accumulation and transport of space charges. At last, the possible mechanisms of the accumulative effect are discussed in this article based on the experimental results. [ABSTRACT FROM AUTHOR]

This paper presents the results of experimental studies about the prebreakdown phenomena in mineral oil under lightning impulse. Experiments were carried out in a non-uniform field setup with HV electrode covered by crepe paper. The results of the experiments were compared with the results of research performed for bare HV electrodes. The onset voltage, propagation velocity, time to initiation, shadowgraph photos of discharges and light emission were respectively measured or recorded during the experiments. Analysis of the research outcomes indicates that the fundamental role of paper insulation is to raise the onset voltage by shoving away the onset sites out of the region of high field stress. The insulation wrapping does not have an influence on the shape of discharge patterns and on the propagation velocity. Some of the results connected with the times to initiation points that the mechanism of discharge initiation at the HV electrode surface does not depend on the fact it is insulated. The source of ?weak points? of the paper-oil insulation system is the oil, not the surface of insulation wrapping or metal of electrode. Observation of the oscillograms of light emitted by discharge channels indicates the difference in the mechanism of discharge propagation. The surface and space charge carriers left in the interelectrode space after discharge flash play an important role. In the case of insulated electrodes, the surface charge remains longer on the surface of electrode, reducing the electric field stress which is responsible for reignition. Simultaneously, the space charge reduces the degree of channel ramification. Both the observations are clearly visible on the oscillograms of light as differences in the frequency of light impulses and their peak values. [ABSTRACT FROM AUTHOR]

We are pleased to announce that the February 2018 issue of the IEEE Transactions on Dielectrics and Electrical Insulation (TDEI) will be a special issue on High Voltage Direct Current (HVDC) Transmission System Insulation topics. [ABSTRACT FROM PUBLISHER]

The purpose of dissolved gas analysis (DGA) in utilities is to help detect the presence of abnormality within transformer. When faults occur in transformers, transformer engineers need to determine the location and risk of the faults. Ultimately, they have to decide, based on DGA, whether to continue operating or not, perform internal inspection, or dispose the transformer. In this study, the fault and failure types in the transformer are suggested to determine the location and risk of the faults. In particular, turn-to-turn insulation faults are classified as degradation and breakdown. These faults are difficult to identify during internal inspection, and have a high possibility of failure. Urgent decision and action are thus required to avoid failure. In degradation of turn-to-turn insulation faults, failures may occur by generating thermal gases in paper during a long period of time. In breakdown of turn-to-turn insulation faults, thermal gases are not generated in paper, and failures are rather due to sudden breakdown of insulation. This study also presents a typical example of a turn-to-turn insulation fault. This example shows the progress of the fault from thermal to discharge, which is common phenomena in winding fault. Based on the findings of this work, transformer engineers can determine by DGA if transformers can be operated with or without internal inspection, or disposed when the fault has not been identified during internal inspection. [ABSTRACT FROM AUTHOR]

An closed oil circulating system with plane electrode was built to study the effect of space charge on oil flow electrification under DC voltage. The results reveal that with the increase of applied voltage, the streaming current was always enhanced. However, the polarity of streaming current depends on the sample configuration. If the negative high voltage electrode was exposed to oil layer, streaming current was negative. Otherwise streaming current had a tendency to be positive as applied voltage increased. In experimental model without exposed electrode, the polarity of streaming current was always positive. And the amplitude of streaming current was related to the number of insulating papers placed under electrode. More craft papers placed under negative high voltage electrode led to the increase of streaming current. Based on the bipolar transport model, a new simulation method is proposed to study the effect of charge injection and transportation on flow electrification under DC voltage. The simulation results agree well with the experimental result. [ABSTRACT FROM AUTHOR]

Describes the above-named upcoming special issue or section. May include topics to be covered or calls for papers. [ABSTRACT FROM PUBLISHER]

When an alternating voltage is applied to a dielectric material, dipoles in the material try to align in the direction of the applied voltages and polarization process starts. For multi-dielectric media especially in which a liquid interface is involved, such as oilpaper insulation, interfacial polarization is predominantly active among all polarization processes. This interfacial polarization is a very slow process and is effective in lower frequency range. This interfacial polarization reflects the inherent condition of the oilpaper insulation. Due to this reason, frequency domain spectroscopy is performed in lower frequency range to assess the condition of oil-paper insulation. So, the time required for frequency response measurement using conventional frequency domain spectroscopy is quite long. Considering the aforesaid facts, a different procedure has been taken here to measure the dielectric response in a significantly lesser time keeping the entire frequency range of conventional frequency domain spectroscopy. Instead of using pure sinusoidal excitation voltages over wide frequency range, dielectric response current has been measured using excitation having time growing frequency sweep (TGFS) signal. For this purpose, test samples that emulate the oil-paper insulation of real-life transformer have been prepared in the laboratory. The moisture contents of the test samples have been kept different to investigate the reliability of the proposed method. A setup has been made in the laboratory to measure dielectric response currents using the both excitations. In order to investigate the applicability of the proposed method, experiment has also been performed on actual transformers. It has been observed that the use of excitation signal having time growing frequency sweep provides almost similar information regarding insulation condition as obtained by the use of conventional sinusoidal excitation voltages. Hence, the proposed method is capable of reduction in time in FDS nearly by an amount of 85%. [ABSTRACT FROM PUBLISHER]

This paper presents an experimental study on the characteristics of creeping discharges propagating over pressboard immersed in olive oil, rapeseed oil, tetra-ester, methyl oleate, and mineral oil, under both negative and positive DC voltages. The investigated characteristics are mainly the shape of discharges and their stopping lengths. Two experimental test cells are used: in the first one, the pressboard is inserted between pointe and plane electrodes so that the tip is perpendicular and at the center of the pressboard sample; and in the second one, the pressboard is placed between a point and bar electrodes in a way that allow a propagation in one direction (tangentially to pressboard). It is observed that the creeping discharges under DC voltage are not radial. Their shapes are different from those observed in previous work under AC and lightning impulse voltages. On the other hand, for given voltage and pressboard thickness, the stopping length Lf is shorter when the point is negative than when it is positive indicating that the flashover voltage will be greater with a negative point This also evidences a difference in the processes involved in each polarity especially the space charge resulting of the injected charges and the charge accumulated at the interface (including the charges resulting of double layer) as well as the evolution of the electric field in the vicinity of the point electrode and along the pressboard surface. For a given pressboard thickness, Lf increases with the voltage; it decreases when the thickness of pressboard is reduced. [ABSTRACT FROM AUTHOR]

The insulation performance of an oil-immersed power transformer against lightning surges is verified by applying a lightning impulse (LI) voltage alone. However, the surge voltage generated at the transformer terminal in an actual system is superimposed over the ac operating voltage. Particularly for UHV- and 500 kV-class, where the ratio of ac operating voltage to the LI withstand voltage is high, the insulation characteristics for this LI and ac superimposed voltage are crucial factors to study the insulation design. Accordingly, this paper reviewed the research results of insulation characteristics of transformers for the superimposed voltage. Subsequently, these results were evaluated from perspectives of an insulation coordination and transformer insulation test. The insulation elements of transformers covered were the turn-to-turn and section-to-section insulation, for which the LI test is key for insulation design. As a result, for the section-to-section insulation for a shell-type transformer and the turn-to-turn insulation, the breakdown (BD) voltage for the superimposed voltage remained unchanged from the result when the LI was applied alone. Conversely, for the section-to-section insulation of a continuous disk winding for a coretype transformer, the BD voltage attributable to the superimposed voltage declined by about 10% to 20% from that when the LI was applied alone if the oil-impregnated paper component ratio compared to the oil-gap length was low. This is because the oil gap was primarily responsible for the BD and flaws were generated there when the ac voltage was applied. On the other hand, when this ratio was increased, the oil-impregnated paper part was primarily responsible for BD and the decline in BD voltage could be suppressed. For the section-to-section insulation for the interleaved disk winding, the BD voltage attributable to the superimposed voltage remained constant from that when the LI was applied alone. This is because the BD was determined by the LI partial discharge inception at the turn-to-turn part of the section-to-section insulation, where the superimposed voltage made little impact on BD. Based on the above, the LI test is considered adequate to verify the insulation performance of the section-to-section insulation for a shell-type transformer and for an interleaved disk winding and the turn-to-turn insulation against surge overvoltage. Conversely, for the section-to-section structure of a continuous disk winding, the influence of the ac voltage must be taken into account. These results are key findings for rationalizing transformers while maintaining their insulation reliability. [ABSTRACT FROM AUTHOR]

This paper presents a comparative analysis of the creeping discharge characteristics on mineral oil-impregnated pressboard (OIP) under combined AC and DC voltages. The creeping discharge behavior at various voltage levels to failure were analyzed using partial discharge (PD) measurements, camera observations, and electric field simulations. It was found that the creeping discharge distribution was significantly influenced by the DC component, and it was difficult to induce failure even on OIP with high moisture under combined AC and DC voltage. The flashover along the free-oil boundary layer above the OIP during creeping discharge occurred much more easily under combined AC and DC than under AC voltage, and the quality of the oil was important to avoid flashover. These differences were attributed to the combined influence of the electric field distribution under DC and space charge accumulation. [ABSTRACT FROM AUTHOR]

The aim of this special issue, entitled HVDC Insulation and Diagnostics, is to provide a state-of-the-art of all aspects related to the design, manufacturing, operation and diagnosis of all HVDC components (transformers, valves, insulators, cables, GIS). These topics are covered by a total of 25 papers, 11 from China, 10 from Europe, one from Canada and Korea, two from IEEE Dielectrics and Electrical Insulation Society (DEIS) technical committees. In the call, the suggested topics were: ? New materials for HVDC insulation systems ? Design of HVDC and UHVDC components: challenges related to space charge, voltage polarity reversal, repetitive fast transients, mixed AC+DC waveforms ? Space charge measurements including measurements on full-size equipment ? Partial discharge detection and analysis under dc and mixed AC+DC waveforms ? Innovative diagnostic tools [ABSTRACT FROM AUTHOR]

This paper presents an evaluation of the performance of different methods for estimating the parameters required for the application of the Disruptive Effect Model to the analysis of the breakdown characteristics of 15 kV distribution insulators subjected to lightning impulses. The tests involved the application of both the standard (1.2/50 μs) and non-standard lightning impulse voltage waveshapes (1.2/4 1.2/10 3/10 μs) and 7.5/30 μs)of positive and negative polarities. The obtained Vxt characteristics are compared with those predicted by the Disruptive Effect Model using the most common procedures for estimating the parameters necessary for its application, including a novel method which is presented in the paper. An analysis of the results relevant to the various procedures shows that the parameters obtained according to the proposed method lead to a better agreement between theoretical and experimental results. [ABSTRACT FROM AUTHOR]

According to the IEC Standard, the insulation performances of power apparatus under lightning impulse voltages are evaluated by the withstand test of standard lightning impulse voltage, with the wave front/tail time of 1.2/50 ?s. However, the standard lightning impulse voltage originally obtained from the transmission lines or triggering lightning towers, is quite different from the invading lightning impulse voltage in the power transformers. Hence, in this work, an attempt has been made to obtain the waveform characteristics of invading lightning impulses in power transformers using an overvoltage online monitoring system. Energy Method was then proposed to convert the recorded oscillatory waveform into double-exponential waveform defined by IEC Standard. And the statistical waveform parameters of recorded invading lightning impulses, namely, the non-standard lightning impulses, were determined based on the Energy Method. Current insulation assessment methods, for example, the V-t characteristics, are far from satisfactory to meet the demand of insulation design for the power apparatus, whereas the impulse breakdown voltage-number of voltage applications characteristics (V-N characteristics) could well evaluate the long term performance of insulation system. Therefore, we experimentally identified the V-N characteristics of oil paper insulation under impulse voltage with different waveforms, including non-standard lightning impulse voltage, and then mathematically analyzed the obtained data. At last, the influence of wave front/tail time on the V-N characteristics of oil paper insulation was experimentally studied. The test results indicate that the insulation performances of oil paper insulation under repeated nonstandard lightning impulse voltages and under standard switching impulse voltages were better than that under repeated standard lightning impulse voltages. And the capacity to withstand repeated voltage became increasingly stronger with the increase in wave front time or the decrease in wave tail time. The breakdown voltage, however, was only related to the wave front time of applied impulse voltage. [ABSTRACT FROM AUTHOR]

Converter transformers in HVDC system use oil-pressboard as the main insulation. The insulation structures of valve side need to withstand combined AC-DC voltage. Oil-pressboard insulation shows distinctive PD characteristics under combined AC-DC voltage which are the bases of insulation assessment and fault diagnosis. In this paper, PD characteristics are investigated with the established experimental platform of ballplate electrode under 1:1 combined AC-DC voltage. The behavior of experimental results is explained trough discharge mechanism. To reflect the characteristics of the whole PD development process without the influence of test conditions on PD magnitude, Φ-ΔT-N pattern is used to establish the 3D spectrum Hn (ΔT,Φ) based on time interval, phase and repetition rate of PD pulses. Barycenter C (xc, yc) is calculated as a comprehensive feature parameter. In order to exclude the effect of oscillation components on real-time assessment, variational mode decomposition (VMD) is used to extract the primary development trend of the feature parameter. The difference value R (X, Y) between the value of each point in the trend and the reference target decreases in the whole PD development process. And R can assess the PD development degree continuously based on the historical trend without the normalization problem of PD magnitude. [ABSTRACT FROM AUTHOR]

In recent years, dielectric relaxation methods in time and frequency domains have been evolved as a tool for the assessment of transformer insulation. However, it has been observed that the results of these tests are highly influenced by several factors viz. the temperature of the insulation, the magnitude of the test voltage, the geometry of transformer insulation structure. In the present work, a detailed analysis of the influence of the above-mentioned factors on the dielectric response data is presented by performing measurements on a prorated transformer model. A novel experimental procedure is suggested to acquire a database to study the effect of insulation duct geometry on the relaxation measurements. Possible corrective actions are suggested to improve transformer insulation diagnosis. [ABSTRACT FROM PUBLISHER]

This paper discusses electrical insulation characteristics of liquid nitrogen (LN2) / polypropylene (PP) laminated paper composite insulation system for the practical electrical insulation design of high temperature superconducting (HTS) cables. Partial discharge (PD) inception, propagation and breakdown (BD) characteristics under ac voltage application were obtained and discussed at atmospheric and pressurized conditions. Experimental results were analyzed quantitatively and revealed that void-type discharge was categorized as an important form of discharge to characterize the transition of PD activity leading to BD. [ABSTRACT FROM AUTHOR]

Since there exist discharge problems in gas insulated metal-enclosed switchgear due to internal metal particle movement, the study in this paper thus, conducted an experiment and simulation research on the movement characteristics of particles, where the charge on particle and the collision recovery coefficient (CRC) was obtained. When the electrode is brass, the charge on the particle is close to the theory one and the CRC of aluminum, stainless steel and particle are 0.52, 0.75 and 0.79 respectively. Accordingly, the study, in order to take the particle charge and its collision process into consideration, established a particle movement model, and conducted a test has thereof. The results show that there exists excellent consistency between charge and its collision. According to the research, the relationship between particle size, material and collision characteristics was revealed. The result shows that under DC voltage the metal particle collision frequency decreases with the increase of particle size when the material of the particles is the same. The time difference between particles' moving to high voltage electrode and ground electrode becomes greater with the increase of the size of the particle. When the particle material is different, the collision characteristics are influenced by its density and collision recovery coefficient of restitution. Particle motion velocity loss becomes less before and after the collision when its collision recovery coefficient of restitution is larger. So the collision frequency is higher. The research results in this paper can offer some reference value to particles suppression measures in gas insulated metal-enclosed transmission line. [ABSTRACT FROM PUBLISHER]

IEC 60156 and IEC 60897 were standardized with the purpose of evaluating the breakdown voltage of liquid dielectrics both under AC and impulse voltages. AC breakdown voltage determination is a simple method but only able to provide information on the presence of contaminants. The limit of this test is the large scatter in the results and, for this reason, this IEC standard has been recently revised. Impulse breakdown voltage determination allows discrimination of different dielectric liquids, also in terms of their chemical composition. This test has only limited application because of the dimensions of the high voltage generator. The present paper shows a comparison among breakdown voltages of various mineral oils and natural esters under both AC and impulse voltages. The paper presents also an effort to describe how these two tests may reveal the electrical performance of the dielectric liquid under test. [ABSTRACT FROM AUTHOR]

The choice of the materials used as the dielectric media in large transformers continues to be a combination of pressboard and mineral oil. However, a number of failures in the field have been attributed to the surface discharge or creeping discharge over the oil/pressboard interface. This paper reports on an experiment to study surface discharge over oil/pressboard. The TJ (triple-junction) model was used to study the characteristic of partial discharge (PD) from inception to flashover. The max PD charge, pattern, frequency and current pulse were measured during the flashover process. Four oil/pressboard specimens with different aging degrees were used and their influence on the flashover process over oil/paper interface was studied in this experiment. The results indicate that the max PD charge and PD frequency increase with the increase of applied voltage. The oil aging has great influence while the pressboard aging has little influence on PD characteristic over oil/pressboard. The PD pattern and current pulse shape show great difference for various applied voltages. The results therefore suggest that the PD detection on the oil/pressboard can be used effectively to implement the early monitor of surface discharge thereby to avoid the formation of surface flashover. [ABSTRACT FROM AUTHOR]

This paper deals with a comparative study of the negative and positive DC breakdown voltages of different insulating liquids namely two natural esters (an extra virgin olive oil (OO) without additive and rapeseed oil), two synthetic ester fluids (a methyleolate obtained by transesterification of olive oil and tetraester), and a naphthenic mineral oil (MO); an oils mixture namely 80% OO + 20% MO is also investigated. Statistical analysis of the experimental data is also performed. The obtained results are also compared with those obtained in our previous work under AC and lightning impulse voltages. It is observed that the negative and positive DC breakdown voltages of natural esters are higher than those of mineral oil. On the other hand, the DC breakdown voltage of (80% OO + 20 % MO) mixture shows comparable behavior to that of mineral oil. The DC negative breakdown voltages are higher than the positive ones in the tested liquids. It is also shown that the DC breakdown voltage values of the investigated liquids generally obey the normal distribution. [ABSTRACT FROM AUTHOR]

Cellulose impurities seriously threaten the insulating performance of oil-pressboard insulation. In this paper, we investigate the bridging and discharge mechanisms of cellulose impurities in transformer oil under DC voltage. The results show that under the non-uniform DC electric field, the cellulose impurities are mainly subjected to forces including gravity, buoyancy, dielectrophoretic force and drag force, among which the dielectrophoretic force dominates the bridging of the impurities. By studying the leakage current of dry (moisture 8 ppm) and wet (moisture 26 ppm) oil samples with and without cellulose bridges, deductions are made that the coexistence of cellulose and water in oil leads to the formation of a cellulose-water bridge, which increases the leakage current within the oil, causing a heating effect and gasifying the water on the bridge. Further analysis indicates that when the amount of gasified water exceeds the permeation channel threshold, the cellulose-water bridge transforms into a cellulose-gas bridge. Final oil breakdown happens along this gaseous bridge. [ABSTRACT FROM AUTHOR]

To reduce the electric field (E-field) distortion in a gas-solid insulating system, this paper proposes a centrifugation technique to fabricate interfacial E-field regulating (IER) insulators. During the curing process of the liquid epoxy/SiC mixture in the mold, a centrifugal force was used to force the SiC particles to the insulator surface, forming a uniform surface layer with nonlinear conductivity. After fabrication, electrical evaluations, including electrical simulations and flashover tests, were conducted to verify the E-field regulating effect of novel insulators. As the surface layer thickness of the novel insulator increases, the maximum E-field strength in the flashover region declines and converges to a stable value, but the conduction loss continues growing in the surface layer. The E-field concentration in the flashover region can be greatly relaxed by these novel insulators with 0.1 and 0.25 mm surface layers. Flashover test results show that the novel insulators have better insulation performance than that of the conventional insulator under different voltage waveform applications, i.e., gradually increasing voltage, DC voltage superimposed with impulse voltage, and polarity reversal voltage. By applying such novel insulators, a higher reliability and compacter structure can be realized for the DC-GIL. [ABSTRACT FROM AUTHOR]

Evolution and operation of modern grids and networks necessitate reconsideration of several characteristics. For example transformers operating in power grid or in industrial environment are subjected nowadays also to transients or stimulus with different waveforms. Traditionally, in case of power transformers the stresses are related to lightning and switching phenomena. In industrial networks distribution transformers are exposed to many switching operations generating often overvoltages. Transformer windings subjected to such system disturbances are stressed due to the non-linear transient voltage distribution along the winding and potential resonance phenomena. Initial voltage distribution, reflects mainly the capacitive coupling between turns, windings, core and tank. The non-uniform distribution of voltage results in concentred stress in certain parts of the winding. Winding resonance might occur, if the frequency spectrum of incoming surge matches the winding natural frequencies. Therefore external transients occurring in power systems might trigger internal overvoltages with large maximum value in transformer windings. The frequency characteristic of the transformer winding may determine those regions in the frequency spectrum. The paper presents results of voltage distribution measurements performed on model windings with different constructions subjected to voltages with different rise time. The experimental setup allows for investigation of influence of the dielectric (air/oil) and temperature on the stress distribution along the winding. Also the impact of oil and temperature on frequency characteristics of internal voltages are shown. The presented results might be used both for design and optimization of transformers windings. [ABSTRACT FROM PUBLISHER]

This paper reports on an investigation into the surface flashover of an insulator in a C4F7N/CO2 mixture which is used as an alternative to SF6. To date, no studies on this aspect have been reported in the literature. This study was done for a uniform field using a 220 kV AC experimental platform. The influence of molar fraction, gas pressure and creepage distance is reported and compared to that in SF6. The results show that small quantities of C4F7N mixed in CO2 can increase the surface flashover voltage but, as the fraction of C4F7N increases, saturation of the surface flashover occurs. The relative surface flashover voltage of 13.0% C4F7N/87.0% CO2 is approximately 80% that of SF6. The empirical formulae on the critical flashover electrical field has been obtained, which may be referenced in the design of environment friendly high voltage equipment. [ABSTRACT FROM AUTHOR]

Field grading materials (FGMs) with nonlinear conductivity, which can form a uniform local field in high-voltage direct current (HVDC) cables, have received considerable attention. This paper focuses on the temperature-dependent charge transport property of an FGM based on high-temperature vulcanized (HTV) silicone rubber (SiR). A nonlinearity in the conductivity is achieved by mixing silicon carbide (SiC) particles with the SiR matrix. The conductivities of the samples are measured at temperatures of 30–90 °C. In addition, the surface potential decay is analyzed. The experimental results show that the nonlinear conductivity is suppressed at high temperatures. Increased temperature can cause a lattice vibration, which scatters carriers and reduces the conductivity. With an increase in the homopolar impulse voltage, all samples experiences a rise in the initial surface potential. The increase in SiC content and the increase in impulse voltage can both accelerate the decay process. The heteropolar impulse voltage not only intensifies the accumulation of surface charge but also slows down the decay process. Neutralization occurs immediately between heterocharges. The positive charges are captured by traps, and the attraction between heterocharges is thought to be the reason for the observed phenomena. [ABSTRACT FROM AUTHOR]

The epoxy resin insulation in saturable reactors in a HVDC transmission converter valve is subjected to a bipolar exponential decay pulse voltages under normal operating conditions and its aging mechanism is still unclear. In this paper, an electrothermal aging experiment of epoxy resin was carried out under bipolar exponential decay pulse voltages at high temperature. The dielectric properties of epoxy resin before and after aging were measured using frequency-domain spectroscopy. To quantitatively study the aging characteristics, the double-relaxation Cole-Cole model was used to fit the dielectric spectra. As a result, an exponential function relationship between the relaxation time constant and the aging time of epoxy resin under various aging conditions was obtained. This relationship can be used as an empirical formula to evaluate the degree of aging. Furthermore, a comparison of the aging life characteristics of epoxy resin under pulse and sinusoidal voltages with the same peak value was performed. The results show that the electrothermal aging rate of epoxy resin under a pulse voltage is lower than under a sinusoidal voltage. The effect of the measurement temperature on the insulation life evaluation of epoxy resin was also analyzed. [ABSTRACT FROM AUTHOR]

Nonlinear conductivity composites can be applied to modify the local electric field distribution and accelerate the surface and space charge dissipation in HVDC cables. This paper focuses on the effect of the nonlinear conductivity on the charge transportation under impulse superimposed on DC voltage. The surface-charge-accumulation property of silicone rubber (SiR)/SiC composites under impulse superimposed on DC voltage is studied. The surface potential decay (SPD) is measured after the sample is stressed by impulse superimposed on DC voltage. The effect of nonlinear conductivity on carrier mobility is analyzed. The results show that under DC voltage, the decay rate increases for the rise in the initial surface potential (ISP). Higher surface potential caused by higher impulse voltage leads to a larger nonlinear conductivity, which immediately dissipates the surface charge. The increase in DC voltage is shown to be more effective in accumulating surface charge than impulse. The vertical movement to the ground and the horizontal movement of surface charges are two factors affecting the ISP and SPD, which are both affected by the nonlinear conductivity of SiR/SiC composites. [ABSTRACT FROM AUTHOR]

In order to scale down the dimensions of gas insulated line (GIL) insulators and to improve their reliability at ultra high voltage (UHV), considerable research has been conducted to raise the flashover voltage of the insulators. Usually, a non-uniform electric field distribution is an important factor triggering flashovers. Under AC and impulse voltages, the application of functionally graded materials (FGM) with spatial distribution of dielectric permittivity (ε-FGM) has been shown to be an effective solution. But at DC, the electric field distribution depends not only on the conductivity of the dielectrics but also the charge distribution on the surface. Based on the cone-type insulator, this paper reports on the surface charge accumulation and DC flashover of epoxy (EP)/graphene (GR) coated insulators in air. The EP/GR coated insulators with different filler amounts were prepared to test their surface charge accumulation and flashover characteristics. The results show that the uniformly distributed homopolar surface charge is helpful in reducing the peak electric field thereby raising the flashover voltage. The 0.1% EP/GR composite has the slowest surface potential decay process. Accordingly, the 0.1% coated insulator has the highest flashover voltage among the other EP/GR coated insulators and uncoated insulator. The bipolar surface charge regions caused by wire-type metal particles distort the electric field distribution. Flashovers of the 0.1% coated insulator occurs at lower voltage than the uncoated insulator when a wire-type particle attaches to the surface. [ABSTRACT FROM AUTHOR]

C5-PFK has attracted much attention as one of the most promising SF 6 substitutes for medium voltage (MV) power equipment because of its high insulation strength and low global warming potential (GWP). In this paper, the dielectric properties of C5-PFK-Air and C5-PFK-CO 2 mixtures are experimentally investigated, to explore the most effective solution for C5-PFK mixtures for application in MV power equipment. The basic physical properties of C5-PFK and its mixtures are calculated and compared with those of C4-PFN based gases and SF 6 -N 2 mixtures. The GWP and saturated vapor pressure for gas mixtures are calculated, and a relative value R-GWP is defined to evaluate the greenhouse effect at specific pressure. The 50% breakdown voltages of C5-PFK-Air and C5-PFK-CO 2 mixtures under lightning impulse voltage are determined experimentally. The mixture ratio and filling pressure are determined taking into account of the ambient temperature and pressure for MV apparatus. The possibility of practical applications for these mixtures are analyzed by combining the saturated vapor pressure characteristics and the breakdown electric field strength. The synergy effects of the C5-PFK-CO 2 and SF 6 -N 2 mixtures are greater than those of the C5-PFK-Air and C4-PFN-CO 2 mixtures. The C5-PFK-Air and C5-PFK-CO 2 mixtures have higher dielectric strength than SF 6 at 0.1 MPa when the C5-PFK is about 28% under the temperature limitation of −5°C. The pressure increments of 0.014 and 0.066 MPa are required for C5-PFK-CO 2 and C5-PFK-Air mixtures, respectively to reach the dielectric strength of pure SF 6 at 0.1 MPa under the temperature limitation of − 25°C. [ABSTRACT FROM AUTHOR]